Wheel alignment is the proper adjustment of ALL the interrelated suspension angles. In alignment terminology, these adjustment angles are called caster, camber, toe-in, steering axis inclination (SAI), vehicle ride height and toe-out on turns.
Camber: the inward or outward tilt of the wheel measured from top to bottom. Camber is adjusted to prevent excessive tire wear and to enhance straight ahead stability. It is measured in degrees and has several methods of adjustment.
Toe: is the difference in the distance measured on any axle between the leading edges and trailing edges of the wheel and tire assemblies at spindle height. This is the most critical tire wearing angle. The angle formed by a horizontal line through the plane of one wheel versus a centerline is called the individual toe.
Individual Toe: When a horizontal line is drawn through the plane of each wheel, and they intersect in front of the wheels, this is called toe-in or positive (+0 toe. When they intersect behind the wheels, this is called toe-out, or negative (-) toe.
Caster: forward or rearward tilt of the steering knuckle pivot points, is also called the steering axis. Caster is measured in degrees, from the steering axis to vertical, as viewed from the side. It is computed in degrees by looking at the camber change of each wheel through a specified steer. There are three types of caster settings.
Positive caster angle is when the top of the steering axis it tilted rearward. Negative caster angle is when the top of the steering axis is tilted forward. Negative caster can cause difficulty in returning out of a turn and wandering & weaving at high speeds. If the vehicle has unequal caster, it can cause the vehicle to pull from side to side. And zero degree caster is when the top of the steering axis is exactly vertical.
Set-Back: Front wheel set-back is the angle formed between the geometric centerline and a line perpendicular to the front axle. It is measured in degrees. Excessive set-back is normally created by frame or chassis errors. These errors are brought about in most cases by front end collision and in some cases by a manufacturing tolerance error. On cars designed with engine cradles, the cradle placement will affect set-back. If the cradle isn't square with the chassis, it must be straightened.
Set-back, if extreme, could bring about adverse handling effects, along with incorrect alignment measurement if overlooked. This may cause the vehicle to lead to the side where set-back exists; also a pull during braking can occur. On FWD vehicles it can also cause increased torque steer.
Thrust Angle: is the angle formed by the thrust line and the geometric centerline. The geometric centerline is a line drawn between the mid-point of the front axle and the mid-point of the rear axle.
When toe is off on either of the rear wheels, it creates a thrust angle that causes rear axle steer. The thrust line dictates the position of the front wheels when driving straight ahead. It is therefore the most accurate reference when measuring or adjusting the front wheels.
Inspection of the tires can help in diagnosing some wheel alignment failures. The tire wear patterns associated with improper alignment include single shoulder wear, cupping and feather edging.
that's all,
robert
