For 1990-2009 cars

Caster is a measure of the angle between the steering axis and vertical when viewed from the side of the vehicle with the wheel in the straight ahead position. The steering axis is determined by the location of the lower control arm ball joint and the center of the strut-to-body attachment. A line drawn through the center of the ball joint and the strut-to-body joint represents the steering axis. When the center point of the strut-to-body attachment is rearward of the ball joint, the caster is positive. When the center of the strut-to-body attachment is forward of the ball joint, the caster is negative.