Chain Tensioner

Sheldon Brown's Bicycle Glossary · letter C
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1. A device used to adjust the chain tension when a bike with vertical dropouts is converted to use a single-speed or internal-gear drive train. Some of these use two pulleys, like a rear derailer that doesn't move from side to side, others use a single pulley. If used with a front derailer, a chain tensioner must be spring-loaded like a rear derailer. A chain tensioner cannot be used with a fixed gear, though other solutions exist -- see my article on fixed-gear conversions.
2. An idler pulley or sprocket fixed rigidly to the frame, generally on a slotted bracket, and used to adjust the slack (not actually the tension) of a tandem sync chain. This system is used on inexpensive tandems, as a cheap substitute for an eccentric bottom bracket. The pulley should be in the return (slack) run of the chain, for minimal wear and drag. See also John Allen's article on do-it-yourself tandems.
3. A device used on some one-speed bicycles to adjust the chain tension by pulling back on the rear axle with a screw thread. One popular type used on BMX bicycles is called a "banjo bolt"

Copyright © 1996, 2008 Sheldon Brown

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