As metal wears out of each connection in the chain, the chain is elongated. This is sometimes called ‘chain stretch’ even though the metal has not stretched it’s worn away. The elongated chain wears the cog wheels on which the chain runs. If the chain extends too much the cogs will become so worn that a new chain won’t run on the old cogs. If the chain is too worn the cogs have to be scrapped along with the old chain. Changing the chain at the right time means the cogs last much longer, through the life of many chains.
Measure Chain Wear
If the chain is on the bike hold the chain in tension by gently loading the pedals with the back brake locked ‘on’ with a toe strap. If the chain is off the bike hang it up. Hold a ruler or metal tape-measure with the ’0′ against a pin. One foot will measure 12 full links of new chain. When the chain is worn the 24th pin will be more than 12 inches away. If the extension is less than 1/16 of an inch the chain is OK. If it’s between 1/16 and 1/8 of an inch then the chain needs to be replaced but the cogs are OK. If the extension is more than 1/8 of an inch the chain and the cogs need to be replaced. The cogs are too worn and a new chain will skip if it runs on them.
Gauge Is Easier
You can buy special gauges that measure chain wear easily and have clear indicators of the significant chain wear thresholds.