magazine article

sPeciaL / APRILIA RS485 YPVS
44
Performancebikes.co.uk | OCTObeR 2014
THE LIT TLE
L ARGE SHOW
A N D
With a 485cc stroker motor packed into the chassis of an RS250, this home
brewed GP replica has a feisty surprise hidden under its fairing
Words Gary Inman / Photography: Paul bryant
WANTED TO give the bike an engine
it deserved,’ explains Dave Bimson
of his 1996 Aprilia RS250 Mk1. ‘I’ve
owned the bike nine years and love the
handling, it worked impeccably, but
lacked a bit of power.’ Dave, who has
owned GSX-R1000s, VFR400s, Aprilia
SXV550s, Futuras and Milles, continues,
‘being a road rider, not a fan of riding
on track, it meant staying with mates on
Fireblades and GSX-Rs was not an easy task.
The 250 had the edge on entry, mid-corner
and really tight stuff, but it was always a
struggle to keep up on the exits. So, four years
ago, my mate Steve came up with the idea of
fitting a bigger engine.’
Four years… That’s how long it took to
finish, on and off, stop-start. ‘I ended up
going around and around and around,
because I’d fit one thing and it would make
the part next to it look scruffy, so I had to
replace so many different parts.’
Putting big engines in small two-stroke
‘I chassis is nothing new, the British are
masters at it, but this concoction – of bigbore RD350LC in an RS250 is much rarer.
Apparently there are another two or three
running in the UK, with another two in
development hell. There are only a handful
elsewhere in the world.
Aprilia RS250s are such a rare sight now,
I haven’t seen on one the road or at a bike
meeting for years, that it’s tricky for me
to spot the changes forced on the bike by
the new engine. The bottom of the fairing
is cut and widened to fit in the huge Tyga
expansion chambers while nearly everything
else looks close to factory.
Sat in isolation with no other bikes around
it, the RS250 (Dave’s ‘kiddie dream bike’ that
he bought years after his adolescence, when
he could afford such toys) looks big. Large
capacity sportbikes have shrunk so much,
and so incrementally, that this pre-millennial
tiddler has as much presence as an RSV4.
The long seat dates the design, but still floats
the boats of men of a certain age.
A gentle swing on the kickstart changes
everything though. There’s no other word to
describe the noise than crackle. Maybe add
and adjective - evil crackle. It’s an exhaust
note full of portent. It makes my nerves
jangle like wind chimes in a tornado. It’s
impossible not to blip the Domino twin-cable
throttle, imagining you are a nonchalant,
Italian mechanic in crowded pit-lane.
Swarthy and you know it.
Even though I’m the correct age, I’m not
one of those people who has a special place
in their heart for two-strokes. I spent too long
pushing and fixing the lousy ones I bought
for that. I bought a CBR, then a GSX-R and
didn’t look back. But that doesn’t mean
blipping this reimagined Aprilia’s twistgrip
isn’t doing something to me.
I pull on my helmet and prepare to roll
out of the sleepy West Country village, that
Dave’s bodyshop and paintwork business is
based in. ‘It’s feisty,’ warns Dave...
OCTObeR 2014 | Performancebikes.co.uk
45
tyga cans emit an evil crackle - and lots of smoke
The bike is seriously cramped compared
to the four-strokes I’ve been riding recently.
It’s made for teens with more-meat-on-abutcher’s-shoe physiques. I suck in my protogut, pull out of the yard and realise the sideto-side steering is tighter than my leathers.
The engine feels unsophisticated. That,
for many, is the attraction. It’s as simple as
a ditch pump. Minimal number of moving
parts for maximum impact. The Hinson
clutch is the only heavy thing on the bike.
After a while the temperature gauge is
showing 55˚, the self-imposed limit Dave has
set his engine to run at. I turn onto a straight
lined with trees that are so green they seem
fluorescent. It’s a perfect English summer’s
day, but no longer peaceful. Tugging on the
throttle the engine wakes up with all the
grace and manners of Godzilla dealing with
a TNT enema. I forgot my earplugs and I’m
the fairing was widened to fit the huge spannies
regretting it. This thing is scaring the life out
of me, but I know I would be going at least as
fast on an S1000RR. This isn’t a racetrack, it’s
not my bike, no one has a stopwatch on me,
no one’s even looking, I’m just trying to get
a taste of ‘what this thing will do.’ And I’ve
been given the answer, loud and clear.
The trees to my left are a TT-onboard blur
and I’m in a bubble of noise. I know if I sat
further back and tugged the bars it’d lift in
third. I’m finding it quite easy to reign my
urges in. I admit it, tuned two-strokes are out
of my comfort zone, but this ride is pressing
some buttons I didn’t know were still wired
up. The powerband rips. It would take a long
time to get bored of this bike.
‘I like the prettiness of it,’ says Dave, rather
unexpectedly, when I return it in one piece.
He’s kept everything that was good about the
original and improved the rest. ‘I added parts
Dave took the paintjob
from the Aprilia RSW500,
Aprilia’s frst attempt at
premier class competition.
After winning numerous 125
and 250 world titles, Aprilia
employed lateral thinking
to chase glory in the 500cc
class. They were up against,
among others, Rossi and his
NSR500. Yes, quite.
The project was headed
by Jan Witteveen, later
joined by Gigi Dall’Igna (now
top man at Ducati’s MotoGP
effort). The thinking was, a
nimble V-twin, with a lower
weight limit, could outfox
the brutish, heavier 500.
In theory, and on a perfect
lap, it could. McWilliams put
the twin on pole in 2000 at
Phillip Island and scored two
46
PReSS ASSOCIATION
what’s an Rsw500?
McWilliams leading
eventual winner Rossi
and kenny Roberts Jr
at Donington 2000
thirds in the same season,
at Mugello and Donington
(where he was just 1s off
the winning race time). The
Ulsterman also crashed 14
times, an illustration of just
how on the edge he was
riding. Ouch.
The main problem, it
seemed, was the Aprilia
could put in blistering laps,
Performancebikes.co.uk | OCTObeR 2014
but if a V4 was parked midcorner, the Aprilia’s lost its
mid-corner speed and it
would be blitzed on the exit
and down the straights.
With the dawn of the
diesels, Aprilia shelved the
project to concentrate on
the MotoGP RS3 Cube.
It was far less successful
than their RSW500.
Maxton modified Fireblade forks slot straight in
that I liked from other bikes. I really liked the
front end feel when I rode a 2006 Fireblade,
so I fitted them.’
Makes sense, I suppose. Handily, the
forks slotted straight into the stock yokes.
The original forks, on this year of RS250,
had minimal adjustment. Dave sent the
Honda Showa forks to Maxton for them to be
modified to suit the weight of bike and rider.
The same decision process led to Dave
mating the Fireblade Tokico front brake
calipers with a master cylinder from a Ducati
999R. The wavy discs are 320mm diameter.
After doing some online research, Dave
let his plans be known on a forum and was
contacted by John Corrin of VSK Corrinaldi,
who said, ‘I’ve done conversions like this, let
me know if you want any tips or advice.’
Dave took John up on the offer and used
his services to rebuild the early YPVS motor.
‘Johnny’s been brilliant,’ says Dave.
The barrels are CPI – Calvin Pollet
Industries, original destined for a Banshee
quad. ‘There’s a massive market for drag and
dune quads in the States, and Calvin Pollet
deserves a lot of credit, but sadly, he passed
away recently,’ says John, before adding, ‘but
it’s not as easy as bolting them on as the ports
need altering to suit a motorbike.’
The long-stroke crank is another US-made
item, designed for quads. ‘There’s a shopping
list of parts you can order from the States,
then it’s just a case of bolting it all together
properly. That’s what I do,’ says John Corrin.
With bigger bores and longer stroke, the
capacity is up to 485cc. The bores are fed
through V Force 3 reedvalves by brand new
38mm Mikuni carbs with Ramair filters.
‘I haven’t had it on a dyno,’ admits
Dave, ‘but I’m expecting 90 horsepower.’
John concurs, saying 80-90 rear wheel
horsepower, based on what his modified
TZR535, featured in PB, makes. ‘But
horsepower figures are bullshit,’ reckons
BSt carbon wheels are half the weight of aprilia’s
the devil is in the detail and Dave’s a perfectionist
Close eyes, blip, imagine you are a GP mechanic...
‘The engine wakes
up with all the grace
and manners of
Godzilla dealing with
a TNT enema’
OCTObeR 2014 | Performancebikes.co.uk
47
one of the coolest frame plates in the bike world
hagon shock is 15mm longer to avoid scrapes
Mikuni 38mm carbs feed v Force 3 reed valves
John, ‘It’s more important if it’s rideable and
this bike is.’
Corrin made the bespoke powervalves and
the Tyga pipes, ordered from the Far East,
and a Hinson clutch finish the package.
Fitting the engine into the frame required
new engine hanger brackets. ‘They’re more
L-shaped than boomerang-shaped like the
originals,’ says Dave. A kit is available, but,
he says, it’s not complete. John gave Dave the
dimensions to make the missing parts.
Dave is a fundamentalist when it comes
to cooling his bike. It’s swallowed a lot of
time and money, and the owner wants to
look after his labour of love. ‘Keeping it cool
sPECIFICatIOn
DAVE BIMSON’S APRILIA RS485
EnGInE ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
485cc Yamaha RD350YPVS; CPI barrels tuned
by John Corrin; CPI pistons; long-stroke crank;
Mikuni 38mm carbs; PMA radiator; Yamaha R6
fan; Zeeltronic programmable ignition; Johnny
Corrin Powervalves; V Force 3 reed-valve petals;
Ramair flters; Suzuki RGV250 thermostat
housing; uprated mechanical water pump;
engine built by John Corrin of vkscorrinaldi.
co.uk; engineering by John Finch, Dunkeswell
(01404 892958)
ChassIs ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
2006 Fireblade forks re-valved by Maxton;
Aprilia RS250 yokes; Ducati 999R master
cylinder; Hagon rear shock; bST carbon wheels;
Talon sprockets; Michelin 2CT Power One
tyres – medium rear, soft front; brackets by Rob
Marsh and owner; bodywork modifed by owner;
TRW rearsets; Nuvo screen; Domino throttle;
paint by Db body’s (01823 681499)
48
Performancebikes.co.uk | OCTObeR 2014
is key,’ Dave tells me. He had a new, larger
and more efficient radiator made by PMA
in Leicester. They made it to suit the gap
between the modified fairing and it fits like
a glove. Dave then searched for a slimline
fan, plumping for one from an R6. ‘It fits with
5mm to spare all round,’ he states, proudly.
It’s linked to a thermostat marketed to kit car
builders and wired into a modified loom.
‘The wheels were the last thing I changed
and they should have been the first,’ says
Dave of the BST carbon wheels. ‘They’re
very light, not far off half the weight of the
originals. When you’re flip-flopping though
tight bends it’s like someone’s helping you.’
Being a painter for a living, that was a job
Dave could easily handle, he just had to make
the time. The factory replica paint dates from
2000. The bands of colour are all paint, the
logos are stickered on, but Dave gave eight
coats of lacquer, each carefully flattened
off, so the surface is like glass. There is no
ridge where the sticker starts, like most race
replicas. Dave also purposely inserted a
couple of mistakes into the paint scheme that
only one person has ever noticed without
prompting. Write in if you spot them.
The first meeting we had planned was
postponed. Dave was cagey about the
reason, but when we do meet he admits it’s
because on the first quick run through his
favourite set of bends he heard a ‘chink!’
as the expansion chamber touched down.
As he now realises, leaning a bike over at
standstill to check ground clearance doesn’t
replicate a bike at speed, under compression.
The Aprilia was still running the stock rear
shock. It was replaced with a 15mm longer
unit, with an additional 20mm of adjustment
should he need it, from Hagon.
With that fly fished out of the ointment
Dave only has one thing on his mind, ‘I spent
four years building it, now I’m going to spend
the next four riding it.’
the slight scuff is due to an initial lack of ride height
Front end
Maxton sticker almost guarantees
a planted front end. 2006 Blade
forks slid straight into 1996 Aprilia
yokes. Easy
Zeeltronic programmable ignition cleans things up
the seat was re-trimmed by Custom Covers Uk
Dave kept the rim widths the same as the originals
paint work
motor madness
radiator
The RS250 has given way to a
RD350 YPVS engine with CPI
barrels, CPI pistons and a long
stroke crank taking it up to
485cc and around 90bhp
A Windermere of two-pack
lacquer was laid on and rubbed
back to guarantee a perfectly
smooth finish
The PMA radiator is a work of
art. VKS Corrinaldi made the
neat V-shaped panel to fit the
wider gap between the bottom
fairing panels
Dave looks on at the
result of a youth’s
dream built with an
adult’s wisdom
OCTObeR 2014 | Performancebikes.co.uk
49