TOTAL 4-WHEEL ALIGNMENT

Motor,  Jun 2007  by Mavrigian, Mike

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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 suspension system. Included angle (IA) is the combination of SAI and wheel camber. Both SAI and IA are measured to verify that the iked 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 die 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.