Checking chain wear before it costs you a cassette

Chains do not stretch. The metal does not get longer. What happens is that the pins and the insides of the rollers wear, and every one of those tiny gaps adds up along the length of the chain. A worn chain is measurably longer than a new one, and that is what we measure.

This matters because the chain and the sprockets wear together. Once the chain has lengthened, it no longer sits correctly in the teeth, and it starts wearing the sprockets into a matching shape. Leave it long enough and the cassette and chainrings are worn to fit that specific worn chain, and replacing the chain alone will make the bike skip.

So the question is not whether a chain wears out. It is whether you replace it before or after it takes the cassette with it.

The numbers

A chain wear indicator drops into the chain and tells you a percentage. The thresholds are simple:

• Under 0.5 percent, the chain is fine, keep riding.
• 0.5 to 0.75 percent, replace the chain now and the cassette will almost certainly survive.
• Over 0.75 percent, replace the chain and expect to need a cassette. On 11 and 12 speed, treat 0.5 as the limit rather than 0.75, because the sprockets are thinner and wear faster.
• Over 1 percent, the chainrings are probably going too, and the bill has roughly tripled.

Doing it without a tool

You can measure with a steel ruler. On a new chain, twelve full links measure exactly twelve inches, pin centre to pin centre.

Line the zero mark up with the centre of one pin and look at the twelfth. If that pin centre is at twelve inches, the chain is new. At twelve and one sixteenth, it is around 0.5 percent worn and due. At twelve and one eighth, it is past a percent and you are into cassette territory.

It is fiddlier than the tool and perfectly accurate if you are careful.

How often to check

Every couple of months for regular riding, and more often over winter. Grit is what actually wears a chain out, and a wet gritty winter can take a chain from new to finished in a single season. There is more on that in winter maintenance that actually matters.

Write down the reading and the date. Two or three readings tell you your own wear rate, and after that you stop guessing.

The economics, plainly

A chain costs somewhere between £20 and £40. A cassette is £40 to £90. Chainrings on top of that are another £40 upwards, and on some modern chainsets you cannot replace the rings on their own at all.

Replace chains on time and you buy three or four chains over the life of one cassette. Ignore it and you buy a chain, a cassette and possibly a chainset, and you do it more often.

This is the single highest return maintenance job on a bicycle and it takes a minute.

One caveat about very worn drivetrains

If your chain is well past a percent and the bike still rides fine, do not assume you have got away with it. It will skip the moment you put a new chain on, and people often conclude the new chain is faulty. It is not, the cassette is simply worn to match the old one.

At that point you replace both together, which is what we would quote you for in a full service. We measure the chain on every bike that comes through and write the number on your job sheet, so you can see it moving year to year rather than taking our word for it.