The usual causes
Chains rarely break for one reason alone. They usually fail where a weak point meets a heavy load. The causes below are widely cited by mechanics and cycling resources.
Shifting under heavy load
Pedaling very hard while shifting, such as on a steep climb or when sprinting, puts large and awkward forces on the chain as it is pushed sideways between cogs. Easing the pressure on the pedals for a moment during the shift reduces this. It also reduces the chance of the chain dropping, as covered in why your bike chain keeps coming off.
A worn or stretched chain
As a chain wears, its pins and bushings loosen and it grows slightly longer. A worn chain meshes poorly with the teeth and can fail more readily under load. A wear gauge is a more reliable guide than judging by eye, and your chain’s manufacturer can tell you the replacement point for your drivetrain. Wear is also a common cause of skipping, discussed in bike chain slipping under hard pedaling.
A poor join or re-used pin
If the chain was previously cut and rejoined by pushing a standard pin back in, that link can be weaker than the rest. Pins and plates can be damaged during this process. Using a quick link or the connecting pin that the maker specifies is generally safer; see how to shorten a bike chain. If a link was joined with a makeshift method, such as the approach in removing a bike chain without a chain tool, consider replacing that section.
Bent or misaligned parts
A bent derailleur hanger, a bent derailleur cage or damaged cogs can push the chain sideways and force it to run at an angle. That twisting load can crack a plate. Crashes and bikes falling on the drive side are common causes of this kind of damage.
Cross-chaining
Running the chain at an extreme angle, such as the big chainring with the biggest rear cog, increases side loads. Using gear combinations closer to a straight line is gentler on the chain.
Debris, chain suck or a jam
Anything that jams the chain, such as a stick, or a chain that wraps around a chainring, can stop it suddenly. The force on the links in that moment can be large.
Poor lubrication and corrosion
A dry or rusty chain wears faster and can develop stiff links. Rust can weaken parts over time.
Manufacturing defects
A brand-new chain that breaks under normal use may have a defect, though this is less common than the causes above. If it happens, it is worth contacting the retailer or manufacturer.
Working out what happened
Ask yourself:
- Was the chain old or new?
- What were you doing at the time: climbing, sprinting, starting from a stop or shifting?
- Where did it break: at a quick link, at a pin, or in a plate?
- Is there any sign of damage, such as a bent derailleur, a crash or a stick in the drivetrain?
A break at the join often points to how it was joined. A break in the middle of the chain suggests wear or load.
Getting home with a broken chain
If you have a chain tool and a spare quick link or connecting pin, remove the damaged link and join the chain. The chain will be a link shorter, so avoid the biggest chainring combined with the biggest cog, and shift gently.
Without tools you may be limited to coasting or walking. In some cases, a single-speed bike can be ridden with a shortened chain that has been joined by a quick link. Carrying a small tool kit makes this easier.
Preventing another break
- Replace the chain based on wear, not on appearance.
- Use a quick link or the correct pin when joining.
- Ease off the pedals when you shift.
- Keep the chain clean and lubricated.
- Have the derailleur and hanger checked after a crash.
- Match chain, cassette and chainrings by speed count.
After a break
Inspect the cassette and chainrings, since a worn drivetrain can chew up the new chain. If skipping starts when you install a new chain, wear in the cogs may be the reason.
Frequently asked questions
Does a loose chain affect mileage?
A loose or worn chain can wear itself and the teeth faster, which tends to shorten the life of the drivetrain. For how to fix slack, see how to tighten a bike chain.
How do I fix a broken chain?
Remove the damaged link with a chain tool and rejoin with a quick link or the correct pin. If more than a link is damaged, replace the chain.
Why won’t my bike chain stay on after fixing it?
The chain might be the wrong length, or a stiff link or misaligned derailleur could be the cause; see why your bike chain keeps coming off.