Measure chain wear

Wipe about twenty links of the chain with Clean rags: grime between the rollers throws the measurement off. Shift onto the big chainring and a middle sprocket, so that the top run of the chain is under tension.

Always measure on the top run, between the chainring and the cassette, never on the lower run hanging under the derailleur. A chain wears by getting longer: the pins and rollers develop play, and the pitch between links grows.

Place the Chain checker (or else Steel ruler) on the top run: hook the fixed tooth into a roller and let the other end drop in without pushing. Measure at three different places along the chain.

On an 11- or 12-speed drivetrain, replace the chain as soon as the checker drops in fully on the 0.5 % side. On 10 speeds or fewer, wait until it drops in on the 0.75 % side. If it does not drop in on either side, the chain is fine.

Without a checker, a steel ruler does the job. Put the zero of the Steel ruler (or else Tape measure) on the centre of a pin and count 12 full links, which is 24 pins. On a new chain, the centre of the 24th pin sits exactly at 304.8 mm, one foot.

Read where the centre of that pin sits, to the nearest half millimetre. At 306.3 mm or more, an 11- or 12-speed chain needs replacing; at 307.1 mm or more, a chain of 10 speeds or fewer. Write the value down to follow the wear from one check to the next.

A chain replaced at the wear limit needs nothing more than a new chain. If it went well past the limit, around 1 %, about 307.8 mm on the ruler, the cassette and sometimes the chainrings are worn too: a new chain will skip on the sprockets you use most.

In that case, plan a new cassette along with the chain. To follow wear without surprises, measure every 500 km on a road bike, more often on a mountain bike or in wet weather, and every time you clean the chain.