I PERFORMANCEI ncorrect wheel alignment con-ditions affect tire wear and can cause drifting and/or pulling during cruise, acceleration and braking, plus poor directional control. For

Incorrect wheel alignment con-ditions affect tire wear and cancause drifting and/or pullingduring cruise, acceleration andbraking, plus poor directionalcontrol. For the performance-

minded customer, the need for precisewheel alignment becomes more pro-nounced, due to a number of factors:

•He expects crisp handling and maxi-mum grip/traction.

•Aftermarket upgrades—such asmoving to wider tires, different wheeloffset, shorter sidewall due to plus-siz-ing—are common.

•Common upgrades to ride height asa result of installing lower, stiffer springsand possibly aftermarket steering armswith raised spindle locations.

The goal, regardless of any potentialaftermarket upgrades, is to retain theoriginal wheel alignment specifications,as closely as possible for street-drivenvehicles. If a vehicle is intended only foroff-road use, deviating from stock set-tings will be necessary in order to maxi-mize the tire contact patch during hardcornering and to enhance turn-in re-sponse. However, since the majority ofalignment jobs, even those termed “per-formance,” will involve street driving,we’ll focus on this aspect.

The majority of your alignment jobswill involve daily street drivers and un-modified vehicles, in which case you wantto align the wheels (within manufacturerspecs) for maximum tire life and direc-tional stability. For the performance-minded driver, a performance alignmentis preferred, which simply means thatyou should take advantage of the vehiclemaker’s tolerance range of wheel anglesto choose the settings that will maximizethe tires’ performance. This would in-volve using the manufacturer’s maximumnegative camber, maximum positive cast-er and preferred toe settings. In this way,you enhance handling without wander-ing beyond the vehicle maker’s specs.

Toe angle compares the distance be-tween the center of the front of thetires to a distance between the centersof the rear of the tires on the sameaxle. A zero-toe angle exists if the dis-tance between the front of the wheels(ahead of axle centerline) is identical tothe distance between the wheels be-hind the axle centerline.

A toe-in condition (also referred to asa positive toe angle) is present when thetwo wheels on the same axle are closertogether at the front and wider apart atthe rear. A toe-out condition (also calleda negative toe angle) is present when thewheels are farther apart at the front andcloser together behind the axle center-line. All front suspensions, regardless ofdesign, feature toe angle adjustment, ata location on the steering tie rods/tie rodends. Live rear axles will feature no toeangle adjustment, since this is a fixed an-gle. Independent rear suspensions usu-

ally offer rear wheel toe adjustment.The toe angle is critical in terms of

tire tread life. Ideally, the front steerwheels need to be parallel while cruis-ing to avoid tread scrub. However, toecan also be used to alter a vehicle’s han-dling traits. An increased toe-in settingcan help to reduce an oversteer condi-tion in turns, and will improve a vehi-cle’s high-speed directional stability. Anincrease in toe-out can reduce an un-dersteer tendency and will enhance ini-tial turn-in during cornering. But, a toe-out can also result in a darty, less deci-

34 June 2015

BY JIM GIBSON

Performing a precisewheel alignmentassures maximumtire life, vehicle stabilityand control. Alignmentassumes even greaterimportance as vehicleperformance increases.

PERFORMANCEWHEELALIGNMENT

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sive straight-ahead condition at speed,especially in wet or slippery conditions.

With an excessive toe setting (in orout), each front tire is pointed in a di-rection other than straight ahead. Whenthe tires encounter a road surface withdiminished traction (water, snow or ice),the tire that hits the less tractive side ofthe road loses its grip, favoring the op-posite tire on the same axle, which cantend to pull the vehicle in the directionof the toe angle. For the street, it’s bestto stay within the limit range specifiedby the vehicle maker.

Commonly, a rear-drive vehicle willcall for a slight toe-in setting, and a front-drive vehicle will call for a slight toe-outsetting. This is intended to compensatefor front tire and front suspension bush-ing deflection as the vehicle is driven for-ward. As a rear-drive vehicle moves for-ward, the front wheels may tend to try tocrawl away from each other, while afront-wheel-drive vehicle’s front wheelsmay try to crawl inboard. If the suspen-sion has been modified with the use ofstiffer, less compliant bushings, theamount of toe change under acceleration

likely won’t change as much, so you maybe able to set front toe closer to zero, or abit toward the preferred side, dependingon driver requirements.

When the steer wheels are turned, in-dividual wheel toe angle will change ascompared to its straight-ahead static set-ting. For example, when the steeringwheel is turned to the left, the left frontwheel will exhibit greater toe-out as com-pared to the number of degrees that theright front wheel toes in. This design fea-ture reduces the tendency of tire scrubduring turns and reduces the turning ra-dius of the outboard wheel, reducing thecar’s tendency to turn-in too quickly,while providing reduced recovery effortwhen the vehicle direction changes. Theinside wheel must turn in a tighter radiusthan the outside wheel to allow asmoother turn and reduce tire scrubbing.

While static toe (with wheels aimedstraight ahead) allows both front wheelsto rotate at the same speed and parallelto each other, during a turn the individ-ual wheel toe angles differ when thesteering wheel is turned more than 20°.This is referred to as the Ackermanprinciple, allowing the inside wheel toturn in a tighter radius while the outsidewheel turns at a larger radius.

Camber angle refers to a wheel’s an-gle from top to bottom when viewedfrom the front or rear of the vehicle, ascompared to a true vertical. If the wheelleans out at the top, this is positive cam-ber. If the wheel leans inward at the top,this is negative camber. If the wheel isset at a true vertical, this is zero camber.

For high-performance driving, a neg-ative camber angle is preferred, to com-pensate for the lateral forces experi-enced in cornering. Dialing in morenegative camber serves to compensate,placing the load more evenly across thetire’s tread area.

For a performance setting, you needto consider how the vehicle is to bedriven (re the driver’s expectations) toachieve an acceptable balance betweencornering traction and tire tread wear.As you increase negative camber, the in-side tread area will tend to wear fasterthan the rest of the tread when the ve-hicle is driven in a straight line; but ifthe driver is aggressive in turns, insuffi-cient negative camber will allow the tireto “roll” excessively, reducing the tire

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contact patch. The goal is tocreate an acceptable com-promise between tread wearand cornering grip. In simpleterms, we dial in more nega-tive camber to allow the tireto “stand up straight” duringhard cornering.

Factory camber specs areusually biased toward overalltire tread life for so-callednormal driving. The perfor-mance-minded driver willbenefit from a setting that’snegative. Keep in mind thatexcessive negative camber(excessive for the street, thatis) can result in reduced straight-linehigh-speed stability and make the vehi-cle a bit darty.

Caster angle settings for street-drivenvehicles should provide a good compro-mise to achieve an acceptable steeringeffort, confident high-speed stabilityand turning/cornering performance. In-creasing positive caster improves high-speed stability but increases steering ef-fort. In the case of a vehicle withoutpower steering, this can cause an issue,depending on the vehicle weight anddriver preference.

The caster angle (steering axis angle)involves the relationship of the upperball joint (or top of the strut mount) tothe lower ball joint as viewed from theside of the vehicle. Using a true verticaldrawn through the hub center as a ref-erence, caster angle is represented by astraight line drawn through the upperball joint/pivot location through the low-

er ball joint. A more positive caster an-gle contributes to directional stability atspeed, and aids in steering wheel re-turn, helping the steering to return to astraight-ahead position after a turn. Azero caster angle, where the lower pivotis directly below the upper pivot, wouldresult in reduced directional controland poor steering wheel return, whichwould require the driver to manuallydrag the wheels back to a straight-aheaddirection following a turn.

The steering axle’s caster angle has amajor influence in directional control.Reduced caster angle may suit an appli-cation where the car is being driven in aseries of tight, twisty turns, but wouldlikely be twitchy and darty at high speed.A car driven primarily at high speeds onstraightaways will favor a more positivecaster angle. With that said, for streetdriving, even on a performance vehicleapplication, it’s best to follow the vehicle

manufacturer’s caster spec toobtain the best compromisebetween tight-turn and high-speed control.

Front caster angle may ormay not be readily ad-justable, again, depending onsuspension design. If thefront suspension features up-per and lower control arms,the upper arm will likely beadjustable, either via the ad-dition or removal of shims(between the upper arm andframe) or via eccentric bush-ings. If an upper/lower con-trol arm system is featured,

the two anchoring locations (where theupper arm attaches to the frame) can beadjusted (again, with shims or ec-centrics). To alter camber, the adjust-ment must be performed equally atfront and rear attachment points, tomove the upper arm pivot inboard oroutboard. If caster is to be adjusted, onlyone end would be adjusted.

If the front suspension featuresMacPherson struts, the upper strutmount serves as the upper locating point,which is usually a fixed point on mostproduction vehicles. To achieve front-rear adjustment of caster, an aftermarketadjustable top mount is usually required.

Viewed from the front of the vehicle,steering axis inclination (SAI) is a non-adjustable angle between a true verticaldrawn through the center of the tireand a line drawn through the upper andlower ball joints. The SAI is determinedat the point in which these two lines in-

PERFORMANCE WHEEL ALIGNMENT

During turns, the inside wheel turns at a greater toe-out angle,turning the wheel to follow a tighter radius than the outsidewheel. This reduces tire scrub during a turn, allowing the wheelsto rotate during the turn with less rolling resistance.

Camber angle refers to the lean of the wheel, viewed from thefront. Performance alignments should bias the front wheels to-ward the maximum tolerance of the factory’s negative camberrange to maximize the tire contact patch during hard cornering.

Caster angle refers to the angle created by the location of the up-per ball joint or strut top relative to the lower ball joint. As shownon the left, if the upper point is ahead of the lower, this is nega-tive caster. If the upper and lower follow a true vertical, this is zerocaster. If the upper is behind the lower, this is positive caster. In-creased positive caster aids in directional control at speed and su-perior steering wheel return following a turn.

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tersect. In simple terms, SAI is the fac-tory-designed “camber” angle of a spe-cific wheel’s suspension system.

Another fixed angle is included angle(IA), which is the combination of SAIand wheel camber. Both SAI and IA aremeasured to verify that the fixed-by-design angles are correct. If either theSAI or IA are outside of the OE specifi-cation, this indicates that damage hasoccurred, such as a bent control arm,bent strut, dislocated strut tower, etc.

Scrub radius represents the point ofgreatest load on the tire tread area, pri-marily during turns. As viewed from thefront of the vehicle, this is determinedby considering the distance between thecenter of a front tire tread and the imag-inary SAI line, when measured at theroad surface. Since these two lines willeventually intersect, it’s this intersectionpoint that we’re really interested in.

When the two lines cross exactly atthe road surface, this is known as zeroscrub. When the lines cross above the

road surface, this condition is known asnegative scrub. When the lines intersectbelow the road surface, the condition iscalled positive scrub. An excessivelynegative scrub radius tends to increasesteering effort, while excessive positivescrub radius (where the load of thetread is moved moves further outboard)can not only affect handling and ease ofsteering, but can overstress wheel bear-ings as well.

Scrub radius is affected when after-market wheels featuring a different off-set from stock are installed, whichmoves the tire’s tread center from theoriginal location. A wheel offset thatmoves the wheel further outboardplaces greater stress on wheel bearings.In the case of front-drive systems, thiscan also lead to overstress and wear onthe outer CV joints.

In most cases, a short-arm/long-armsuspension (upper and lower controlarms where the lower arm is longer)will exhibit a positive scrub radius.

Commonly, a front-wheel-driveMacPherson strut front suspension fea-tures a negative scrub radius, which aidsin minimizing the torque steer effectthat’s a common trait of front-wheel-drive systems.

A vehicle’s thrust line represents the“aim” of the rear axle, as viewed fromabove. The thrust line effectively di-vides left and right rear wheel toe. Thethrust line may or may not follow thegeometric centerline.

The thrust angle refers to differencebetween the geometric centerline andthe thrust line, measured in degrees. Asviewed from above, if the thrust angleaims to the right (passenger side), this isa positive thrust angle. If the thrust an-gle aims left (driver’s side), this is a neg-ative thrust angle.

Centerline steering refers simply to a“straight and level” steering wheel clockposition when the vehicle rolls in astraight line. If the steering wheel is notcentered, this may indicate a possiblethrust angle deviation. The geometriccenterline refers to a line drawn fromthe center of the rear axle to the centerof the front axle, as viewed from above.

Types of Wheel AlignmentThe age-old method of centerline two-wheel alignment does not consider therear wheel positions, and should not beemployed because it ignores the thrustdirection of the rear axle. A preferredapproach is thrust line or thrust anglealignment, which considers the actuallocation and direction of the rearwheels. This allows you to adjust thefront wheel angles relative to the rearwheel angles, regardless of the geomet-ric centerline.

If a vehicle features rear wheel toeadjustment, we can achieve optimumwheel alignment using the total four-wheel alignment, which includes adjust-ment of all four wheels, allowing toe ad-justment to bring the thrust angle toideal zero, or as close to zero as possible.

If the thrust angle is off zero, this cancontribute to vehicle dog-tracking(crooked body relative to direction oftravel), increased tire wear and unequalleft/right turning. Total four-wheelalignment allows you to adjust andhopefully correct rear axle thrust angle,

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then adjust the front wheels parallel tothe rear wheels.

Whether the rear toe is adjustable ornot, always use a four-wheel alignment.This approach allows you to refer to andconsider the rear, while a total four-wheel alignment allows adjusting of bothfront wheel angles as well as rear toe.

Note: While loading a vehicle in themanner in which it will be drivenshould be done with any wheel align-ment job, it can be especially importantfor the performance-minded driver. Toobtain the optimum wheel alignmentangles for someone who expects maxi-mized performance, the weight of thedriver should be considered (as well asone or more passengers, depending on

how many people will be riding in thevehicle for the majority of its operation).Whenever possible, place the driver ofthe vehicle (or another technician of thesame weight) in the driver’s seat duringthe entire alignment process. This willallow you to better tune the wheel an-gles in a loaded condition as the car willnormally be driven. This becomes moreof a factor if the driver tends to be onthe heavy side.

Today’s production vehicles tend tooffer a limited range of adjustment for

camber and caster angles. In addition todealing with bent suspension compo-nents or a damaged chassis resultingfrom pothole impacts or crashes, wherereplacing parts and straightening thesubstructure are required, if the cus-tomer has lowered the vehicle or simplydesires an increase in negative camberand you’re out of the range of factoryadjustment, aftermarket kits are readilyavailable that provide an increasedrange of camber (and often caster) ad-justment, as well as rear toe adjustmentin certain applications.

Depending on suspension design,upper and lower control arms may beadjustable for camber and caster viaspacer shims between the upper arm

and frame, or by rotating an eccentricshaft or washers on either arm. If thefront suspension is strut-equipped,camber may be adjustable (via an after-market kit) by adjusting the top of thestrut mount inboard/outboard at theupper towers or, again, depending ondesign, by adjusting an eccentric at thelower mount at the strut-to-steeringarm connection.

If a vehicle is equipped with an inde-pendent rear axle, camber should be ad-justable via either eccentric bushings at

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41June 2015

Steering axis inclination (SAI) is the fixedangle established by a line drawn throughupper and lower ball joints (or center topof a strut to lower ball joint), with refer-ence to a true vertical at the lower joint.Out-of-spec SAI indicates suspension dam-age. Included angle (IA) considers both SAIand the hub and wheel camber angle.

The pivot/load point scrub radius of a tireconsiders the angle created by the wheelcamber and the SAI. An aftermarket wheelwith greater offset from OE that places thewheel centerline further outboard resultsin a more positive scrub radius, which canplace excess strain on wheel bearings.

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the inboard control arm pivot points oran eccentric at the strut-to-rear upright.To aid in maintaining chassis rigidity,

aftermarket strut tower braces can be in-stalled to tie the left and right struts to-gether, preventing the unibody fromflexing during hard turns. This will helpto avoid camber changes during spiriteddriving. Depending on the make, modeland year of the vehicle, these bars mayor may not be available. Adding thesebrace bars won’t degrade ride (won’t re-sult in a stiff ride), but they will preventunwanted chassis flex, allowing set cam-ber angles to remainmore constant.You’re probably all too familiar with

the phenomenon known as tramlining—the tendency for a vehicle to follow irreg-ularities on the road surface such as un-even pavement or severe rut-ting. Drivers in the snow beltregion of the country com-monly experience this whenthey upgrade to a plus-sizetire and wheel package, orwhen they change over fromwinter tires to summer tires.Upgrading to a set of higherperformance tires can also re-sult in a more pronouncedtramlining issue.The more “sensitive” the

tire, the more susceptible thevehicle is likely to become toroad irregularities, sincewider tires with shorter side-walls and/or tires that featurestiffer sidewalls have a greatertendency to “track” the roadsurface. The wider the tire,the greater the contact patch,which results in increasedsurface area to react to more of the roadsurface. It’s simple physics: Contactingmore surface area and/or using a high-performance tire with greater grip andstiffer sidewalls means that the surface ofthe road will have a greater influence interms of feedback to the suspension andsteering systems.

Wheel offset has a direct bearing ontramlining as well. Installing wider tiresor a plus-size tire and wheel packageusually requires using wheels with a dif-ferent offset than the vehicle’s originalwheels. In some cases, the new wheelswill have slightly less offset than the

original and in other cases, slightly more.Usually the amount of offset change iskept to a minimum, and vehicle trackingremains relatively unchanged. However,if the offset is significantly different, itwill alter the way the road forces aretransmitted through the tire and wheelto the suspension. Therefore, largechanges in wheel offset will increase thelikelihood of tramlining.Also consider the condition of suspen-

sion components such as bushings, balljoints and shock absorber mounts. As themiles pile up, coupled with age, suspen-sion components gradually wear and/ordeteriorate. Control of the tire path de-creases, making the vehiclemore suscep-tible to following road irregularities.Imagine a worn suspension that al-

lows a front wheel and tire to swing be-tween the recommended 1⁄16 in. of toe-in and 1⁄16 in. of toe-out when it encoun-ters a rut in the road. This 1⁄8-in. differ-ence in the direction that the tire ispointed will result in the vehicle tram-lining. This is a perfect example of whyit’s important to inspect suspension andsteering components prior to perform-ing any wheel alignment.Wheel camber and toe settings com-

bine to affect a vehicle’s tendency totramline. Extreme positive or negativecamber settings will make a vehiclemore sensitive, especially when only

one wheel at a time encounters a longi-tudinal rut and/or groove. Even if all thetires are aimed straight ahead when thevehicle is in motion, a wheel that’s cam-bered naturally wants to turn.

Camber thrust is generated by aleaning tire. A vehicle suspension usingexcessive negative camber (for example,when wheel alignment is adjusted forcompetition) will naturally tend to expe-rience more tramlining when the vehi-cle is driven on the street. When a chas-sis is set up for competition use, it’scommon to adjust for greater toe-out inorder to achieve more immediate turn-in on corners. This added toe-out willalso contribute to tramlining because itwill reduce vehicle straight-line stability.It should be obvious at this point that

adjusting wheel alignment forthe track will be detrimentalfor street driving, and viceversa. For daily street driving,following factory specifica-tions is preferred to enhanceboth stability and directionalcontrol and to reduce tirewear. Track settings are forthe track, street settings arefor the street. Period.A final word about infla-

tion pressure: While tire in-flation pressure must be ad-justed for any competitionapplication to obtain maxi-mum tire contact patch, es-pecially under severe lateralforce (during turns), usinghigher tire pressures thanrecommended by the vehiclemanufacturer for street driv-ing will unnecessarily stiffen

the tire and make it even more willingto cause tramlining. Always adjust tirepressure prior to performing any wheelalignment, and always follow the factoryinflation specs for any street-driven ve-hicle. If the vehicle owner intends touse the vehicle for occasional autocrossevents, you can increase tire pressure tosuit track conditions, but he must droppressure to factory spec before drivingon public roads.

PERFORMANCE WHEEL ALIGNMENT

TToopp:: The geometric centerline of any vehicle refers to a line drawnfrom the center of the rear axle through the center of the frontaxle. AAbboovvee:: The thrust line refers to the angle at which the rearaxle is aimed. The thrust angle is the difference between the geo-metric centerline and the measured thrust line.

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This article can be found online atwww.motormagazine.com.

Thrust Line & Geometric Centerline

Thrust Line

Geometric Centerline

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