How the Hub Works

The drawing below shows a basic single-speed planetary unit, comparing regular geometry (left) to the equivalent bevel configuration (right). Both configurations give the same gear ratio and, in both cases, the planet gear holder is the input element (the shaft that is driven, either directly, as in a direct-drive bike, or by a chain sprocket, as in a regular chain-driven bike). The ring gear element is held stationary by being fixed to the bicycle frame so that the planet gears drive the sun gear, which is the output drive to the hub shell and wheel.

Bevel planetary advantages

 

From this diagram it can immediately be seen that the bevel configuration (right) allows the planet gears to be much larger, because they are not tightly constrained between the ring and sun gears. Instead, the width of the hub can be employed to expand the size of the planet gears. The larger size allows larger gear teeth for the planet gears and therefore for the entire transmission. The larger gear teeth can take higher forces and so the hub can handle more input torque. Another benefit is that there is now enough space to for friction reducing elements, such as roller teeth – improving the mechanical efficiency of the hub transmission.