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 nickel-plated to prevent rust, or simply for aesthetics.
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.
A bicycle chain can be very energy 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".
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.
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, (or "broken" ) unless the triangle can be split (usually the seat stay). Chain can either be broken with a chain tool or at a master link. 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 Wippermann Connex.
Chain wear, often called chain stretch, becomes an issue with extensive cycling. The wear is removal of material from the bushings and pins (or half-bushings, in the Sedis design) rather than elongation of the sideplates. The tension created by pedaling is insufficient to cause the latter. Because the spacing from link to link on a worn chain is longer than the 1/2" specification, those links will not precisely fit the spaces between teeth on the sprockets, resulting in increased wear on the sprockets and possibly chain skip on derailleur drivetrains, in which pedaling tension causes the chain to slide up over the tops of the sprocket teeth and skip to the next alignment, that reduces power transfer and makes pedaling 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 longer than a chain on a lightly-used city bike that's cleaned less. Depending on use and cleaning, a chain can last only 1,000 km (e.g. in cross-country use, or all-weather use), 3,000 to 5,000 km for well-maintained derailleur 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). 
Nickel-plated chain also confers a measure of self-lubrication to it's moving parts as nickel is a relatively non-galling metal.((cn))
Chain wear rates are highly variable, so replacement by calendar is likely premature or continued use of a worn chain, damaging to rear sprockets. One way to measure wear is with a ruler or machinist's rule. 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.
The chain in use on modern bicycles has a 1/2" pitch, which is ANSI standard #40, where the 4 in "#40" indicates the pitch of the chain in eighths of an inch, and metric #8, where the 8 indicates the pitch in sixteenths of an inch.
Chains come in either 11/128", 3/32", 1/8", 5/32", or 3/16" roller widths, the internal width between the inner plates. 1/8" chains are used on bikes with a single rear sprocket: those with coaster brakes, hub gears, fixed gears such as track bicycles, or BMX bikes. Chains with 3/32" wide rollers are used on bikes with derailleurs such as racing, touring, and mountain bikes. Fixed sprockets and freewheels are also available in 3/32" widths so fixed-gear and single-speed bikes can be set up to use narrow and lighter 3/32" chains. Finally, chains with 5/32" wide rollers are used on freight bicycles and tricycles.
With derailleur equipped bicycles, the external width of the chain also matters, because chains must not be too wide for the cogset or they will rub on the next larger sprocket or too narrow that they might fall between two sprockets. Chains can also be identified by the number of rear sprockets they can support, anywhere from 3 to 11, and the list below enables measuring a chain of unknown origin to determine its suitability.
- 6 speed - 7.8mm (all brands)
- 7 speed - 7.3mm (all brands)
- 8 speed - 7.1mm (all brands)
- 9 speed - 6.6 to 6.8mm (all brands)
- 10 speed - 6.2mm (Shimano, Campagnolo)
- 10 speed(Narrow) - 5.88mm (Campagnolo, KMC)
- 10 speed(Narrow, Direction) - 5.88mm (Shimano CN-5700,CN-6700,CN-7900)
- 11 speed - 5.5mm (Campagnolo, KMC, Shimano CN-9000)
The Wikibook, "Bicycle Maintenance and Repair", has more details on this topic.
New chains usually come in a stock length, long enough for most upright bike applications. The appropriate number of links must be removed before installation in order for the drive train to function properly. The pin connecting links can be pushed out with a chain tool to shorten, and additional links may be added to lengthen.
In the case of derailleur gears the chain is usually long enough so that it can be shifted onto the largest front chain ring and the largest rear sprocket without jamming, and not so long that, when shifted onto the smallest front chain ring and the smallest rear sprocket, the rear derailleur cannot take up all the slack. Meeting both these requirements is only possible if the rear derailleur is compatible with the gear range being used on the bike.
In the case of single-speed bicycles and hub gears, the chain length must match the distance between crank and rear hub and the sizes of the front chain ring and rear sprocket. These bikes usually have some mechanism for small adjustments such as horizontal dropouts, track ends, or an eccentric mechanism in the rear hub or the bottom bracket. In extreme cases, a chain half-link may be necessary.
In order to reduce weight, chains have been manufactured with hollow pins and with cut-outs in the links. Chains have also been made of stainless steel for corrosion resistance and titanium for weight reduction, but they are expensive. A recent fad is chains of various colors, as can be seen in the lead image above. KMC offers a chain model specifically for electric bicycles, and won the iF Product Design Gold Award for its BMX ‘Kool Knight Chain’, designed to withstand the rigors of extreme stunts, freestyle, gliding.
Notable bicycle chain manufacturers include:
|This article needs additional citations for verification. (February 2008)|
- Spicer, James (1999-08-19). "Pedal Power Probe Shows Bicycles Waste Little Energy". Archived from the original on 6 March 2008. Retrieved 2008-02-13.
- "Sheldon Brown: Chain Maintenance". Archived from the original on 5 December 2008. Retrieved 2008-12-03.
- Brandt, Jobst. "Chain care, wear and skipping". Archived from the original on 9 December 2008. Retrieved 2008-12-03.
- "Sheldon Brown: Chains from Harris Cyclery". Archived from the original on 16 December 2008. Retrieved 2008-12-03.
- Sheldon Brown. "Chain Maintenance: Chain "Stretch"". Retrieved 2013-08-20.
- "KMC: How to minimize chain wear". Archived from the original on 23 April 2013. Retrieved 2013-07-27.
- "KMC: When to maintain your chain". Archived from the original on 23 April 2013. Retrieved 2013-07-27.
- "How to use ruler to measure chain wear". RoadBikeReview components forum. Retrieved 2012-04-24.
- Sheldon Brown. "Chain". Retrieved 2012-05-14.
- Warren Rossiter (1 Jun 2007). "KMC x10sl Gold road chain". BikeRadar.com. Retrieved 2013-03-26. "KMC have achieved this with the pared down profile and slotted plates (something Campag also do with the Ultra 10-speed chain) and hollow pins connecting everything together."
- "Interbike Tech: Retro Fondriest, solar hydration pack, organic nutrition and more". VeloNews. Sep. 20, 2012. Retrieved 2013-03-26. "Wippermann claims that its Black Edition stainless-steel ConneX chain is the toughest, most corrosion-resistant derailleur chain on the market."
- Daniel Carruthers (Jan 4, 2010). "Taichung Bike Week round-up". BikeRadar.com. Retrieved 2013-03-26. "Titanium chain from YBN ... at US$180 (approx £110) you're unlikely to see too many of these around."
- "KMC Launched Components for Bosch e-Bike Drive System". Bike Europe. January 27, 2012. Retrieved 2013-02-06.
- "KMC Stars Award Winning BMX Chain at Taipei Show". Bike Europe. March 11, 2010. Retrieved 2013-02-06.
- 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
- How to calculate chain length (video tutorial)
- Animation of Shimano gearing system