When to replace a bike chain: 3 signs and how to measure

A chain is one of the cheapest parts on a bike, but a worn one destroys the most expensive ones — the cassette and the chainrings. Replaced on time, a chain costs a few tens of euros. Replaced late, several times that.

Here is how to tell when it is time.

Why a chain "stretches"

Strictly speaking, a chain does not stretch. Its pins and bushings wear, so the distance between links slowly grows. A new chain has an exact 12.7 mm pitch; once that grows by more than 0.5 %, the chain no longer matches the sprocket teeth and starts cutting into them.

That is why wear is expensive: the chain itself is cheap, the cassette and chainrings are not.

3 signs your chain is worn

1. Gears shift unevenly. Especially under load — climbing, or pushing away from the lights. The chain skips across the teeth.

2. You hear rattling or surging. A rhythmic noise appears at a steady speed that was not there before.

3. The chain lifts off the chainring. Pull the chain away from the largest chainring with your fingers. If a whole tooth gap shows, the chain is well worn.

These signs are late ones. The reliable method is to measure.

How to measure accurately

Use a chain wear indicator — a simple tool that drops between the links and reads the wear as a percentage. What the reading means:

• under 0.5 % — the chain is fine, keep riding;
• 0.5–0.75 % — replace the chain now, the cassette is still healthy;
• over 0.75 % — you will probably need a new cassette too;
• over 1 % — replace chain, cassette and often the chainrings.

Without a tool, a ruler works: 12 links on a new chain measure exactly 304.8 mm. If you measure 306 mm or more, it is time.

How often to check

A practical rule is every 500 km. Check more often on muddy trails or in winter: grit and salt wear a chain several times faster than dry tarmac.

A chain that is cleaned and lubricated regularly lasts 2000–3000 km. One that is never cleaned can be finished in 800 km.

What to choose

The key figure is the number of speeds. An 11-speed chain will not work on a 9-speed cassette — they differ in width. Count the sprockets on your cassette and buy a chain to match.

When replacing a chain, we suggest you also:

• check the cassette teeth — if they look "wavy", replace it at the same time;
• clean the derailleur jockey wheels;
• pick up chain lubricant — dry or wet, to suit the season.

Replacing a chain takes about 15 minutes and needs one tool, a chain breaker. It is one of the simplest repairs worth learning to do yourself.