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Planetary Gearing
In planetary gearing, the hub has a single gear attached to the middle of the axle around which three or four other gears mesh, like a sun with planets rotating around it. The planet gears in turn mesh with a hollow gear ring, which has teeth on its inner surface.
Drive Sprocket
Most hub gears have three speeds, which depend on the part of the planetary gear system to which the drive sprocket is attached. The drive sprocket is the toothed wheel that drives the bicycle chain (3). Depending on the arrangement of the drive sprocket, gear ring and planet gears, the bicycle chain can have a long or short distance to travel before it can turn the bicycle's wheel.
Three-Speed Hub Gears
If the drive sprocket is connected to the gear ring, the bicycle chain will have farther to travel and the wheel will turn more slowly than the sprocket, providing a lower gear. If the drive sprocket is connected to the planet cage, it will provide a higher gear. The bicycle shifts into the middle gear courtesy of direct drive, wherein the drive sprocket meshes directly with the gear ring, and the gear ring meshes with the central hub.