This technical article is for the automotive owner who may not understand what wheel alignment is all about. What I hope to do is provide some insight and alignment terminology to help you understand this particular automotive service procedure.
First, I will tell you that wheel alignment does not include wheel balance. Wheel alignment provides proper tire wear, ease of steering and proper steering tangibility. The wheel alignment settings will affect either tire wear or setting. Years ago, there were only front end alignment machines and in the case of cars with independent rear suspension - like C2 Corvettes which had 2 alignment settings on the rear of the car - they would back the car onto the alignment machine. Those problems are taken care of today with the 4 wheel alignment settings. Some settings are adjustable and others are not.
Caster affects steering and is usually adjustable. Caster is the forward or rearward tilt of the steering axis in reference to a vertically line. The angle is measured in degrees. Some early Corvettes used kingpins later upper and lower ball joints which provide the steering axis. Today’s vehicles use a lower ball joint and a strut system.
Caster refers to the angle of the steering axis in relation to an imaginary vertical line through the center of the wheel when viewed from the side. "Positive caster" is the term used when the verticle line is tilted back towards the rear. If it's tilted forward we call it "negative caster". The proper caster angle stabilizes your car for better steering.
Camber is the angle formed by the inward or outward tilt of the top of the tire as referenced to a vertical line. This angle is measured indegrees and is adjustable.
Camber refers to the tilt of the wheels toward or away from one another when viewed from the front. Wheels that tilt in toward the vehicle have "negative camber". Wheels that tilt away from the vehicle have "positive camber".
Toe is defined as the difference in distance measured across the front and across the rear of the tires either at the front or back wheels. Toe is measured in inches or milliliters and is adjustable. Toe refers to the tilted direction of the wheels toward or away from one another when viewed from the top. Toe is the most critical tire wearing angle. Tires that "toe-in" point toward one another. Tires that "toe-out" point away from each other.
- Steering Axis Inclination (SAI)
SAI is the inward tilt of the kingpins, ball joints or strut at the top. The purpose of SAI is to aid steering tangibility and establish a pivot point about which the wheel can be turned easily. This angle helps to bring the steering wheel back to the straight ahead positron after a turn and is not squalidly adjustable.