A bicycle chain is a roller chain that transfers power from the pedals to the drive-wheel of a bicycle, thus propelling it. Most bicycle chains are made from plain carbon or alloy steel, but some are chrome-plated or stainless steel to prevent rust, or simply for aesthetics.
History
Obsolete chain designs previously used on bicycles included the block chain, the skip-link chain, and the Simpson lever chain. Most modern bicycle chains used with a single chainring and single rear sprocket are conventional industrial bushing chain. Until the 1980s, most derailleur chains were also bushing chains, but today, virtually all derailleur chains are of the "Sedis" bushingless design. Compared to a bushing chain, a bushingless chain is cheaper to make, is less likely to break under shifting load, promotes better lubricant flow inside the rollers, and creates more lateral flexibility for multi-geared bicycles. However, it also wears much faster and has slightly worse mechanical efficiency than a bushing chain.
Before the safety bicycle, bicycles did not have chains and the pedals were typically attached directly to the drive-wheel, thus limiting top speed by the diameter of the wheel and resulting in designs with front wheels as large as possible. Various linkage mechanisms were invented to raise the effective gear ratio, but with limited success. 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.
Efficiency
A bicycle chain can be very efficient: one study reported efficiencies as high as 98.6%. The study, performed in a clean laboratory environment, found that efficiency was not greatly affected by the state of lubrication. A larger sprocket will give a more efficient drive, reducing the movement angle of the links. Higher chain tension was found to be more efficient: "This is actually not in the direction you'd expect, based simply on friction".
Maintenance
How best to lubricate a bicycle chain is a commonly debated question among 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, are transported by an evaporating solvent, and 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. The chain should be wiped dry after the lubricant has had enough time to penetrate the links. 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. On recumbent bicycles the chain is often run through tubes to prevent it from picking up dirt, and to keep the cyclist's leg free from oil and dirt.
Removal
On most upright bicycles, the chain loops through the right rear triangle made by the right chain stay, right seat stay, and seat tube. Thus a chain must be separated, "broken" is a term commonly used, with a chain tool or at a master link. (Care must be taken not to push the rivet all the way out, as it is very difficult to reinsert. See "Removing a Bicycle Chain" in the Wikibook on bicycles.) A master link, also known as a connecting link, allows the chain to be inserted or removed with simpler tools, or even no tools, for cleaning or replacement.
Some newer chain designs, such as Shimano and Campagnolo 10-speed chains, require a special replacement pin to be used when installing or reinstalling a separated chain. An alternative to this process is to install a master link, such as a SRAM Power Link or a Wipperman Connex.
Wear
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.
Since chain wear is strongly aggravated by dirt getting into the links, the lifetime of a chain depends mostly on how well it is cleaned (and lubricated) and does not depend on the mechanical load. Therefore, well-groomed chains of heavily used racing bicycles will often last much longer than those of a lightly used, but not so well cleaned city bike. Depending on use and cleaning, a chain can last only 1,000 km (e.g. in cross-country use, or all-weather abuse), 3,000 to 5,000 km for well-maintained derailer chains, or more than 6,000 km for perfectly groomed high-quality chains, single-gear, or hub-gear chains (preferably with a full cover chain guard).
Chain wear rates are highly variable, so replacement by calendar is likely to cause either needless chain replacement or continued use of a worn chain, damaging rear sprockets. One way to measure wear is with a ruler. Another is with a chain wear tool, which typically has a "tooth" of about the same size found on a sprocket. They are simply placed on a chain under light load and report a "go/no-go" result - if the tooth drops in all the way, the chain should be replaced.
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 (0.5% 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. In this case, the 'skipping' mentioned above is liable to continue even after the chain is replaced, as the teeth of the sprockets will have become unevenly worn (in extreme cases, hook-shaped). Replacing worn sprocket cassettes and chainrings after missing the chain replacement window is much more expensive than simply replacing a worn chain in the first place.