The Comprehensive Suspension Tuning Guide
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TheComprehensive SuspensionTuning Guide BY MIKE KOJIMA PHOTOGRAPHYBYMIKE KOJIMA ANDJ0SH JACQU0T ILLUSTRATIONSBYTI TONG BMWs,Mk I andllToyota Supras and Frontof the Datsun510 have severe toe steer. TOESTEER IS the car Becauseof thisgeometry, some semi- INDUCEDINSEMI- TRAILINGARM trailingarm suspensions toe out under SUSPENSIONSAS ro[[,which can cause severe traiting- THESUSPENSION throttleoversteer. Others, Iike those COMPRESSES foundon the Porsche928 and FC RX-Z BECAUSEOFTHIS use[inks or bushingsof different GEOMETRY,THE hardnessto forcethe suspension to REARWHEEL TOES passivelytoe in underrolt, which OUIDURING COMPRESSION createsundersteer at the limit. Thesedesigns work wel[ if they're operatingthe way the engineers intendedthem to. lt's when speed junkies[ike us try to out-engineerthe Stockride height:trailing arms engineersthat probtems devetop. This Fig.1 parrallelto ground.A and B usua[[yhappens when lowering the car. are equal. Oneeasy solution to reducetoe changein [owered,semi-trailing-arm suspensionsisto increasethe spring rateto reducewheeltrave[ (effectively increasingthe overat[ wheel rate). The lessthe wheel moves, the less toe can change.0bviousty, overstiffen ing the rearsuspension has its own drawbacks, butit workedwetlon the Datsun 5ro and BMW3 Series,both of which had a [ong andsuccessfuI racing heritage. Manycurrent sport compacts use multilinkrear suspensions designed to sweepthrough complex camber and toe anglesto maximizestreet performance, loweredride height: trailing comfortand stability. However, this Fi1.2 armstilted up in rear.A is combinationof camberand toe can greaterthan B. conspireagainst an enthusiastwho's wittingto acceptthe compromises of a moreaggressive suspension setup. Aftermarkettuners tike Whitetine offeroffset bushings for cars tike the EVOthat relocate the pivot points to reducetoe steer. lf kitsare unavailabtefor your apptication, a racecarfabricator can reposition the traitingarms and other Iinks to a correctedposition on severetylowered cars.lt's also common to stotcontrol armmounting points to altertheir pathof traveland correct toe on loweredcars. FIG.1 INA TRAILING ARM BEAM AXLE Excessivelowering can make toe SUSPENSION,THETRAILING TRAVELINGUPPULLS THE BEAM AXLE ARMSMOVE INOPPOSING steerworse by ptacingthe suspension FURTHERTHAN THE SIDE TRAVELING DIRECTIONSASTHE CAR ROLLS AND DOWNWARD. linksin a staticposition THISDIFFERENCE IN andrange of PULLTHE BEAM AXLE EQUALLY WITH MOVEMENTCREATES TOESTEER trave[they were never designed for. A NOCHANGE INTOE. 98 .I beamaxle suspension with traiting arms,as found in the rear of many COMPANIES sma[[front-whee[-drivecars is a good LIKEWHITELINE exampteof wherethis might occur. OFFERPIVOT OFFSET Atstock ride height,the trailing BUSHINGSFOR armsare usua[[y parattel to the MULTI-LINK ground.When the car rolts, the REAR outboardand inboard arms swing in SUSPENSION differentdirections. The resulting arc- CARSLIKE THE shapedaxle path shortens the car's EVOVIII, WHICH wheelbaseduring compression and ALTERTHE PIVOTPOINT IO droop.Since each arm swings equa[[y MINIMIZETOE in differentdirections, the axle's toe STEER doesn'tchange because each end of thebeam axle is pul[ed the same distanceforward. lf thecar is loweredtoo much, angle,the arms don't move equa[[y in axlerearward while the outer arm bothtrailing arms point downward opposingdirections. Under ro[[, the wittputt its side of the axle forward, towardthe front of the car.At this insidearm wi[[ push its side of the causingundersteer. ANTI-DIVEAND ANTI-LIFT Anti-diveand anti-lift are tricks that canbe apptied to a car'sfront WHITELINE suspensiongeometry to OFFERS controlbrake WRXANTI-LIFT diveand acceteration [ift. BUSHINGSTHAT Liftand dive can be mitigated by LOWERTHE REAR carefu[[ylocating suspension pivot PIVOTPOINT ON pointsto takeadvantage of the"force THEFRONT reaction"on the chassiscreated by CONTROLARMS. acceterationor deceleration.This THISCHANGES THE INSTANT avoids .. CENTER theneed to increasespring OFTHE FRONT ratesto reducepitching-an important SUSPENSION, issuefor ridequatity in a[[street cars. WHICHREDUCES Anti-divehetps prevent the noseof ANTI-LIFL thecar from diving during hard braking.Most cars have some degree of anti-divedesigned into the stock frontsuspension to stiffenwhite brakingor acceterationforces. Most suspensiongeometry. Anti-dive brakingfor a corner,which can cause kitsavailable for seriouswrenchers to utilizesdeceleration forces to increase understeer.ln extremeamounts, anti- alteranti-dive and anti-tift work to thefront wheelcompression rate and divegeometry witt cause wheel hop reducethese factors. reducebrake dive. andcaster changes under braking. 0n front-and a[[-wheet-drive cars, Bychanging the angle of the Mostracecar suspensions have thissame geometry also resists front- suspension[inks, the amount of muchless anti-dive and anti-lift than endtift underacceleration. Subaru's anti-dive canbe maniputated. streetcar suspensions. 0n racecars, WRXis notoriousfor largeamounts of However,excessive anti-dive can the stiffsuspension is usedto control anti-diveand anti-[ift, causing nontinear hinderperformance by causing the bodymotion instead of redirecting steeringresponse. Bushings reducing i i i i i i i i i i i i anti-dive andanti-[ift are designed to andbushings to tunethe anti-tift givemore naturaI steering i responseand andanti-dive out of manypoputar improvecorner-exit traction on i WRXsby a[[-wheet- and front-wheet-d rive eliminating i thesecompromises. cars.Kits are available for a[[WRXs, i Anti-tiftgeometry greatlyaffects theSentra SE-R, many Celicas, the i launch traction for front-wheet-drive Mazda323 and the Gatant/ Ectipse. A dragcars, yet ontyrecently have fewJapanese tuners offer subframe i tube-frame proctass front-wheet- mountsforthe NissanSt3,St4,Z3z drivedrag racers begun to consider i 3ooZX,R32,33 and 34 Skytine to usingfront-end anti-tift geometry. i changethe height of thefront Whitelinehas suspension mounts instantcenter. i i CatcutatingAnti-Squat i Heightof the C.G. i ,) a i Anti-squatis 25%because this Distanceto InstantCenter distance is one.quarterof the heightof ..........- the C.G. t i \-Qftsnging this distancewiil arter the squattingreaction of the car i i Heightof the G.G. J Anti-squatis 50%because this dlstancels hatfthe helghtof the C.G. 1O2 | orcrueen2o0s r trlyr,y.smnrcoupAgrcAnwEB.coM is notoriousfor [aunchingpoorty at thedragstrip. Rear-whee[-drivedrag racers have madea scienceof anti-squattuning to ANTI-SOUAT havea greatdeaI of anti-squatin maximizerear-wheel traction. Drag Anti-squatisthe exact same theirrear suspension geometry. carsoften have so much anti-squat geometry apptiedat the backaxle Thismakes them transition to on- geometrythat the backof thecar actuatly of a rear-wheel-drivecar. Unlike throttleoversteer very rapidly liftswhen [aunching, driving the tires thefront of thecar, smallamounts becauseanti-squat, [ike anti-dive, intothe ground. Drag cars have of anti-squatare generally a good significantlyincreases the wheel adjustablefour-[ink rear suspensions, thingfor a rear-drivecar since it rate.This is whythe Sr3works so sothis percentage istunabte for the a[]owsfor softerrear suspensions we[[for drifting. amountof bitedesired in different withoutthe excess squat. Extremeanti-squat can cause conditions(see sidebar on page102 to Cars[ike the NissanR3z Skytine wheelspinand rear wheel hop learnhow to calcutateanti-tift, anti-dive GT-R,Z3z 3ooZX and Sr3 24oSX underpower, which is why the 23z andanti-squat). B-. AngleA is smaller \ thanangle B. .}z ACKERMANSTEERING ISA DYNAMICTOE OUT THAT COMPENSAIES FORTHE DIFFERENCEINTURNING RADIUS OF THE INSIDE AND OUTSIDE WHEELS ACKERMAN ANGLEIS THE DIFFERENCE INSTEERING ANGTE BETWEEN THE INSIDE AND OUTSIllEWHEET INA CIIRNER. toed-outalignment white turning without ACKERMANANGLE the handlingand tire-wear drawbacks of Whencornering, the insideand statictoe-out in a straightIine. outsidewheels have to traveldifferent Justabout attcars have Ackerman buitt distancesand different arcs. lf both intotheir steering geometry. lt is,for a[[ wheelsare at thesame steering angle, practicaIpurposes, non-adjustabte. The thenone or bothtires would be Ackermanangte can only be changed by scrubbing. movingthe steering rack. A simpler Thinkof Ackermanas dynamic toe- solutionis to justdiat in somestatic toe- out,which increases toe-out as the out,which witt muttiptythe Ackerman steeringwheelis turned. This gives effectthat's atready engineered in the thequick turn-in advantage of having car's steeri ng geometry. CAMBERCURVE thewheelgain negative camber as the ro[[centerlocation. Fortunately, it's easy 0ned rawbacioi ndependent suspenstoncompresses. to finda goodcompromise between roll suspensiondesign is its inability to However,there can be drawbacks. centerlocation, negative camber gain maintaina consistentcontact patch as Toomuch camber gain can cause side andminimaI side scrub. Companies [ike thecar's body rolls in a turn.Suspension scrub.Side scrub occurs as a control SPLoffer adjustable suspension [inks, engineerscounter with designs that gain armsweeps through its rangeof motion whichatlow for camber curve and ro[[ negativecamber under ro[1, which andpulls the tire lateratly,which adds center[ocation adjustments. increases cornering grip. furthertraction load on the tire. This 0n MacPhersonstrut-equipped cars, Multitinkand A-arm suspensions are cancause instability over bumps, thewheelwittgain negative camber designedwith shorter upper links or especia[[yif the bumpsonly affect one underro[[