I know the theory of caster, but the mesurement is what I'm talking about. If the ball joint is stationary why take 20 degree turns to find caster off of the ball joint studs since the joints don't move through steering. But, if the above posts is talking about taking chamber difference measurements from the wheel it self while turned 20 degrees each way that would make sense to me. Is he taking caster measurements by the rim at 20* or ball studs at 20*?