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This article is about the drive chain on a bicycle. Chain locks for bicycles are discussed in the Bicycle lock article.

Roller chain and sprocket

A bicycle chain is a chain that transfers power from the pedals to the drive-wheel of a bicycle thus propelling it.

The chain in use on modern bicycles is a roller chain with a 1/2" pitch. Chain comes in either 5/32", 1/8", 3/32" or 3/16" roller widths. 5/32" is used on cargo bikes and trikes, 1/8" is used on the common low cost coaster (back pedal brake) bike, and on three speed, fixed gear and track bicycles, and 3/32" is used on derailleur-equipped road bicycles such as racing and touring models.

Obsolete chain designs previously used on bicycles included the block chain, the skip-link chain, and the Simpson lever chain. Virtually all modern chains are of the "Sedis" bushingless design, which is cheaper to make, promotes better lubricant flow inside the rollers, and has more lateral flexibility for multi-geared bicycles.

Before the safety bicycle, bicycles did not have chains and the pedals were attached directly to the drive-wheel, thus limiting top speed by the diameter of the wheel and resulting in very dangerous designs with front wheels as large as possible. Using chain drive allowed the mechanical advantage between the drive and driven sprockets to determine the maximum speed, thereby enabling manufacturers to reduce the size of the driving wheel for safety. It also allowed for the development of variable gearing, allowing cyclists to adjust their gearing to the difficulty of the terrain, on the fly.

Close-up of a touring bicycle's chain protected by a chain guard

Chain lubrication is a common problem for cyclists. Liquid lubricants penetrate to the inside of the links and are not easily displaced, but quickly attract dirt. "Dry" lubricants, often containing wax or Teflon, have poor penetrating qualities unless carried in an evaporating solvent, but stay cleaner in use. The cardinal rule for long chain life is never to lubricate a dirty chain, as this washes abrasive particles into the rollers. Chains should be cleaned before lubrication. An alternative approach is to change the (relatively cheap) chain very frequently; then proper care is less important. Some utility bicycles have fully-enclosing chain guards which virtually eliminate chain wear and maintenance.

Chain wear, or chain stretch, becomes an issue with extensive cycling. Although the overall effect is often called "stretch", chains generally wear through attrition of the bushings (or half-bushings, in the Sedis design) and not by elongation of the sideplates. The tension created by pedaling is insufficient to cause the latter. Because an old chain is longer than needed, its links will not precisely fit the spaces between teeth in the drivetrain, making gear shifts a problem and possibly resulting in a 'skipping' chain that reduces power transfer and makes pedalling very uncomfortable.

Twenty half-links in a new chain measure 10" (254 mm), and replacement is recommended before the old chain measures 256 mm (0.7% wear). A safer time to replace a chain is when 24 half-links in the old chain measure 12¹/₁₆ inches (0.4% wear). If the chain has worn beyond this limit, the rear sprockets are also likely to wear, in extreme cases followed by the front chainrings. Replacing worn sprocket cassettes and chainrings after missing the chain replacement window is much more expensive.

Bicycle chains are made by companies such as:

• Campagnolo
• Rohloff AG
• KMC
• Shimano
• SRAM

• Bicycle gearing
• Chain tool
• Other Chainless bicycle like Hydraulic bicycle

• Bicycle chain length calculator and stretch/wear test
• Wikibooks Bicycle Maintenance and Repair - see the section on Chains
• The Complete Guide to Chain - general, not specific to bicycles