A worn chain will struggle to change gears and will wear out the cassette and chainring prematurely.You can check how worn your chain is by using a wear indicator. Simply place the indicator's notch between two links and let the other end drop onto the chain. If it fits neatly between two links, the chain is worn and it's important to change it soon, before it damages the cassette and chainring.For 11- or 12-speed cassettes, we recommend changing your chain when it's 50% worn. Above 75%, you should change the chain, cassette AND chainring, which is why it's best to regularly change your chain: this stops you having to replace your entire drivetrain.