Caster Alignment

Caster is the steering angle that uses the weight and momentum of the car's chassis to lead the front wheel in a straight path. In wheel alignment, caster is the backward tilt or the forward tilt of the steering axis that tends to stabilize steering in a straight direction. It places the weight of the vehicle either ahead of or behind the area of tire-to-road contact. "Positive" caster is the angular amount that the upper ball joint is farther back than the lower joint. "Negative" caster is the condition when the upper ball joint is farther ahead than the lower one. The "caster angle" is the number of degrees (or fraction of a degree) that the steering axis is tilted backward or forward from the vertical axis of the front wheels.