Bicycle Chain - Wear

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 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 in the first place.