PIN-IT QUICK CHANGE SPROCKET SYSTEM, Fits 50mm axles and below. (**NOTE-50mm installations will require the separate 50mm attachment hub. 40mm and below can use standard wheel hubs), Pin-It Sprocket Hubs allow for gear changes in seconds. Features quick-change pins and a built-in chain guard! Mounts to any Metric or American/Standard rear wheel hub (wheel hub and gears sold separately).