If your chain has started to come off frequently when it didn’t in the past, something in your bike setup has changed. The first thing to do is check that all of your drivetrain bolts are tight, that nothing has moved or got whacked, and to make sure your rear wheel is correctly seated in the dropouts.
Let’s take a look at how you deal with other possible reasons your chain is coming off.
If your chain comes off at the front
Adjusting your front derailleur limit screws
If your chain is coming off the chainset, make sure that the two limit screws on your front derailleur are properly adjusted.
One of the screws – sometimes, but not always, marked H for ‘high’ – limits movement of the front derailleur cage outwards.
The other screw – sometimes, but not always, marked L for ‘low’ – limits movement of the front derailleur cage inwards.
If your chain regularly comes off the inside of your chainset, it could be that the L screw needs adjusting. Put the chain on the small chainring and the largest sprocket at the back. The inner plate of the front derailleur cage should nearly but not quite touch the chain; we’re talking about a gap of 1-2mm. If the gap is larger than that, turn the L screw clockwise to move the inner plate of the front derailleur cage closer to the chain.
If your chain frequently comes off the outside of your chainset, it could be that your front derailleur H screw needs adjusting. Put the chain on the big chainring (up front) and the smallest sprocket (at the back). The outside plate of the front derailleur cage shouldn’t quite touch the chain. If there’s a gap larger than 1-2mm, turn the H screw clockwise and you’ll see the cage move inwards.
