PERFORMANCE WHEEL ALIGNMENT

I

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

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

•Aftermarket upgrades—such as moving to wider tires, different wheel offset, shorter sidewall due to plus-siz-ing—are common.

•Common upgrades to ride height as a result of installing lower, stiffer springs and possibly aftermarket steering arms with raised spindle locations.

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

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

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

A toe-in condition (also referred to as apositive toe angle) is present when the two wheels on the same axle are closer together at the front and wider apart at the rear. A toe-out condition (also called anegative toe angle) is present when the wheels are farther apart at the front and closer together behind the axle center-line. All front suspensions, regardless of design, feature toe angle adjustment, at a location on the steering tie rods/tie rod ends. Live rear axles will feature no toe angle 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 steer wheels need to be parallel while cruis-ing to avoid tread scrub. However, toe can also be used to alter a vehicle’s han-dling traits. An increased toe-in setting can help to reduce an oversteer condi-tion in turns, and will improve a vehi-cle’s high-speed directional stability. An increase 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-BY

JIM

GIBSON

Performing a precise

wheel alignment

assures maximum

tire life, vehicle stability

and control. Alignment

assumes even greater

importance as vehicle

performance increases.

PERFORMANCE

WHEEL

ALIGNMENT

PERFORMANCE

WHEEL

ALIGNMENT

P h o to il lu stra ti o n : Ha ro ld A. P e rry; ima g e s: Hu n te r E n g in e e ri n g & T h in k sto ck

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

With an excessive toe setting (in or out), each front tire is pointed in a di-rection other than straight ahead. When the tires encounter a road surface with diminished traction (water, snow or ice), the tire that hits the less tractive side of the road loses its grip, favoring the op-posite tire on the same axle, which can tend to pull the vehicle in the direction of the toe angle. For the street, it’s best to stay within the limit range specified by the vehicle maker.

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

likely won’t change as much, so you may be able to set front toe closer to zero, or a bit toward the preferred side, depending on driver requirements.

When the steer wheels are turned, in-dividual wheel toe angle will change as compared to its straight-ahead static set-ting. For example, when the steering wheel is turned to the left, the left front wheel will exhibit greater toe-out as com-pared to the number of degrees that the right front wheel toes in. This design fea-ture reduces the tendency of tire scrub during turns and reduces the turning ra-dius of the outboard wheel, reducing the car’s tendency to turn-in too quickly, while providing reduced recovery effort when the vehicle direction changes. The inside wheel must turn in a tighter radius than the outside wheel to allow a smoother turn and reduce tire scrubbing. While static toe (with wheels aimed straight ahead) allows both front wheels to rotate at the same speed and parallel to each other, during a turn the individ-ual wheel toe angles differ when the steering wheel is turned more than 20°. This is referred to as the Ackerman principle, allowing the inside wheel to turn in a tighter radius while the outside wheel turns at a larger radius.

Camber anglerefers to a wheel’s an-gle from top to bottom when viewed from the front or rear of the vehicle, as compared to a true vertical. If the wheel leans out at the top, this ispositive cam-ber. If the wheel leans inward at the top, this isnegative camber. If the wheel is set at a true vertical, this iszero 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 more negative camber serves to compensate, placing the load more evenly across the tire’s tread area.

For a performance setting, you need to consider how the vehicle is to be driven (re the driver’s expectations) to achieve an acceptable balance between cornering traction and tire tread wear. As you increase negative camber, the in-side tread area will tend to wear faster than the rest of the tread when the ve-hicle is driven in a straight line; but if the driver is aggressive in turns, insuffi-cient negative camber will allow the tire to “roll” excessively, reducing the tire

contact patch. The goal is to create an acceptable com-promise between tread wear and cornering grip. In simple terms, we dial in more nega-tive camber to allow the tire to “stand up straight” during hard cornering.

Factory camber specs are usually biased toward overall tire tread life for so-called normal driving. The perfor-mance-minded driver will benefit from a setting that’s negative. Keep in mind that excessive negative camber (excessive for the street, that

is) can result in reduced straight-line high-speed stability and make the vehi-cle a bit darty.

Caster anglesettings for street-driven vehicles should provide a good compro-mise to achieve an acceptable steering effort, confident high-speed stability and turning/cornering performance. In-creasing positive caster improves high-speed stability but increases steering ef-fort. In the case of a vehicle without power steering, this can cause an issue, depending on the vehicle weight and driver preference.

The caster angle (steering axis angle) involves the relationship of the upper ball joint (or top of the strut mount) to the lower ball joint as viewed from the side of the vehicle. Using a true vertical drawn through the hub center as a ref-erence, caster angle is represented by a straight line drawn through the upper ball joint/pivot location through the

low-er ball joint. A more positive castlow-er an-gle contributes to directional stability at speed, and aids in steering wheel re-turn, helping the steering to return to a straight-ahead position after a turn. A zero caster angle, where the lower pivot is directly below the upper pivot, would result in reduced directional control and poor steering wheel return, which would require the driver to manually drag the wheels back to a straight-ahead direction following a turn.

The steering axle’s caster angle has a major influence in directional control. Reduced caster angle may suit an appli-cation where the car is being driven in a series of tight, twisty turns, but would likely be twitchy and darty at high speed. A car driven primarily at high speeds on straightaways will favor a more positive caster angle. With that said, for street driving, even on a performance vehicle application, it’s best to follow the vehicle

manufacturer’s caster spec to obtain the best compromise between tight-turn and high-speed control.

Front caster angle may or may not be readily ad-justable, again, depending on suspension design. If the front suspension features up-per and lower control arms, the upper arm will likely be adjustable, either via the ad-dition or removal of shims (between the upper arm and frame) or via eccentric bush-ings. If an upper/lower con-trol arm system is featured, the two anchoring locations (where the upper arm attaches to the frame) can be adjusted (again, with shims or ec-centrics). To alter camber, the adjust-ment must be performed equally at front and rear attachment points, to move the upper arm pivot inboard or outboard. If caster is to be adjusted, only one end would be adjusted.

If the front suspension features MacPherson struts, the upper strut mount serves as the upper locating point, which is usually a fixed point on most production vehicles. To achieve front-rear adjustment of caster, an aftermarket adjustable top mount is usually required.

Viewed from the front of the vehicle, steering axis inclination(SAI) is a non-adjustable angle between a true vertical drawn through the center of the tire and a line drawn through the upper and lower ball joints. The SAI is determined at 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 outside wheel. This reduces tire scrub during a turn, allowing the wheels to rotate during the turn with less rolling resistance.

Camber angle refers to the lean of the wheel, viewed from the front. Performance alignments should bias the front wheels to-ward the maximum tolerance of the factory’s negative camber range 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 shown on 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 zero caster. 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.

Toe-Out Toe-In Il lu stra ti o n s co u rte sy Hu n te r E n g in e e ri n g

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

Another fixed angle isincluded angle

(IA), which is thecombinationof SAI

and wheel camber. Both SAI and IA are measured to verify that the fixed-by-design angles are correct. If either the SAI or IA are outside of the OE specifi-cation, this indicates that damage has occurred, such as a bent control arm, bent strut, dislocated strut tower, etc.

Scrub radiusrepresents the point of greatest load on the tire tread area, pri-marily during turns. As viewed from the front of the vehicle, this is determined by considering the distance between the center of a front tire tread and the imag-inary SAI line, when measured at the road surface. Since these two lines will eventually intersect, it’s this intersection point that we’re really interested in.

When the two lines cross exactly at

the road surface, this is known aszero

scrub. When the lines cross above the

road surface, this condition is known as

negative scrub. When the lines intersect below the road surface, the condition is

calledpositive scrub. An excessively

negative scrub radius tends to increase steering effort, while excessive positive scrub radius (where the load of the tread is moved moves further outboard) can not only affect handling and ease of steering, 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, which moves the tire’s tread center from the original location. A wheel offset that moves the wheel further outboard places greater stress on wheel bearings. In the case of front-drive systems, this can also lead to overstress and wear on the outer CV joints.

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

Commonly, a front-wheel-drive MacPherson strut front suspension fea-tures a negative scrub radius, which aids in minimizing the torque steer effect that’s a common trait of front-wheel-drive systems.

A vehicle’sthrust line represents the

“aim” of the rear axle, as viewed from above. The thrust line effectively di-vides left and right rear wheel toe. The thrust line may or may not follow the geometric centerline.

Thethrust anglerefers todifference

between the geometric centerline and the thrust line, measured in degrees. As viewed from above, if the thrust angle aims to the right (passenger side), this is apositive thrust angle. If the thrust

an-gle aims left (driver’s side), this is a

neg-ative thrust angle.

Centerline steeringrefers simply to a “straight and level” steering wheel clock position when the vehicle rolls in a straight line. If the steering wheel is not centered, this may indicate a possible

thrust angle deviation. Thegeometric

centerlinerefers to a line drawn from the center of the rear axle to the center of the front axle, as viewed from above.

Types of Wheel Alignment

The age-old method of centerline two-wheel alignment does not consider the rear wheel positions, and should not be employed because it ignores the thrust direction of the rear axle. A preferred

approach isthrust lineorthrust angle

alignment, which considers the actual location and direction of the rear wheels. This allows you to adjust the

front wheel anglesrelativeto the rear

wheel angles, regardless of the geomet-ric centerline.

If a vehicle features rear wheel toe adjustment, we can achieve optimum

wheel alignment using thetotal

four-wheel alignment, which includes adjust-ment of all four wheels, allowing toe ad-justment to bring the thrust angle to ideal zero, or as close to zero as possible.

If the thrust angle isoff zero, this can

contribute to vehicle dog-tracking (crooked body relative to direction of travel), increased tire wear and unequal left/right turning. Total four-wheel alignment allows you to adjust and hopefully correct rear axle thrust angle,

then adjust the front wheels parallel to the rear wheels.

Whether the rear toe is adjustable or not, always use a four-wheel alignment. This approach allows you to refer to and consider the rear, while a total four-wheel alignment allows adjusting of both front wheel angles as well as rear toe.

Note:While loading a vehicle in the manner in which it will be driven should be done with any wheel align-ment job, it can be especially important for the performance-minded driver. To obtain the optimum wheel alignment angles for someone who expects maxi-mized performance, the weight of the driver should be considered (as well as one or more passengers, depending on

how many people will be riding in the vehicle for the majority of its operation). Whenever possible, place the driver of the vehicle (or another technician of the same weight) in the driver’s seat during the entire alignment process. This will allow you to better tune the wheel an-gles in a loaded condition as the car will normally be driven. This becomes more of a factor if the driver tends to be on the heavy side.

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

camber and caster angles. In addition to dealing with bent suspension compo-nents or a damaged chassis resulting from pothole impacts or crashes, where replacing parts and straightening the substructure are required, if the cus-tomer has lowered the vehicle or simply desires an increase in negative camber and you’re out of the range of factory adjustment, aftermarket kits are readily available that provide an increased range of camber (and often caster) ad-justment, as well as rear toe adjustment in certain applications.

Depending on suspension design, upper and lower control arms may be adjustable for camber and caster via spacer shims between the upper arm

and frame, or by rotating an eccentric shaft or washers on either arm. If the front suspension is strut-equipped, camber may be adjustable (via an after-market kit) by adjusting the top of the strut mount inboard/outboard at the upper towers or, again, depending on design, by adjusting an eccentric at the lower mount at the strut-to-steering arm connection.

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

PERFORMANCE WHEEL ALIGNMENT

Steering axis inclination (SAI) is the fixed angle established by a line drawn through upper and lower ball joints (or center top of 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 SAI and the hub and wheel camber angle.

The pivot/load point scrub radius of a tire considers the angle created by the wheel camber and the SAI. An aftermarket wheel with greater offset from OE that places the wheel centerline further outboard results in a more positive scrub radius, which can place excess strain on wheel bearings.

Camber Kingpin or Steering Axis Inclination Scrub or Pivot Angle Radius IA SAI

the inboard control arm pivot points or an 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 from flexing during hard turns. This will help to avoid camber changes during spirited driving. Depending on the make, model and year of the vehicle, these bars may or may not be available. Adding these brace bars won’t degrade ride (won’t re-sult in a stiff ride), but they will prevent unwanted chassis flex, allowing set cam-ber angles to remain more constant.

You’re probably all too familiar with the phenomenon known astramlining— 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 belt region of the country com-monly experience this when they upgrade to a plus-size tire and wheel package, or when they change over from winter tires to summer tires. Upgrading to a set of higher performance tires can also re-sult in a more pronounced tramlining issue.

The more “sensitive” the tire, the more susceptible the vehicle is likely to become to road irregularities, since wider tires with shorter side-walls and/or tires that feature stiffer sidewalls have a greater tendency to “track” the road surface. The wider the tire, the greater the contact patch, which results in increased

surface area to react to more of the road surface. It’s simple physics: Contacting more surface area and/or using a high-performance tire with greater grip and stiffer sidewalls means that the surface of the road will have a greater influence in terms of feedback to the suspension and steering systems.

Wheel offsethas a direct bearing on

tramlining as well. Installing wider tires or a plus-size tire and wheel package usually requires using wheels with a dif-ferent offset than the vehicle’s original wheels. In some cases, the new wheels will have slightly less offset than the

original and in other cases, slightly more. Usually the amount of offset change is kept to a minimum, and vehicle tracking remains relatively unchanged. However, if the offset is significantly different, it will alter the way the road forces are transmitted through the tire and wheel to the suspension. Therefore, large changes in wheel offset will increase the likelihood of tramlining.

Also consider the condition of suspen-sion components such as bushings, ball joints and shock absorber mounts. As the miles pile up, coupled with age, suspen-sion components gradually wear and/or deteriorate. Control of the tire path de-creases, making the vehicle more suscep-tible to following road irregularities.

Imagine a worn suspension that

al-lows a front wheel and tire to swing be-tween the recommended1_⁄

16 in. of

toe-in and1_⁄

16in. of toe-out when it

encoun-ters a rut in the road. This1_⁄

8-in.

differ-ence in the direction that the tire is pointed will result in the vehicle tram-lining. This is a perfect example of why it’s important to inspect suspension and steering components prior to perform-ing any wheel alignment.

Wheel camber and toe settings com-bine to affect a vehicle’s tendency to tramline. Extreme positive or negative camber settings will make a vehicle more sensitive, especially when only

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

Camber thrust is generated by a

leaning tire. A vehicle suspension using excessive negative camber (for example, when wheel alignment is adjusted for competition) 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’s common to adjust for greater toe-out in order to achieve more immediate turn-in on corners. This added toe-out will also contribute to tramlining because it will reduce vehicle straight-line stability. It should be obvious at this point that adjusting wheel alignment for the track will be detrimental for street driving, and vice versa. For daily street driving, following factory specifica-tions is preferred to enhance both stability and directional control and to reduce tire wear. Track settings are for the track, street settings are for the street. Period.

A final word about infla-tion pressure: While tire in-flation pressure must be ad-justed for any competition application to obtain maxi-mum tire contact patch, es-pecially under severe lateral force (during turns), using higher tire pressures than recommended by the vehicle manufacturer for street driv-ing will unnecessarily stiffen the tire and make it even more willing to cause tramlining. Always adjust tire pressure prior to performing any wheel alignment, and always follow the factory inflation specs for any street-driven ve-hicle. If the vehicle owner intends to use the vehicle for occasional autocross events, you can increase tire pressure to suit track conditions, but he must drop pressure to factory spec before driving on public roads.

T

Toopp:: The geometric centerline of any vehicle refers to a line drawn from the center of the rear axle through the center of the front axle. AAbboovvee:: The thrust line refers to the angle at which the rear axle is aimed. The thrust angle is the difference between the geo-metric centerline and the measured thrust line.

This article can be found online at www.motormagazine.com.

Thrust Line & Geometric Centerline

Thrust Line Geometric Centerline