To propel most bicycles, the rider straddles the saddle and uses his or her legs and feet to rotate the pedals around the crank axle. The pedals, in turn, are fixed to a chain ring (sprocket) with teeth that engages the bicycle’s continuous chain.
The chain then transmits the pedaling action to a cog on the hub of the rear wheel, causing the rear wheel to rotate and drive the bicycle forward. On most bicycles, cogs are mounted on a freewheel or cassette mechanism that allows the rear wheel to continue turning even if the rider stops pedaling and coasts.
A shaft-driven bicycle is a chainless bicycle that uses a driveshaft instead of a chain to transmit power from the pedals to the wheel. Shaft drives were introduced over a century ago, but were mostly supplanted by chain-driven bicycles due to the gear ranges possible with sprockets and derailleurs. Recently, due to advancements in internal gear technology, a small number of modern shaft-driven bicycles have been introduced.
Shaft-driven bikes have a large bevel gear where a conventional bike would have its chainring. This meshes with another bevel gear mounted on the driveshaft. The use of bevel gears allows drive from the pedals to be turned through 90 degrees. The driveshaft then has another bevel gear near the rear wheel hub which meshes with a bevel gear on the hub where the rear sprocket would be on a conventional bike. The 90-degree change of the drive plane that occurs at the bottom bracket and again at the rear hub requires the use of bevel gears. Bevel gears are the most efficient way of turning drives 90 degrees as compared to worm gears or crossed helical gears. The driveshaft is often mated to a hub gear which is an internal gear system housed inside the rear hub. Today, there are three significant manufacturers of internal hubs suitable for use with shaft drive systems, including Shimano Nexus, SRAM and Sturmey-Archer.