This wheel has a 3/4-inch aluminum hub with 1/4 and 3/16-inch keyways. It comes with two steel discs for holding the wheel in place on the end of a 3/4-inch shaft. This makes it easy to mount these wheels directly onto the output shaft of an AmpFlow gearmotor. The bore diameter is 3/4-inch. The width of the tread is 1. 5 inches These rugged wheels are ideal for robotics and other industrial and mobile applications. Other sizes, including 3, 4, 5, 6, 8, 10, and 12. 5-inch wheels, are also available. Search for "BattleKits Wheels" or "AmpFlow Wheelmotors"