Chain Wear As chains are used, the friction between the pins & side plates causes wear. Eventually, enough wear takes place which results in the chain getting slightly longer. This lengthening is known as "chain stretch."

Why Chain Stretch is Bad Actually, lots of chain stretch can take place, & you’ll never know there’s a problem. Your shifting, both on the front chain-rings, & on the rear cassette, will work fine. But a worn chain will greatly accelerate wear on both the chain-rings & the cassette. A badly worn chain will eventually not shift properly. When the worn-out chain is finally replaced, the cyclist often finds that the old cassette is also so badly worn that the new chain skips on the old cassette, forcing the replacement of the cassette, too, and sometimes even the front chain-rings.

Benefit of Chain Maintenance While chains cost about $25, cassettes cost $40-$180 & chain-rings cost $50-$125 (depending on the model). Most bike mechanics agree that if chains are replaced on an appropriate schedule, then frequent replacement of the more costly cassettes & chain-rings can be avoided.

Replacement Schedule The problem is, until the release of the new Park Chain Checker #CC-3, there’s never been an economical & easy way to check for chain wear. Shimano suggests 2000 miles as a maximum, but this is only for a chain that’s never ridden in the rain. It turns out using a chain in the rain greatly accelerates chain wear, even if the chain is lubricated immediately after the ride is over. Park has had available a tool that measures chain stretch, but at $25, most home mechanics wouldn’t buy it.