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Published by Martin Zustak 2013
Copyright © Martin Zustak 2013
All rights reserved. This book or any portion thereof may not be reproduced or used in any manner whatsoever
without the express written permission of the publisher except for the use of brief quotations in a book review.
First published in Ireland in December 2013
Second edition, August 2014 (version 2.8)
ISBN 978-0-9575657-0-8
www.martinzustak.com
twitter.com/mzustak
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There is no end to the knowledge you can get
or the understanding or the peace
by going deeper and deeper.
Ayrton Senna
2
Preface
I never met Ayrton Senna but I was a fan. Since his untimely death on May 1, 1994 at Imola, every book
imaginable has already been written about him, except for one. The subject of that missing book is contentious
because it addresses the fundamental question of what caused Senna’s car to veer off the track at 300km/h on
Lap 7 of the San Marino Grand Prix.
Tamburello was conceived more than fifteen years ago as a private project to fulfil my own need for
understanding. The original project had grown in fits and starts and lay dormant for over a decade until its
resurrection when I was approached to publish again my old material on Imola ’94. What began as minor revision
of a short article has metamorphosed into a book.
I’m painfully aware of the fact that - for many understandable reasons - the cause of Ayrton Senna’s accident
remains a sensitive subject, but I feel an obligation towards the wider world to share what I’ve discovered.
Any inaccurate assumptions or misinterpretations of proprietary design specifications, measurement
inaccuracies, errors and omissions are entirely mine.
Martin Zustak
December 2013
3
Acknowledgements
My foremost thanks to the late Christopher Hilton, whose numerous books on Senna and motor sport have been a
source of inspiration. I still fondly remember our phone conversations and the visit to his family home in Hearts,
UK, back in the spring of 2000.
In compiling this book I have drawn background and quoted from several published works. All the relevant
credits are indicated in the text and referenced in the Bibliography. For permission to quote, I formally thank
Helen Wilson of The Guardian newspaper, and Graham Cook of Haynes Publishing.
My most grateful thanks to Paul-Henri Cahier, who supplied the cover photo; to Remi Humbert and Joachim
Kutt of GurneyFlap.com, who kindly allowed me to reproduce the close-up shots of Williams’ 1994 car; to Giorgia
Buselli, Darragh Colgan, Tycho Schenkeveld, Dalin Vyskovsky, and Martin Kyncl for their generous assistance;
and to all those who willingly helped but did not wish to be named in the book.
Thanks, also, to the many motor racing enthusiasts whose videos on YouTube have helped massively in research.
I am especially indebted to Mark Clair, the founder of RaceRecall in Victoria, Australia, for allowing me to
reproduce and annotate the Mustang 350GT accident sequence.
I’m most grateful to Patrizia Coluccia, the friendly Press Officer at CINECA in Italy, who kindly gave me
permission to enhance the original CINECA material and videos. The book would not have been possible without
her support.
I owe a particular debt to Rini Ruitenschild, CEO of Hydroline Powersteering B.V. in the Netherlands, for his
valuable comments and for putting me in touch with the incredible Tony Woodward, CEO of Woodward Machine
Corporation in Casper, USA. Tony generously gave his time and expertise and dealt patiently with my questions
about the ins and outs of power steering. This book would have been hollow without him.
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Fact file
AYRTON SENNA
1960 – Born on March 21 in Sao Paolo, Brazil, to Neyde and Milton da Silva.
1964 – Gets a go-kart from his father and inadvertently discovers a world which fulfils his inner needs for control
and detail.
1979 – Go-karting race in Jesolo, Italy. Overtaken by his experienced teammate Terry Fullerton and finishes
second. Back in the hotel, pushes Fullerton into the swimming pool.
1981 – Norfolk, England. Moves to single-seaters, winning two Formula Ford championships. Unexpectedly decides
to quit at the end of the season and flies home to Brazil.
1982 – Returns to England, divorced and fully committed to being a professional racing driver.
1982 –Rejects an offer from Ron Dennis of McLaren to race in the junior F3 category because the contract doesn’t
guarantee him a future seat in F1.
1983 – Graduates to F3 on his own terms and wins the first nine races.
5
Fact file
AYRTON SENNA
1984 – Arrives to F1 with the midfield Toleman team. Makes his mark in the rain-soaked Monaco Grand Prix by
finishing second to Alain Prost in the McLaren.
1985 – Estoril, Portugal. Scores his maiden F1 win in torrential rain in only his second race with Team Lotus,
almost lapping the whole field in the process.
1986 – Jerez, Spain. Amazes the Lotus engineers by being able to recall the car’s speed and revs at every corner
around the circuit. In qualifying, predicts his own time to a fraction of a second.
1986 – Detroit, USA. Skips the mandatory press conference after setting pole position in qualifying in order to
watch Brazil lose in the football World Cup against France. Wins the race the next day and waves the Brazilian flag
on his victory lap.
1987 – Monza, Italy. Announces a deal with Ron Dennis to join McLaren alongside two-time World Champion Alain
Prost. Stays for the next six seasons.
6
Fact file
AYRTON SENNA
1988 – Monte Carlo, Monaco. Fastest in qualifying by an astounding margin, 1.427s clear of teammate Alain Prost.
Later confesses to having an out of body experience on the record lap. Crashes out of the race while in an
unassailable lead and storms off straight into his nearby apartment.
1988 – Suzuka, Japan. Clinches his first World Championship by charging from fourteenth to first.
1989 – Monte Carlo, Monaco. Refuses to speak to Alain Prost following a controversial overtaking move on his
teammate at the previous race at Imola, and Prost’s subsequent comments in the press.
1989 – Suzuka, Japan. Has to win to keep his title hopes alive. With six laps to go, overtakes Alain Prost for the lead
at the chicane but Prost turns into him and the two McLarens collide. Senna re-joins the race and goes on to win,
only to be disqualified for missing the chicane and causing the collision.
1990 – Suzuka, Japan. Prost – now driving for Ferrari - has to win to keep his title hopes alive. Senna takes justice
into his own hands and deliberately crashes into Prost at the first corner to clinch his second World Championship.
1991 – Sao Paolo, Brazil. Maiden win on home soil despite being stuck in sixth gear and suffering unbearable
muscle cramps in the last ten laps. Goes on to win the Championship for the third time at the end of the season.
7
Fact file
AYRTON SENNA
1992 – Spa, Belgium. First on the scene of Erik Comas’ 300km/h accident. Stops the car and rushes to help the
briefly unconscious Frenchman. Attends to Comas and holds his head until the paramedics arrive.
1992 – Plays second fiddle to Nigel Mansell in the Championship as his McLaren is no match for the technologically
superior Williams car.
1993 – Donington, England. Excels in the rain in the underpowered McLaren and delivers the most memorable
opening lap in motor racing’s history by going from fifth at the start to first at the end of Lap 1.
1993 – Estoril, Portugal. Confirmed as Prost’s replacement at Williams for the following season.
1993 – Adelaide, Australia. Wins his 41st F1 race in his final outing for McLaren, and afterwards is invited by
singer Tina Turner on stage for ‘Simply the Best.’
1994 – Asks sister Viviane to set up what would become the Ayrton Senna Foundation, which helps millions of
underprivileged children in Brazil.
1994 – Imola, Italy. Crashes fatally while leading the San Marino Grand Prix.
1994 – Sao Paolo, Brazil. Laid to rest at Morumbi Cemetery on May 5. An estimated three million people attend his
funeral procession.
8
Prologue
Ayrton Senna has been dead for a generation, yet the
cause of his accident remains a mystery. The question
is - does it matter?
Common sense says let him rest in peace:
establishing the cause is a mission impossible. The
safety lessons have been learned and since the
tragedy, F1 enjoyed its longest ever spell without a
fatality. Further analysis is not going to bring Senna
back or help his family, and the Italian legal system
had its day in court but grudgingly conceded that
racing cars are prototypes just like the space shuttles
Challenger and Columbia: when people and
components are pushed to the limit, mistakes and
accidents are inevitable.
information. Would he accept that the world of motor
racing has shied away from trying to resolve the
conundrum of his death, or would he insist on going
through every number, every telemetry trace, and
every piece of information again until he was
convinced there was no new knowledge to be gained?
Curiosity is fundamental to the human condition,
and the surest way of attracting someone’s attention
to a problem is to tell them it’s not solvable.
Gigabytes of data about the accident are scattered
across cyberspace and in print but, at least outside
the courtroom, they’ve never been brought together
in a comprehensive fashion. This book is an attempt
to achieve that, and to do Senna’s legacy justice by
approaching the subject with single-mindedness and
attention to detail of which the late Brazilian would
hopefully approve.
On the other hand, Senna never gave up. Suzuka
1988, Suzuka 1989, Donington 1993, and Sao Paolo
1991 particularly spring to mind. He was also the
ultimate perfectionist who left no stone unturned in
his quest to understand the car and its behaviour on
the track - often to the point of mentally exhausting
his engineers – and it is a cruel twist of fate the last
seconds of his life remain obscured by imperfect
9
CHAPTER 1
The driver
At the age of 34, Senna was at his peak as a racing
driver. He was fit and in perfect health: the tests
ruled out a blackout and confirmed that he hadn’t
taken any drugs or banned substances. According to
comments in the press, he went into the 1994 San
Marino Grand Prix with a higher than usual heart
rate, which was almost certainly an effect of his
troubled frame of mind.
Senna had been working hard to setup his emerging
business empire, and due to the time zone difference
between Europe and Brazil, he was often on the
phone late at night. He was in love with model
Adriane Galisteu, whom he’d been dating since the
1993 Brazilian Grand Prix, but their relationship met
with the disapproval of his family and Senna was
under pressure to split with her.
After six glorious years at McLaren, which yielded
three world championships, 35 wins and 46 pole
positions in 96 races, Senna was still going through
the induction process at Williams, where he found
the culture colder and much different from what he
had been used to. Having forced his arch-rival Alain
Prost into retirement at the end of 1993 by joining
the Williams team, Senna now also lost his main
target and source of professional motivation. He
appeared softer, as if he had lost his killer instinct.
The new Williams car proved troublesome and its
unpredictable handling and constantly shifting
balance forced Senna to dig deep in order to squeeze
competitive lap time out of it. He spun out of the first
race of the season in Brazil and was punted off in the
first corner in Japan while his younger rival Michael
Schumacher driving for the Benetton team won both
races. Senna was heading to Imola for the San
Marino Grand Prix on the back of his worst ever start
to a championship: zero points against Schumacher’s
perfect twenty.
After his retirement in Japan, Senna closely observed
the behaviour of Schumacher’s car on the track and
became convinced that the Benetton exploited illegal
traction control and launch control systems. He was
incensed that the battle was not fought on equal
terms. He complained bitterly to several close friends
during the fateful Imola weekend and, as stated by
former McLaren principal Ron Dennis, the prospect
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of racing an illegal car was at the forefront of Senna’s
thoughts immediately before the start1.
And then there were the feelings of his own fragility
and mortality, awakened by the death of fellow racing
driver Roland Ratzenberger during the second
qualifying session the day before. It was the first
fatality during a Grand Prix weekend in twelve
years and sadness, doubt, premonition, and fear
must have, at least temporarily, entered Senna’s
mind. But the most plausible scenario is that once the
lights turned green, Senna’s famous powers of
concentration prevailed and when he met his destiny
in the Tamburello corner he was fully focused on one
thing and one thing only – winning.
Figure 1 - Ayrton Senna
minutes before the start
of his last race (photo
Paul-Henri Cahier).
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CHAPTER 2
The car
Due to regulation changes mandated by the motor
sport federation FIA, the new Williams car designated FW16 - was forced to shed most of the
electronic gizmos that had made its predecessors so
dominant, and initially it struggled to adapt to life
without active suspension, traction control, launch
control, and ABS brakes. It featured pushrod
operated ‘passive’ suspension with double wishbones,
a torsion spring at the front and a coil-spring at the
rear. The car was powered by a normally aspirated
3.5-litre Renault V10 engine that revved to
14,300rpm6 and produced around 790hp6. Senna had
been driving chassis number 02 since the beginning
of the season. At Imola, he ran the same basic set-up
as teammate Damon Hill7 and he was fuelled to stop
twice during the race. Senna’s main rival Michael
Schumacher planned three stops, which made his car
lighter and potentially faster in the early stages of the
race.
The FW16 ran on Goodyear tyres fitted to 13’’ wheel
rims, 13’’ wide at the front and 15’’ wide at the rear.
Like all high performance tyres, the Goodyears were
sensitive to temperature fluctuations and even a
slight change in the ambient temperature or a sudden
cloud could result in a 1.5psi drop in tyre pressures8.
The 1994 San Marino Grand Prix took place on a
warm spring day in dry conditions, and at racing
speeds Senna had no trouble keeping his tyres in the
optimal temperature window in which they stay
properly inflated and provide the best possible grip.
Paradoxically, the aerodynamics were the FW16’s
Achilles heel despite Adrian Newey’s presence as
chief designer. The team ran active suspension in the
previous two years and developed the aerodynamics
around it, so the new FIA regulations that stipulated
passive springs and dampers were a major setback.
Shifts in aerodynamic balance on the passive car
were much more pronounced than on its active
predecessor and the design team didn’t fully
compensate for that. The FW16 ended up being
hypersensitive to changes in ride height and
aerodynamically unstable9. Also, the sidepods were
too long85 and the front wing sat too close to the
ground10. To address these aero issues, the team
introduced a raft of modifications for Imola,
12
Figure 2 – Ayrton Senna at speed in his Williams FW16-Renault, Imola 1994 (photo Paul-Henri Cahier).
13
including a raised front wing, slightly taller front
wing end-plates, reshaped cockpit, and a revised
wheelbase.
During the pre-season testing of the new Williams, it
quickly dawned on Senna that the car’s ergonomics
had Nigel Mansell’s DNA written all over it - the
monocoque was an evolution of the highly successful
FW14B model that Mansell won the world title with
in 1992. Senna preferred to sit as low as possible in
the cockpit to have less turbulence around his head11;
and because he sat low the steering wheel needed to
be positioned lower, too. He drove with his fingertips
and liked a big steering wheel with a thin rim, which
offered the best grip for his small hands. The bigger
steering wheel (280mm in diameter as opposed to
Mansell’s customary 240mm) couldn’t be
accommodated because of the tightly packaged
cockpit of the FW16, but at least the Williams
engineers agreed to alter the steering position.
The requested changes necessitated redesign as the
old specification would have protruded to the area
forbidden by the FIA regulations and would have
obstructed the driver’s timely exit from the cockpit in
an accident. The column was cut in half and a
machined piece was inserted in the middle so the
design now consisted of three elements rather than
just one. The whole steering assembly – nine
components in total – went to production after
March 10, 1994, and took the different departments
within Williams two to three days to make. The
assembly was inspected to ensure it had been
produced as per design specification and it also
underwent quality checks before being assigned a
part number and stored in the warehouse. Three new
columns were then shipped to Brazil for the first
Grand Prix of the season and put not only on Senna’s
car but also on Damon Hill’s and the spare12.
After Senna’s collision with Nicola Larini during the
second race in Japan at Aida - where Larini’s Ferrari
T-boned the stranded Williams into retirement - the
steering column was subjected to tests by penetrating
liquids in the Williams factory. No cracks were found
and the column was put back on the car for the San
Marino Grand Prix3.
According to Williams, the steering column design
involved a considerable amount of flexing and the
steering wheel (made of carbon fibre) would also
flex13. Three years after the accident, the engineers
conducted a demonstration using one of the FW16s
on display in the team’s museum, with Senna’s
replacement David Coulthard behind the wheel.
In the video footage later made public by National
14
Geographic Channel in their TV documentary series
Seismic Seconds14, Coulthard is seen bending the
steering wheel by several centimetres both vertically
and horizontally.
Little information has surfaced about the power
steering system since the accident. Williams drew on
their experience with active suspension and in 1994
pioneered electrohydraulic power steering. The
system sensed strain momentarily existing in the
steering assembly and converted it to an electrical
signal, which was then suitably amplified and fed into
a solenoid, which in turn operated a linear hydraulic
servo valve. In response to this signal, the valve’s
ports controlled the hydraulic fluid flow to and from
the pistons in the steering rack and assisted the
driver.
The team couldn’t change any settings remotely by
issuing commands while the car was on the track.
The main data logging unit (the black box) collected a
range of parameters about the state of the car’s vital
systems including dynamic behaviour and
environmental variables such as g-forces. The
content of the black box could be downloaded to a
computer using a three-pin connector and a data
card. Engine supplier Renault operated their own
data logging unit - installed behind the cockpit - and
Williams used its spare capacity to piggyback on
some of the main unit’s telemetry channels. Data
from the previous lap was beamed every time the car
passed the pits, the memory banks then reset and
started recording data from the next lap.
Electronics on the FW16 were largely confined to the
engine management system, semi-automatic gearbox
(six forward gears and a reverse), cockpit dashboard,
radio communication with the pits, electrohydraulic
actuators, sensors, and car telemetry. Williams
employed one-way telemetry, which meant that data
from the different sensors installed around the car
was collected and radio-transmitted in regular
intervals to the team’s computers in the pits.
15
CHAPTER 3
modifications were only carried out in time for the
1995 race as a result of Senna’s death.
The track
The track at Imola had been used for F1 in the same
configuration since 1981 when it hosted the inaugural
San Marino Grand Prix. Over the years, four highspeed accidents similar to Senna’s took place in
Tamburello, although not at exactly the same spot:
Nelson Piquet (Williams) suffered concussion after a
left rear tyre failure during qualifying for the 1987
San Marino Grand Prix, spinning twice before hitting
the wall sideways; Gerhard Berger (Ferrari) was
knocked unconscious and received second-degree
burns in the 1989 race when his car burst into flames
after the front wing had failed; Michele Alboreto
(Footwork) ended up with several stitches in his leg
after a test shunt before the 1991 race when his front
wing collapsed; and Riccardo Patrese (Williams)
suffered concussion and minor lower spine injuries
during a test crash before the 1992 race, which was
caused by a right rear tyre failure. Despite all these
accidents (whose footage can be viewed on YouTube),
the corner remained a flat out left-hander and
Figure 3 - Imola circuit, Autodromo Enzo e Dino Ferrari.
Darker circuit outline represents the eight strips of new
tarmac laid between the start-finish line and the exit of
Tamburello.
The track surface was old and bumpy, and to improve
the situation eight short strips of new tarmac had
been laid after the 1993 race between the start-finish
line and the exit of Tamburello. Nonetheless, during
the test two weeks prior to the 1994 race drivers still
complained about a small dip in the middle of the
track, which created an unevenness of three to four
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centimetres, and Senna personally alerted circuit
director Giorgio Poggi. It wasn’t feasible to resurface
this section of the track so close to the race, but Poggi
at least managed to get it smoothed out3. The repairs
only slightly improved the situation and there still
was a bump. According to former Minardi driver
Pierluigi Martini, there was only one line through
Tamburello and the bump could not be avoided; the
cars touched the ground and were unsettled, but the
bump effect was normal and the driver just had to
hold his line3,4.
FIA inspections two months prior to the race and
then again on the Wednesday before the race.
Nothing of concern was reported3.
The run-off area on the outside of Tamburello
consisted of a narrow strip of grass followed by
concrete that extended to an unprotected wall. This
limitation was dictated by nature as immediately
beyond the perimeter wall ran the Santerno River
and its course could not be easily altered. There was
an angle between the track and the run-off: the
average gradient of the track was 3.1 percent,
whereas the average of the run-off area was only 2.1
percent5, which made braking on the concrete less
effective.
The wall was not protected by old tyres because at the
time the FIA considered a concrete wall the safer
option in corners where impacts were likely to occur
at a shallow angle. The Imola circuit passed official
17
CHAPTER 4
The race
Sixty-one laps of the 5,040km Imola circuit awaited
the drivers as they lined up in sombre mood for the
start of the 1994 San Marino Grand Prix. For the
third race in succession, Ayrton Senna qualified his
Williams on pole position, with Michael Schumacher
in the Benetton yet again alongside him. It was a
variation on one of life’s universal themes: the
established star resisting a challenge from the young
pretender.
When the lights turned green, Senna made a perfect
getaway, leaving two thick layers of rubber and the
other twenty-four competitors behind him. But
farther down the field, there was mayhem. JJ Lehto
(Benetton) in fifth stalled on the grid and was struck
by the unsighted Pedro Lamy (Lotus). Wheels and
bodywork were flying everywhere, injuring spectators
in the grandstands and littering the straight with
sharp pieces of carbon-fibre. The stricken Lotus
ricocheted down the track, but somehow both Lehto
and Lamy escaped unharmed and the race control
decided to deploy the safety car. The race would have
been stopped in similar circumstances in the past,
but the safety car alternative was introduced in 1993
to avoid disruption to TV schedules and to spice up
the often processional racing.
Although the marshals worked hard to clear the
track, the cars had to weave through the carnage on
four separate occasions. While most drivers chose the
middle of the track or the relatively cleaner side by
the pitwall, on Laps 2, 3, and 4, Senna drove on the
outside line where the chances of picking up some
debris were the highest. The slow pace of the Opel
saloon used as safety car prevented Senna from
generating enough heat into the tyres, and the tyre
pressures on his Williams must have dropped
considerably.
On Lap 5, the safety car extinguishes its roof lights.
Senna’s radio crackles with the voice of race engineer
David Brown confirming the imminent restart. Senna
acknowledges the message; there are no complaints
about the car. He holds back while the Opel peels off
into the pits, then floors the throttle and accelerates
hard onto the start finish straight, catching every
driver other than Schumacher napping. The timing
screens update with fresh gaps at the start of Lap 6:
Schumacher -0.556s in second, Gerhard Berger
18
(Ferrari) -2.586s in third, Damon Hill (Williams)
-5.535s in fourth. It’s looking good. Get more heat
into the tyres. Build a gap. Focus on the track ahead.
Ignore Schumacher in the mirrors.
Senna clears the new tarmac past the control tower,
the skid-plates under his Williams producing a
shower of sparks on the first and third strips. He
takes the tight inside line through Tamburello and
the rear of the car kicks up five ominous plumes of
molten yellow sparks as he rides over the longest
seventh strip. There is another flash on the exit of the
eighth strip: the tyres are cold but he has navigated
Tamburello and rushes towards the Tosa hairpin,
past the scene of Roland Ratzenberger’s fatal
accident the day before.
to 0.675s over Schumacher. Deft flicks of the paddle
under the steering wheel engage the semi-automatic
gearbox into third, fourth, fifth, and sixth. He will not
change the gear again. The Renault black box beams
the latest set of data to the computers on the pitwall
and the telemetry clock resets to 0.00s. It is May 1,
1994, 14.17hrs, the beginning of Lap 7.
At four seconds, Senna passes under the start lights
gantry and sweeps past the control tower, its red and
white colour scheme synonymous with his best years
at McLaren. He is now entering the parkland of
Imola, a solitary place where tall trees outnumber the
spectators and where the afternoon sun casts long
shadows across the track.
The technical section of Piratella, Acque Minerali,
Variante Alta, and Rivazza is next, the corners
coming at the driver in quick succession. The
Williams handles surprisingly well considering that
the tyres must be still getting up to temperature;
Senna’s hand movements are smooth and there is no
snap oversteer or understeer that would require
significant correction with the steering.
At four-and-a-half seconds, Senna flattens a piece of
blue debris from Lehto’s Benetton2, which is still
lying in his path despite the extensive clean-up
operations. He lets the Williams drift towards the
outer edge of the track in order to minimize tyre
scrub and straighten the upcoming Tamburello
corner. The skid-plates graze the tarmac on the first
dark strip, spewing sparks back at the pursuing
Schumacher.
He threads his way through Variante Bassa and
Traguardo at the end of the lap and extends his lead
Between six and eight seconds, the picture from the
on-board camera mounted above Senna’s left
19
shoulder gets briefly distorted as the car shudders
crossing the third strip of new tarmac. Behind
Senna’s back, the Renault black box goes impassively
about recording data arriving from the different
sensors installed around the car. He is on the short
straight before Tamburello, the mouth of the corner
beckoning and approaching at eighty metres per
second.
At 8.22s, Senna has travelled 582 metres since
breaking the start-line timing beam, his foot full
down on the throttle, the car doing 302km/h. As he
begins the initial turn-in to Tamburello, he places the
left front wheel by the white inside kerb and the
lateral force builds gradually from 0.37G to 2.45G
(at 9.30s).
tyres while cornering. Engine revs have been rising
gradually and now plateau at nearly 14,000rpm; the
speed touches 307km/h. The longest seventh strip of
new tarmac is about 60 metres away.
Since 10.00s, the lateral force has been decreasing
gradually from 2.92G to 2.01G (at 10.80s), indicating
reduced turning left. Shortly before entering the
seventh dark strip, however, Senna makes a
correction towards the left (10.90s) and the lateral
force starts climbing steeply until it reaches the
maximum recorded by the telemetry (3.52G at
11.20s).
The Williams travels across the fifth dark strip of new
tarmac (9.56s), the skid-plates churning sparks both
on the entry and exit. The floor of the car hits the
ground – ‘bottoms out’ (revs jump to 14,204rpm at
9.68s) - and then the car recovers grip (revs drop to
13,561rpm at 9.72s). Moments later, there are more
sparks on the sixth strip (10.26s).
At 11.08s, Senna has travelled 825 metres from the
start line. His Williams flings back a flurry of sparks
as he hits the surface change from old to new tarmac
and the first bump on strip seven. The on-board
images are distorted and their quality is getting
progressively worse until 11.16s. The speed is up to
309km/h and the car keeps turning left, so much so
that by 11.16s it is facing the grass verge that
separates the track from the Armco barrier on the
inside of the corner.
Senna’s foot is still full down on the throttle but the
car has stopped accelerating because of aerodynamic
drag imposed by the rear wing and the scrub of the
In the cockpit, Senna realises something has gone
wrong. There is no margin for error in Tamburello:
he is travelling at 86 metres per second, yet a mere
20
100 metres separate him from a concrete wall. Does
he have time to grasp instinctively what is happening
to him?
Assuming the reaction time of top F1 drivers is
around one tenth of a second, 11.10s is the moment
he decides to lift and at 11.20s he is already
responding. For the first time on Lap 7, his right foot
is not on full throttle (99.4 percent), the engine revs
drop by 350rpm to 13,445rpm (at 11.24s), and the car
begins to slow down. Simultaneously, the Williams
straightens its trajectory.
There are two short flashes from the underbody
which correspond to the remaining two bumps on
strip seven, and the on-board images are distorted
accordingly at 11.22s, 11.26s, and 11.28s. The
Williams bottoms out on the third, biggest bump
(revs spike to 14,077rpm at 11.26s); at this point,
however, the car is already out of control. It stays on
the track for another 65 metres, lasting less than one
second.
At 11.30s, Senna has covered 10 metres since he
began to lift, and his speed reduces to 306km/h. He
hesitates with the throttle for a couple of tenths of a
second - the values oscillate around 50 percent until
11.42s – and then completes the lift. At 11.50s, the car
is at zero throttle after the throttle damper has done
its work. The revs dip below 13,000rpm and the rear
wheels lock up temporarily as they rotate notably
slower than is the speed of the car (the difference is
17km/h).
Now Senna has abandoned the curve, deviating some
nine degrees from the normal racing line, and the
lateral force keeps falling from the maximum
recorded by the telemetry to virtually zero. His
helmet lunges forward and tilts slightly to the left,
which is confirmed by the angle of the Nacional
banner and the visor in the rear-view mirror.
During 11.50-11.68s, Senna is shifting his right foot
from the throttle to the brake pedal and hydraulic
pressure is still building in the braking system. In the
middle of this lull in driver input, he exits the seventh
strip of new tarmac and the floor of the car generates
another flash of sparks (11.58s).
At 11.68s, he has covered 41 metres since he began to
lift, and is about to enter the final eighth strip of new
tarmac. No brake pedal telemetry exists, but the
steep increase in deceleration (up to 4.7G by 12.08s)
and the rapid fall in engine revs (down to 10,200rpm
by 12.10s) reveal the onset of hard braking: from this
21
moment on, Senna is trying to reduce the speed of
the inevitable impact.
He releases the brakes briefly during 11.74-11.76s,
which is visible on the telemetry as a blip in
otherwise steeply increasing deceleration, and his
head is moving back towards the headrest. The onboard picture gets distorted and the rear of the
Williams sparks again as it exits the eighth strip at
11.98s.
In the eight-tenths of a second since he began to lift,
Senna has travelled 62 metres and has managed to
slow down from 310km/h to 260km/h. His head
moves forward again due to hard braking, which
stops only when he runs out of road and enters the
grass (12.16s). The dynamic weight of the car shifts
forward onto the front axle, and the Williams is
pulling gently right in the last three tenths of a
second on the track (the lateral force turns negative,
-0.15G).
At 12.16s, about 25 metres separate him from the
wall. Senna leaves the track and drops three wheels
on the grass. The change in surface makes the
braking less effective (deceleration drops to 1.5-3G).
The car gets momentarily airborne and bounces from
side to side, inducing jolts that produce spikes in the
lateral force (from -0.12G to 1G to 0.33G). Senna’s
feet bounce about the footwell, which shows on the
telemetry as a squirt of throttle (9 percent at 12.12s)
and modulation of the clutch. Simultaneously, the
engine almost stalls (revs drop dramatically from
10,750rpm to 4,200rpm within just 0.24s).
At 12.46s, about 15 metres separate him from the
wall. Senna exits the grass strip and enters the
concrete run-off area. The car twitches from left to
right (there is a 1.8G lateral jolt during 12.50-12.58s),
but once it lands fully on the concrete, the tyres grip
again and the braking improves. Senna’s foot
touching the throttle means the revs fluctuate but are
no longer falling (4,900-6,600rpm after 12.42s).
Two-tenths of a second to impact. Either accidentally
or intentionally, Senna tries one more desperate
attempt to steer away from the wall by pressing
harder on the throttle pedal (7 percent at 12.60s, then
18 percent at 12.80s) and by further modulating the
clutch (up to 8.7mm). The car straightens (lateral
force falls to 0.4G) and reduces its speed to an
estimated 216km/h81.
At 12.80s and 930 metres into Lap 7, the telemetry
goes blank. Senna hits the unprotected concrete wall
at a shallow angle of twenty-two degrees3,5 about
22
three metres past the ‘I Pilotissimi’ sign. He suffers
serious head injuries and dies four hours later in the
Maggiore Hospital in Bologna.
When the first track marshals and doctors reach him,
they notice the steering column dangling uprooted in
the cockpit. To extricate the deeply unconscious
driver from the wreck, they remove the steering
wheel and leave it lying beside the cockpit with the
upper section of the column and one of the cables
still attached to it. But did the broken column trigger
the accident, or did it only break on impact and had
no role to play?
Figure 4 – Imola, Tamburello corner, Lap 7.
23
CHAPTER 5
Video footage
TV cameras captured the accident from three
different angles but the footage is of relatively poor
quality and doesn’t paint a complete picture as to
what happened to Senna. Images from Schumacher’s
on-board camera are grainy, showing the Williams
heading for the wall and disappear out of sight as the
Benetton successfully navigates the corner. The
stationary camera of Italian network RAI positioned
past the exit of Tamburello catches the Williams
already out of control but, thanks to the curvature of
the run-off area, it mercifully misses the full impact
of the right hand side of the car against the wall; and
the on-board tape from Senna’s car ends abruptly
0.9s before the impact.
The on-board footage was first publicly broadcast on
Brazilian channel TV Globo using a VHS quality tape
‘discovered’ by Brazilian journalist Roberto Cabrini.
It ended 1.4s before the impact. Later, an additional
0.5s of footage appeared when a Betacam quality tape
was supplied by Bernie Ecclestone’s FOCA TV
company. Crucially, both tapes end before Senna
veers off the track, leaving question marks over his
last actions in the cockpit.
In 1994, 20 out of 26 cars carried miniature on-board
cameras but live feed was received only from four
cars at a time. On Lap 6, three of the four were
Senna’s Williams, Schumacher’s Benetton, and
Martin Brundle’s McLaren. As Senna was leading and
facing a clear track, ten seconds before the accident
the director decided to switch to the Tyrrell of Ukyo
Katayama. There was a delay of several seconds
because RAI had to be notified about the switch and
Katayama’s camera had to be activated3.
By sheer coincidence, the switch to Katayama’s
Tyrrell took place exactly at the moment when
Senna’s Williams was about to leave the track.
However, the next frame on the tape is not an image
from Katayama’s car but 14 seconds of blurred
pictures and grey lines. According to the testimony of
FOCA TV personnel in court, the switcher pressed
the wrong button and transferred to Berger’s Ferrari
instead. As Berger’s camera wasn’t activated, the tape
only contains interference until the right button was
pressed and footage from Katayama’s car appears17.
Very limited amateur footage of the race exists, but
the images do not reveal anything new.
24
CHAPTER 6
Image processing
CINECA ANALYSIS
Shortly after the accident, the State Prosecutor in
Italy contracted CINECA consortium to process all
available TV footage and telemetry from Senna’s car.
CINECA (Interuniversity Consortium of North
Eastern Italy for Automatic Computing) is a large
scientific computing centre for public and private
research and is comprised of 13 universities. Their
software engineers digitized the footage and
synchronized images from Senna’s car with those
from Schumacher’s and also from the stationary RAI
camera on the exit of Tamburello. Former Ferrari
chief designer Mauro Forghieri then synchronized
the digitized video stream with telemetry data from
the Renault black box3. CINECA prepared three
versions: composite footage from the different
camera angles, on-board footage that incorporates
the track and the surroundings, and on-board footage
that zooms in on the cockpit. The end product was
presented in court as prosecution evidence18. You can
watch the videos on CINECA’s website.
CINECA also attempted to analyse Senna’s steering
wheel movement. It used the yellow button on the
left spike of the steering wheel and the serigraphed
letter ‘V’ (Figure 5) to determine the amount of
flexing and steering lock applied.
Figure 5 - Ayrton Senna’s steering wheel, Williams
FW16/2-Renault, Imola 1994. It differs from the steering
wheels seen on the car later in the season (drawn to scale
based on CINECA’s website19).
25
Figure 6 - Cockpit of the Williams FW16-Renault. The Senna Exhibition, Autosport International ‘98.
26
The yellow button’s ‚normal‘ position is 83mm from
the centre of the wheel (green arc in the videos), the
‚normal‘ position of the ‘V’ is 55mm from the centre
of the wheel (red arc in the videos). The two reference
points were triangulated with the rim of Senna‘s
cockpit and their baseline movement was established
by tracking steering wheel rotation during the safety
car period when both objects describe regular arcs
dictated by the laws of geometry. The actual position
of the yellow button in the video sequence against the
green arc reveals the relative movement of the
steering wheel between the safety car period and Lap
7.
The cockpit is in the shade and the bumpy track
surface creates interference, which makes the yellow
button hard to distinguish in some frames. For the
purpose of our analysis, the original CINECA
sequence was deconstructed frame by frame and a
yellow dot superimposed on the actual yellow button.
No yellow dot was added to frames where the exact
position of the button is uncertain or where it is
completely off the screen. The individual frames were
then merged into a new on-board sequence.
Enhanced yellow button sequence, Lap 7
As CINECA used the safety car period for
establishing the baseline position of the yellow
button, it could be argued that it only captured
steering wheel behaviour at relatively low speeds
whereas Senna’s accident occurred at 310km/h on a
bumpy section of the track. To understand the
behaviour it is therefore important to study not only
the accident itself but also the car’s handling under
normal conditions in the first three races of the 1994
season.
Hours of publicly available footage from Eurosport,
BBC, RAI, TV Globo, TF1, RTL, FOCA TV, and Ayrton
Senna Online Channel20 yield twelve minutes of
images from Senna’s car. On-board footage exists
from qualifying and race in Brazil, from qualifying
and the warm-up session in Japan, and also from the
Imola weekend: the lap from Friday morning practice
during which Senna unexpectedly greeted his old
nemesis Alain Prost, a couple of laps from Friday
qualifying, brief sequences from the Sunday morning
warm-up, four short stints at low speed behind the
safety car, last seven seconds of lap 5, complete lap 6,
and the tragic lap 7.
To analyse Senna’s steering inputs in those first three
races, a white circle of low opacity was superimposed
on the footage (Figure 7) in such a way that the edge
27
was determined by comparing the ‘virtual’ distance of
the yellow button from the serigraphed ‘V’ on the
screen with the actual distance in the real world
(which is known – 44mm, Figure 5). The serigraphed
‘V’ does not appear on the footage from Brazil and for
that reason the rivet below the ‘DATA’ button was
used instead as the reference point (the actual
distance is known as well - 53mm, Figure 5).
Figure 7 - A white circle of low opacity was superimposed
on twelve minutes of on-board footage from Senna’s car to
track steering wheel movement. This demo image shows
two examples of the yellow button flexing with deviations
‘x’ upwards and ‘y’ downwards (background image used
with CINECA’s permission).
of the white circle tracks the movement of the two
yellow buttons located on the spikes of Senna’s
steering wheel. In addition, the white circle was also
triangulated with the rim of the cockpit.
The footage was studied frame by frame and
deviations from the white circle were measured on
the computer screen and then converted to real
distances using a scaling factor. The scaling factor
Analysis of the season opener in Brazil (total footage
duration 3min 20s) reveals two brief instances (0.2s
each) where the yellow button deviates 4-5mm from
the white circle of low opacity. The story is very
similar during the race weekend in Japan (2min 34s
of footage): there are four instances of flexing that
stay within 4–5mm.
At Imola, the footage from the Friday morning lap
(1min 35s) shows two deviations (0.5s each) that
peak at 6-8mm in Tamburello and on the exit of
Piratella. Laps from Friday afternoon qualifying
(58s in total) produce three deviations of 6-8mm on
the exit of Tosa, Piratella and in Tamburello.
Footage from the Sunday morning warm-up (1min
20s) is in line with our observations so far and
reveals two peaks at 6-8mm on the exit of Rivazza
(0.2s) and on the entry to Variante Bassa
28
(1s duration). There is no perceptible flexing during
race laps 2 - 5 (1min 7s) as Senna is going slowly
behind the safety car and is weaving from side to side
in preparation for the imminent restart. In
conclusion, the twelve minutes of on-board footage
from the beginning of the 1994 season until the end
of race lap 5 at Imola show no or minor flexing
(peaks reaching 6-8mm) regardless of speed or track
conditions, and support the reliability of CINECA’s
methodology. The same is also true of the limited
footage available from Damon Hill’s sister car.
ACCIDENT TIMELINE, YELLOW BUTTON MOVEMENT, LAP 7
8.20s: Senna gradually turns into Tamburello. The yellow button begins to descend at a 45 degree angle.
9.86s: Within 1.5 seconds, the yellow button deviates to the level of the serigraphed ‘V’ (the red arc), some
25-27mm from its customary position on the green arc.
10.74s: The yellow button stops dropping and hovers on the edge of the screen.
10.74-11.20s: The yellow button remains on the edge of the screen or just under it, but it is moving from right to
left. It is during this period that Senna loses control.
11.22-11.30s: The yellow button jumps up just above the red arc, and then nearly reaches the original green arc
position in a vertical upward movement.
11.32-11.42s: There is another vertical steering movement, this time in the opposite direction (downwards) –
the yellow button goes from being nearly at the green arc to being completely off the screen at 11.44s.
11.44s: The yellow button drops below the edge of the screen.
11.52s: The yellow button reappears just above the edge of the screen and then is gone again.
11.74s: The yellow button reappears for an instant on the red arc level, but quickly moves down below the screen
after 11.76s. It is not seen until the end of the footage at 11.84s.
29
BOTTOMING EFFECT
Even at rest, the ride height of an F1 car at the rear is
only a few centimetres, and at top speeds it is not
uncommon for the bottom of the car to literally
scrape the ground. The FW16’s underbody was
protected by skid-plates made of titanium, which
generated spectacular showers of sparks every time
the chassis made contact with the ground. This
phenomenon was widespread in the late ‘80s and
early ‘90s, especially in the opening laps of the race
when the cars were still on full tanks and also in
qualifying when teams tried to get away with the
lowest possible ride height in order to maximize
downforce over a single lap (watch some great
examples on YouTube from the Japanese GP 1987,
Belgium GP 1992, or Brazilian GP 1994).
TV footage from the Imola weekend reveals that the
cars were prone to bottoming at four different parts
of the track (Figure 8): through Tamburello, past
Tosa towards Piratella, in the downhill section before
the Acque Minerali chicane, and on top of the hill
approaching Rivazza. Most drivers were affected,
including Senna’s teammate Damon Hill, Pierluigi
Martini (Minardi), and the Ferraris of Gerhard
Berger and Nicola Larini. Berger’s Ferrari
in particular bottomed heavily at Tamburello during
the Sunday warm-up and also following the second
start after Senna’s accident.
The ride heights and tyre pressures at the time of the
accident aren’t known, but it is possible to deduce
their relative changes from the extent to which
Senna’s car bottomed and generated sparks from its
titanium underbody. All things being equal, if the
tyres are cold their circumference is smaller, the ride
heights are lower, the bottoming effect is stronger,
and the car should generate a higher amount of
sparks compared to another lap on which the tyres
are already warmer. That assumes the driver hasn’t
suffered a puncture or a suspension failure, that he
takes a similar racing line through the corner, and
that he doesn’t suddenly lift or brake (which pushes
the front of the car to the ground and raises the rear).
All through Friday practice, qualifying, and Sunday
warm-up, Senna’s car was visibly touching the
ground while riding over the eight dark strips of new
tarmac laid between the start line and exit of
Tamburello (strips two and four were very narrow).
The Williams consistently generated sparks on the
exit of the fifth strip (sometimes also on the entry);
on the exit of the sixth strip (sometimes also on the
entry); it produced five distinct showers of sparks on
30
the longest, seventh strip (with sparks on the exit
always being the strongest); and it usually cleared the
eight strip without visibly touching the ground.
These observations imply that the bottoming on
strips one, three, six, and eight was caused by
elevation changes between the old and new surface
whereas on strip seven there were not only elevation
changes on entry and exit but also three bumps in
between. It was strip seven where Senna lost control
on Lap 7.
Figure 8 - Four sections of the Imola circuit where the cars regularly experienced bottoming during the
1994 race weekend (yellow numbers 1-4).
31
CAR TRAJECTORY
The process of establishing Senna’s exact trajectory is
challenging because the telemetry wasn’t recording
the actual position of the car (front and rear) relative
to the track, and conclusions based on watching the
TV footage provide only qualitative insight.
One option is to reconstruct the trajectory by
measuring the relative movement of the car against
stationary objects in the background such as kerbs
and advertisement hoardings. Senna’s Williams hit
the wall about three metres to the left of Agip’s
advertisement hoarding ‘I Pilotissimi,’ which makes
it the perfect candidate for a reference point as that’s
where the car is irrevocably heading to. Alas, even
this approach has its limitations because frame
distortion blurs the stationary objects and prevents
highly accurate measurement.
Two additional yellow lines define the momentary
angle between Senna’s car and the inside kerb in
Tamburello. The absolute values themselves are not
that significant, but the relative change in the angle
from frame to frame is a good approximation of the
actual change in the attitude of Senna’s car as it is
negotiating the corner.
The first line is defined by three points on the car that
form a straight line - the outer blue edge of the
cockpit rim, the black point that fixes the windshield
to the monocoque, and the bottom end of the
antenna. The second line is defined as the line of best
fit that runs alongside the top edge of the kerb.
A vertical yellow line in Figure 9 tracks the leading
edge of the ‘I Pilotissimi’ sign; its relative shift from
frame to frame indicates change in the direction of
travel (if the car keeps turning left, the sign continues
moving right until it disappears from the screen).
Car trajectory analysis
32
Figure 9 – Car trajectory analysis
33
Figure 10 - Direction of travel analysis based on the ‘I Pilotissimi’ sign. This graph expresses the position of the yellow
vertical line as a percentage of the overall frame width. The measurement is taken from left to right (i.e. when the left edge
of the ‘I Pilotissimi’ sign just comes into view, the result is 0%; when the left edge of the sign reaches the right end of the
frame, the result is 100%). Blue data points denote values from race Lap 7, green points from a Friday qualifying lap, and
red data points from the Friday morning practice lap during which Senna greeted Prost. The three laps are synchronized
using timing and distance markers of Lap 7 (synchronization is necessary because the car travels through the same place
on the circuit at slightly different times).
34
Figure 11 - Momentary angle between Senna’s car and the inside kerb in Tamburello. Change from lower to higher values
means the car is heading more towards the inside kerb; change from higher to lower values means the car is moving away
from the inside kerb. Blue data points denote values from race lap 7, green points from the Friday qualifying lap, and red
points from the Friday morning practice lap.
35
ACCIDENT TIMELINE – CAR TRAJECTORY
10.52-10.74s: Until now, Senna has driven on a nearly identical racing line compared with his laps from Friday.
From 10.52s, however, the car’s rate of direction change is lower, and the Williams runs slightly wide towards the
middle of the track. The angle between the car and the kerb is decreasing as well.
10.76-10.90s: The car’s direction change is still less than during the Friday laps but Senna has recovered some of
the ground, as indicated by the increased angle between the car and the kerb.
10.90-11.10s: The car’s rate of direction change is greater than usual and the Williams recovers to its regular
trajectory (the blue, green, and red lines meet again in Figure 10).
11.10-11.18s: The car’s rate of direction change becomes even greater and it keeps increasing to the point where
the Williams is visibly facing the inside of the corner (the blue line crosses over and above the green and red lines in
Figure 10). The angle between the car and the kerb also increases considerably. It is during this period that Senna
loses control.
11.18-11.26s: The rate of direction change is still slightly higher than during the Friday laps, but the car is
straightening up and the angle between the car and the kerb is decreasing.
11.26-11.40s: The blue line flattens and stays horizontal, which means the car is heading dead-straight for the ‘I
Pilotissimi’ sign. There is no perceptible direction change. The Williams has abandoned its regular trajectory (the
green & red lines are now above the blue line). The angle between the car and the kerb keeps decreasing.
11.40-11.56s: The blue line is going down, which suggests the car is moving back towards the right end of the ‘I
Pilotissimi’ sign and that the Williams is heading gently right.
11.56-11.70s: The car’s direction change is gently left – about half the rate required to navigate the corner at this
point (the green and red lines keep going up at roughly double the rate of the blue line).
11.70-11.84s: Until the end of the on-board footage, the Williams is heading straight for the ‘I Pilotissimi’ sign.
36
LEFT FRONT TYRE ANGLE
Because the Williams black box was damaged in the
crash, we’re in the dark with respect to the
correlation between Senna’s steering angle and the
directional change of his front wheels. The closest
suitable proxy is the incremental change in the angle
of the left front tyre relative to the monocoque, which
can be measured manually frame by frame and then
plotted in time sequence (Figure 14).
Angle of the left front tyre
The accuracy of this method is limited: Tamburello
required only a shallow steering lock to navigate the
corner and the incremental tyre movement is quite
small; the tyre undergoes deformation under
cornering and deceleration forces (although the right
front carried the bulk of the lateral load in
Tamburello), and some of the key frames are
distorted by bumps, which hampers highly accurate
measurement. Thanks to the large number of data
points, however, the method yields a higher-order
trend that indicates whether the tyre is turning left,
turning right, or aiming straight.
It is also possible to analyse the rotation of the left
front tyre by superimposing the complete video
sequence on one reference frame in which the tyre is
pointing straight – for instance when Senna is on the
short straight before Tamburello. The relative
changes in tyre shape against this reference frame
will show on the composite footage as white space (or
black outline) in those areas where the superimposed
images differ (Figure 13). This method helps validate
tyre angle during 10.90-11.18s when the tyre rotation
left reaches its peak.
Independent validation of tyre angle
It’s even harder to quantify the directional changes of
Senna’s left front tyre as his out of control Williams
skims across the run-off area. The quality of the TV
images is poor and the car turns into a smudge in the
critical moments before the impact. Still, shortly after
the car lands on the concrete the tyre’s sidewall
comes distinctly into view, suggesting the front wheel
is turned right, and the sidewall appears to be facing
skywards. Moments later, the tyre seems to wobble
right and left; nonetheless, the images are so blurred
that this could be just a mirage. One observation can
be made though: at no point did Senna’s tyre keep
pointing left in an attempt to steer the car away from
the wall.
Williams skims across the run-off area
37
Figure 12 - The width of Senna’s left front tyre sidewall as a percentage of a control height measured from the top of the tyre.
Wider tyre profile corresponds to a higher percentage in Figure 14 (left front wheel turning right), narrower tyre profile
corresponds to a lower percentage (left front wheel turning left).
38
Figure 13 - Difference in the shape of the left front tyre (white space) relative to the reference frame.
39
Figure 14 - The width of Senna’s left front tyre measured as a percentage of the control height (Lap 7).
40
ACCIDENT TIMELINE - LEFT FRONT TYRE ANGLE
3.98-4.74s: The left front tyre is turning left (Senna drives under the gantry and past he control tower).
4.76-5.62s: Tyre is aiming straight (Senna lets the car drift to the outside).
5.64-6.22s: Tyre is turning right (Senna guides the car towards the edge of the track to minimize tyre scrub and to
straighten the upcoming corner).
6.24-8.20s: Tyre is aiming straight (Senna is on the short straight and approaching Tamburello).
8.22-9.08s: Tyre is turning gently left (initial turn-in to Tamburello).
9.10-9.66s: Tyre is aiming straight.
9.68-10.48s: Tyre is turning left.
10.50-10.72s: Tyre is aiming straight (Senna’s Williams drifts slightly wide).
10.74-10.88s: Tyre flicks left.
10.90-11.18s: Tyre rotation left reaches its maximum, with three observable culminations around 10.94s, 11.06s
and at 11.18s. It is during this period that Senna loses control.
11.20-11.26s: Tyre returns swiftly to the neutral position.
11.28-11.52s: Tyre is aiming slightly right.
11.54-11.66s: Tyre is aiming straight or slightly left.
11.68-11.80s: Tyre is turning imperceptibly right.
11.82-11.84s: Tyre is aiming straight.
41
CHAPTER 7
Car telemetry
The Williams black box was damaged in the crash: of
the twenty memory chips it contained, eighteen
would lose data once the power supply failed while
two chips were able to retain data even in case of a
power failure. Only two chips were damaged, the two
being those two that were capable of storing data
after the power had gone15.
The Renault black box survived intact. Information
from the incomplete Lap 7 was copied onto a floppy
disk on the day of the crash, and the black box then
got erased during tests on an engine bench in Paris,
France, a few days later16. The computers in the pits
had received and probably retained data from the
previous six laps of the race as well, but those
telemetry traces have never been made public.
Williams haven’t disclosed the exact definitions of
the telemetry channels, either. Most of them are selfexplanatory except for the steering data, which calls
for very specific knowledge of the system’s design
and operation.
The graphs used in our analysis are based on
telemetry values from Lap 7 and are derived from the
computer synchronization prepared by CINECA in
1997. The numerical values end at 11.98s at the same
time as Senna’s on-board tape, values from 12.00s
until 12.80s are therefore extrapolated from
CINECA’s original telemetry graphs (which run in the
computer synchronization until the impact, Figure
17).
All parameters except engine revs, throttle pedal,
throttle valve position, gear, clutch and longitudinal
acceleration were read from the sensors every tenth
of a second. Distance travelled by Senna’s car from
the start line was also refreshed every tenth of a
second and the actual distance may not be completely
accurate in case of those parameters that were
refreshed more frequently.
No data was captured regarding suspension travel,
ride heights, tyre pressures, temperatures within the
car, brakes, brake pedal position, steering angle, and
downforce levels.
42
Channel
Freq
Colour
Definition
Unit
TIME
0.02s
Grey
Time
S
SPACE
0.10s
Cyan
Distance
M
RODSP
0.10s
Red
Car speed
km/h
REARSP
0.10s
Yellow
Rear wheels speed
km/h
LATACC
0.10s
Blue
Lateral acceleration
G
LONACC
0.02s
Green
Longitudinal acceleration
G
N
0.01s
White
Engine revs
RPM
GEAR
0.01s
Green
Gear engaged
#
CLUTCH
0.01s
Red
Clutch position
mm
PEDAL1
0.02s
Brown
Pedal 1 position
%
PEDAL2
0.10s
Blue
Pedal 2 position
%
POTG
0.01s
Yellow
Butterfly position (L.H.S.)
%
POTD
0.05s
Grey
Butterfly position (R.H.S.)
%
STGPR
0.10s
Green
Steering pressure
PSI
STGSTN
0.10s
Grey
Steering strain
N/m2
STGTGT
0.10s
Blue
Steering target
PSI
STGACT
0.10s
Cyan
Steering pressure difference
PSI
STGERR
0.20s
Red
Steering error
Dec
Figure 15 - Telemetry parameters recorded by the Renault black box as presented by CINECA in 1997 (reproduced with
CINECA’s permission).
43
Figure 16 - Computer synchronization showing telemetry values from the Renault black box
(reproduced with CINECA’s permission).
44
Figure 17 - Telemetry graphs from the CINECA computer synchronization (reproduced with CINECA’s permission).
45
Figure 18 – RODSP. Actual speed of Senna’s car on the road.
46
Figure 19 – REARSP. Rotation of the rear wheels. If the speed of the rear wheels is higher than the car’s speed on the
road, it indicates loss of traction (for example, wheelspin under acceleration, on bumps or kerbs). If the speed of rear
wheels is lower than the car’s speed on the road, it indicates braking, wheels locking under braking, or recovery of grip
after wheelspin.
47
Figure 20 - Speed difference between the car and the rear wheels, extrapolated from the telemetry as REARSP minus
RODSP.
48
Figure 21 – Engine revolutions per minute . A sudden rise in RPM at constant throttle corresponds to the rear of the
car losing traction or touching the ground (riding kerbs or hitting a bump). A drop in RPM at constant throttle
indicates that the rear has regained traction and the tyres bite harder into the tarmac.
49
Figure 22 – LONACC. Acceleration and deceleration of Senna’s car in a straight line. Positive figures indicate that
the car is speeding up (driver is on the throttle), negative values indicate that the car is slowing down (driver lifts
or is braking).
50
Figure 23 – LATACC. Lateral acceleration or cornering force on the car when it’s not travelling in a straight line,
commonly referred to as g-force. Positive values indicate turning left, negative values turning right.
51
Figure 24 – PEDAL1. Throttle pedal application, from 0 percent (no throttle) to 100 percent (full throttle). Pedal1
data was ready every 0.02s, unlike Pedal2, which was read only every 0.1s. Pedal2 is less accurate for the purpose
of the analysis and can be ignored.
52
Figure 25 – POTD/POTG. Position of the butterfly valve (throttle plate)78 that regulates the amount of air flowing into
the engine - and hence the power of the engine - based on throttle pedal input from the driver , from 0 percent (valve
fully closed) to 100 percent (valve fully open). L.H.S. denotes measurement taken in the intake manifold on the left
hand side, R.H.S. on the right hand side. POTG data was read every 0.1s. It is displayed on CINECA’s telemetry as a
graph without numerical values. POTD data was read every 0.05s but otherwise it looks identical to POTG.
53
Figure 26 – CLUTCH. Clutch barrel position. The semi-automatic gearbox engaged the clutch automatically; the FW16
however had a clutch pedal in the cockpit (to the very left) that was used by the driver during the start and pitstops.
54
CHAPTER 8
Steering telemetry
The steering telemetry cannot be interpreted
unambiguously without the original technical
specifications; unfortunately, the spectre of the
manslaughter trial and potential jail sentences for
senior Williams personnel in the late ‘90s meant that
the team didn’t divulge information about how the
steering worked, and the facts that are in the public
domain today have come indirectly from witnesses
giving evidence during the trial or from reports,
drawings, and photos leaked to the press.
Still, certain fundamental principles apply. Since the
FIA ban on electrohydraulically assisted steering in
the early ‘00s the majority of F1 teams have been
using hydro-mechanical power steering valves. These
valves are either rotary or linear systems that direct
the flow of hydraulic fluid based on mechanical
input. In the rotary system, for instance, this input
comes from a small torsion bar that senses strain
exerted on the steering column. The valves are
passive in the sense that they do not respond to any
electrical commands and are fundamentally the same
as hydraulic power steering on your road car –
they’re just much more compact and responsive22-28.
Back in 1994, however, Williams did not operate a
purely mechanical system and in fact became the first
F1 team to develop power steering that exploited
electrohydraulic servo valves. The valves direct the
flow of hydraulic fluid based on electrical rather than
mechanical signals. They were originally used in the
aerospace industry and, by the late ‘80s, found their
way into F1 as actuators in active suspension when
supplier Moog entered the scene with series 30 and
later series 50 valve29-30. Their use quickly spread to
other areas of design until by 1993 every car would
feature as many as ten valves controlling anything
from the clutch, differential, gearbox and throttle
actuation, to the engine and ABS brakes27. The main
benefit of electrohydraulic servo valves is their
incredible power density: they can handle the same
loads as electric or pneumatic motors yet they are
faster, smaller and lighter in comparison – all
qualities much desired in F1 regardless of cost.
However, before trying to interpret the steering
telemetry from Senna’s car, we need to explain how
this type of power steering works. While the
remainder of this chapter is inevitably a little
55
technical, it helps in following the rest of the book
and doesn’t require any special knowledge.
ELECTROHYDRAULIC STEERING
Electrohydraulic power steering assembly consists of
several components (Figure 27). The steering column
(1) typically runs through a support strut and bushing
(2), and connects at the lower end to the pinion (3).
Pinion is a circular gear used to convert rotational
motion originating in the column into linear motion
of the steering rack (4) by engaging the teeth on the
rack (5) and causing it to slide left or right. In designs
where the pinion engages the rack from above rather
than underneath as would be normal for a car with
the rack ahead of the front wheels, the pinion is
attached to the steering rack through a reversing gear
(6). Without this extra gear to reverse the direction
the assembly would otherwise steer backwards. The
steering rack houses the rack teeth and a hydraulic
cylinder (7) with integral piston (8) at each end.
The twisting force on the steering column is sensed
by one or more strain gauges (9), which are small
electrified patches glued to the steering column31,32 or
tie rods. When the column is put under strain (i.e.
driver turns the wheel, the front wheels hit a bump,
or the combination of both), the length distortion of
the tiny conductors in the patches causes a change in
electrical resistance, which is then measured,
amplified, and converted into a suitable electrical
signal. This signal is fed into an electrohydraulic
servo valve (10) where it controls the opening of two
hydraulic ports (11) which direct the flow of fluid to
the cylinders at each end of the steering rack. The
steering rack works as a double-acting hydraulic
cylinder and assists the driver to turn the front
wheels.
The electrohydraulic valve is the heart of the power
steering system. It is quite a small device (about
65x40x40mm, 400g) typically made of stainless steel
and aluminium alloy30. It is a valve, and therefore it
doesn’t control the hydraulic pressure – it controls
the direction and rate of the fluid flow. It must be
capable of supplying adequate volume of fluid so it
can respond to the greatest demand coming from
either the steering or the wheels. The quicker the
driver turns the steering wheel, the more rapidly the
valve’s control ports must fill and drain the hydraulic
cylinders in the steering rack in order to assist the
direction change of the front wheels.
56
Figure 27 - Schematic diagram illustrating the basic function of electrohydraulic power steering components: (1) column,
(2) support strut with bushing, (3) pinion, (4) steering rack, (5) rack teeth, (6) reversing gear, (7) hydraulic cylinder, (8)
piston, (9) strain gauges, (10) electrohydraulic servo valve, (11) servo valve’s hydraulic control ports C 1 and C2.
57
Figure 28 - Schematic diagram of an electrohydraulic servo valve29. (1) Permanent magnet, (2) Flapper, (3) Armature,
(4) Solenoid, (5) Left nozzle, (6) Right nozzle, (7) Sliding valve spool, (8) Cantilever spring.
58
Detailed technical description of the valve’s operation
is beyond the scope of this book, but it can be found
in the literature28-29 and a simplified version will
suffice here (Figure 28).
In the electrical part of the valve, a flapper (2) is
attached to armature (3) rotating in magnetic field.
When twist is applied to the column by the driver and
is resisted by the wheels, or when twist is applied by
the wheels and is resisted by the driver, the electrical
current circulating in the solenoid (4) changes and
deflects the flapper, which restricts fluid flow from
one nozzle (5) while accelerating the flow from
another (6).
As a result, intermediate pressure difference builds
between the two sides of a linear valve spool (7),
causing it to slide left or right from the centred
position. The sliding of the spool also creates a
restoring torque on the flapper via a cantilever spring
(8). Once the restoring torque equals the torque
generated by the current in the solenoid, the flapper
returns to its neutral position and the valve spool
stops at a point proportional to the electrical current.
The movement of the spool right opens the supply
pressure port (PS) to one control port (C2) while
simultaneously opening the return port (R) to the
second control port (C1). Pressure rises within line C2
and exerts force against the piston in the steering
rack until it performs the desired power assist. At the
same time, fluid escapes from the other side of the
steering rack via line C1 and flows to the return port
(R). When steering in the opposite direction, the
process is reversed: the spool slides left and line
C1 exerts force against the piston while line
C2 exhausts fluid to the return port (R).
The greater the difference between steering effort
and resistance of the wheels, the greater the degree of
twist which momentarily exists in the steering
column. The more twist, the greater the movement of
the valve spool and the higher the rate of fluid flow in
and out of control ports C1 and C2, up to the
maximum available at the end of a steering stroke or
when the wheels hit an obstruction.
When the car is not being steered, the valve spool
remains in the centred position in which it fully
covers ports PS and R, and the supply pressure fluid
flows directly from the two nozzles to port R. Ports C1
and C2 and the respective lines to the pistons in the
steering rack are still filled with fluid, but C1 and C2
provide equal pressure to both sides of the steering
rack, the pistons don’t move, and no power assist
occurs.
59
The system is closed-centre, which means that it is
pressurized all the time with constant pressure while
the valve selects only the direction. There is no
dedicated hydraulic pump and the supply pressure
comes from the car’s main circuit that serves other
hydraulic-based components as well. The pump is
never in a relieved state like in a road car where the
valve not only selects the direction but also demands
supply pressure from the pump (open-centre
system).
WILLIAMS INSTALLATION
When it comes to the actual power steering assembly
used by Williams on the FW16 model, it was similar
to the generic version described in Figure 27. A rare
photo of the footwell33 shows the lower end of the
steering column connecting to an aluminium case,
which protrudes through a cut-out in the frontal area
of the chassis and integrates on the other side with
the steering rack34-35. The drawings released by
CINECA19 suggest that this aluminium case
contained the pinion and reversing gear.
steering ratio if both gears are of identical pitch. The
latter offers a quicker steering ratio as more teeth on
the pinion means more rack travel per degree of
steering wheel rotation; however, it is physically
more demanding on the driver and puts more stress
on the steering column. In Williams’ case, the 10
tooth pinion gear was usually fitted only for the
qualifying, but at Imola, apparently, Senna drove
with it in the race as well.
Williams opted for Moog E050 electrohydraulic servo
valve with three strain gauges sensing strain in the
steering system and with three sensors per cylinder
in the steering rack measuring hydraulic pressures
(three of each to ensure redundancy). The supply
pressure of its hydraulic circuit is unknown, but the
Moog valves usually operated at up to 3,000psi30 and
the values recorded by the Renault black box are well
within that range. We’re now ready to return to
Senna’s telemetry and take a closer look at the five
steering traces STGPR, STGACT, STGTGT, STGERR,
and STGSTN.
According to one source83, the FW16 could
accommodate either a 7 tooth or 10 tooth pinion
gear, although this represents a radical change in the
60
Figure 29 - Steering rack on the Williams FW16B-Renault, which raced from the German Grand Prix 1994. This photo
was taken during a historic race meeting some years later, but the rack assembly is fundamentally the same as the one
used by Senna in the first three races of the season (courtesy of Remi Humbert and Joachim Kutt, GurneyFlap.com).
61
Figure 30 - Frontal view of the steering rack assembly – sensors inside the hydraulic cylinders measured pressures at
both ends of the rack, but only pressure on the left hand side was reported by the telemetry (STGPR). Pressure
difference between the two cylinders was reported as STGACT (based on photos from the 1994 Brazilian and San
Marino Grand Prix34-35).
62
POWER STEERING TELEMETRY
STGPR (Steering pressure) is the hydraulic pressure
measured inside the cylinder on the left hand side of
the steering rack as seen from the driver’s viewpoint.
The pressure is lower or falling when the front wheels
are turning left (the cylinder on the right provides the
desired power assist); the pressure is higher or rising
when the front wheels are turning right (the cylinder
on the left provides the desired power assist). When
the wheels are centred the pressures in both cylinders
in the steering rack are very similar (around 400psi).
In addition, transient pressure fluctuations can also
be triggered by shocks propagating into the cylinders
through the wheels.
The trends observed on the telemetry back this
interpretation up: STGPR drops from 317psi to
188psi as Senna begins to ease his car into
Tamburello, and the pressure drops even lower to
164psi between 11.00-11.18s when the car’s trajectory
turns sharply left. In comparison, STGPR is generally
higher once the car has abandoned its normal
trajectory and is heading for the ‘I Pilotissimi’ sign
(around 400psi after 11.30s).
Pressure in the left cylinder (STGPR) is linked to the
pressure in the right cylinder (not shown on the
original telemetry) via STGACT – Steering pressure
difference. That is obvious once the two telemetry
traces are plotted together on one graph (Figure 32).
Hence STGACT is the pressure difference between
the ‘turning’ side and the ‘exhausting’ side of the
steering rack (Figures 27, 30), and it is defined as
STGPR (right cylinder) minus STGPR (left cylinder).
Zero to low values are expected with the steering
centred or unloaded; maximum values are expected
at the end of a steering stroke or at highest load (i.e.
shocks) coming through the front wheels. Positive
values indicate steering left (pressure against the
piston is higher in the right cylinder), negative values
indicate steering right (pressure against the piston is
higher in the left cylinder).
63
Figure 31 - Steering pressure STGPR in the cylinder on the left hand side of the steering rack.
64
Figure 32 - Correlation between Steering pressure and Steering pressure difference.
65
Figure 33 - Steering pressure difference.
66
Figure 34 - Steering pressure in the cylinder on the right hand side of the steering rack (not measured by the original
telemetry), extrapolated from Steering pressure difference values.
67
ACCIDENT TIMELINE – STEERING PRESSURES
7.98-9.08s: Pressure difference across the steering rack is building gradually from 58psi to 764psi as Senna eases
his car into Tamburello. Pressure in the cylinder on the left hand side (L.H.S.) drops slightly while most of the
difference comes from pressure rising against the piston in the cylinder on the right hand side (R.H.S.). This
corresponds to smooth turning & steering assist left. From 9.08s, however, the pressure difference trace switches
into alternating peaks and valleys, which points to the rack being repeatedly loaded and unloaded.
9.10-9.48s: Valley One. Pressure difference drops by 40 percent (from 764psi to 435psi), chiefly as a result of the
R.H.S. cylinder becoming unloaded, although it still continues to offer reduced steering assist left.
9.50-9.88s: Peak One comes shortly before Senna enters the fifth strip of new tarmac. Pressure difference doubles
(from 435psi to 929psi) as the R.H.S. cylinder is reloaded either through increased steering effort left or through the
surface change from old to new tarmac.
9.90-10.18s: Valley Two. The R.H.S. cylinder becomes unloaded again and pressure difference drops by 50
percent (from 929psi to 411psi). That said, sufficient difference exists to continue steering assist left.
10.20-10.38s: Peak Two. Pressure difference increases by 60 percent (from 411psi to 694psi) as the R.H.S.
cylinder is loaded harder, probably because Senna takes a bite left with the steering or because the surface change
on the sixth strip disturbs the right front wheel.
10.40-10.98s: Valley Three starts at 10.40s and is characterized by a lull period of low pressure difference
(around 294psi) between 10.60s and 10.98s. There is a noticeable pressure increase in the L.H.S. cylinder (from
176psi to 247psi) and, at the same time, the pressure in the R.H.S. cylinder is at its lowest since Senna began
turning into Tamburello (529psi). This indicates minimal turning effort left.
11.00-11.08s: Peak Three is the steepest increase in pressure difference recorded by the telemetry (from 294psi to
788psi). Pressure in the L.H.S. cylinder is the lowest recorded as well (164psi), suggesting a sharp turn left with the
steering. At this point Senna hasn’t entered the seventh strip of new tarmac yet.
68
ACCIDENT TIMELINE – STEERING PRESSURES
11.10-11.28s: As Senna enters the dark surface of the seventh strip and rides over the three bumps, substantial
pressure in the R.H.S. cylinder and the very low pressure in the L.H.S. cylinder remain, although the pressure
difference begins to taper off (from 788psi to 623psi). This implies that, at least until the next readout at 11.20s, the
front wheels keep turning left. It is during this time interval that Senna loses control.
11.30-11.68s: Pressure difference falls off the cliff, with pressures in the L.H.S. and R.H.S. cylinders all but equal
(around 400psi). No steering assist occurs and the front wheels are pointing straight.
11.70-11.88s: Pressure difference rises imperceptibly to the level last seen when Senna started easing his car into
Tamburello at 8.10s (223psi). This is a sign of tentative steering effort left, a shock from the road, or disturbance
from the onset of hard braking reaching the cylinders via the front wheels.
11.90-12.38s: For half a second, pressure difference turns negative (-190psi) and for the first time the pressure in
the L.H.S. cylinder is higher than that in the R.H.S. cylinder – in Senna’s final metres on the track, the front wheels
are steering right.
12.40-12.48s: A huge transient spike (1,520psi). This is almost certainly provoked by a shock propagating
through the steering system as the front wheels land hard on the concrete before the wall. As the pressure difference
is positive, it indicates that the right front wheel received the shock or the wheels bounced left.
12.60-12.68s: Another huge transient spike (1,170psi) and another hit propagating from the road through the
front wheels to the cylinders and steering rack. The pressure difference is again positive, indicating that the wheels
bounced left.
12.70-12.80s: Third transient spike, this time in the opposite direction (-590psi). It implies the front wheels
bounced right immediately before the impact.
69
The next trace, Steering target (STGTGT), is the
differential pressure between control ports C1 and C2
of the electrohydraulic servo valve (Figure 35),
technically referred to as Load Pressure Drop.
Steering target stays within ±250psi of Steering
pressure difference values (STGACT), as could be
reasonably expected in conditions where the primary
source of pressure difference across the steering rack
comes from the steering column rather than shocks
from the road.
Plotting Steering target values against Steering strain
values (STGSTN) reveals a near-linear relationship
between the two parameters (Figures 36 and 37),
which would suggest that the power steering valve
responded in an approximately linear fashion to the
strain exerted on the steering column. The data
points don’t fall into a perfect line and are scattered
around the line of best fit. This could be due to either
measurement accuracy, the inherent characteristics
of the valve, or some other factor.
Figure 35 - Steering target,
defined as the differential
pressure between ports C1
and C2. The white arrows
signify the fluid flow and
valve dynamics when
steering left.
70
The final power steering trace, Steering error
(STGERR), is hard to decipher without having access
to the original design specifications, but this
telemetry channel probably relates to the integral
spool position sensor used to monitor correct servo
valve operation. The values are zero except for a
single moment at 11.74s where the graph shows a
vertical line.
Figure 36 - The relationship
between Steering target
(STGTGT) and Steering
strain (STGSTN) is almost
linear, with correlation
coefficient of -0.92 (as
opposed to -1.00 for a
straight line). Three orange
‘outliers’ highlight the
biggest deviations from the
line of best fit at times
8.60s, 11.20s, and 11.30s.
71
Figure 37 - Correlation between Steering target and Steering strain.
72
Figure 38 - Steering target.
73
Figure 39 - Steering error.
74
ACCIDENT TIMELINE - STEERING TARGET
8.60s: The first ‘outlier’ where the relationship between Steering target and Steering strain deviates from the line of
best fit. Considering the relatively low strain on the steering column, the target pressure is higher than expected.
10.70-10.98s: A decrease in Steering target (from 505psi to 294psi) corresponds to the spool in the servo valve
sliding back towards the centred position and therefore to reduced steering assist left.
11.00-11.08s: The steepest increase in Steering target recorded on the telemetry (from 368psi to 764psi),
suggesting a strong steering assist left immediately before Senna loses control.
11.20-11.30s: The second and third ‘outliers’ – considering the high strain in the steering column, pressure
difference between the control ports of the servo valve is appreciably lower than expected.
11.30-12.80s: The Williams has abandoned its regular trajectory and is heading more or less straight for the ‘I
Pilotissimi’ sign. Steering target pressure falls dramatically at 11.30s (to 175psi) and stays low until the impact
(below the level required for the initial turn-in to Tamburello).
12.40-12.48s: Steering target remains minimal (under 100psi) while Steering pressure difference spikes (to
1,520psi). This confirms there is little fluid flow through the control ports of the valve and the pressure spike in the
cylinders comes from the wheels rather than from the steering, otherwise Steering target would also have spiked.
12.60-12.68s: Steering target again stays minimal despite another Steering pressure difference spike. The reason
is another shock propagating through the front wheels.
75
STEERING COLUMN TELEMETRY
The telemetry records Steering strain (STGSTN) in
units of torsional stress N/m2 (Newton per square
meter). When a column is subjected to torque or
twisting, torsional stress is produced in the column,
with values ranging from zero in the column’s axis to
a maximum at the outside surface. In this context,
STGSTN is the stress momentarily generated in the
steering column by the continuously varying
opposition of inputs from the driver and the front
wheels.
The three strain gauges measuring torsional stress
cannot be seen on any of the drawings19 or photos33,
and it wasn’t until May 2014 that Adrian Newey
alluded to their exact position in an interview for
auto motor und sport85: the sensors were located ‘in
the struts of the front axle’, which most likely refers
to the tie rods linking the front wheels to the steering
rack because a strain gauge measuring the tension
and compression of a tie rod is the same as one
measuring torsional stress in a column. No strain
gauges were found in the upper section of the
steering column that broke off, and it is unclear
whether the third gauge was in fact positioned at the
lower end of the column near the pinion or not
(Figure 40).
Figure 40 - Strain gauges measuring torsional stress on the steering column
76
Figure 41 - Steering strain.
77
ACCIDENT TIMELINE - STEERING STRAIN
7.98-9.08s: Steering strain is building in the column as Senna starts easing his Williams into Tamburello, reaching
-19.15N/m2 at 9.08s (from -4.79N/m2 at 7.98s).
9.10-9.68s: A period of relatively stable strain in the region of -16 to -17N/m2 that is a sign of steady turning left.
9.70-9.78s: In the middle of the fifth dark strip, strain drops from -17.95N/m2 to -11.97N/m2. This is consistent with
the steering becoming lighter as the car hits an undulation and experiences transient oversteer.
9.80-9.98s: A sharp increase in Steering strain indicates the oversteer has gone and Senna continues steering
effort left (-23.34N/m2).
10.00-10.48s: The steering is still loaded but the strain is gradually decreasing (from -23.34N/m2 to -14.96N/m2),
which suggests lighter steering effort left.
10.50-10.58s: Another quick steering correction left (strain reaches
-22.14N/m2).
10.60-10.78s: Sharp drop in Steering strain (from -22.14 to -10.17N/m2) as steering effort declines. This is similar
to the drop experienced on the fifth dark strip, but this time the car is not on the new tarmac and the decrease is not
caused by transient oversteer.
10.80-11.08s: The steepest increase in Steering strain recorded on the telemetry. It keeps rising from -10.17N/m2 at
10.78s up to -26.93N/m2 at 11.08s. This corresponds to a brusque steering effort (left) exerted by Senna immediately
prior to losing control.
78
ACCIDENT TIMELINE - STEERING STRAIN
11.10-11.28s: There is sustained high Steering strain (between -25.13 and -26.33N/m2) in the critical period during
which Senna loses control. There is no sudden drop in the strain that would signal that the steering got light on the
first two bumps.
11.30-12.38s: From this point onwards, Steering strain falls to virtually zero (0.60N/m2) in a close to quadratic
curve.
12.40-12.48s: There is a blip in Steering strain that mirrors the huge spike in Steering pressure difference. The
value (-7.40N/m2) is low and equivalent to the strain exerted by Senna on the initial approach to Tamburello.
12.60-12.68s: Second blip in Steering strain (-4.20N/m2) that again mirrors the huge spike in Steering pressure
difference.
79
CHAPTER 9
The cause
There are several reasons why Ayrton Senna’s
accident has never been satisfactorily explained.
Every unnatural death in Italy warrants an
investigation and Senna’s was no exception. The
police impounded the car at the circuit and the
authorities denied Williams unreserved access to the
wreck in the belief that this could potentially
compromise the integrity of court evidence. In fact,
technical director Patrick Head was granted just two
ten-minute visits3,95. Likewise, the Williams team had
no incentive to publicly release the full technical
details because their primary motivation was to
protect their staff.
In February 1997, three Imola track officials and
three senior Williams figures – team owner Frank
Williams, technical director and shareholder Patrick
Head, and chief designer Adrian Newey – were
charged with involuntary manslaughter. The
prosecution alleged that the steering column design
was to blame; the defence maintained that Senna lost
control on a bump. When the original trial ended in
November 1997 (the proceedings are
comprehensively covered on the S-Files website33),
the three track officials and Frank Williams were
acquitted, but the case against Newey and Head
dragged on until May 2005 and April 2007,
respectively86. In the end, the appeal court ruled that
Adrian Newey was innocent of involuntary
manslaughter and the case against Patrick Head was
dropped under a statute of limitations77.
The trial meant years of stress and sleepless nights
for Newey and Head36, who build racing cars with the
intention to win races, not to get their drivers killed.
Losing one of the greatest drivers of all time in one of
their designs must have been a ‘life sentence’ in itself,
so the threat of a suspended jail term was seen by
many as wholly unnecessary.
Finally, some of the vital information is either
missing or did not exist in the first place: telemetry
data from the damaged Williams black box (Laps 1 to
7); telemetry data from Renault’s black box (Laps 1 to
6 and the first eight seconds of Lap 7); tyre pressures
and tyre temperatures; on-board footage through
Tamburello from Schumacher’s camera on Lap 6;
and the missing 0.9s of on-board footage from
Senna’s car until the impact from Lap 7.
80
Over the past twenty years, the different protagonists
have voiced contradicting opinions about the cause of
Senna’s accident: Patrick Head hasn’t pointed the
finger squarely at the aerodynamics but every now
and then has re-emphasized that the steering worked
until the car hit the wall; David Coulthard, who has
never discussed his role in the steering wheel flexing
demonstration, believes the steering column broke
on impact87; Adrian Newey, now at Red Bull, openly
admitted in May 2011 that the design of the steering
column was very poor36 but otherwise he remains
non-committal on the cause of the accident (steering
failure94, aerodynamics94, tyre failure36); Damon Hill
is convinced that Senna made a driver error, pushing
too hard too soon in a difficult car37,84,92 ; while Frank
Williams has maintained silence since the end of the
trial.
failure, tyre failure, aerodynamic instability, and
steering column failure.
The cause may never be confirmed beyond
reasonable doubt, but that doesn’t automatically
mean that no further insights are possible. In the
second part of this book, we’re going to build on the
information presented in the first and take a fresh
look at five hypotheses generally accepted as
plausible explanations of what happened to Senna on
Lap 7 of the 1994 San Marino Grand Prix. The five
hypotheses are: suspension failure, power steering
81
CHAPTER 10
Suspension failure
HYPOTHESIS
The accident was caused by a non-catastrophic
suspension failure, which impaired the car’s handling
but didn’t cause the suspension to collapse outright.
EVIDENCE
In the aftermath of the accident, unconfirmed reports
talked about a long score mark on the concrete area
before the wall apparently caused by a piece of
suspension38, but otherwise there is little to support a
load-bearing suspension failure.
Had the rear suspension collapsed outright, the
substantial aerodynamic loads acting on the car in a
corner like Tamburello would have slammed the
chassis hard against the ground in the area where the
suspension failed, which would have resulted in some
sparks from the underbody as the car ran off the
road. But the only sparks visible are those induced on
the bumps and on the undulations between the old
and new surface (for comparison, watch Rubens
Barrichello’s left rear suspension failure during the
2003 Hungarian Grand Prix).
According to Patrick Head21,38 the downforce
generated at 300km/h was about 2,000kg plus
640kg to account for the car, driver, and fuel. In a
long left hander like Tamburello, about 65 percent of
this weight would be on the right hand side, where a
suspension failure would most likely have occurred.
Under the load of at least 1,700kg it would have
pitched the Williams sharply left or into a spin.
Intriguingly, though, later that year at Silverstone
Senna’s teammate Damon Hill suffered a front
suspension failure where the upper suspension
mounts were both pushed up and out of the chassis
purely due to high load, causing the car to porpoise
badly (see YouTube). Williams blamed a
manufacturing fault rather than component failure
but, in principle, Senna could have suffered the same
problem: the dynamics of his accident are consistent
with a right front suspension failure (compare with
Kimi Raikkonen’s crash at the Nurburgring in 2005),
and photos93 showing the upper right front
suspension arm of Senna’s car apparently pulled out
82
from the mount rather than snapped give this
hypothesis some credibility.
A non-catastrophic suspension failure in a
component such as damper or spring is a possibility,
but it would have been difficult to detect if the
component got destroyed in the crash, and it’s been
so long since the accident that no new evidence can
be extracted from the wreck. The mangled Williams
FW16/2 was returned back to the team in April 2002,
and although some sources maintain that it remains
safely locked in a place known to only a few
insiders39,40, the car - minus its engine, which was
returned to Renault - was in fact cut up to sheets of
carbon fibre and set alight in Frank Williams’ back
garden95.
83
CHAPTER 11
Power steering failure
HYPOTHESIS
A mechanical failure or electronics glitch in the
power steering system.
EVIDENCE
Williams first equipped its cars with power steering
in 199412 and it is reasonable to expect that the new
electrohydraulic system was still relatively unproven
three races into the season. It is therefore logical that
after Senna’s accident teammate Damon Hill was told
on the grid before the second start to switch it off.
Tony Woodward, CEO of Woodward Machine
Corporation in Casper, USA, brings more than thirty
years’ experience in hydraulic power steering (do not
confuse him with Gary Woodward, who was
responsible for mechanical components on Senna’s
car in 1994). According to Woodward, “the telemetry
traces show no sudden drop in steering pressures or
flow interruptions. The hydraulic circuit remained
intact until Senna hit the wall. Electronics can cause
problems if the system decides to reboot while at
speed but, based on the telemetry, that didn’t happen
on Senna’s car.”
Had the telemetry recorded abnormalities in steering
pressures, the most likely cause would have been a
burst pressure line or fitting, or blown piston seal in
the steering rack. As Woodward points out, however,
none of them would have been catastrophic: “There
was little power steering [in a corner like
Tamburello], and if that failed Senna would have
been able to continue. A sudden pressure loss would
just double the force required to steer. I think his
reaction would be different.”
There is no indication that Senna suffered a power
steering failure, and Williams deactivated the system
on Hill’s sister car as a precautionary measure rather
than in response to a serious issue they noticed on
Senna’s car.
84
CHAPTER 12
Tyre failure
HYPOTHESIS
The accident was caused by a cracked tyre sidewall or
slow puncture that wasn’t visible to the naked eye but
was serious enough to disrupt the car’s handling.
EVIDENCE
A broken wheel rim forced Senna out of the San
Marino race four years earlier; nonetheless, no hard
evidence exists to support a rim failure or debris in
the rim and it would have been difficult to detect,
especially on the right hand side where the car
smashed against the wall.
Slow motion replay of the out of control Williams
broadcast on BBC reveals porpoising which is
consistent with a tyre problem - the car isn’t
bouncing up and down because it is braking on the
bumpy track, but because two of its four wheels are
no longer in proper contact with the ground.
Lower pressure in one of the tyres was the most likely
culprit. The tyre either failed to warm-up at the same
rate as the other three tyres because of a cracked
sidewall, a phenomenon that tyre supplier Goodyear
reportedly experienced with several teams during the
Imola ‘94 weekend41, or Senna suffered a slow
puncture when he drove through the debris on the
main straight during the safety car period. Both
scenarios would have had a similar effect on the car.
It is unlikely he would have felt the debris, especially
if it was a flat or squashable piece. F1 steering is more
direct compared to a road car, but there is so much
activity going through the steering wheel that it
would have gone unnoticed against the background
of other feedback. It is also not certain that Senna
actually drove over the debris, it could have just
made contact with the front wing or the underbody
and then flipped at the rear wheel42.
In principle, the slow puncture or cracked sidewall
could have affected any of the four tyres, but
considering the dynamics of the accident and Senna’s
initial twitch left before straightening the car, the
problem must have been triggered by the right rear
or, which is less likely, by the right front. This means
that the blue debris from Lehto’s Benetton that
85
Senna flattened shortly after entering Lap 7 is a red
herring, because he drove over it with his left wheels.
deflated tyre whereas Senna according to the
telemetry held the steering wheel centred.
To understand what happens when a car suffers a
tyre failure it helps to study high speed accidents
where the exact cause is known. Two relevant cases
include Lewis Hamilton’s crash at the Nurburgring in
2007 (right front tyre failure in a fast left hand
corner) and Sebastian Vettel’s retirement from the
Abu Dhabi Grand Prix in 2011 (right rear tyre failure
in a fast left hand corner).
Equally, there are uncanny similarities between
Senna’s accident and Sebastian Vettel’s exit from the
Abu Dhabi Grand Prix in 2011: Vettel suffers a right
rear puncture while on full throttle in the fast left
hander Turn 2. His Red Bull turns sharply left distinctly pointing towards the Armco barrier on the
inside of the corner exactly like Senna’s Williams and then straightens up momentarily before the tyre
deflates and the car spins off.
In Hamilton’s case, the right front tyre delaminates
as he is pointing his McLaren at the apex of Turn 8 at
an estimated 260km/h. Slow motion analysis reveals
that just before the tyre thread peels off the wheel
rim, the car’s rate of turning left increases for a split
second in a fashion not dissimilar from Senna’s,
although the effect is not as pronounced. The
McLaren then darts right and continues more or less
straight towards the tyre barrier. It does not wobble
or spin because the rear tyres still maintain good
traction while Hamilton is not making any major
steering movements and simply brakes hard - he
knows instantly he has a big problem. Again, his
reaction resembles Senna’s. One weakness in this
argument is the observation that Hamilton keeps a
steady steering lock left to compensate for the
The right rear tyre failure hypothesis has a lot going
for it, including the backing of former Williams chief
designer Adrian Newey who in an extensive interview
for The Guardian36 in May 2011 admitted that “if I
was pushed into picking out a single most likely cause
that [right rear puncture] would be it.”
Newey is of the opinion that Senna’s car bottomed
much harder on Lap 7 than on Lap 6, which seems
counter-intuitive considering that the tyres should
have come up to their operating temperature by then.
On-board footage from Lap 6 through Tamburello –
released in 2014 after 20 years in the FOCA archives
thanks to Roberto Cabrini’s efforts82 –contradicts
Newey’s view. The bottoming on Lap 6 is so hard the
86
camera can’t cope with the violent shaking and
records only a few usable frames during the six
seconds it takes Senna to navigate the corner. By
contrast, the picture from Lap 7 remains relatively
clean until 10.56s and only then does it get
marginally distorted. The analysis of the bottoming
effect discussed in the first part of the book also
indicates that the tyre pressures did in fact improve
as the sparks coming off the rear of the Williams look
much stronger on Lap 6.
When the engineers inspected the cars after the race,
they discovered that the front skid-plates on Senna’s
car were heavily worn and in the footwell area were
even pushed into the chassis, whilst Hill’s identically
set-up car was found almost intact7,95. That said, the
damage to the front skid-plates could have occurred
on Lap 6 rather than Lap 7, or once the car left the
road. No other evidence points to harder bottoming
on Lap 7.
If the right rear tyre failure hypothesis is correct, the
slow puncture (or cracked sidewall) made Senna’s
right rear wheel to drop and the left front to lift
moments before the Williams entered the seventh
strip of new tarmac in Tamburello. Most of the load
had already been on the right front tyre, but now the
difference became even bigger as the left front lost
some mechanical grip. Senna put on more steering
lock to navigate the final part of the corner just as the
imbalance in tyre pressures kicked in and the car
turned sharply left (time interval 10.90-11.18s). This
marks the beginning of the accident and it is well
documented on the telemetry as steep increase in
Steering strain, lateral g-force, and Steering pressure
difference. Moreover, it is also present in the graphs
comparing the car’s direction change relative to the ‘I
Pilotissimi’ sign, in the car’s angle against the kerb,
and in the rotation of the left front tyre.
Senna tried to straighten the car by applying opposite
lock (11.20-11.26s) and by reducing throttle to 50
percent (11.20-11.44s). The drop in pressure on the
right rear however meant that the car didn’t react to
his steering input in the usual and expected way and
swung right. The situation wasn’t helped at 11.26s
when the Williams drove over the third and biggest
bump. Senna realised he was a passenger and went
hard on the brakes (11.68-12.16s).
Apart from the question mark over the degree of
bottoming experienced by Senna in his last seconds
on the track, four additional factors cast doubt on the
rear tyre failure hypothesis: the fact that the car did
not spin and that it retained full braking capability,
statements from Goodyear regarding Senna’s tyres
87
after the accident, and the unusual movement of the
steering wheel.
damage inflicted by the impact masked any traces of
a minor defect in the right rear tyre.
If the right rear tyre had failed, the handling and
traction on the rear axle would have got progressively
worse. Senna would have been fighting the bouncing
car like Vettel in Abu Dhabi - although in Vettel’s
case the tyre deflated abruptly – and this would have
shown as wilder steering movements on the on-board
footage and telemetry. Senna did not attempt to
avoid hitting the wall by forcing the car into a spin,
either. Furthermore, the car’s braking ability would
have reduced if it had lost significant contact patch
with the ground. The data suggests this was not the
case – deceleration still peaks at a healthy 4.4-4.7G.
This brings us to the final point: the tyre failure
hypothesis doesn’t elaborate on the movement of the
yellow button on the steering wheel during the last
three seconds before the crash. A puncture alone
cannot account for the observed phenomenon
because in order to lower the apparent position of the
steering wheel by 25-27mm relative to a camera
viewpoint fixed in space, the car would have had to
assume a severe pitch attitude in excess of its static
ride height. The hypothesis therefore implies that the
flexing was normal.
Immediately after the accident, Goodyear technicians
inspected all four tyres and ruled out a tyre failure.
According to a statement made by Barry Griffin,
there was no tyre delamination and no sign of
damage caused by a foreign object that could have led
to deflation. Three tyres had cuts, but all were
consistent with the impact rather than a puncture43.
A former Goodyear employee told writer Christopher
Hilton that there was never any suggestion of a slow
puncture as it would have become visible at some
stage3. Still, it remains a possibility that the severe
88
CHAPTER 13
Aerodynamic instability
HYPOTHESIS
The trigger for the accident was some external factor
that unexpectedly disturbed the car’s aerodynamics
as Senna entered Tamburello; or, the inherent
aerodynamic instability of the FW16 was exposed in
the wrong place at the wrong time by a set of
unfavourable circumstances: the long period behind
the safety car, low tyre pressures at the restart,
bumpy track surface in Tamburello, the FW16’s
aerodynamic weaknesses, and Senna’s unwillingness
to back off even in tricky conditions.
On Lap 7 Senna entered Tamburello flat out,
travelling much faster than Damon Hill in the sister
Williams and using the tighter inside line which was
quicker but bumpier. As he crossed the seventh dark
strip of new tarmac (11.08s) and rode over the first
bump, the rear of the car bottomed and stepped out
(oversteer) due to the combination of low tyre
pressures, bumpy surface, and a sudden loss of
downforce.
The oversteer pointed the Williams towards the
inside of the track (11.10-11.18s). One tenth of a
second later (11.20s), Senna started applying
opposite lock and reduced throttle to 50 percent to
stabilize the sliding car. With less acceleration
pushing the car forward, the rear grip recovered but
was now applied sideways as the car was still being
corrected to the right. The very next moment
(11.26s), the car struck the third, biggest bump in
Tamburello.
The car bottomed out heavily at the front, with much
of the weight sitting on the suspension mounts
instead of the tyres. This provoked a sudden loss of
front downforce and heavy understeer, which in
conjunction with the rear grip acting sideways
prompted the car to slap back and turn right, so
much so that Senna’s head swung out of the cockpit
(11.48s).
Senna now found himself nine degrees off the racing
line, but the change relative to the car’s axis was even
greater due to the initial oversteer. He realised he
couldn’t recover the situation and decided to keep the
steering centred for optimal braking. He lifted
completely at 11.50s and began a desperate attempt
to slow the car down at 11.68s7.
89
Figure 42 - Oversteer (left) means that the front of the car continues on the racing line (white) while the rear steps
out because of a lack of rear grip induced by either wheelspin or loss of downforce. Understeer (right) means that
the rear of the car sticks to the racing line while the front of the car suffers from a lack of grip and struggles to make
the corner despite increased steering lock.
90
EVIDENCE
In the opinion of French TV reporter Jean-Louis
Moncet, who studied the view from Schumacher’s onboard camera, a small piece of debris was dangling
from underneath Senna’s car and upset its
aerodynamics in Tamburello just enough for Senna
to lose control. The debris flew off the track
immediately afterwards21,44. Beside the grainy TV
footage, however, Moncet’s theory has little else to
rely on. Aerodynamic instability, on the other hand,
is the official hypothesis put forward by Williams
during the manslaughter trial in the late ‘90s. Its
origins go back to the dark days and weeks following
the tragedy when the engineers in the factory pored
for hours over the telemetry data and video footage.
The sequence of events began to unravel with the
start line collision between Lehto and Lamy, which
brought out the safety car and prevented Senna from
building a comfortable margin over Schumacher in
the Benetton. The Opel saloon used in Imola as safety
car was too slow for F1 speeds and by the end of Lap
5 the tyre temperatures (ditto tyre pressures and ride
heights) dropped dramatically - perhaps by as much
as 25 percent45 - leading to a 7psi tyre pressure loss95
and ride height drop of about 4-5mm14.
When the race resumed with Lap 6, it is likely Senna
wasn’t aware that Schumacher was planning three
refuelling stops to his two, and that he was trying to
keep at bay a lighter car. He had zero points in the
championship and simply had to win the race.
Making a break from his pursuers on the first lap had
always been his trademark and he wasn’t going to
back off now just because the car was a handful on
cold tyres.
There is also every reason to believe that Senna was
well equipped to cope with low tyre temperatures. As
he revealed in an interview for Autosport11 in early
1993, he had a secret system for warming up his tyres
and the first lap after the restart backs this claim up:
on Lap 6, he managed to navigate Tamburello on cold
tyres and clocked 1min 24.887s, which was a very
competitive lap time that remained the third fastest
lap of the whole race. Only towards the end of the
race did Hill and Schumacher better his time.
Although we do not have precise information about
the state of Senna’s tyres going into Lap 7, the logical
conclusion from the above must be that the ride
heights had gone up as the tyres got warmer and as
the car burned one more lap worth of fuel. That
assertion is reinforced by the severe distortion of onboard images from Lap 6 and by the amount of
91
sparks generated by the rear of the Williams in
Tamburello: the sparks do not look as strong on Lap
7 as they do on Lap 6 (although the footage is taken
from Schumacher’s on-board camera rather than a
stationary RAI TV).
A lot has been written about Senna’s state of mind
going into the race but as explained earlier, in all
likelihood he was fully focused on one thing –
winning. Few are better placed to comment on
Senna’s psyche than his last teammate Damon Hill.
Hill remained tight-lipped about the accident until
the tenth anniversary of Senna’s death, when he
suddenly decided to speak out in a column headlined
‘The harsh fact is that Ayrton made a mistake’,
published by The Times37 in April 2004.
In the article Hill didn’t mince his words, arguing
that Senna’s fiercely competitive nature and desire to
cultivate a ‘demigod’ image made him disregard the
risks of taking Tamburello flat out in circumstances
in which the sensible thing was to lift. Hill is the only
living person who knows what it felt like to drive the
Williams FW16 on May 1, 1994 through Tamburello
on cold tyres, and the memory is enough to convince
him that Senna made a mistake.
Hill has a point, but it was easier for him to lift and
take it easy through Tamburello. He was running in a
distant fourth place and didn’t have to content with
Schumacher breathing down his neck. This was a
motor race, after all, and Senna’s ability to transcend
the ordinary was what set him apart from his peers.
The next best qualified person to voice an opinion is
Michael Schumacher, who had seen the accident
unfold before his eyes and it was an experience that
affected him profoundly. Schumacher noticed that on
the lap prior to the accident Senna’s car touched the
ground in Tamburello and appeared skittish, and that
on the next lap, in Schumacher’s words, “he just lost
it.”
By contrast, former rival-turned-TV-commentator
Martin Brundle years later (2011, 2014) noted46,88
that the on-board footage from Lap 6 on cold tyres
looks serene, with smooth steering inputs and no
wild oversteer or understeer moments. Senna looks
fully in control, not like a driver pushing the limits.
Although Senna had made plenty of mistakes in his
career, many find it hard to believe that the trigger
for the accident wasn’t some kind of a mechanical
failure. Indeed, all previous crashes in Tamburello
were caused by a technical problem. The same
92
opinion was voiced during the trial by rival F1 drivers
Michele Alboreto and Pierluigi Martini, and designer
Adrian Reynard; and recently in the critically
acclaimed movie Senna also by Alain Prost and
McLaren chairman Ron Dennis1. Former McLaren
Team Coordinator Jo Ramirez muses in his Memoirs
of a Racing Man (Haynes Publishing 2005)47 that
“despite all the investigation, deliberations and
conclusions over the years, I remain unconvinced
that the best racing driver in the world could make a
mistake at Tamburello.”
If driver error came into the equation, it was an error
of judgment rather than a driving mistake. Senna
must have been convinced he could keep the FW16
under control on the bumps in Tamburello regardless
of its iffy aerodynamics. After all, he had been doing
exactly that lap after lap during the Imola weekend,
up to and including race Lap 6.
UNDERBODY STALL
As mentioned in the first part of the book, the FW16
was an aerodynamically troublesome car. Both Senna
and Hill got caught out on several occasions in the
first three races of the 1994 season, notably in Japan
where, bizarrely, they both lost control at exactly the
same spot on the circuit. In addition to that, Senna
spun out of the race in Brazil and then in Friday
qualifying at Imola, while Hill spun again during the
race in Japan and also in qualifying at Imola. The
spins happened in low speed corners and usually on
the exit. The front wing was to blame: it sat too close
to the ground and once the car hit a bump exiting the
corner, the front suddenly gripped and the driver lost
the rear10.
The FW16 was also prone to severe underbody
stall9,85. The floor of a modern F1 car at the rear is
designed to exploit the Venturi effect, which says that
air accelerates when passing through a constricted
channel while its pressure drops. This is dictated by
the law of conservation of energy: the total energy
within a fluid system must remain constant and if the
potential energy (air pressure) is reduced by a
constricted channel, the kinetic energy of the fluid
particles (air velocity) must increase accordingly so
that the total energy is conserved. In F1’s parlance,
this constricting channel is called diffuser (Figure 43)
and it is the diffuser that in certain circumstances
induces underbody stall.
Diffuser on the Williams FW15D-Renault
93
Figure 43 - Diffuser channels at the rear of the Williams FW16B-Renault, which replaced the unloved FW16 from the
German Grand Prix 1994 (photo courtesy of Remi Humbert and Joachim Kutt, GurneyFlap.com). Regulation changes
introduced after Imola forced fundamental modifications and the diffuser raced by Senna at Imola was larger and looked
different80 .
94
the ground (downforce) and makes it go faster
around corners. The diffuser further increases
downforce by sucking the air out from underneath
the car48.
Figure 44 - The diffuser (in red) creates an area of faster
flowing air at lower pressure between the underbody and
the track (2). The different colours represent different
airflow velocities, from the peak velocity where the flat
bottom of the car transitions into the diffuser (violet, 4),
to gradually reducing velocities (5, 6, 7) until the air
reaches the ambient pressure (red, 8)49.
Diffuser speeds up the airflow underneath the whole
car, creating an area of low air pressure due to the
Venturi effect (Figure 44). This low pressure causes
pressure difference to build between the car’s
underbody and bodywork, which pushes the car to
The diffuser channels gradually increase in volume
and provide space in which the faster air flowing
underneath the car is allowed to slow down and
expand49. The net result is a smooth transition from
high to normal air velocity and from low to high air
pressure that maximizes the downforce (Figure 44).
The slope of the diffuser is very important and it
must be designed to match closely the airflow
velocities through the channels otherwise the air will
separate from the diffuser’s roof and sides50. This
separation is known as underbody stall.
Underbody stall differs from aerodynamic stall,
which affects wings on aircraft and F1 cars alike
(Figure 45). A wing is stalled once a certain angle of
attack is exceeded, whereas diffuser’s angle of attack
stays almost constant and airflow separation takes
place when the air velocity through the channels
drops enough for the air to detach from the diffuser’s
surfaces.
That can happen if the car’s ride heights change
abruptly (lifting, braking, acceleration). Under
95
braking, for instance, the front of the car dives down
while the rear rises up, allowing more air in from the
sides. The air pressure underneath the car increases
and the air velocity decreases to the point where the
diffuser stalls. The diffuser can also stall on a bump
because once the chassis hits the ground (bottoms
out) the air flow underneath the car is significantly
reduced or completely blocked51.
A sudden loss of downforce ensues and if the car is
going round a corner the rear may step out and the
driver will experience oversteer.
Tamburello was bumpy: there were three bumps on
the apex and if the driver went straight over them the
car travelled a slightly shorter distance but it was
running on the ground and the aerodynamic loads
Figure 45 - Aerodynamic stall explained - a wing generates downforce when the air flows smoothly and remains ‘attached’
to the upper and lower surfaces (left). The curvature of the wing bends the streamlines and deflects masses of air upwards,
causing air compression and higher pressure at the upper surface. Correspondingly, with less air to fill the space at the
lower surface the pressure drops and the whole wing is pushed down. Once a certain angle of attack is exceeded (right),
however, the air becomes turbulent and air flow separation occurs. The wing ‘stalls’ and loses its downforce52.
96
were going through the chassis rather than the tyres.
The car was no longer sprung by the suspension and
it was effectively steered by the skid-plates instead of
the rear wheels. During the Imola weekend, Senna
warned Hill about the tight inside line as it made
their Williams feel unstable, whether out of genuine
concern or because it gave him an additional
advantage over his less experienced teammate. Hill
accepted Senna’s advice and avoided the bumps by
driving in the middle of the track, especially on cold
tyres and heavy fuel. It cost him a few metres but the
car felt more predictable. Senna stuck to the bumpy
tighter line.
From the distortions visible on the on-board footage,
we can establish that Senna’s car hit undulations
during 11.08-11.16s (entry onto the seventh dark strip
and the first bump), at 11.22s (second, minor bump),
and 11.26s (third, biggest bump). This fits the
aerodynamic instability theory well; nevertheless, it
doesn’t answer the key question whether the
undulations gave rise to just bottoming, or bottoming
that resulted in underbody stall and oversteer.
Bottoming at Tamburello was the norm and Senna’s
car invariably generated five distinct showers of
sparks on the seventh strip throughout the whole
weekend. The front skid-plates were heavily worn7,95,
but it’s unclear whether the damage occurred on Lap
6, Lap 7, or in the course of the accident.
There is a textbook example of bottoming that led to
oversteer on the fifth dark strip: the revs spike by
400rpm to 14,204rpm as the chassis touches the
ground (9.68s) and then, as the tyres grip fully again,
fall down to 13,561rpm (9.72s). Simultaneously, the
steering feels lighter (strain drops from -18 to
-12N/m2 at 9.70s) and subsequently recovers (strain
doubles from -12 to -22N/m2 at 9.80s).
The pattern observed in the critical period when
Senna loses control (11.10-11.18s) is somewhat
different. First of all, the engine revs rise steadily in
an almost linear fashion until 11.10s (14,014rpm) and
then plateau until 11.22s (13,800-13,900rpm). There
is no observable spike in revs that would indicate
heavy contact of the chassis with the ground. Second,
unlike on the fifth dark strip, there is no decrease in
Steering strain that would signify that the steering
suddenly got light due to oversteer: between 10.8011.08s the telemetry shows a steep increase in the
strain on the column (from -10 to -26.9N/m2)
consistent with substantial steering effort left, and
the high load on the column doesn’t drop and is
sustained until at least 11.20s (-26.3N/m2).
97
Lateral g-force and speed of the rear wheels are
compatible with oversteer and aerodynamic
instability: g-force keeps increasing from 2G (10.80s)
to 3.27G (11.10s) to the maximum recorded by the
telemetry 3.52G (11.20s). Also, the Williams must
have lost some traction on the undulations because
the rear wheels are rotating 4km/h faster than is the
speed of the car (11.10-11.18s).
The car’s direction change tells a similar story. Based
on the ‘I Pilotissimi’ reference point the car’s rate of
turning left against the static background is greater
since 10.76s as the Williams nearly recovers to the
racing line taken on the two Friday laps used as
comparison (blue line meets the red and green lines
in Figure 10).
The rate of turning left then further increases in the
critical period 11.10-11.18s (the blue line overtakes
the red and green lines) and the momentary angle
between the car’s reference axis and the inside kerb
also shoots up (Figure 11). All these observations are
symptoms of oversteer, although the brusque
direction change seems to have been initiated before
Senna entered the seventh strip of dark tarmac and
the bumps (around 10.90s).
The neat explanation also gets complicated by
Steering pressure difference (STGACT), which
measured hydraulic pressure difference across the
steering rack. The difference more than doubles in
period 11.00-11.08s (substantial turning left) and it
stays high during 11.10-11.18s (continued turning and
power assist left). Correspondingly, pressure in the
hydraulic cylinder on the left hand side of the
steering rack (STGPR) is at the lowest level recorded
on the telemetry.
The movement of the left front tyre casts further
doubt on the aerodynamic instability theory (Figures
13 and 14). The tyre is pointing more or less straight
during 10.50-10.72s but then it starts turning
progressively left until a peak at 10.94s. As the
Williams enters the seventh dark strip of new tarmac
the tyre flicks further left around 11.06s before it
reaches what appears to be the third peak at 11.18s
just when Senna loses control. If the rear has stepped
out due to underbody stall and the car is
oversteering, Senna is making the oversteer worse by
simultaneously turning in the direction of the slide.
That is atypical because usually the front wheels do
not move while the rears break traction and start
sliding.
98
A slow motion analysis of oversteering cars backs this
observation up: the random sample studied includes
Nick Heidfeld at Sepang 2008, Nico Rosberg in
Monaco 2008 and 2011, Kimi Raikkonen at Spa and
crashing into the back of Adrian Sutil in Monaco
2008, Lewis Hamilton at Silverstone 2008 and in the
last corner in Valencia 2010, Pastor Maldonado
crashing out in Australia 2011, and Daniil Kvyat and
Adrian Sutil in Monaco 2014. Unless the slide at the
rear ‘overtook’ Senna’s increased steering input left,
it is likely his Williams turned sharply left not
because of oversteer alone but because the steering
also pointed the car that way.
At 11.20s, Senna starts applying opposite lock to
correct the slide. That is not always the best response
to high-speed oversteer. On American ovals, where
high-speed oversteer is common, drivers learn
quickly not to apply opposite lock because once the
car grips again the correction spits them
unceremoniously to the outside wall. Drivers instead
steer into the slide so the car spins harmlessly down
the track, avoiding a potentially massive accident7.
Senna was conditioned by years of experience to
apply opposite lock in such circumstances and unlike
on ovals, where there is plenty of room down the
track, he would have gone head-on into the Armco
barrier on the inside of Tamburello.
The car has now recovered full traction as there is
minimal speed difference between the car and rear
wheels. For the first time on Lap 7, Senna lifts
(throttle goes down to 50 percent between 11.2011.26s) and the revs drop by 500rpm accordingly. He
must have decided to lift around 11.10s or earlier,
either in response to the oversteer or because he
anticipated the car to get unsettled based on his
experience from Lap 6. Steering strain remains high
at 11.20s (-26.3N/m2) as he is about to apply opposite
lock, while the Steering pressure difference across the
steering rack starts to drop drastically (STGACT goes
from 623psi at 11.20s to -12psi at 11.30s).
In the brief period between 11.20s and 11.26s, the left
front wheel returns swiftly to the neutral position and
points straight, the angle between the car’s axis and
the inside kerb returns to a level observed just before
the car entered the seventh dark strip, and the rate of
direction change against the ‘I Pilotissimi’ sign
reduces so that by 11.26s the car is heading straight
for the sign. All of the above indicates the car is
responding instantaneously and that its direction
change doesn’t lag behind the rotation of the left
front tyre.
99
By contrast, the slow motion analysis of oversteering
cars discussed earlier reveals a delay between the
application of opposite lock and the subsequent
correction in the car’s trajectory. The pendulum
effect of the slide keeps the car oversteering in one
direction even though the driver has already pointed
the front wheels the other way, because the front
tyres need some time to bite and regain sufficient
grip in the new direction before they can correct the
slide.
This delay is even more dramatic in accidents in
which the driver loses the car on a bump (the rear
bottoms out). Four confirmed examples in recent
times include Alex Wurz’s crash in Monaco 1998,
Allan McNish’s crash in Suzuka 2002, David
Coulthard’s crash in Monaco 2008, and Sergio
Perez’s crash in Monaco 2011. In all four instances,
the direction change induced by the bottoming is so
forceful and so sudden that the drivers have no hope
of catching the slide despite the swift application of
opposite lock, and the cars stay on the crash course
despite the fact that the front wheels are clearly
pointing the other way. This indirect evidence adds
weight to the argument that had Senna lost the car
purely as a result of bottoming on a bump, he would
have ended up crashing head-on into the Armco on
the inside of Tamburello (like Wurz, Coulthard, or
Perez did), or he would have experienced a major
‘tank-slapping’ moment similar to McNish’s at
Suzuka. Instead, Senna’s car straightens within
0.08s.
There is a heavy bottoming moment at 11.26s (the
third bump), which is visible on the telemetry as a
600rpm spike in revs (from 13,445rpm to 14,077rpm)
and as severe picture distortion on the on-board
footage. However, Senna is already out of control,
there is no oversteer, and the steering doesn’t go light
(Steering strain remains relatively high at -18.5N/m2
at 11.30s).
According to the aerodynamic instability theory, the
Williams now understeers heavily at the front and the
rear grip is acting sideways, prompting the car to
turn right. In terms of the car’s trajectory against the
‘I Pilotissimi’ reference point, the Williams initially
rushes straight for the sign (11.26-11.40s), then pulls
gently right (11.40-11.56s), then gently left (11.5611.70s), and finally it heads straight for the sign until
the on-board video ends (11.70-11.84s).
The left front tyre follows a similar pattern: it turns
slightly right (11.28-11.52s), then straight or slightly
left (11.54-11.66s), then slightly right or straight
100
(11.68-11.84s). Correspondingly, lateral g-force falls
to virtually zero (11.30-11.58s), then pulls gently left
(11.60-11.88s), and finally gently right (11.90-12.18s).
In just over half a second, the strain on the steering
column drops to zero in a quadratic curve (11.3011.90s) and Steering pressure difference across the
steering rack also falls off the cliff to zero (11.3011.68s). There is a minor pressure difference (11.7011.88s) equivalent to the effort exerted by Senna
when he was easing his car into Tamburello, and then
the pressure difference becomes negative, implying
that the wheels are turned slightly right (11.9012.38s).
The above data confirms there is no or very little
direction change after 11.26s, mainly because the
sharp direction change right was accomplished
during the brief correction period (11.20-11.26s).
Senna hesitates with the throttle (around 50 percent
during 11.24-11.44s), then lifts completely (11.50s),
and after switching his right foot to the brake pedal
begins desperate braking (11.68-12.16s).
because ‘keeping the steering straight’ more often
than not involves a strain condition in the steering.
What is puzzling is that unlike all the drivers studied
in our oversteer analysis, Senna didn’t fight the car
and just kept the front wheels approximately straight.
He had a full second to take the corner, to react and
to wrestle the car like the other drivers – apply
steering lock left, force the car into a tank-slapper or
into a spin. But he didn’t; either he chose not to or he
wasn’t able to.
The aerodynamic instability hypothesis is silent on
the movement of the yellow button on the steering
wheel in the last three seconds before the crash other
than saying that the flexing observed was normal as
demonstrated by David Coulthard in the team’s
museum. The hypothesis would gain further strength
if it were able to reproduce the observed flexing
under a defined set of racing conditions, especially
because no such flexing exists in the twelve minutes
of on-board footage analysed from Brazil, Japan, and
Imola (see Chapter 6).
The power steering and Steering strain telemetry
correlates well with the hypothesis’ claim that from
11.26s on Senna kept the steering centred for optimal
braking, although it would be next to impossible to
distinguish that from any other condition of no input
101
CHAPTER 14
Broken steering column
HYPOTHESIS
The accident was caused by material fatigue in the
redesigned section of the steering column. The
column partially broke and felt ‘mushy’ but
continued transmitting sufficient torque for getting
the car round Tamburello (‘partial column failure’
version), or it broke off and stopped transmitting
torque even before Senna left the track (‘complete
column failure’ version).
The tragic events were set in motion once the safety
car peeled off into the pits and the race resumed with
Lap 6. The initially bumpy ride on cold tyres
accelerated fatigue in the steering column near the
support strut and a crack propagated through its
upper crescent (the crack may already have
developed earlier during the weekend). Three
seconds before Senna crashed in Tamburello (8.20s
into Lap 7), the fatigue extended far enough to impair
the steering. The combination of high downforce and
Senna’s arms resisting lateral forces bent the steering
column to such degree that the steering wheel began
falling into his lap. This caught him by surprise and
momentarily upset his concentration.
In the ‘partial failure’ version of the hypothesis,
Senna could no longer steer the car with the usual
precision as he tried to hang on to it at 300km/h. He
initiated a horizontal correction left with the steering
at 10.74s just before he entered the seventh dark strip
of new tarmac, probably to bring the car back to the
normal racing line from which he had started to
deviate after 10.10s, or because he finally noticed the
unusual position of the steering wheel. As his
fingertips couldn’t feel the tyres through the steering
anymore, the wheels turned more than was necessary
and the direction change towards the inside of the
corner continued unchecked until 11.18s.
He began to lift at 11.20s but he must have made that
decision at 11.10s or earlier; in other words even
before he experienced the sudden twitch left. He bent
the faltering steering column back up (11.20-11.30s),
and the sharp vertical movement straightened the
car’s trajectory. The car received a massive jolt on the
third bump and Senna was now expecting the worst.
He decided to keep the abnormally responding
steering straight although the partially severed
column was still capable of handling enough steering
102
torque to get him round a long left hander like
Tamburello53. The steering wheel oscillated up and
down as Senna tried to keep it more or less straight.
He lifted completely by 11.50s and then went hard on
the brakes at 11.68s. The column broke off once the
right front wheel hit the wall and bounced back
against the cockpit54.
In the ‘complete failure’ version of the hypothesis, the
column broke off before Senna left the track. The
severe flexing observed after 8.20s meant that the
column became distorted beyond its rotating
clearance in the bushing, and the distortion dragged
Senna’s input enough to prompt the sudden twitch
left. The column then seized in the bushing, which
provided the resistance to complete the break and at
the same time registered steering strain on the
telemetry just as if the driver was resisting the front
wheels.
103
STEERING COLUMN FATIGUE
As stated earlier, the steering column was altered at
Senna’s behest before the start of the season and
regardless of the pros and cons of the new design, the
modification by default increased the number of
possible failure modes because of the system’s
inherent complexity.
The column assembly now measured 910mm and
consisted of three elements rather than one: the
original tube (steel T45, diameter 22mm, wall
thickness 0.9mm), attached at the upper end to the
steering wheel; a smaller tube (steel EN14, diameter
18mm, wall thickness 1.2mm), fitted in the middle;
and the original tube again (steel T45, diameter
22mm, wall thickness 0.9mm), connected at the
lower end to the pinion and steering rack.
T45 and EN14 are compatible steel made of carbon
manganese that conform to the Aerospace
Specification laid down by British Standards (BS).
T45 is commonly used in motorsport due to its high
strength-to-weight ratio55 whereas EN14, used by
Williams for the newly inserted piece of smaller
diameter, has a medium tensile with excellent
resistance to shock and its chemical composition
makes it suitable for welding56.
The machined part (red colour in Figure 46) was
inserted into the original tube towards the steering
wheel but fit over the original tube where it went
through the support strut. There were two
circumferential welds (red triangles in Figure 46) –
one at the point where the machined part went inside
the original tube, and the second just past the
support strut where the machined part fit over the
original tube.
The modification called for welding as the tubes used
were of different material and different diameter. All
publicly available drawings19 show the tube joints as
fitting size-to-size while simply annotating the outer
diameter (OD) and wall thickness. When the
assembly is drawn strictly to the annotated
dimensions, however, it shows the dimensions
cannot be accepted uncritically because the upper
joint has plenty of wobble room (Figure 47). One can
safely assume the splice part would not have been
machined for such a loose fit.
104
Figure 46 - Steering column specification derived from three (contradictory) drawings used during the manslaughter trial
and later released by CINECA on their website. Red colour represents modified material inserted before the start of the
season. Blue colour represents the bushing and support strut. The orange line denotes the point where the column broke off.
105
Figure 47 - Detail of the steering column, drawn strictly to annotated dimensions. Orange line denotes the break.
Note the wobble room in the upper joint, which most likely existed only on paper (nominal dimensions).
Tony Woodward makes the following observation:
“It is likely the part sketches presented in court were
gross approximations giving nominal dimensions
rather than actual dimensions. The part was surely
intended to fit the ID [inside diameter] of the tubing.
Tubing is manufactured oversize on the outer
diameter and can vary somewhat in wall thickness.
As a result, fitting machined pieces to it for welding is
not predictable to the extent that they can be preprogrammed on CNC machine tools with confidence.
The actual insertion invariably requires some hand
work and it’s unlikely that the modified column
would have been created entirely by computercontrolled methods, which means the actual
dimensions were arrived at during fit-up and as such
did not need documentation.”
The quality and durability of the weld would have
been heavily influenced by the welding procedure
employed, and any minor process non-adherence
could have resulted in a somewhat embrittled weld in
106
the heat-affected zone. Even so, it is important to
emphasize that no issue was found with the weld
itself – it was on the other side of the support strut
and nowhere near the break.
As part of the investigation conducted by the Italian
authorities in 1995, the steering column underwent
metallurgical analysis in Pratica di Mare, the Italian
military aerospace laboratory, and also in one other
independent laboratory. The analysis revealed a
fatigue-related failure caused by a downward bending
force. The fatigue occurred in the top semi-circular
half of the modified section of the column and
extended around its circumference. It was exactly at
this point where the column broke off - at the
reduction of diameter where the filleted corner of the
modified section transitioned into the smaller
straight diameter. All of the smaller diameter section
stayed with the upper column.
Both the prosecution and Williams agreed that
sooner or later the column would have snapped; what
they couldn’t agree on was the extent of the fatigue
(40-60 percent prosecution figures, 21–40 percent
Williams figures), and whether it weakened the
steering enough to hinder Senna’s ability to steer7,57.
The fatigue is clearly visible in the photos33,81, 91 and
illustrations (Autosprint90, L’Automobile79): the
surface of the fatigue zone - the area of the initial
slow crack propagation - exhibits a smooth rubbed
and velvety appearance which extends from nine
o’clock position to about one o’clock at minimum
(Figure 48).
Figure 48 - Driver’s view of the cross section of the
modified steering column. Red colour denotes the
minimum area of crack propagation (fatigue zone),
black colour the area of final failure (instantaneous
zone), grey colour the disputed area of fatigue.
Drawn to scale.
107
Conversely, the area of final failure (the rupture
zone) where the column broke off is jagged and
brittled. The face of the fracture looks typical of a
part that ruptured in stages. Higher resolution
photos would reveal more about the forces involved –
whether they were predominantly torsional or in
bending, their magnitude, fluctuations, and the
estimated length of time from crack initiation to
rupture58,59.
The modified section of the column had an
inadequate fillet radius and the crack propagated
from there3,60. There were also tool marks left on the
piece near where it fit over the original tube3,61,62.
Unaware of the official report’s findings, Tony
Woodward took one look at the drawings and the
photos of the broken column and observed that “the
piece that broke had a stepped cross section. This is a
textbook illustration of a sectional discontinuity
creating a stress concentration. It begs the question
how large and smooth (or small and rough) was the
fillet radius in that corner, and did the crack
propagate from a stress raiser such as a tool mark.”
The fatigue was most likely caused by a severe stress
concentration59,63-66 at the change in diameter
because the original part of the column extending
from the support strut was disproportionally stiffer
than the new element that fit over it. The stiffness of
a shaft increases as the fourth power of its diameter
(for tubing one subtracts the inside diameter’s
equivalent stiffness) and this property is not
necessarily obvious at first glance. Tony Woodward
again: “Imagine the failure mode of a fishing rod if it
were tapered in the wrong direction, or had a
discontinuity like this column. The column protruded
quite a distance from its bearing point, and the
bending load on a steering column is more than
people think. The localized stress concentration
would just about guarantee Senna’s upper body
would break it off there.”
Although Williams accepted the results of the
metallurgic analysis, they contested its interpretation
and disagreed that the accident was caused by the
broken column54. The team argued that the rupture
zone was more consistent with a heavy impact
brought on by the right front wheel hitting the wall
and then the side of the cockpit67.
The fatigue zone estimates vary from 21 to 60 percent
of the column’s circumference, but that in itself says
little about the likelihood of the column snapping
because the size of the fatigue zone depends on how
heavily the part is stressed at the time of rupturing.
If a given material is highly overstressed, the fatigue
108
zone is very small compared with the rupture zone, if
the overstress is low the fatigue zone is much bigger
than the rupture zone58,68. What the 21–60 percent
figure tells us is that Senna’s column was exposed to
medium to high overstress when it snapped.
Williams also reported that a fatigue failure would
only have arisen after 350,000 stress cycles (a cycle is
one application of load on a component), but the
steering column had undergone a mere 27,000 cycles
when inspected after the Pacific Grand Prix in
Japan69.
Stress tests are usually performed on servo-hydraulic
rigs using load that varies randomly in amplitude and
frequency70,71 and the method assumes the material
sample is entirely representative. The column raced
by Senna bore a tool mark, whereas the column that
withstood 350,000 cycles on a test rig before failing
did not, and once a stress raiser in the form of a tool
mark enters the picture, predictions about fatigue life
become irrelevant53. More importantly, low cycle
fatigue can occur near stress concentration points
such as holes and fillets63,71 and tool marks in the
surface greatly accelerate the time to rupture because
such microscopic discontinuities are natural starting
points (stress raisers) for the propagation of cracks64.
Senna’s steering column cracked near a fillet and
near a tool mark.
Forced to choose between the ‘partial failure’ and
‘complete failure’ hypotheses, most experts are
inclined to favour the former. Adrian Reynard, who
participated in the trial on behalf of Senna’s family,
thinks the lower crescent of the column (the rupture
zone) held out until the impact7. Aerospace engineer
Augusto Suppo, called by the defence, testified that
the lower crescent of the column was strong enough
to get Senna round Tamburello72. Tony Woodward is
of a similar opinion: “Unless the column twisted
apart, Senna would have retained rotational input.
Examination of the column would have revealed a
failure in torsion and there does not appear to have
been one. It looks like a tensile failure in bending
with just a hint of torsion. I think it wasn’t broken all
the way through, and I would expect a low-carbon
steel component of that diameter and wall thickness
in a partially severed condition to remain capable of
transmitting 25 inch-pounds of torque, or enough to
retain practically full steering input as long as Senna
held the wheel so as to keep the column
approximately straight. But once you lose it at that
speed you quickly run out of room to gather it back
up, steering or no steering.”
109
BOTTOMING EFFECT & OVERSTEER
Contrary to some of the theories discussed earlier,
this hypothesis argues that Senna’s Williams didn’t
bottom out heavily and didn’t oversteer because of
underbody stall. Detailed arguments for and against
are covered in the section dealing with aerodynamic
instability; a brief summary will suffice here to
demonstrate that the evidence for oversteer is
inconclusive: bottoming at Tamburello was the norm
and Senna had no major issue with it throughout the
weekend or on Lap 6; tyre pressures must have
markedly improved as he set the third fastest lap of
the race on Lap 6; the on-board picture distortion is
much more pronounced on Lap 6; the sparks flying
from underneath the Williams don’t look as strong on
Lap 7 as they do on Lap 6; there is no sudden spike
and drop in revs, suggesting the car bottomed out
and then recovered grip; there is no considerable
drop in steering strain signalling that the steering got
light; and the video analysis of oversteering cars
indicates that the dynamics were different in Senna’s
case. The on-board footage does become distorted
and the rear wheels do rotate 4km/h (1.1m/s) faster
than is the speed of the car (11.10-11.18s), but this is
not abnormal in the context of values observed since
7.98s, which range from -2km/h to +4km/h. The
difference in speeds could represent just a minor loss
of traction on the bumps and not necessarily a heavy
bottoming event accompanied by underbody stall.
Another argument against oversteer is the fact that
Senna decided to lift even before his car turned
sharply left (11.10s or earlier, if we consider 0.10s
reaction time), and that he decided to stop the car
rather than just lift and gather it up again. He had a
full second to steer the car left in what was a shallow
corner, and he was already at the geometrical apex.
On-board footage from the Imola weekend shows
him at this point of the circuit running the car wide
towards the outside of the track. As Mauro Forghieri
points out in the National Geographic documentary,
the likely explanation for Senna’s decision to lift and
start braking is that he’d realised there was
something wrong with the car.
During the trial7, Williams stated that the telemetry
isn’t compatible with a steering failure because Senna
would have decided to lift and brake around 11.36s
(0.10-0.15s after losing the steering) rather than at
11.20s, and that he would not have hesitated with the
throttle for another 0.20s. Not necessarily. The brief
hesitation looks on the telemetry more like an
uncontrolled stab on the throttle, and it coincides
with the front of the car bottoming out on the third
110
bump: the shock could have disturbed Senna’s foot
on the throttle pedal just enough to interrupt the lift.
Likewise, it would have taken him until 11.36s to
react only if the steering column had snapped
without warning. In Senna’s case, however, the
steering wheel began to flex at 8.20s and that gave
him at least three seconds to suspect something was
amiss and convinced him to stop the car.
TRAJECTORY & DIRECTION CHANGE
Car trajectory and direction change were
comprehensively covered in the section on
aerodynamic instability; nonetheless, both fit the
steering failure hypothesis as well: the left front tyre
rotates left from 10.74s, it reaches the first peak at
10.94s just before the car crosses the dark strip, and
then it rotates left again at 11.06s and 11.18s. The
car’s rate of turning left increases from 10.76s and
culminates at 11.18s, while the momentary angle
between the car’s reference axis and the inside kerb
also goes up. The interpretation of these observations
is that the car turned towards the inside of the corner
because the front wheels pointed it that way - either
due to loss of accurate steering input or because the
wheels moved uncontrollably on the bumps once they
stopped being firmly restrained by the steering
column.
During 11.20-11.26s, the left front tyre returns swiftly
to its neutral position and the rate of direction
change against the ‘I Pilotissimi’ sign and the angle
between the car’s axis and the inside kerb reduce
accordingly. Again, the conclusion is that the car
straightened its trajectory because the front wheels
pointed it that way – either due to Senna’s correction
with the imprecise (but still partially functional)
steering, or because the front wheels centred back to
their neutral position once they were no longer
restrained by the steering column. There is little or
no observable direction change after 11.26s.
Direction changes against the ‘I Pilotissimi’ sign after
10.52s (Figure 10) reveal an unusual pattern that
hints at the steering becoming imprecise: while the
two sample laps from Friday display gradual
direction change as Senna keeps constant steering
lock to navigate Tamburello (the green and red lines
follow a near-perfect straight line), the blue line from
race lap 7 looks erratic and wayward in comparison.
There is less direction change (10.52-10.76s), then
more (10.76-11.10s), and then significantly more
(11.10-11.18s).
111
One of the arguments put forward by Williams
against the steering failure theory has been the
evidence that Senna’s car didn’t leave the track on a
tangent but instead straightened (turned right) after
the initial twitch left. Adrian Newey made this point
on at least two separate occasions, back in 2000 for
Autosport10, and in May 2011 in the interview for
The Guardian36.
“The honest truth is that no one will ever know
exactly what happened. There's no doubt the steering
column failed and the big question is whether it
failed in the accident or did it cause the accident? It
had fatigue cracks and would have failed at some
point. There is no question that its design was very
poor. However, all the evidence suggests the car did
not go off the track as a result of steering column
failure. The telemetry we have, and the view from the
on-board cameras, show that what happened does
not fit with a steering column failure. The car didn’t
go straight on as most people believe, it physically
turned right.”
I have great respect for Adrian Newey’s genius and
his towering achievements over the past thirty years,
but there is the possibility that a car veers right when
it experiences a steering failure. A case in point is the
steering failure suffered by a Shelby Mustang GT350
driver during the 2008 Phillip Island Classic in
Australia (see video and Figure 49).
Shelby Mustang GT350 steering failure
This unique footage was captured by Mark Clair, the
founder of RaceRecall, and it is reproduced here with
his kind permission. RaceRecall specializes in onboard video systems and for the race at Phillip Island
it equipped the Mustang with three cameras mounted
inside the car. The synchronized feed from the three
different angles not only allows studying the
trajectory as the car is leaving the track, but it also
pinpoints the exact moment when the steering fails.
When the steering wheel detaches from the column,
the Mustang doesn’t leave the track in a straight line:
it actually keeps turning left, then it veers right, and
only then does it plough straight on. The accident is a
textbook demonstration that Senna’s twitch left
(11.10-11.18s) and the subsequent turn right (11.2011.28s) could have taken place under broken steering
column conditions.
We can also plot direction change of the Mustang
against a fixed point in the background, using as a
reference the black and white pattern on the bridge
that crosses the track (as we did with Senna’s car
position against the ‘I Pilotissimi’ sign, Figure 50).
112
Figure 49 – Top left: the exact moment when the steering
wheel comes off the Mustang GT. The yellow line tracks the
car’s direction change against the black and white pattern on
the bridge towards which the Mustang is travelling, the orange
line is ‘fixed’ to the bonnet.
Bottom left: three frames since the steering wheel has come off.
Notice that the car is still turning left (the orange line ‘fixed’ to
the bonnet has moved away from the yellow reference line).
Bottom right: seven frames since the steering wheel has come
off. The car turns sharply right compared with the previous
frame (the orange line ‘fixed’ to the bonnet moves back very
close to the reference yellow line). Courtesy of Mark Clair,
enhanced by author.
113
Figure 50 - Direction of travel based on the ‘I Pilotissimi’ sign (Williams FW16) and the pedestrian bridge (Mustang GT).
The graph for Mustang GT is offset by 20 percent to bring the two lines closer together for easier comparison. The orange
vertical line pinpoints the moment when the steering wheel comes off on the Mustang.
114
When the position change is tracked over time and
adjusted for the speed differential of the two cars, we
get strikingly similar graphs. The Mustang accident
was clearly caused by a steering failure and its
dynamics can explain the directional changes of the
Williams as well.
STEERING TELEMETRY
Within a month of the accident, Williams produced a
comprehensive technical report on the steering
telemetry that according to Patrick Head
demonstrated to the Italian authorities that steering
data couldn’t be recorded with the wheel physically
separated from the column3,74. Because the report
was never published, it is unclear whether ‘steering
data’ represents steering angle applied on the
steering wheel, power steering pressures, steering
strain, or all of the parameters.
Steering angle is measured by a potentiometer that
tracks either linear or angular movement. It can be
installed on the steering rack (linear), on the end of
the pinion (angular), or under the steering wheel
(angular). Tom Rubython in his book The Life of
Senna states21 that Williams deployed the last option
and that the data was captured by the black box
damaged in the crash, which means the steering
angle applied by Senna during the accident cannot be
determined. On top of that, Adrian Newey recently
confirmed that in fact there was no position sensor85.
Power steering pressures would be recorded
regardless of the state of the steering column as long
as the hydraulic circuit remained intact. All evidence
points to that being the case – there is no
catastrophic drop in steering pressures and no flow
interruptions. The Mustang accident proves that even
without steering, a car can still twitch left or right,
and for that reason the pressure in the steering rack
cylinders (STGPR) doesn’t reveal whether the column
failed or not. What the STGPR trace does say is that
the pressure is at its lowest during the critical period
when Senna loses control (a sign that the wheels are
turning firmly left), and that after 11.30s the pressure
stays around 400psi (wheels pointing more or less
straight) except for period 11.68-11.88s (wheels
gently left, possibly due to the onset of hard braking).
This brings us to Steering pressure difference
(STGACT), which measured the difference in
pressures between the left and right hand side of the
steering rack (Figure 33).
115
Until 9.10s everything looks ordinary – Senna eases
his Williams into Tamburello at 7.98s and the
pressure difference builds smoothly and gradually
from 58psi (the previous straight) to 764psi (steering
slightly left). This is consistent with the constant
steering lock Senna habitually applied through
Tamburello. Having said that, it would be easier to
spot normal and abnormal trends on the telemetry if
the data was available over a longer time span, i.e. at
least from the beginning of Lap 7.
From 9.10s, pressure difference across the steering
rack begins to fluctuate. This can be seen on the
telemetry as a series of peaks and valleys (Figure 51)
that continue until 11.28s when Senna loses control.
The oscillations in pressure difference originated
either from steering input or road surface or from the
combination of both, and they repeatedly loaded and
unloaded the pistons inside the rack that assisted the
front wheels. If the primary source of these
fluctuations was steering input rather than bumps, it
would indicate that Senna experienced imprecise
steering from about two seconds before the crash.
Pressure difference then falls to zero at 11.30s and
stays close to zero until 11.68s, which signals that in
that period the steering rack is unloaded and the
wheels are steering straight. This data is compatible
with both versions of the broken column theory.
During 11.70-11.88s, the pressure difference raises
momentarily to 223psi (equivalent to the initial
smooth turn-in to Tamburello): if the ‘partial failure’
theory is correct, the imprecise steering further
wobbled as Senna tried to stop the car; if the
‘complete failure’ theory is correct, this minor
pressure difference came from the unrestrained front
wheels as they hit undulations or as they twitched
under the onset of hard braking (which started
around the same time at 11.68s).
In the final quarter of a second on the track (11.9012.14s) and then on the grass (12.16-12.38s), the
pressure difference actually turns negative (front
wheels turning right). The remastered footage that
appeared in the movie Senna1 bears that out – the
left front wheel visibly turns right just as the left rear
wheel drops on the grass. This gives weight to both
versions of the hypothesis because had Senna
retained full steering capability it’s unlikely he would
have turned towards the wall in the last moments on
the track.
116
Figure 51 - Peaks and valleys observed in Steering pressure difference (STGACT) two seconds before losing control
(9.10-11.10s).
117
Two pressure difference spikes at 12.40s and 12.60s
most certainly weren’t prompted by driver input.
Tony Woodward expands on this: “the steepness of
these spikes is considerably more than I would expect
from even the most panicked driver input, so my
conclusion is that they were oscillations produced by
external impact on the wheels, thence on the
hydraulic pistons within the steering rack, within a
sufficiently brief timespan as to not necessarily
involve the steering wheel. Such shocks can occur
independently of steering wheel input - it’s often
possible to hammer the steering hard enough to burst
something before the driver gets involved.”
The final power steering trace is Steering target
(STGTGT, Figure 38), which measured pressure
difference across the control ports of the
electrohydraulic servo valve (Figure 35). Again,
STGTGT offers no proof that the steering column
suffered either partial or complete failure, but the
graph is consistent with both versions of the
hypothesis.
the target pressure would be zero or whatever
Williams had under a no-load or residual load
condition.
The telemetry registers a steep increase in Steering
target during 11.00-11.08s, suggesting strong power
assist left immediately before Senna loses control.
STGTGT then begins to fall at 11.10s when the car
starts turning sharply left and it reaches what could
be interpreted as residual load condition at 11.30s
(175psi). At this point Senna is already a passenger
and is heading more or less straight for the ‘I
Pilotissimi’ sign. The target pressure stays low and
never recovers above 170psi.
Then there are the three outliers where the steering
target pressure notably deviates from the expected
values. If the observed near-linear relationship
between Steering strain and Steering target holds
true, there should be an explanation why two out of
the three outliers transpire at exactly the point when
Senna pulls the steering wheel sharply up and loses
control (11.20s and 11.30s).
If the ‘partial failure’ hypothesis is correct, the target
pressure in the servo valve after 11.30s would be
minimal because Senna kept the steering
approximately straight; if the ‘complete failure’
hypothesis is correct and the column broke off, then
118
The situation is more complex when it comes to
analysing strain present in the steering column
during the accident, because even the knowledge that
all three strain gauges were located below the break
is insufficient for determining which of the
competing theories is correct.
The sharp increase in Steering strain during 10.8011.08s (from -10 to -26.9N/m2) could have been
brought on either by brusque but regular steering
input (column intact), or by a sudden bite of an
abnormally responding steering column (‘partial
failure’ hypothesis), or by a column in the process of
seizing in the bushing (‘complete failure’ hypothesis).
Equally, the release of the strain after 11.28s is
reconcilable with either letting go of the wheel
(highly unlikely), returning the steering to its centred
position (column intact or the ‘partial failure’
hypothesis), or with having the steering wheel
detached from a seized column (‘complete failure’
hypothesis).
Figure 52 - Schematic diagram of the lower end of the steering column that remained in the bushing and connected
to the pinion and steering rack.
119
If the steering column remained intact, the strain
reported by the telemetry would be the actual strain
present in the column during the accident. The same
applies if the column partially broke but continued
transmitting enough torque to get Senna round
Tamburello (‘partial failure’ hypothesis), and also if
the column held together but was so close to
rupturing that it stopped transmitting sufficient
torque (imagine two sections of a snapped curtain
rail that haven’t twisted apart yet). All three scenarios
are compatible with the observed Steering strain
values (Figure 41).
driver of the Mustang GT did by forcing it back in
where the modified piece changed in diameter. The
outer diameter of the modified piece (18mm) could
technically fit in the inside diameter of the original
column (20.4mm) provided that the deformation at
the filleted corner allowed it. But considering the
little time Senna had to react and the dimensions of
the parts involved, the chances of even partial success
are virtually zero.
The final possibility is that the column physically
separated from its lower end (‘complete failure’
hypothesis, Figure 52). Under normal circumstances,
no strain would be logged because the lower end of
the column attached to the pinion would rotate
unrestrained. Although an electrical cable ran inside
the hollow tube of Senna’s column and stayed
connected until the impact (it was cut with pliers by
the driver of the medical car Mario Casoni) , it is
difficult to imagine that a cable alone could generate
measurable strain.
Residual strain would also be recorded if there were
some resistance in the section of the column that
remained in the bushing and connected to the pinion
(Figure 52). Strain would be generated from friction
of this ‘amputated’ column getting partly stuck or
jammed in the bushing, especially if that part rattled
or had become ovaled in the course of rupturing. The
amputated end in the bushing looks slightly
deformed in the photos33,91 although the resolution of
the images is too poor to be certain. The drag from
even minor deformation such as this can be
significant and it could have provided enough
resistance to seize the column in the bushing and
read on the telemetry as steering strain.
Still, it is possible to record strain with the top end of
the column disconnected: in theory, Senna could
have tried to reattach the broken column just like the
“It would be good to know what the rotating
clearance was in the bushing and what it was made
of,' says Tony Woodward. “It’s black so my guess
120
would be DuPont Black Delrin, Celcon, or even black
Nylon. Black Delrin is so slick it will run against steel
under water, and is rigid and accurately machinable.
However, it is subject to thermal growth and, in my
experience [from NASCAR series], when used as a
steering column bushing it needs about 0.125mm
clearance [to permit free rotation]. Therefore the
modified piece of Senna’s column running in it would
have had to be distorted by about that much, minus
any existing ovality. The expected ovality in that area
due to the variable contraction stress of the weld, if
the weld were performed manually, would be around
0.025 - 0.050mm.”
So under such circumstances, the column could have
seized once it became distorted beyond
approximately 0.1mm. But without being able to
measure that part of the column and the effective
inside diameter of the bushing, we can’t know what
happened. If, however, the piece of the column
running in the bushing still exists and is frozen in
place or nearly so, it would be a major coup for the
‘complete failure’ version of the hypothesis.
completely when Senna pulled the steering wheel up
(11.20-11.30s). Once deprived of driver input, the
‘amputated’ end of the column got jammed or rattled
in the bushing and offered residual drag that
registered on the telemetry as gradually decreasing
strain.
It is the way Steering strain falls in the next second
(11.30-12.30s) that adds to the intrigue: the trace
follows a quadratic curve (Figure 41), and when the
square root of strain is plotted instead of the actual
Steering strain, the trace turns almost into a straight
line (Figure 53). This relationship hints at a natural
process at work rather than human intervention, and
if a meaningful physical property can be assigned to
the square root of strain, then that property was
falling all but linearly between 11.30s and 12.30s.
When Senna arrives on the grass (12.16s), the strain
momentarily turns positive (0.6N/m2). This signals
that the front wheels turned right on the surface
change, briefly exerting strain in the opposite
direction against the residual resistance of the seized
column.
The unusual direction changes experienced by Senna
during 10.74-11.30s are very similar to those of the
Mustang GT driver. They could have been triggered
by a column seizing in the bushing that broke off
121
Figure 53 - The square root of Steering strain is reducing almost linearly in Senna’s final second on the road
(11.30-12.30s). The orange line represents the line of best fit.
122
There are two final blips in Steering strain at 12.40s
and 12.60s, with amplitudes even lower than the
modest strain levels recorded during Senna’s initial
turn-in to Tamburello. They were induced by the
bouncing front wheels as the car landed on the grass
and concrete before the wall because they coincide
with the two massive spikes in Steering pressure
difference (STGACT).
Regardless of the state of the steering column, the
strain values could be low simply because the shocks
from the bouncing wheels were so short in duration
that they occurred independent of driver input. The
strain could also be low because the condition of the
partially severed column further deteriorated after
the car’s hard landing on the grass and concrete (for
comparison, watch Vitaly Petrov’s crash in Malaysia
2011), or because the drag of the ovaled column
rotating in the bushing wasn’t able to offer more than
just residual resistance against the bouncing wheels
(‘complete failure’ hypothesis).
video evidence indicates that Senna gripped the
steering wheel until the impact.
The emotionally charged footage shown in the movie
Senna1 incorporates a remastered sequence taken
from the helicopter hovering over the scene of the
accident. The improved images allow to study, for the
first time, the grim reality inside the cockpit of the
Williams before the arrival of the medical crew.
Senna sits motionless except for a slight reflexive
head movement to one side. His right hand is
outstretched alongside his thigh and is clearly
identifiable by the white contours of his racing glove.
But Senna’s left glove – a fuzzy patch of white and
blue against the darker background of the cockpit – is
still clutching the rim of the steering wheel, which
has been pulled out and is now resting in his lap.
Finally, low strain values would have been recorded if
Senna had let go of the steering wheel and the front
wheels had turned unrestrained. This may sound
counter-intuitive, but racing drivers sometimes
‘brace for impact’ to avoid injuring their hands. Such
a scenario can be ruled out, however, because recent
123
STEERING WHEEL FLEXING
The starting point for studying the behaviour of
Senna’s steering wheel in the seconds leading to the
accident is the CINECA analysis18 from 1997
discussed in Chapter 6. The other vital ingredient is
the digitally remastered on-board footage from Laps
6 and 7 that recently appeared in the movie Senna1.
The painstaking work of the producers sheds new
light on the movement of the steering wheel,
particularly in frames where the yellow button is not
clearly distinguishable on the earlier CINECA
footage.
Nothing out of the ordinary happens as Senna
crosses the line to start Lap 7 and the yellow button
follows its customary trajectory. Then, 8.20s into the
lap and three seconds before Senna loses control at
310km/h in Tamburello, the steering wheel begins to
drop steadily at a 45-degree angle, so much so that by
10.74s the button has descended to the edge of the
screen some 25-27mm from its regular position.
After 10.74s, the behaviour of the yellow button
changes. It is no longer dropping and instead it is
moving horizontally from right to left until 11.20s just
as Senna’s car suddenly twitches left. At 11.20s,
Senna pulls the steering wheel sharply up and the
yellow button almost reaches the expected position as
defined by the reference green arc on the CINECA
video (11.30s). Nonetheless, this correction is only
temporary and the button is immediately dropping
again in vertical fashion (11.32-11.42s) until it
disappears from the screen (11.44s). There is a
glimpse of the button at 11.52s before its final
appearance at the edge of the screen during 11.7411.76s. It is gone until the end of the footage (11.84s).
The enhanced cockpit zoom sequence, which is based
on the original CINECA analysis, affirms that the
steering is pulled downwards and forwards after
8.20s. The rim of the steering wheel - outlined by
blue dots where the dark contours of the rim are
discernible against the lighter background – clearly
drops at 10.48s, at 10.74s, and then at 11.40s.
Enhanced cockpit zoom sequence, Lap 7
The late Ferrari driver Michele Alboreto testified in
court that the stress on the steering column at a
circuit like Imola would normally generate flexing in
the order of two or three millimetres, not centimetres
as seen on the footage, and that the flexing would
depend on the bending force inflicted by the arms of
the driver, the composition of the materials, and the
124
distance of the steering wheel from the support
strut75.
Williams’ answer to Alboreto’s assertion was the
video showing David Coulthard sitting in an FW16
car in the museum and replicating Senna’s steering
wheel movements from the last three seconds before
the crash. Williams didn’t disclose the methodology
used in this demonstration, and it remains unclear
how the demonstration was put together, which part
of the steering assembly was responsible for the
flexing observed, and under what circumstances
would such flexing actually materialize on the race
track. The fact that the dashboard and steering
column on the video are hidden from view by white
space doesn’t help either.
From the other side of the pond, Tony Woodward
offers the following perspective on the Williams
demonstration: “Until we introduced the SCA827
column (1.25’’ OD x 0.065’’ wall), many a US stockcar racer would bend his 0.75’’ OD x 0.120’’ wall
steering column while climbing into the car, and then
have to bend it back straight in order to drive. I’m not
making this up.”
Despite that, it is hard to believe that any F1 driver let alone Senna - would accept flexing of the
magnitude shown in the demonstration. Senna’s
strive for perfection was legendary and he forced the
designers to alter the steering wheel position before
the start of the season just to get more feel and
performance out of the car. Had he experienced
similar flexing on regular basis in the previous races,
he would have demanded design modifications
immediately.
In the closing trial statements on November 18, 1997,
Williams’ lawyers argued that the oscillations of the
yellow button cannot be relied upon due to optical
illusions because the on-board camera was not of
fixed rigidity69. No part on a racing car is of fixed
rigidity, but that doesn’t mean that, for instance, the
flexing of the front wing on Red Bull’s model RB6
from 2010 was just an optical illusion. The reality is
that all publicly available on-board footage from
Senna and Hill’s 1994 races shows no or just minor
flexing, and the yellow button describes regular
circles that do not deviate considerably from the
white circle of low opacity (Figure 7).
As we discussed in Chapter 6, there is literally no
flexing observable on the footage from Brazil despite
the track’s notoriously bumpy and relatively highspeed nature (two brief instances, 4-5mm); there is
barely noticeable flexing on the footage from Japan
125
(four instances, 4–5mm); and several minor
deviations during Imola practice, qualifying, and
warm-up sessions (peaks at 6–8mm). The story is the
same in the race until 36 seconds into Lap 6.
finish straight, his upward steering movement not
circular but almost linear. There is one more jitter
(9mm) before Senna crosses the line and the button
then settles on the white circle.
But then something unusual happens, visible once a
white circle of low opacity is superimposed on the onboard footage of Lap 6. Only the overlay of steering
movements can be published here; however, the
original sequence on which the overlay is based
appeared in the official review of the 1994 season76
and also features in the movie Senna1.
This evidence is significant for two reasons: first, it
suggests that the flexing progressively worsened over
the course of the first three races, and second, it
reveals that substantial flexing arose not just once,
but on several occasions in the last sixty seconds
before the crash. Moreover, the flexing becomes
visibly worse towards the end of Lap 6 after Rivazza
corner. It strongly supports the assertion that at this
stage Senna was already driving with a partially
severed steering column.
Overlay of steering movements, Lap 6
36.54s after the restart on Lap 6, the yellow button
on Senna’s steering wheel jitters substantially and
drops from the white circle on the exit of the fast lefthander Piratella. The peak deviation is twice as high
as anything observed previously (15–16mm) and the
angle of the deviation is approximately 40 degrees
(Figure 54).
1:13.60min into Lap 6, the yellow button drops after
exiting Rivazza (10-11mm), it shoots up above the
white circle (8-9mm) shortly afterwards on the entry
to Variante Bassa, and then it falls again on the exit
(11–12mm). It stays markedly below the white circle
(10-11mm) as Senna turns right and enters the start-
The final deviation begins 8.20s into Lap 7 when the
yellow button strays by 25-27mm and never recovers
to the level of the white circle. Three distinct phases
are recognisable: the dropping of the button at a 45degree angle towards the centre of the steering wheel
(8.20-10.74s), the slight horizontal movement from
right to left during which the car turns sharply left
(11.74-11.20s), and then vertical oscillations up and
down once Senna loses control after 11.20s.
Overlay of steering movements, Lap 7
126
In comparison, Senna’s (and Hill’s) on-board footage
through Tamburello earlier during the Imola
weekend looks very different and the yellow button
barely moves because both Senna and Hill keep
constant steering lock all the way round the corner.
And even if Senna is holding the steering straight
after 11.26s to achieve optimal braking, the yellow
button should still remain visible and it should stay
near the reference green arc because that’s how the
button behaves under braking on all footage from the
1994 season.
Figure 54 - Steering wheel movements during the last 60
seconds before the crash. Grey arc denotes the regular
trajectory of the yellow button, the white line represent
the distance of the yellow button from the serigraphed ‘V’.
Top: deviation on the exit of Piratella corner towards
Acque Minerali.
Bottom: deviation on the exit of the Variante Bassa
towards Traguardo (this overlay is based on remastered
on-board footage from Lap 6).
.
127
Tamburello wasn’t an acceleration or braking zone
where strong forces could pull the steering column in
different directions under hard throttle or braking, or
over the kerbs. So why would Senna exert effort
strong enough to pull the steering down and right
after 8.20s?
According to the broken column hypothesis, the
fatigue crack in the upper crescent meant the
weakened column could no longer withstand the
combined stress of high downforce, lateral g-force,
and the bending force applied by Senna’s arms, and
the steering dropped along the line of least
resistance. If a shaft develops a crack that extends
from nine o’clock to one o’clock position33,79, it is
most susceptible to bending on the opposite side
between three o‘clock and seven o’clock (Figure 55).
This explains why the steering wheel dropped
towards the bottom right hand corner of the cockpit
at a 45-degree angle, and why the severe flexing
observed on Lap 6 after Piratella, Rivazza, and
Variante Bassa exhibits a similar pattern.
footage suggests that the lower crescent of the
column held together at least until 11.20s. While
between 8.20s and 11.20s the yellow button either
drops steadily or moves across, once Senna pulls the
steering wheel sharply up during 11.20-11.30s the
button starts jumping up and down in a vertical line,
indicating that the column may have sheared near
the support strut.
There is video footage that demonstrates what the
accident may have looked like in this scenario. A
dragster driver suffers a column failure in a straight
line at an estimated 350km/h73. When observed in
slow motion, the dragster’s column wobbles left to
right by several centimetres for about three seconds
until it reaches the breaking point and shears
completely. The movement illustrates why Senna’s
steering wheel could drop by 25-27mm moments
before the crash and then oscillate up and down.
Dragster driver suffers a column failure
Although the flexing indicates a steering problem, it
doesn’t clarify whether the column continued
transmitting sufficient torque through the lower
crescent or whether it broke off and was dangling in
Senna’s hands. On closer inspection, the on-board
128
Figure 55 - Position of the yellow button at 10.74s on three different laps during the Imola weekend (timing based on
race Lap 7). The orange dot sitting on the circle of low opacity marks the yellow button position at exactly the same
part of the circuit on two Friday laps (morning practice and qualifying). The yellow dot is the actual button position on
race Lap 7.
.
129
At what point would Senna realise that the column
was rupturing? His reaction time, measured as the
time elapsed since 8.20s when the steering wheel
started to drop towards the bottom right hand corner
of the cockpit, is similar to that of the dragster driver
– about three seconds. Senna was famous for his
ability to detect even the slightest changes in his car’s
behaviour; in this instance, however, he would have
lacked a suitable reference point because he hadn’t
experienced a steering failure behind the wheel of an
F1 car before. This assertion is supported by the
frightening tale of former F1 driver David Brabham,
who raced at Imola 1994 for the beleaguered Simtek
team. Having witnessed his teammate Roland
Ratzenberger’s fatal crash on the Saturday, Brabham
bravely decided to race on the Sunday, only to suffer
a steering failure not long after Senna’s accident. The
Simtek’s steering column detached completely from
the steering rack on the short straight before the
Variante Bassa chicane, yet Brabham didn’t notice
the problem until he tried to turn in at the end of the
straight, and initially he suspected that he had lost
the front wing89.
So it is by no means certain that Senna would have
diagnosed the severity of the situation accurately and
immediately. And even if he had already felt
something unusual towards the end of Lap 6, he may
have decided to continue for another while – he was
leading the race and was under massive pressure to
win. Diving into the pits for a precautionary check
was not an option.
The accident happened incredibly fast and therefore
it is hard to account for time lags or to separate cause
from effect. Still, the movements of the yellow button
closely mirror those of Senna’s helmet and the
helmet’s movements are in turn dictated by the laws
of gravity as Senna stayed on partial throttle, lifted,
braked hard, or did nothing.
Steering and helmet synchronization, Lap 7
In period 11.24-11.40s – either intentionally or
accidentally - Senna applies more or less 50 percent
throttle. Correspondingly, no shifts in longitudinal
force act on the car or the driver’s body and the
yellow button is visible after Senna has pulled the
steering up. He lifts completely between 11.42-11.50s
and his helmet lunges forward. The yellow button
drops and disappears from the screen, in sync with
the movement of the helmet.
130
At 11.52s, the initial deceleration phase is complete
and throttle is now zero. Senna’s helmet is moving
back towards the headrest and his arms holding the
steering wheel go up as well. The yellow button
reappears accordingly at the bottom of the screen.
Between 11.54-11.68s, Senna’s right foot switches to
the brake pedal, his helmet is leaning slightly
forward, and the yellow button dips below the edge of
the screen again. Hard braking begins at 11.68s
(longitudinal force is rising to 1.3G, to 1.8G, and then
to 3.1G) and there is no sign of the yellow button.
From 11.74 to 11.76s, Senna temporarily releases
pressure on the brake pedal (longitudinal force is
reduced from 3.1G to 2.3G) and his helmet moves
back towards the headrest. This is exactly the
moment when the yellow button makes its final brief
appearance.
The explanation is that as Senna’s head and body
jolted forwards and backwards in response to the
varying forces, his arms holding the steering wheel
did likewise because they were no longer supported
by the resistance of the steering column, which is
under normal circumstances firmly anchored to the
steering assembly.
131
CHAPTER 15
The verdict
There is no smoking gun, but also no underlying flaw
in the ‘partial column failure’ hypothesis. A great deal
of disparate evidence points to Senna losing precision
and feel as he bent the faltering steering column,
which led to his Williams twitching left and right in
the critical moment. And because he retained some
torque in the column, steering strain values were still
recorded.
wheel began to flex substantially only in the last sixty
seconds and never before that.
The quest to solve the mystery of Senna’s death is no
longer a mission impossible, but nor is it a mission
accomplished. On balance, the evidence tips the
scales in favour of a steering failure, although the
only person who knows the whole truth took it with
him to the grave. But at least Ayrton Senna can rest
in peace knowing that the fundamental human desire
to go deeper and deeper in the search for
understanding is alive and well.
There is intriguing evidence supporting the ‘complete
column failure’ hypothesis as well; however, without
knowing exactly how the power steering system
worked, and without inspecting the amputated end of
the steering column, it is not possible to complete the
puzzle and make a definite judgment.
The tyre failure and aerodynamic instability
hypotheses look persuasive; nonetheless, the
contradictory evidence is difficult to ignore, and
some of the evidence can be explained via a more
straightforward route - for instance, why the steering
132
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