Hub gear
（重定向自Gear hub）
![Epicyclic gears as implemented in common bicycle 3-speed gear hubs: the sun gear (yellow) is fixed, the sprocket drives either the ring gear (red) or the planet cage (green), and the hub is driven by either the ring gear (red) or the planet cage (green), depending on the gear selected.[1]](/uploads/202501/15/Bicycle_hub_gear2723.png)
![This belt-drive on a Trek Soho is fitted with a sprocket too.[2]](/uploads/202501/15/Belt-drive_internal-geared_multi-speed_rear_hub2723.JPG)
A hub gear, internal-gear hub, or just gear hub is a gear ratio changing system commonly used on bicycles that is implemented with planetary or epicyclic gears. The gears and lubricants are sealed within the shell of the hub gear, in contrast with derailleur gears where the gears and mechanism are exposed to the elements. Changing the gear ratio was traditionally accomplished by a shift lever connected to the hub with a Bowden cable, and twist-grip style shifters have become common.