Bicycle chain
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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 all bicycles today is a roller chain with a 1/2" pitch and either a 1/8" or 3/32" width, the former being used on the common low cost coaster, three speed and track bicycles, and the latter being used on derailleur-equipped road, racing, and touring bicycles. 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.
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 chaincases which virtually eliminate chain wear and maintenance.
In all cases a new chain should be fitted before 24 half-links in the old chain measure 12 1/16 inches. If the chain has worn beyond this limit the rear sprockets must also be changed, which is considerably more expensive. Although the overall effect is still 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.
Bicycle chains generally available today are made by: