What: Caster is the backward or forward tilt of the wheel spindle support. On a conventional suspension system, the upper and lower ball joints form the wheel spindle support. A line drawn through the centers of the two ball joints, as seen from the side of the vehicle, illustrates the tilt of the wheel spindle support—the wheel caster.
How: The caster angle is measured between the centerline of the wheel spindle support and a vertical line through the center of the wheel. The caster angle is positive when the centerline of the spindle support intersects the road in front of the tire's point of contact with the road. Caster is negative when the spindle support centerline meets the road behind the tire's point of contact. Zero caster occurs when the spindle support centerline is vertical, meeting the road at the tire's point of contact. Most vehicles are designed with positive caster. Positive caster provides good directional stability by tending to return the front wheels to the straight-ahead position. It also helps the front wheels maintain the straight-ahead position, which is why too much positive caster causes hard steering.
Where: More information is available from www.hotrodlibrary.com