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.
A valid comparison of shaft vs. chain drives can only be made if both bikes use the same type of gearing, whether single-speed or with an internal gear system. Most of the advantages claimed for a shaft drive can be realized by using a fully-enclosed chain case.
Some of the other issues addressed by the shaft drive, such as protection for clothing and from ingress of dirt, can be met through the use of chain guards. The reduced need for adjustment in shaft-drive bikes also applies to a similar extent in chain drive hub geared bikes. Not all hub gear systems are shaft compatible.
Bicycles typically employ more torque (an adult man can easily generate more than 100 lb/f) but at much lower rotational speed than motorcycles and cars This mitigates against shaft drive as the shafts must be large to cope with the torque, which adds to their mass. This is particularly noticeable when pedalling hard, when there can be noticeable torsional springing in the drive train.
The shaft-driven bicycle was originally developed in the late 1800s. The French company L'Acatane did most of the development in the 1890s. During this period, Columbia aggressively marketed the chainless bicycle in the USA. Chainless bicycles were moderately popular in 1898 and 1899, although sales were still much smaller than regular bicycles, primarily due to the high cost. The bikes were also somewhat less efficient than regular bicycles -- there was roughly an 8 percent in the gearing, in part due to limited manufacturing technology at the time (although the efficiency loss was not huge -- Major Taylor won several races on a shaft drive bicycle). The rear wheel was also more difficult to remove to change flats. Many of these deficiencies have been overcome in the past century.
While a small number of chainless bicycles were available, for the most part, shaft-driven bicycles disappeared from view for most of the 20th century. There is, however, still a niche market for chainless bikes, especially for commuters, and there are a number of manufacturers who offer them either as part of a larger range or as a primary specialisation.