Set of 3; Polished Ebony With Felt; Inner Diameter: 2.5″, Outer Diameter: 4″; Benchworld’s Premium Medium Sized Solid Wooden Caster Cups in polished ebony finish are the best choice for a Grand Piano with smaller wheels. The new design has three ripples on the side and a wider inner diameter for the wheel. The outer diameter is about 4″ and the inside diameter is 2.5″. This item comes in a set of 3 with black adhesive felt. The felt will be easy to slide whereby the rubber makes it harder to slide the piano.