If you’re chain keeps coming off your bike, there are a few things to check:
- Is the chain worn? – you can use a chain measuring tool to see if it needs changing or you can do a simple test with your thumb and see how far it comes off front chain ring. A worn chain is more likely to come off.
- Check indexing of gears. Chains often slip off during bad gear changes, if the indexing system is misaligned it can more easily come off.
- Check limits on derailleurs – especially front. The derailleurs have (H) high limits and (L) low limits (red arrows below). This set the amount by which you can move the derailleur. If the (H) value is too big, you can knock your chain off when moving into the outer chain ring. You need a screwdriver to reduce the high limit until the derailleur keeps the chain over the chainring. The best thing is just to see how the derailleur moves as you turn the screwdriver.
high end derailleurs (like Shimano Dura Ace and Campag) often don’t mark which is high and low, but you can soon find out through trial and error)
- Also check the vertical height of the front derailleur.
- Chain Too loose. Look at the chain in different gears, if it is slack, the chain is too long it will need some links breaking. see: Chain length for bike
- Front Derailleur. If you have removed front dérailleur for a single chain ring, the chain is much more likely to jump off. The front dérailleur helps keep it in place.
- Make sure back wheel is fully pulled back into rear stays to keep chain stretched.
General Maintenance for Chains
Keep lubricated with something like GT-80 or a specific chain lubricant.