Principle of Operation
The simplest 3-speed hubs use a single planetary epicyclic gearset. The sun gear (in yellow above) is mounted solidly to the axle and is thus fixed relative to the bicycle frame.
- In low gear, the sprocket drives the annulus (in red above) and the planet carrier (in green above) drives the hub, giving a gear reduction.
- In middle gear, the sprocket drives the hub directly.
- In high gear, the sprocket drives the planet carrier and the annulus drives the hub, resulting in a gear increase.
The hub axle of a hub gear (unlike that of a derailleur system) will carry torque in all gears except direct drive, and so must be securely braced against rotation. While anti-rotation washers between the dropout and axle nut have often proved adequate, wider-ranging modern systems use a reaction arm affixed to the chain stay. Rear wheels with drum brakes (a feature on some commuter bicycles) require a reaction arm anyway.
Most hub gears are operated in a similar manner, with a single twist, trigger or thumb-shifter. An exception is the older style of Sturmey-Archer 5-speed, which used a second shift cable to change between close and wide-range sun gears, effectively giving two 3-speed hubs in one unit. The middle gear in both ranges was direct drive, so there were five distinct gears. They could either be controlled with a special 5-speed shifter which operated both cables, or with a regular 3-speed shifter and a friction shifter.
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