Don, smaller tires will change the caster. If you think about the axle and how its mounted to the springs, the king pins are angled back so the truck will track straight. The angle is set with the truck setting a few degrees lower in the front. When you change tires, the front is now much lower so the king pins are not nearly as angled back as they were, so you have to add more shim to get the king pins back where they were originally.
The way I understand it is, the king pins always have to stay at the same angle in relation to the road surface, so if the truck is lowered in front or raised in the rear, you have to maintain the king pin angle.
Joe
