Caster measures the steering axis centerline angle--the angle of a line drawn through the ball joints or kingpin--compared to vertical. Most suspensions incorporate a few degrees of positive caster to help stabilize steering. Just like the caster on a shopping cart, the tire wants to trail behind the pivot point on the ground because that's where the weight is concentrated. The end result is that the wheels want to go straight when traveling forward, and turn when moving in reverse.