A casing has an inner end, an outer end, and an internal drive mechanism. The internal drive mechanism has an outer wheel mounting hub at the outer end of the casing and an inner wheel mounting hub at the inner end of the casing, both supported for rotation about an axis. A motor operates the internal drive mechanism to impart rotation to the wheel mounting hubs with equal force whilst allowing the wheel mounting hubs to rotate at different speeds. The internal drive mechanism further has a carrier that is disposed between the inner wheel mounting hub and the outer wheel mounting hub and that is rotated about the axis by the motor. The carrier contains a differential gear mechanism having an outer output shaft for imparting rotation to the outer wheel mounting hub and an inner output shaft for imparting rotation to the inner wheel mounting hub.