Description 4-inch diameter wheel with 8mm bore Tread material: Thermoplastic polyurethane Body material: Polycarbonate Coefficient of friction: 0.9 on carpet, 0.63 on FTC soft tiles View more Dimensions Diameter: 4'' Weight: 0.24 pound Specifications Bore: 8mm Bolt pattern 3.5mm holes on a 16mm circle (Inner) 6 holes around a 1.875'' circle (Outer)