Set of 3; Inner Diameter: 3″ Outer Diameter: 6″. Benchworld’s X-Large Shock Absorbent Caster Cups are the best solution for customers who do not want the vibrations of their grand piano to be transferred to the floor below. These heavy duty caster cups have a thick felt under them which makes it easier to move your piano while they are sitting on the felts. They also protect your floor from being scratched. The caster cups are made out of heavy duty rubber and in the middle of caster cup there is a thick steel plate which absorbs the vibrations even more.