suspension tuning guide, part 4

TheComprehensive
Suspension
TuningGuide
BY MIKE KOJIMA
P H O T O G R A PBHYYM I K EK O J I M A
A N DJ 0 S HJ A C Q U 0 T
I L L U S T R A T I OBNYS
TITONG
B M W sM
, k I a n dl l T o y o t aS u p r aas n d
the Datsun510haveseveretoesteer.
Because
of thisgeometry,
somesemit r a i l i n ga r ms u s p e n s i otnoseo u t u n d e r
ro[[,whichcancauseseveretraitingthrottleoversteer.
Others,
Iikethose
foundon the Porsche
928and FCRX-Z
use[inksor bushings
of different
hardness
to forcethesuspension
to
passively
toe in underrolt,which
creates
understeer
at the limit.
Thesedesigns
workwel[if they're
operating
thewaytheengineers
intended
themto.lt'swhenspeed
junkies[ikeustry to out-engineer
the
engineers
thatprobtems
devetop.
This
usua[[y
happens
whenlowering
thecar.
Oneeasysolution
to reduce
toe
change
in [owered,
semi-trailing-arm
suspensions
isto increase
thespring
rateto reducewheeltrave[(effectively
increasing
theoverat[
wheelrate).The
lessthewheelmoves,
the lesstoecan
change.
0bviousty,
overstiffen
ingthe
rearsuspension
hasitsowndrawbacks,
butit workedwetlonthe Datsun5roand
BMW3 Series,
bothof whichhada [ong
andsuccessfuI
racingheritage.
Manycurrentsportcompacts
use
m u l t i l i n kr e a rs u s p e n s i o d
ne
s s i g n etdo
sweepthroughcomplex
camberandtoe
anglesto maximize
streetperformance,
comfortandstability.However,
this
c o m b i n a t i oonf c a m b ear n dt o ec a n
conspire
againstan enthusiast
who's
wittingto acceptthe compromises
of a
Front of
the car
F i g .1
Fi1.2
TOE
STEER
IS
INDUCED
INSEMITRAILING
ARM
SUSPENSIONS
AS
THESUSPENSION
COMPRESSES
BECAUSE
OFTHIS
GEOMETRY,
THE
REAR
WHEEL
TOES
O UD
I URING
COMPRESSION
Stockride height:trailingarms
parrallelto ground.A and B
are equal.
loweredrideheight:trailing
armstiltedup in rear.A is
greaterthanB.
moreaggressive
suspension
setup.
Aftermarket
tunerstikeWhitetine
offeroffsetbushings
forcarstikethe
EVO
thatrelocate
thepivotpoints
to
reduce
toesteer.
lf kitsare
u n a v a i l a b ft o
e r y o u ra p p t i c a t i o an ,
racecar
fabricatorcanreposition
the
t r a i t i n ga r m sa n do t h e rI i n k st o a
corrected
positionon severety
lowered
cars.lt's alsocommonto stotcontrol
a r m m o u n t i n gp o i n t st o a l t e rt h e i r
pathof traveland correcttoe on
loweredcars.
Excessive
loweringcanmaketoe
steerworseby ptacing
the suspension
linksin a staticpositionandrangeof
trave[theywereneverdesignedfor.A
98
.I
FIG.1 INATRAILING
ARMBEAM
AXLE
SUSPENSION,
THETRAILING
ARMS
MOVE
INOPPOSING
DIRECTIONS
ASTHECARROLLS
AND
PULL
THEBEAM
AXLE
EQUALLY
WITH
NOCHANGE
INTOE.
TRAVELING
UPPULLS
THE
BEAM
AXLE
FURTHER
THAN
THE
SIDE
TRAVELING
DOWNWARD.
THIS
DIFFERENCE
IN
MOVEMENT
CREATES
TOE
STEER
b e a ma x l es u s p e n s i owni t ht r a i t i n g
a r m sa, sf o u n di n t h e r e a ro f m a n y
sma[[front-whee[-drive
carsis a good
exampte
of wherethis mightoccur.
At stockrideheight,
thetrailing
parattel
armsareusua[[y
to the
ground.
Whenthecarrolts,the
outboard
andinboardarmsswingin
different
directions.
Theresulting
arcshaped
axlepathshortens
thecar's
wheelbase
duringcompression
and
droop.Sinceeacharmswingsequa[[y
in different
directions,
theaxle'stoe
doesn'tchange
because
eachendof
the beamaxleis pul[edthesame
distance
forward.
l f t h e c a ri s l o w e r e dt o o m u c h ,
b o t ht r a i l i n ga r m sp o i n td o w n w a r d
towardthe front of the car.At this
COMPANIES
LIKEWHITELINE
OFFER
PIVOT
OFFSET
BUSHINGS
FOR
MULTI-LINK
REAR
SUSPENSION
CARS
LIKETHE
EVOVIII,WHICH
ALTER
THE
PIVOT
POINT
IO
MINIMIZE
TOE
STEER
a n g l et,h e a r m sd o n ' tm o v ee q u a [ [ yi n
o p p o s i nd
g i r e c t i o n sU. n d e rr o [ [ ,t h e
i n s i d ea r mw i [ [ p u s hi t ss i d eo f t h e
axl erearw ard
w hi l etheouterarm
w i ttputti tssi deoftheaxl eforw ard ,
causi ng
understeer.
ANTI-DIVE
ANDANTI-LIFT
A n t i - d i v ea n da n t i - l i f ta r et r i c k st h a t
c a nb e a p p t i e dt o a c a r ' sf r o n t
s u s p e n s i ogne o m e t rtyo c o n t r o bl r a k e
d i v ea n da c c e t e r a t i o[ inf t .
Liftanddivecanbe mitigatedby
carefu[[y
locatingsuspension
pivot
pointsto takeadvantage
of the "force
reaction"
on the chassis
createdby
acceteration
or deceleration.
This
a v o i d st h e n e e dt o i n c r e a ssep r i n g
ratesto reducepitching-an important
issuefor ridequatityin a[[ streetcars.
A n t i - d i v eh e t p sp r e v e ntth e n o s eo f
t h e c a rf r o md i v i n gd u r i n gh a r d
braking.Mostcarshavesomedegree
o f a n t i - d i v ed e s i g n e idn t ot h e s t o c k
s u s p e n s i ogne o m e t r yA.n t i - d i v e
utilizesdeceleration
forcesto increase
t h e f r o n tw h e e l c o m p r e s s i o
r ant ea n d
reducebrakedive.
B y c h a n g i ntgh e a n g l eo f t h e
s u s p e n s i o[ni n k st,h e a m o u n o
tf
a n t i - d i v ec a nb e m a n i p u t a t e d .
However,
excessive
anti-divecan
h i n d e rp e r f o r m a n cbey c a u s i n g
the
..
front suspension
to stiffenwhite
b r a k i n gf o r a c o r n e rw, h i c hc a nc a u s e
understeer.
ln extremeamounts,antiy i t t c a u s ew h e e lh o p
d i v eg e o m e t r w
a n dc a s t e cr h a n g eus n d e rb r a k i n g .
M o s tr a c e c asru s p e n s i o nhsa v e
m u c hl e s sa n t i - d i v ea n da n t i - l i f t h a n
streetcarsuspensions.
0n racecars,
the stiff suspension
is usedto control
bodymotioninsteadof redirecting
WHITELINE
OFFERS
WRX
ANTI-LIFT
BUSHINGS
THAT
LOWER
THE
REAR
PIVOT
POINT
ON
THE
FRONT
CONTROL
ARMS.
THIS
CHANGES
THE
INSTANT
CENTER
OFTHE
FRONT
SUSPENSION,
WHICH
REDUCES
ANTI-LIFL
brakingor acceteration
forces.Most
kitsavailablefor seriouswrenchers
to
a l t e ra n t i - d i v ea n da n t i - t i f w
t o r kt o
reducethesefactors.
0n front-anda[[-wheet-drive
cars,
this samegeometry
alsoresistsfrontendtift underacceleration.
Subaru's
WRXis notorious
for largeamountsof
anti-dive
andanti-[ift,
causing
nontinear
steeringresponse.
Bushings
reducing
i
i
i
i
i
i
i
i
i
i
i
i
i
i
i
i
i
i
i
i
anti-dive
andanti-[iftaredesigned
to
givemorenaturaIsteeringresponse
and
improve
corner-exit
tractionon WRXsby
eliminating
thesecompromises.
Anti-tiftgeometrygreatlyaffects
launchtractionfor front-wheet-d
rive
dragcars,yet onty recentlyhave
t u b e - f r a m pe r oc t a s sf r o n t - w h e e t d r i v ed r a gr a c e r sb e g u nt o c o n s i d e r
u s i n gf r o n t - e n dan t i - t i f tg e o m e t r y .
Whitelinh
e a ss u s p e n s i omno u n t s
a n db u s h i n gtso t u n et h ea n t i - t i f t
a n da n t i - d i v oe u to f m a n yp o p u t a r
a[[-wheetandfront-wheet-d
rive
c a r sK
. i t sa r ea v a i l a b lfeo r a [ [ W R X s ,
the SentraSE-R,manyCelicas,
the
M a z d a3 2 3a n dt h eG a t a n tE/ c t i p s A
e.
few Japanese
tunersoffersubframe
mountsforthe NissanSt3,St4,Z3z
3ooZX,R32,33and34 Skytineto
changethe heightof the front
instantcenter.
i
i
C a t c u t a t i nAgn t i - S q u a t
i
Height of
the C.G.
i
,)
a
i
Distance
to InstantCenter
t
i
..........-
\-Qftsnging
Anti-squat
is 25%because
this distance
is one.quarter
of the heightof the C.G.
this distancewiil arter the squattingreactionof the car
i
i
Height of
the G.G.
J
Anti-squat
is 50%because
this
dlstancels hatfthe helghtof the C.G.
1O2 | orcrueen2o0sr trlyr,y.smnrcoupAgrcAnwEB.coM
ANTI-SOUAT
Anti-squat
istheexact
same
g e o m e t r ay p p t i e da t t h e b a c ka x l e
o f a r e a r - w h e e l - d r icvaer .U n l i k e
t h e f r o n to f t h e c a r ,s m a l l a m o u n t s
o f a n t i - s q u aat r eg e n e r a l lay g o o d
t h i n gf o r a r e a r - d r i vcea rs i n c ei t
a [ ] o w sf o r s o f t e rr e a rs u s p e n s i o n s
w i t h o u t h e e x c e ss q u a t .
C a r s[ i k et h e N i s s a nR 3 zS k y t i n e
GT-R,Z3z3ooZXandSr324oSX
AngleA is smaller
thanangleB.
h a v ea g r e a td e a Io f a n t i - s q u ai n
t
t h e i rr e a rs u s p e n s i ogne o m e t r y .
T h i sm a k e st h e mt r a n s i t i o tno o n throttleoversteer
veryrapidly
b e c a u saen t i - s q u a[ti,k ea n t i - d i v e ,
s i g n i f i c a n t il ny c r e a s et h
s ew h e e l
rate.Thisis why the Sr3worksso
we[[for drifting.
Extreme
a n t i - s q u acta nc a u s e
wheelspin
a n dr e a rw h e e lh o p
, h i c hi s w h y t h e 2 3 z
u n d e rp o w e rw
i s n o t o r i o ufso r [ a u n c h i npgo o r t ya t
thedragstrip.
Rear-whee[-drd
i vrea gr a c e r sh a v e
m a d ea s c i e n coef a n t i - s q u at ut n i n gt o
maximize
r e a r - w h e et rl a c t i o nD
. rag
c a r so f t e nh a v es o m u c ha n t i - s q u a t
g e o m e t rtyh a tt h e b a c ko f t h e c a ra c t u a t l y
l i f t sw h e n[ a u n c h i n gd,r i v i n gt h e t i r e s
. r a gc a r sh a v e
i n t ot h e g r o u n dD
a d j u s t a b lfeo u r - [ i n kr e a rs u s p e n s i o n s ,
s ot h i s p e r c e n t a gi set u n a b t ef o r t h e
amouno
t f b i t ed e s i r e di n d i f f e r e n t
c o n d i t i o n s( s e es i d e b aor n p a g e1 0 2t o
l e a r nh o wt o c a l c u t a taen t i - t i f ta, n t i - d i v e
a n da n t i - s q u a t ) .
B-.
\
.}z
ACKERMAN
STEERING
ISA DYNAMIC
TOEOUT
THAT
COMPENSAIES
FOR
THE
DIFFERENCE
INTURNING
RADIUS
OFTHEINSIDE
ANDOUTSIDE
WHEELS
ACKERMAN
ANGLE
ISTHEDIFFERENCE
INSTEERING
ANGTE
BETWEEN
THEINSIDE
AND
OUTSIllE
WHEET
INA CIIRNER.
t o e d - o uat l i g n m e nwt h i t et u r n i n gw i t h o u t
t h e h a n d l i n ga n dt i r e - w e adr r a w b a c kosf
W h e nc o r n e r i n gt h, e i n s i d ea n d
s t a t i ct o e - o u it n a s t r a i g hIti n e .
o u t s i d ew h e e l sh a v et o t r a v e l d i f f e r e n t J u s ta b o u ta t t c a r sh a v eA c k e r m a bn u i t t
d i s t a n c easn dd i f f e r e nat r c s l. f b o t h
i n t ot h e i rs t e e r i n g e o m e t r yl t. i s ,f o r a [ [
w h e e l sa r ea t t h e s a m es t e e r i n g
a n g l e , p r a c t i c apIu r p o s e sn,o n - a d j u s t a b T
t eh. e
t h e no n eo r b o t ht i r e sw o u l db e
A c k e r m aann g t ec a no n l yb e c h a n g e b
dy
s c r u b bni g .
m o v i n gt h e s t e e r i n gr a c kA
. simpler
T h i n ko f A c k e r m aans d y n a m i ct o e - s o l u t i o ni s t o j u s td i a ti n s o m es t a t i ct o e o u t ,w h i c hi n c r e a s et so e - o u a
t st h e
o u t ,w h i c hw i t t m u t t i p t yt h e A c k e r m a n
s t e e r i nw
g h e e l i st u r n e dT. h i sg i v e s
e f f e c t h a t ' sa t r e a d ye n g i n e e r eidn t h e
t h e q u i c kt u r n - i na d v a n t a goef h a v i n g car's steering geometry.
ACKERMAN
ANGLE
CAMBER
CURVE
0ne drawbacioi ndependent
suspension
designis itsinabilityto
maintain
a consistent
patchas
contact
thecar'sbodyrollsin a turn.Suspension
engineers
counterwithdesigns
thatgain
negative
camberunderro[1,
which
increases
corneri
nggrip.
M u l t i t i n ka n dA - a r ms u s p e n s i o a
n rse
d e s i g n ewd i t hs h o r t eur p p e rl i n k so r
differentrotationpointssothe upper
components
sweepin a tighterarcthan
the [ower[inks.Thedifferentarcsmake
t h ew h e e l g a i n e g a t i vcea m b ear st h e
s u s p e n s t oc no m p r e s s e s .
However,
therecanbe drawbacks.
T o om u c hc a m b egr a i nc a nc a u s es i d e
scrub.Sidescruboccursasa control
armsweeps
throughits rangeof motion
andpullsthetire lateratly,
whichadds
furthertractionloadon thetire.This
cancauseinstability
overbumps,
especia[[y
if the bumpsonlyaffectone
sideof the car.
Placing
thesuspension
[inksfor
negative
cambergainalsoaffects
the
ro[[center
location.
Fortunately,
it'seasy
to finda goodcompromise
betweenroll
centerlocation,
negative
cambergain
a n dm i n i m asI i d es c r u bC. o m p a n i e
[ isk e
SPLofferadjustable
suspension
[inks,
whichatlowfor cambercurveandro[[
center[ocation
adjustments.
0n MacPherson
strut-equipped
cars,
t h ew h e e l w i t t g a i n e g a t i vcea m b e r
underro[[as [ongasthe lowercontrol
arm is positioned
lessthan!o degrees
relative
to the strutaxis.Unfortunatety,
m a n ye n t h u s i a sw
t si t h M a c P h e r s o n
strut-equipped
carslowertheircarstoo
m u c ha n dm a k et h i sa n g t eg r e a t etrh a n
Beyond9o degrees,
!o degrees.
the
s u s p e n s i owni [ [g a i np o s i t i vcea m b e r
DOUBLE-WISHBONE
insteadof negative
as it compresses,
SUSPENSIONS
USE
A
s
i g n i f i c a n tcl yo m p r o m i s i g
n rgi p( s e e
SHORTER
UPPER
ARMOR
i[[ustration
below).
DIFFERENT
MOUNTING
Tocounterthisproblemthereare
POINTS
T(lALLllW
THE
WHEEL
T(|GAIN
NEGATIVE aftermarket
controIarmsthat lowerthe
CAMBER
ASTHE
outerpivotto restorea propercamber
SUSPENSI(lN
C(lMPRESSES. curveandro[[centeron an aggressively
THIS
ALLOWS
THECONTACT
lowered,
strut-equipped
car.SPLsetts
PATCH
TOREMAIN
controlarmsfor thefrontsuspension
of
CONSISTENT
ASTHE
the 5t3and5r4.SeveraI
suppliers
CAR
ROLLS
sell
blockspacers
to placebetweenthe
lowercontrolarmandbattjointto
correct
the rot[centerandcambercurve
on MacPherson
strut-equipped
cars.
I
I
;
i
I
=
I
T
Stock rlde height:
less than 90o
Loweredwlth
adjustable
controlarm:
About90o
MACPHERS(|I{
STRUT
SUSPENSII|I{S
ARE
DESIONED
TOGAII{
NEGATIVE
CAMBER
ASIOI{G
ASTHE
LOWER
C(INTR(IT
ARM
A]{I|STRUT
AXIS
F0RM
AtlAllGtE
0FIESS
THAI{
gOotChriS,rHrSUSpTNSIoN
90OEGREES.
WHEN
ACAR
lSL0WERED
T0MAKE
THIS
ANGLE
MOnr
tHnru
WILL
GAIN
POSITIVE
CAMBER.
SEVERAL
C0MPANIES
MAKE
BLOCK
SPACERS
0RREPLACEMENT
CoNTnOt
nnrvrS
rOsRrNG
THrAlrrbiignCK
r0 LESS
THAN
90DEGREES.
E
H
<Eg{
2005r wtyw.spoRrcoMpAcrcAnms.com
106 I orceiileen
*
-..:
CORNERBALANCING
Cornerweightsaresetby adjusting
C o r n ebr a l a n c i nigs t h e a d j u s t m e notf w e i g h t
t h es u s p e n s i or ind eh e i g hat t e a c h
d i s t r i b u t i oant e a c hw h e e [ l.d e a [ t yt h
, e c r o s s - w e i g h t c o r n ew
r h i l et h ec a ri s o n f o u r[ i n k e d
p e r c e n t a gi set h e s a m ed i a g o n a l tbye t w e e n
t h e c a r ' s electronic
scales.
Thescalesdisptaythe
c o r n e r sT.h i si s d o n es oa c a r ' su n d e r s t e e r / o v e r s t e eweight
r
supported
by eachwheet.With
balance
i s t h e s a m ei n a r i g h t -o r [ e f t - h a ncdo r n e r .
t h ed r i v e ri n t h ec a r t, h es p r i n gp e r c h e s
C o r n ew
r e i g h tc a nb e a d j u s t e o
d n a n yc a rw i t h a
areadjusted
to achieve
the desired
height-adjustab
l es p e n s i oTnh. e r ea r e
su
c r o s s - b a t a nR
c ea.i s i ntgh e p e r c h
a p p l i c a t i o nf os r n e a r t ya [ [ p o p u l apr e r f o r m a n ccea r s increases
the weightat thatcorner;
n o w a d a y sF.o ro l d e ro r n o n - m a i n s t r e acm
ars,
loweringdecreases
it
c o m p a n i eIsi k eG r o u n dC o n t r o l s e [ [ p a rttosm a k e
W e i g h at l s ot e n d st o b et r a n s f e r r e d
a n yc a r ' sc o i l - o v esru s p e n s i ohne i g h t - aj ud s t a b l e .
d i a g o n a [ [ayc r o s tsh e c a rw h e n
c h a n g i n pg e r c hh e i g h tT. h r o u g h
triat
a n de r r o rt,h e w e i g h t ss h o u t db e
a d j u s t e tdo b e a s c l o s et o e q u a Ia s
p o s s i b tfer o ms i d et o s i d ea n d
diagonally.
Thecomptexities
of modern
s u s p e n s i odne s i g nf r, o ma n t i - [ i f t
geometry
to cambercurves,
are
f u n d a m e n t at lhsa tm u s tb e u n d e r s t o o d
beforemakingeffective
changes
to a
vehicle's
suspension.
Formore
i n f o r m a t i oonn s u s p e n s i odne s i g na n d
t u n i n gl,o o kf o rt h e" M a k i n gl t S t i c k "
serieson the Webat
www.spo
rlcompactca
rweb.com.
Nextw
, e ' [ [ d i v ei n t ot h e m y s t e r y
o f d a m p e r sd,i s c u s s i ndge s i g n ,
o p e r a t i oa
n n dv a l v i n gs t r a t e g i et sh a t
w o r kt o g e t h etro m a k eo r b r e a ka
c a r ' sh a n d l i n gr .
sportcompact
carweb.com
lune 2fi)5
Part1:Four
steps
t0better
handling
Sticlry
tires,reduce
roll,diveandsquat,
balance
thechasis,
reduce
weight
transfer
luh 2(X)5
Part
2:Four
more
steps
tobetter
handling
Addnegative
your
camber,
tune
car's
toe,add
chasis
stiffnes,
adjust
caster
August2(X)5
Part3:lt'sallinthegeometry,
rollcenter,
bumpsteer,
overlowering
S O U R C E S
Raceadjustable
suspension
links,subframemourtingbushingsfor Nissans
SPL Parts
(866)SPL-PART
w\,vw,splparts.c0m
AdvancedDe.signhigh-techshock
abso6ers,coil-overconversionkits,
generalsuspension
consulting
Ground
Control
Inc.
(530)
677-8600
www.gr0u
nd-c0ntrol.c0m
s
Geomefoy
conec,tingkib
GtobatPerformance
Parts.
NorthAmericandistributors
for Whitetine
(616)
399-9025
globa
www,
Iperforma
ncepa
rts.com