A well-maintained bicycle can last a lifetime, but you can bet the bike chain won’t. There's no predicting how often you’ll need to replace your chain, since too many variables -- from riding conditions to riding style -- affect the chain’s lifespan. Even a broken or bent chain does not necessarily need replacement if you can repair it correctly. An elongated chain, however, is always in need of replacement.
When It Breaks -- Sometimes
A chain that breaks can be in need of replacement -- or not. It depends on the cause of the breakage. If your chain breaks from being worn down and you see obvious cracks in the links, then it’s time for a new chain. Even if you’re able to temporarily repair your chain by removing the broken link and thus shortening the chain, you’ll still need to replace the chain, or at least install a new link to get a chain that’s the correct length for your bike. If your chain breaks because one of the pins in the links comes loose or pops out, you can keep the chain as long as you insert a new pin. The pins tend to pop out if you don’t install them correctly when you put on a new chain.
When It's Bent -- Sometimes
A bent link in a chain can result in the chain skipping, feeling rough when you pedal and eventually breaking. Chain parts can bend if a stick or other object gets sucked into the drivetrain and you end up pedaling along with the debris. Although you can twist the bent portion back into its original shape to continue your ride, you’ll need to replace the chain, or at least the links that were bent. Any type of damage weakens the links and makes them more susceptible to breaking.
When It's Elongated (Always)
Chains don’t actually stretch out per se, but they do get longer when the metal around the rivets in the links wear down. Measuring your chain determines if it’s become elongated and needs replacement. Check the chain by holding a ruler up to the chain while the chain is still on your bike. A chain that is in good shape will measure 12 in. from the middle of one chain pin to the middle of another chain pin. If the measurement to the middle of the second chain pin is at or above 12 1/8 in., it’s time for a new chain.
What Happens If You Let It Go
If you continue to ride with a chain that needs replacement, you can end up needing to replace more than just the chain. Worn chains will wear down the sprockets that keep the chain in place, sprockets that can become too worn to properly support a new chain once you finally do replace it. Checking your chain every three months for elongation is the most effective way to ensure you’ll replace it when necessary, rather than when it’s already damaged other parts of your bike.