TOTAL 4-WHEEL4- ALIGNMENT BY MIKE MAVRIGIAN Modern equipment greatly simplifies the wheel alignment process. But it’s still important to understand how alignment angles are measured, as well as their effect on driveability, braking, tire wear and handling.

he old method of wheel alignment allows you to adjust and alignment, called center- hopefully correct rear thrust angle, line two-wheel align- then to adjust the front parallel ment, should now be to the rear wheels. considered obsolete. •If the vehicle does not allow rear This method does not wheel angle adjustment, take advantage consider the rear wheel positions, and it of a four-wheel thrust line alignment Tsimply isn’t effective, because it ignores approach. the thrust direction of the rear axle. •If the vehicle does allow rear wheel A much more effective method is angle adjustment, perform a total four- called thrust line or thrust angle align- wheel alignment. ment, which considers the actual loca- Granted, a state-of-the-art computer- tion and direction of the rear wheels. ized wheel alignment system will walk a This allows you to adjust the front wheel technician through the steps, perform all angles relative to the rear wheel angles, necessary calculations and instruct the regardless of the geometric centerline. technician to adjust angles in order to If the vehicle in question features meet an OE specification for a specific rear wheel toe adjustment, you can production vehicle. However, it’s also im- achieve optimum wheel alignment using portant to understand wheel angles and the total four-wheel approach, by refer- what these angles represent in terms of ring to and adjusting the vehicle thrust driveability, braking, tire wear and han- angle to as close to zero as possible. dling. In other words, it’s helpful to un- If the thrust angle is off zero, it can derstand wheel alignment theory. Only contribute to vehicle dog-tracking by grasping the rudiments of wheel an- (crooked body relative to direction of gles will you be able to appreciate how travel), increased tire wear and unequal these angles affect a vehicle in motion.

left/right turning. Total four-wheel A number of dimensional angles are Engineering Hunter courtesy photo Harold A. Perry; Photoillustration:

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Before any alignment work begins (especially if the wheels/ tires are being installed during the job), it’s critical to fol- low correct wheel fastener tightening procedures in terms of both torque and sequence. While a problem with either may not directly influence wheel angle, warpage of the

wheel or rotor can result in dynamic balance issues and Mavrigian Mike Photos: pedal pulsation. And don’t forget the basics: Always Some exotic ultra performance vehicles, such as this check tire inflation and measure vehicle ride height before Porsche Carerra GT (featuring pin-drive wheels), require measuring or correcting wheel angles. special adapters to allow attachment of alignment heads. involved in wheel alignment theory, but control of a vehicle, turning response variances in suspension and steering only three are considered adjustable. and tire tread life. Toe-related tread systems, the goal is to establish a static These are wheel toe, camber and caster. wear will cause a feathering wear pat- toe angle that will result in a zero toe We’ll explain each of these adjustable tern across the tread. With too much condition when the vehicle is driven in angles, then follow with an overview of toe-in, the feathering will angle in- a straight line. how these wheel angles affect optimum ward, toward the center of the vehicle; Speaking in general terms, a rear- handling, braking and tire life. too much toe-out causes feathering drive vehicle would likely require a front that angles outward. wheel toe-in (positive) setting, and a Wheel Toe Because of the compliance in con- front-drive vehicle would likely require a Wheel toe describes the relationship of trol arm bushings and other dynamic slight toe-out (negative) setting. The rea- the right and left wheels on the same son? Commonly, as a rear-drive vehicle axle, as viewed from overhead. It’s mea- moves forward, the front wheels tend to sured by comparing the distance be- try to push outward (to crawl away from tween the center of the front of the tires each other); and a front-drive vehicle’s to a distance between the centers of the front wheels tend to try to crawl inward. rear of the tires on the same axle. Toe-out (negative toe) is present Camber when the wheels are farther apart in As viewed from the front or rear of the front of the axle centerline and closer vehicle, camber refers to the “lean” of together behind it. Toe-in (positive toe) the wheel from top to bottom. A wheel is when the two wheels on the same that leans outward at the top (compared axle are closer together at the front and to true vertical) has positive camber. wider apart at the rear. When the mea- One that leans inward at the top has sured distance between the front of the negative camber. If the wheel is oriented wheels (ahead of the steering axle cen- in a true vertical, it’s called zero camber. terline) is identical to the distance be- Camber must always be adjusted to tween the wheels behind the axle cen- maximize the tread contact patch terline, the condition is called zero toe. based on the driving requirements. In All front suspensions, regardless of most cases, OE specifications will rec- Photo courtesy Hunter Engineering Hunter courtesy Photo design, feature toe angle adjustment, at Measuring angles on all four wheels ommend a slightly positive or zero a location on the steering tie rods or tie (even if the rears are not adjustable) camber to maximize tire wear and trac- rod ends. Live rear feature no toe permits a four-wheel alignment, al- tion, and to provide easier steering and angle adjustment, but independent rear lowing front wheel alignment adjust- greater resistance to directional darting suspensions usually do offer the feature. ments to compensate for a slightly in a straight line. The toe angle affects the directional incorrect rear thrust angle. If the wheel features a static negative

32 June 2007 TOTAL 4-WHEEL ALIGNMENT camber angle (vehicle sitting idle), this places more tread load at the road sur- face on the inner shoulder and tread area. Negative camber is regularly em- ployed on performance vehicles (espe- cially race cars on road courses) in order to increase the tire contact patch during hard turns. Since lateral loading (when a car goes into a hard turn) will try to push the top of the inside tire outward, an adequate negative camber angle may be dialed in to compensate for this. So, while the front wheels may display neg- Fig. 1: Examples of camber angles are (from left): ative camber as the vehicle rolls negative camber, zero camber and positive camber. straight, when it goes into a hard turn, the wheel facing the direction of the lower arm would move further outward. with an independent rear axle, camber turn will try to “straighten up,” achiev- If more positive camber is needed, the should be adjustable either via eccentric ing maximum tread contact with the upper arm would move outward, or the bushings at the inboard control arm piv- road. If camber isn’t sufficiently nega- lower arm would move inward. ot points or by means of an eccentric at tive, this tire would lean too far, causing On strut-equipped vehicles, camber the strut-to-rear upright. If adjustment the inside of the tread to lift and placing can be adjusted in one of two ways—by is available (either through OE design excess stress and load only on the out- adjusting the top of the strut mount in- or with the use of aftermarket ad- side of the tread and outer shoulder. ward or outward at the upper towers or justers), it’s best to always adhere to OE Either by using OE adjustment provi- by adjusting an eccentric bolt at the specifications for street driving. sions or aftermarket custom adjustment lower mount, where the strut attaches Camber directly affects tire wear, components, all front suspension cam- to the steering knuckle upright. If the since an improperly adjusted camber ber angles are adjustable. If an upper or vehicle’s OE design provides no adjust- angle may contribute to excessive inner lower control arm is involved, the arm ment, aftermarket adjusters and kits are or outer tire tread wear. will be adjustable by either adding or re- readily available for either top-strut or moving adjuster shims between it and bottom-strut applications. Caster the frame, or by rotating an eccentric Rear camber may or may not be ad- The steering axle’s caster involves the re- shaft or eccentric washers. In some cas- justable, depending on the type of rear lationship of the upper ball joint (or top es, the lower arm may be adjustable via suspension on the vehicle. If a live axle of the strut mount) to the lower ball an eccentric shaft or washers. If more is present (a rigid one-piece axle hous- joint as viewed from the side of the vehi- negative camber is required, the upper ing on a rear-drive vehicle), camber cle. Using a true vertical drawn through arm would move further inward, or the likely won’t be adjustable. However, the hub center as a reference, caster an-

Original equipment MacPherson struts are notorious for Some OE suspensions feature a handily adjustable ec- lacking a provision for camber adjustment. This can be centric bolt and washer that can be rotated to adjust overcome easily with the use of replacement lower strut camber, caster or a combination of the two. bolts equipped with eccentric washers.

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While strut tower bars (like the one seen here connecting The rear suspension of this street rod features a the left and right front strut towers) are commonly added live rear axle, located by adjustable-radius rods. Ad- to enhance appearance and to reduce body/chassis flex, justable rods (usually featuring RH/LH threaded ends they maintain wheel alignment angles by reducing or elimi- and spherical radius rod ends) allow thrust angle, as nating dynamic changes in camber during severe cornering. well as pinion angle, to be tuned to exact dimensions. gle is represented by a straight line shims (between the upper arm and mounts are available that allow you to drawn through the upper ball joint/pivot frame) or the use of eccentric bushings. move the top of the strut fore/aft and location through the lower ball joint. If an upper/lower control arm system is inward/outward for caster or camber. The vast majority of vehicles should fea- featured, the two anchoring locations While caster is not a direct tire-wear ture positive caster, where the upper (where the upper arm attaches to the factor, improper caster angle can con- suspension pivot point is located behind frame) can be adjusted (again, with tribute to excessive tire wear in con- the lower pivot point (again, as com- shims or eccentrics). To alter camber, junction with improper camber and pared to true vertical). If the caster an- the adjustment must be performed toe angles. gle were zero (where the lower pivot is equally at the front and rear attachment directly below the upper pivot), direc- points, in order to move the upper arm Other Wheel Angles tional control would suffer, and there pivot inward or outward. If caster is to Steering axis inclination (SAI) repre- would be little if any steering wheel re- be adjusted, it’s done only on one end. sents a predetermined and fixed angle turn, requiring the driver to manually If the vehicle features MacPherson between a true vertical drawn through drag the wheels back to a straight-ahead struts, the top strut mount serves as the the center of the tire and a line drawn direction following a turn. upper locating point, and the top of the through the upper and lower ball joints Caster is a major contributor to di- strut can be moved forward or rear- when viewed from the front of the vehi- rectional control. A too-small (not posi- ward to alter the caster angle. Com- cle. Where these two lines intersect, the tive enough) caster angle may make the monly on most strut-equipped vehicles, resulting angle is the SAI. vehicle too twitchy (but would require no OE caster adjustment is offered. In a way, you can view SAI as the less driver input to turn the wheel), es- However, aftermarket adjustable strut built-in-by-design “camber” of the sus- pecially as speed increases. In theory, the greater the caster, the more direc- tional control the driver has at higher Fig. 2: As viewed from speeds (which also requires slightly the side of the vehicle, more driver input at the steering the caster angle wheel). However, all suspension sys- represents the location tems are designed to perform best at a of the upper ball joint specific caster angle, so always follow (or upper pivot point) in the OE specification to achieve the cor- relation to the lower ball joint. When the upper rect balance between turning effort and point is located ahead of vehicle directional control. the lower joint (left), it’s Front caster may or may not be read- called negative caster; ily adjustable, again, depending on sus- when it’s behind the pension design. If the front suspension lower joint (right), it’s features upper and lower control arms, positive caster. the upper arm will likely be adjustable, either via the addition or removal of

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Fig. 5: The thrust line notes Fig. 6: The thrust angle is the difference Thrust the direction of the rear wheels, between the geometric centerline and the Angle in terms of toe angle. thrust line. A thrust angle that aims the Is 0° rear axle to the right is a positive thrust angle, to the left a negative thrust angle. Negative Thrust Angle Positive Thrust Angle Is 0° Thrust Angle

Left Rear - Toe-Out Left Rear - Toe-In Left Rear - Toe-In Left Rear - Toe-Out Right Rear - Toe-In Right Rear - Toe-In Right Rear - Zero Toe Right Rear - Toe-Out Thrust Line to the Left Thrust Line Centered Thrust Line to the Right Thrust Line Centered Illustrations: Harold A. Perry Illustrations: pension system. Included angle (IA) is will exhibit a positive scrub radius. part to the caster angle and SAI), indi- the combination of SAI and wheel cam- Commonly, a MacPherson strut front vidual wheel toe will change as com- ber. Both SAI and IA are measured to suspension features a negative scrub ra- pared to its straight-ahead static setting. verify that the fixed angles (those angles dius, which aids in minimizing torque For example, when the steering wheel that exist by design) are correct. If either steer, a common trait of front-drive ve- is turned to the left, the left front wheel SAI or IA is outside of the OE specifica- hicles, where the front wheels tend to will exhibit greater toe-out as compared tion, it’s apparent that a chassis location pull in one direction under hard accel- to the number of degrees that the right has been damaged—for example, a strut eration, especially from a standing start. front wheel toes-in. This phenomenon, is bent, the strut tower has deformed, a The geometric centerline is a line called toe-out on turns, is designed in- lower control arm is bent, etc. drawn from the center of the rear axle to the suspension system to decrease The pivot point created by the force to the center of the front axle, as viewed the turning radius of the vehicle and of the load and the steering axis is the from above the vehicle. prevent tire scrubbing during turns. scrub radius. As viewed from the front The thrust line represents the for- The bottom line is that even the most of the vehicle, this is determined by con- ward direction of the rear axle wheels, expensive high-tech alignment equip- sidering the distance between the center as viewed from above—in other words, ment won’t allow you to achieve proper of a front tire tread and the imaginary the direction in which the rear axle wheel angle adjustment if the “stan- SAI line, when measured at the road aims. Not to be confused with the geo- dards” of the chassis are not correct. surface. Since these two lines will even- metric centerline, the thrust line effec- Before attempting any wheel alignment tually intersect, it’s this intersection point tively divides left and right rear wheel measurement or correction, first verify that we’re really interested in. toe. The thrust line may deviate from vehicle ride height, condition of wheel When the two lines crisscross exactly the geometric centerline. bearings, ball joints, tie rods, tire out- at the road surface, it’s known as zero The thrust angle is defined as the side diameter and tire inflation. Worn scrub. When the lines crisscross above difference between the geometric cen- and sloppy pivot joints will allow wheel the road surface, it’s called negative terline and the thrust line, measured in angles to change from their adjusted scrub. When the lines intersect below degrees. If the thrust angle bears to the static positions on the alignment rack, the road surface, the condition is called right, it’s called a positive thrust angle. If once the vehicle is driven off the rack positive scrub. An excessively negative it bears to the left, it’s a negative thrust and onto the road. Finally, if suspension scrub radius will require greater steering angle. height is uneven (due to worn or dam- effort, while excessive positive scrub ra- Centerline steering is simply a term aged springs, improperly inflated tires dius (where the tread center essentially that refers to a “straight and level” steer- or mismatched tire diameters on the moves further outward) can not only af- ing wheel clock position when the vehi- same axle), you won’t be able to achieve fect handling and steering effort, but can cle rolls in a straight line. If the steering an accurate wheel alignment. overstress wheel bearings as well. wheel is not centered, this may indicate In most cases, a short-arm/long-arm a possible thrust angle deviation. Visit www.motor.com to download suspension (upper and lower control Because of the travel arc that occurs a free copy of this article. arms where the lower arm is longer) when the wheels are turned (due in

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