Why does my bike chain keep coming off?

A chain that drops off the front is nearly always a limit screw, a bent or low front derailleur, or a worn chainring — and each one has a different fix.

Difficulty: beginner
Time: 30 min
Read: 5 min

Short answer

Note where it goes: off the inside of the chainrings towards the frame means the front derailleur low limit screw is too loose; off the outside means the high limit screw is. If it comes off in every gear, look for a worn chainring with hooked teeth, a slack chain, or a bent derailleur cage. On a single-ring bike, a chain that drops means the chain is worn or the chainring is not a narrow-wide.

The front derailleur is a fork that pushes the chain sideways, and two screws stop it pushing too far in either direction. Almost every dropped chain is that mechanism, its adjustment or its alignment. The rest are worn parts. This is a fifteen-minute job with one screwdriver.

What you'll need

A cross-head or flat screwdriver
Allen keys
A rag
A chain wear checker or a ruler

Front derailleur alignment and the two limit screws

Step by step

Work out which side it drops off, and in which gear.Inboard, between the small chainring and the frame, is the low limit. Outboard, past the big chainring onto the crank, is the high limit. In any gear, without shifting, points at wear or alignment rather than adjustment.
Check the front derailleur cage is straight and parallel.Looking down from above, the outer cage plate should be parallel with the chainrings. A knock, a fall or a bike carried on a boot rack can twist it. Slacken the clamp and rotate it back until it lines up.
Check the cage height.With the chain on the big ring, the bottom of the outer cage plate should sit roughly one to three millimetres above the tallest teeth. Too high and the cage cannot control the chain; too low and it fouls the ring.
Set the low limit screw.Shift to the small chainring and the largest rear sprocket. The inner cage plate should be about half a millimetre from the chain. Turn the screw marked L clockwise to bring the derailleur outwards, away from the frame, until the chain can no longer drop inside.
Set the high limit screw.Shift to the big chainring and the smallest rear sprocket. The outer cage plate should clear the chain by about half a millimetre. Turn the screw marked H clockwise to stop the derailleur travelling further out.
Check the chain for wear.A worn chain sits deeper in the teeth and jumps. Use a chain wear checker, or measure: twelve full links should measure twelve inches, and if that has stretched noticeably the chain is finished. Riding on a worn chain also destroys the chainrings and sprockets.
Look at the chainring teeth.Healthy teeth are symmetrical. Worn teeth are hooked, like a shark fin, and the chain will not stay on them regardless of adjustment. Chainrings are replaceable separately on most cranks.
On a single-ring setup, check the ring and the tension.One-by drivetrains rely on a narrow-wide chainring, where alternate teeth are thicker to fill the wide links, and on a clutch rear derailleur to keep tension. A worn narrow-wide ring, a clutch switched off or a knackered clutch will throw the chain off over bumps.
Test it deliberately before you trust it.In a stand or holding the bike, shift into every combination, then ride and shift under light load. A chain drop under power at a junction is how people fall off.

Tips

Chain, chainrings and cassette wear as a set. Replacing a very worn chain on its own often makes shifting worse, because the new chain does not match the worn teeth.
A bent derailleur hanger causes rear shifting problems that look like everything else. It is a cheap sacrificial part and a shop can check and straighten it in minutes.
If a chain has come off and jammed hard between chainring and frame, do not lever it out with a screwdriver against the frame. Slacken the rear wheel or lift the derailleur to create slack first.
Mark limit screws with a dab of paint once set, so you can see if one has vibrated loose.

Common mistakes

Adjusting cable tension to fix a dropped chain — Cable tension controls where the derailleur sits in the middle of its travel. The ends of the travel are the limit screws, and only they stop a chain going too far.
Shifting the front chainring under full power — The derailleur has to lift the chain sideways off a loaded tooth. Ease off for half a pedal stroke and it will change cleanly every time.
Ignoring a chain that only drops occasionally — It will eventually drop at the worst moment, usually pulling away at a junction, and a foot slipping off the pedal at that point is how people end up in front of traffic.

If it doesn't work

Chain drops inside onto the bottom bracket when changing down

Cause: Low limit screw too far out — Fix: Turn the L screw clockwise a quarter turn at a time until the drop stops but the shift is still crisp.

Chain drops outside onto the crank when changing up

Cause: High limit screw too far out, or a rider changing up under heavy load — Fix: Turn the H screw clockwise until it stops, and ease pedal pressure momentarily when shifting the front.

Chain jumps off over bumps on rough ground

Cause: Slack chain, missing or disabled clutch, worn narrow-wide ring — Fix: Switch the clutch on, check chain wear, and replace the chainring if the teeth are hooked. A chain guide or bash ring solves the rest.

Chain comes off the rear sprockets into the spokes

Cause: Rear derailleur low limit screw wrongly set, or a bent hanger — Fix: This is urgent — a chain in the spokes wrecks the wheel and can lock the back wheel. Set the rear low limit and check the hanger is straight.

Chain falls off as soon as you back-pedal

Cause: Worn chain and rings, or a badly adjusted derailleur — Fix: Measure chain wear first. Back-pedalling exposes wear that forward pedalling hides.

Questions people ask

Can I fit a chain-catcher?

Yes — a small plastic or alloy stop clamped to the frame or bolted to the derailleur that physically blocks the chain from going inboard. Cheap and effective, but fix the adjustment first rather than using it as a substitute.

Why does it only happen going uphill?

Hills are where you shift the front under most load, and where the chain is at its most extreme angles. It is the same fault, just exposed.

The chain came off and now the gears do not work at all.

Check the derailleur has not been pulled into the spokes or bent. If the hanger is bent, shifting will be wrong in every gear until it is straightened.