After a while, chains wear. On a simple drive train, such as single speed or hub gear, the bike should work just fine with a slightly worn chain. However, with a complex drive train, such as derailleur, the various components need to be relatively unworn to work properly together. The chain wears before the sprockets so that’s the thing to monitor – and renew when necessary.
How do chains wear?
As the bike is used, water, grit or salt gets inside the chain which, under the force of pedalling, can wear down the metal pins that hold the chain together. Over time the chain elongates and does not mesh with the sprockets properly. As a result a worn chain will grind down the edges of the sprocket teeth and the bike won’t change gear smoothly.
How worn is worn?
A chain is significantly worn if it has elongated by 1 per cent.
The thing to remember is a complete link on a new chain measures exactly 1″ (one inch), so twelve complete links measure 12″ (twelve inches).
- If twelve links of a used chain chain measure less than 12 1/16″ (twelve and one sixteenth inches), the chain is OK.
- If twelve links measure between 12 1/16″ and 12 1/8″ – this is the time to replace the chain.
- If twelve links measure more than 12 1/8″ the chain will have begun to wear the sprocket teeth – so the sprocket will probably need replacing too.
Why bother checking chain wear?
Chains are cheap; sprockets are expensive. Riding a bike with a worn out chain could wear out the sprockets, meaning a big bill when the whole system needs replacing. Replacing the chain when it’s part worn avoids this.
How can you measure chain wear?
- Find a steel ruler and an old rag.
- Place the ruler along the top of the chain.
- Align zero on the ruler with the centre of a chain pin.
- With a rag, grip the ruler against the chain so it doesn’t move.
- Read off the measurement twelve links away.
There is much more information about chain wear and chain maintenance at Sheldon Brown’s website. It’s quite involved.