Shaft-driven bicycle

From Wikipedia, the free encyclopedia

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 spockets and derailleurs. Recently, due to recent 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 would mesh to 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 mesh to a bevel gear on the hub where the rear sprocket would be on a conventional bike. The 90-degree change of plane, for the drive, 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, SRAM and Sturmey-Archer.

Comparison of shaft vs chain driven bicycles

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.

Advantages

Compactness: The driveshaft and bevel gears take up less volume than an enclosed chain.
Ground clearance: The driveshaft system allows greater ground clearance.
Enclosed drive system: The drive shaft and gearing are enclosed which protects moving parts from the elements.
Safety: There is no danger of clothing or fingers being drawn into the chain and sprockets.
Cleanliness: Because a bicycle chain needs to be lubricated, usually with oil, a chain driven bicycle can stain its rider's clothing and hands.
Consistency of performance: Shaft Drives operate at a very consistent rate of efficiency and performance, without adjustments or maintenance.
Suitability to longer drive trains: The shaft-driven system is particularly suitable to many recumbent applications where there is a long distance from the crankset to the driven rear wheel. Maintaining chain tension is often difficult for these applications. Also, the chain line may not be straight, necessitating the use of extra jockey wheels. A drive shaft can relatively easily turn through these slight angles with the use of a universal joint.

Disadvantages

Power loss: Typically, a shaft-driven system runs slightly less efficient than well maintained chain-driven system.
Complexity: Though the shaft-driven system seems simple on the outside, the manufacture of the necessary bevel gears is very complex compared to that of a chain and sprockets.
Cost: Due to the manufacturing complexity of the frame and shaft drive, the manufacturing cost is typically higher.
Gear range: Most 21-speed or greater bicycles will have a gear range that exceeds that offered by most shaft drive bicycles using internal hubs.
Weight: A shaft-driven bicycle typically weighs about 1 pound more than an equivalent chain-driven bicycle.