Start with where the chain falls off
Before changing anything, watch what the chain does. Where it ends up usually narrows the cause quickly.
- It drops off the smallest or largest rear cog, or off the inner or outer chainring: this often points to a derailleur limit screw that needs adjusting, or a derailleur that is not lined up.
- It bounces off on a bumpy road, on a single-speed bike: slack is the likely cause. See how to tighten a single speed chain.
- It skips or jumps under hard pedaling: worn parts or gear indexing are common suspects. There is more on this in bike chain slipping under hard pedaling.
- It falls off when you pedal backwards: this can happen on a geared bike when the chain has slack or the derailleur is set up poorly, and it is not always a sign of a serious fault. On a single-speed or coaster brake bike, check tension first.
The chain is too loose or too long
On a bike with a single gear, the chain stays on only if it has the right amount of tension. Too much slack lets it hop off the teeth over bumps. The fix is moving the rear wheel back in the dropouts, as described in the single speed tightening guide.
On a bike with a rear derailleur, the derailleur’s spring takes up slack, so you cannot tighten the chain by moving the wheel. A chain that is too long for the bike, for example after a change of cassette or because it was fitted without being sized, can sag and drop. A chain that has stretched from wear may behave similarly. If it is simply too long, see how to shorten a bike chain; if it is worn, replace it. If you are unsure which situation you have, how to tighten a bike chain explains the difference by bike type.
Derailleur limit screws and alignment
A rear derailleur has two limit screws, usually marked H and L, which stop it from moving the chain beyond the smallest and largest cogs. The front derailleur has a similar pair. If a screw is set too loosely, the chain can slip over the edge of the cassette or fall off the chainring.
A few things worth checking:
- Look at the bike from behind. The derailleur cage should appear roughly vertical and in line with the cogs. A cage that looks tilted can mean a bent derailleur or hanger, often after a fall or a bike being knocked over on its drive side.
- Shift to the extremes of the gear range while pedaling gently (on a stand, or with the rear wheel lifted) and see whether the chain stops cleanly or tries to go further.
- Adjust limit screws in small turns, a quarter turn at a time, and recheck. The exact procedure varies by brand, so the manufacturer’s instructions are worth consulting.
A bent derailleur hanger is difficult to judge by eye. If the shifting never settles after reasonable adjustment, a bike shop can check alignment with the proper tool.
Worn chain, chainrings or cassette
Chains lengthen slightly as the pins and bushings wear. A worn chain does not mesh well with the teeth, and worn teeth can lose their shape, so the chain can ride up and skip or drop. A common sign is that a new chain skips on an old cassette.
A chain wear gauge gives a more reliable answer than judging by eye. Commonly cited replacement points are in the region of 0.5% to 0.75% stretch, but the right figure depends on the number of speeds and the maker’s guidance, so check what your chain’s manufacturer recommends. Replacing a worn chain early is usually cheaper than replacing a cassette and chainrings later.
Loose or damaged parts
Check for anything that moves when it should not:
- A loose crank, chainring bolt or bottom bracket can let the chain line shift while you ride.
- A bent chainring tooth or bent cog can catch the chain.
- A stiff link, where one link does not flex, can make the chain jump when it reaches the derailleur.
- A rear wheel that is not seated properly in the dropouts changes chain tension and alignment.
Shifting habits
Pushing a shifter further than needed, or shifting while pedaling very hard, can drop the chain. Easing off the pedals for a moment during a shift is gentler on the chain and the drivetrain. The same habit helps avoid breakage, covered in why bike chains break.
Preventing it
- Keep the chain clean and lubricated so links move freely.
- Check chain wear regularly.
- Have limit screws and indexing checked after a crash or after the bike has been transported.
- Make sure the chain is the correct length for your drivetrain.
If you have worked through these checks and the chain still drops, a shop can look at things that are hard to see at home, such as hanger alignment or frame dropouts.
Frequently asked questions
How do I fix a bike chain that keeps falling off?
Work out where it falls, then check the likely cause: tension on a single-speed, limit screws and alignment on a geared bike, and wear on either. Change one thing at a time and test ride after each change.
Why is my bike chain not staying on?
Common reasons are too much slack, a misadjusted derailleur, a worn chain or worn teeth, or a bent part. A chain that was recently joined incorrectly can also cause trouble.
Can I stop it from happening again?
Regular cleaning, lubrication, wear checks and a correctly sized chain reduce the chance considerably, though a crash or bent hanger can cause it regardless.