4-WHEEL ALIGNMENT. two-wheel alignment,

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1 TOTAL 4-WHEEL 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. The old method of wheel alignment, called centerline two-wheel alignment, should now be considered obsolete. This method does not consider the rear wheel positions, and it simply isn t effective, because it ignores the thrust direction of the rear axle. A much more effective method is called thrust line or thrust angle alignment, which considers the actual location and direction of the rear wheels. This allows you to adjust the front wheel angles relative to the rear wheel angles, regardless of the geometric centerline. If the vehicle in question features rear wheel toe adjustment, you can achieve optimum wheel alignment using the total four-wheel approach, by referring to and adjusting the vehicle thrust angle to as close to zero as possible. If the thrust angle is off zero, it 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 to adjust the front wheels parallel to the rear wheels. If the vehicle does not allow rear wheel angle adjustment, take advantage of a four-wheel thrust line alignment approach. If the vehicle does allow rear wheel angle adjustment, perform a total fourwheel alignment. Granted, a state-of-the-art computerized wheel alignment system will walk a technician through the steps, perform all necessary calculations and instruct the technician to adjust angles in order to meet an OE specification for a specific production vehicle. However, it s also important to understand wheel angles and what these angles represent in terms of driveability, braking, tire wear and handling. In other words, it s helpful to understand wheel alignment theory. Only by grasping the rudiments of wheel angles will you be able to appreciate how these angles affect a vehicle in motion. A number of dimensional angles are Photoillustration: Harold A. Perry; photo courtesy Hunter Engineering 30 June 2007

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3 Before any alignment work begins (especially if the wheels/ tires are being installed during the job), it s critical to follow 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 brake pedal pulsation. And don t forget the basics: Always check tire inflation and measure vehicle ride height before measuring or correcting wheel angles. Some exotic ultra performance vehicles, such as this Porsche Carerra GT (featuring pin-drive wheels), require special adapters to allow attachment of alignment heads. Photos: Mike Mavrigian involved in wheel alignment theory, but only three are considered adjustable. These are wheel toe, camber and caster. We ll explain each of these adjustable angles, then follow with an overview of how these wheel angles affect optimum handling, braking and tire life. Wheel Toe Wheel toe describes the relationship of the right and left wheels on the same axle, as viewed from overhead. It s measured by comparing the distance between the center of the front of the tires to a distance between the centers of the rear of the tires on the same axle. Toe-out (negative toe) is present when the wheels are farther apart in front of the axle centerline and closer together behind it. Toe-in (positive toe) is when the two wheels on the same axle are closer together at the front and wider apart at the rear. When the measured distance between the front of the wheels (ahead of the steering axle centerline) is identical to the distance between the wheels behind the axle centerline, the condition is called zero toe. All front suspensions, regardless of design, feature toe angle adjustment, at a location on the steering tie rods or tie rod ends. Live rear axles feature no toe angle adjustment, but independent rear suspensions usually do offer the feature. The toe angle affects the directional control of a vehicle, turning response and tire tread life. Toe-related tread wear will cause a feathering wear pattern across the tread. With too much toe-in, the feathering will angle inward, toward the center of the vehicle; too much toe-out causes feathering that angles outward. Because of the compliance in control arm bushings and other dynamic Measuring angles on all four wheels (even if the rears are not adjustable) permits a four-wheel alignment, allowing front wheel alignment adjustments to compensate for a slightly incorrect rear thrust angle. Photo courtesy Hunter Engineering variances in suspension and steering systems, the goal is to establish a static toe angle that will result in a zero toe condition when the vehicle is driven in a straight line. Speaking in general terms, a reardrive vehicle would likely require a front wheel toe-in (positive) setting, and a front-drive vehicle would likely require a slight toe-out (negative) setting. The reason? Commonly, as a rear-drive vehicle moves forward, the front wheels tend to try to push outward (to crawl away from each other); and a front-drive vehicle s front wheels tend to try to crawl inward. Camber As viewed from the front or rear of the vehicle, camber refers to the lean of the wheel from top to bottom. A wheel that leans outward at the top (compared to true vertical) has positive camber. One that leans inward at the top has negative camber. If the wheel is oriented in a true vertical, it s called zero camber. Camber must always be adjusted to maximize the tread contact patch based on the driving requirements. In most cases, OE specifications will recommend a slightly positive or zero camber to maximize tire wear and traction, and to provide easier steering and greater resistance to directional darting in a straight line. If the wheel features a static negative 32 June 2007

4 camber angle (vehicle sitting idle), this places more tread load at the road surface on the inner shoulder and tread area. Negative camber is regularly employed on performance vehicles (especially 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 negative camber as the vehicle rolls straight, when it goes into a hard turn, the wheel facing the direction of the turn will try to straighten up, achieving maximum tread contact with the road. If camber isn t sufficiently negative, this tire would lean too far, causing the inside of the tread to lift and placing excess stress and load only on the outside of the tread and outer shoulder. Either by using OE adjustment provisions or aftermarket custom adjustment components, all front suspension camber angles are adjustable. If an upper or lower control arm is involved, the arm will be adjustable by either adding or removing adjuster shims between it and the frame, or by rotating an eccentric shaft or eccentric washers. In some cases, the lower arm may be adjustable via an eccentric shaft or washers. If more negative camber is required, the upper arm would move further inward, or the Fig. 1: Examples of camber angles are (from left): negative camber, zero camber and positive camber. lower arm would move further outward. If more positive camber is needed, the upper arm would move outward, or the lower arm would move inward. On strut-equipped vehicles, camber can be adjusted in one of two ways by adjusting the top of the strut mount inward or outward at the upper towers or by adjusting an eccentric bolt at the lower mount, where the strut attaches to the steering knuckle upright. If the vehicle s OE design provides no adjustment, aftermarket adjusters and kits are readily available for either top-strut or bottom-strut applications. Rear camber may or may not be adjustable, depending on the type of rear suspension on the vehicle. If a live axle is present (a rigid one-piece axle housing on a rear-drive vehicle), camber likely won t be adjustable. However, with an independent rear axle, camber should be adjustable either via eccentric bushings at the inboard control arm pivot points or by means of an eccentric at the strut-to-rear upright. If adjustment is available (either through OE design or with the use of aftermarket adjusters), it s best to always adhere to OE specifications for street driving. Camber directly affects tire wear, since an improperly adjusted camber angle may contribute to excessive inner or outer tire tread wear. Caster The steering axle s caster 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 reference, caster an- Some OE suspensions feature a handily adjustable eccentric bolt and washer that can be rotated to adjust camber, caster or a combination of the two. Original equipment MacPherson struts are notorious for lacking a provision for camber adjustment. This can be overcome easily with the use of replacement lower strut bolts equipped with eccentric washers. June

5 While strut tower bars (like the one seen here connecting the left and right front strut towers) are commonly added to enhance appearance and to reduce body/chassis flex, they maintain wheel alignment angles by reducing or eliminating dynamic changes in camber during severe cornering. The rear suspension of this street rod chassis features a live rear axle, located by adjustable-radius rods. Adjustable rods (usually featuring RH/LH threaded ends and spherical radius rod ends) allow thrust angle, as well as pinion angle, to be tuned to exact dimensions. gle is represented by a straight line drawn through the upper ball joint/pivot location through the lower ball joint. The vast majority of vehicles should feature positive caster, where the upper suspension pivot point is located behind the lower pivot point (again, as compared to true vertical). If the caster angle were zero (where the lower pivot is directly below the upper pivot), directional control would suffer, and there would be little if any steering wheel return, requiring the driver to manually drag the wheels back to a straight-ahead direction following a turn. Caster is a major contributor to directional control. A too-small (not positive enough) caster angle may make the vehicle too twitchy (but would require less driver input to turn the wheel), especially as speed increases. In theory, the greater the caster, the more directional control the driver has at higher speeds (which also requires slightly more driver input at the steering wheel). However, all suspension systems are designed to perform best at a specific caster angle, so always follow the OE specification to achieve the correct balance between turning effort and vehicle directional control. Front caster may or may not be readily adjustable, again, depending on suspension design. If the front suspension features upper and lower control arms, the upper arm will likely be adjustable, either via the addition or removal of shims (between the upper arm and frame) or the use of eccentric bushings. If an upper/lower control arm system is featured, the two anchoring locations (where the upper arm attaches to the frame) can be adjusted (again, with shims or eccentrics). To alter camber, the adjustment must be performed equally at the front and rear attachment points, in order to move the upper arm pivot inward or outward. If caster is to be adjusted, it s done only on one end. If the vehicle features MacPherson struts, the top strut mount serves as the upper locating point, and the top of the strut can be moved forward or rearward to alter the caster angle. Commonly on most strut-equipped vehicles, no OE caster adjustment is offered. However, aftermarket adjustable strut mounts are available that allow you to move the top of the strut fore/aft and inward/outward for caster or camber. While caster is not a direct tire-wear factor, improper caster angle can contribute to excessive tire wear in conjunction with improper camber and toe angles. Other Wheel Angles Steering axis inclination (SAI) represents a predetermined and fixed angle between a true vertical drawn through the center of the tire and a line drawn through the upper and lower ball joints when viewed from the front of the vehicle. Where these two lines intersect, the resulting angle is the SAI. In a way, you can view SAI as the built-in-by-design camber of the sus- Fig. 2: As viewed from the side of the vehicle, the caster angle represents the location of the upper ball joint (or upper pivot point) in relation to the lower ball joint. When the upper point is located ahead of the lower joint (left), it s called negative caster; when it s behind the lower joint (right), it s positive caster. 38 June 2007

6 Fig. 5: The thrust line notes the direction of the rear wheels, in terms of toe angle. Fig. 6: The thrust angle is the difference between the geometric centerline and the thrust line. A thrust angle that aims the rear axle to the right is a positive thrust angle, to the left a negative thrust angle. Negative Thrust Angle Thrust Angle Is 0 Positive Thrust Angle Thrust Angle Is 0 Left Rear - Toe-Out Right Rear - Toe-In Thrust Line to the Left Left Rear - Toe-In Right Rear - Toe-In Thrust Line Centered Left Rear - Toe-In Right Rear - Zero Toe Thrust Line to the Right Left Rear - Toe-Out Right Rear - Toe-Out Thrust Line Centered Illustrations: Harold A. Perry pension system. Included angle (IA) is the combination of SAI and wheel camber. Both SAI and IA are measured to verify that the fixed angles (those angles that exist by design) are correct. If either SAI or IA is outside of the OE specification, it s apparent that a chassis location has been damaged for example, a strut is bent, the strut tower has deformed, a lower control arm is bent, etc. The pivot point created by the force of the load and the steering axis is the scrub radius. 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 imaginary 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 crisscross exactly at the road surface, it s known as zero scrub. When the lines crisscross above the road surface, it s called negative scrub. When the lines intersect below the road surface, the condition is called positive scrub. An excessively negative scrub radius will require greater steering effort, while excessive positive scrub radius (where the tread center essentially moves further outward) can not only affect handling and steering effort, but can overstress wheel bearings as well. 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 MacPherson strut front suspension features a negative scrub radius, which aids in minimizing torque steer, a common trait of front-drive vehicles, where the front wheels tend to pull in one direction under hard acceleration, especially from a standing start. The geometric centerline is a line drawn from the center of the rear axle to the center of the front axle, as viewed from above the vehicle. The thrust line represents the forward direction of the rear axle wheels, as viewed from above in other words, the direction in which the rear axle aims. Not to be confused with the geometric centerline, the thrust line effectively divides left and right rear wheel toe. The thrust line may deviate from the geometric centerline. The thrust angle is defined as the difference between the geometric centerline and the thrust line, measured in degrees. If the thrust angle bears to the right, it s called a positive thrust angle. If it bears to the left, it s a negative thrust angle. Centerline steering is simply a term that refers 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. Because of the travel arc that occurs when the wheels are turned (due in part to the caster angle and SAI), individual wheel toe will change as compared to its straight-ahead static setting. For example, when the steering wheel is turned to the left, the left front wheel will exhibit greater toe-out as compared to the number of degrees that the right front wheel toes-in. This phenomenon, called toe-out on turns, is designed into the suspension system to decrease the turning radius of the vehicle and prevent tire scrubbing during turns. The bottom line is that even the most expensive high-tech alignment equipment won t allow you to achieve proper wheel angle adjustment if the standards of the chassis are not correct. Before attempting any wheel alignment measurement or correction, first verify vehicle ride height, condition of wheel bearings, ball joints, tie rods, tire outside diameter and tire inflation. Worn and sloppy pivot joints will allow wheel angles to change from their adjusted static positions on the alignment rack, once the vehicle is driven off the rack and onto the road. Finally, if suspension height is uneven (due to worn or damaged springs, improperly inflated tires or mismatched tire diameters on the same axle), you won t be able to achieve an accurate wheel alignment. Visit to download a free copy of this article. 40 June 2007