Symptoms
- Chain hangs loose off the front chainring or rear cassette
- Happened suddenly, often while shifting under load
- Pedaling produces no resistance and no movement
- Chain may be jammed between the frame and cassette
Likely causes
- CommonShifting under heavy pedaling loadShifting gears while pedaling hard, especially the front derailleur, can throw the chain off.
- CommonFront derailleur limit screws misadjustedIf the derailleur can move too far, it pushes the chain past the chainring.
- PossibleWorn or stretched chainA worn chain doesn't stay seated on the gears as reliably.
- Less commonBent chainring or derailleur hangerImpact damage can cause chronic chain derailment.
What to do
- Shift both shifters to the smallest gear positions before re-threading the chain, which reduces tension.
- Lift the chain and guide it back onto the front chainring teeth by hand, turning the pedal slowly with the other hand.
- Route the chain properly through the rear derailleur pulleys if it's also come off there.
- Spin the pedals slowly to confirm the chain seats properly and shifts through gears without dropping again.
- Check and adjust the front derailleur's high and low limit screws if the chain drops from the same gear repeatedly.
- Ease off pedaling pressure when shifting the front derailleur specifically, since this is the most common cause.
- If the chain keeps dropping despite limit screw adjustment, a bike shop can check for a bent hanger or chainring.
Tools and parts
- Disposable gloves (chains are greasy)
When to call a pro
A bike shop can adjust derailleur limits precisely or replace a bent chainring if the chain keeps falling off.
Safety
Generally low risk
Generally low risk for a careful person. Stop if anything looks different from what this guide describes.