Unshipping your chain can be annoying at best, sometimes dangerous, especially if it happens when you’re in traffic, so you need to minimise the chances of it happening by setting up your bike right.
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.
You can still occasionally unship your chain even if the limit screws are set correctly, particularly if you change gear under load (while standing or pushing hard on the pedals). Try backing off your effort (but still pedaling) when shifting between chainrings.
Another way to keep the chain from coming off the inside of the chainset is to use a chain catcher.
A chain catcher is essentially an arm that acts as a barrier to the chain overshifting inwards. SRAM Red, Force 22 and Rival 22 front derailleurs have an integrated chain catcher.
If your bike doesn’t already have one, you can fit a chain catcher retrospectively. There are different designs out there and they’re all pretty easy to install.
If your chain comes off at the rear
Adjusting your rear derailleur limit screws
If your chain is coming off at the rear, it’s often a simple matter of adjusting the limit screws on your rear derailleur.
If the chain is coming off the inside of the cassette, put the chain onto the smaller chainring (at the front) and the largest sprocket (at the rear). Then turn the L screw clockwise until you see the rear derailleur cage (the section that hangs down) start to move away from the centre of the bike. You need to move it to the point where the chain can move freely into the largest sprocket, but can go no further than that.
If your chain is coming off the outside of the cassette, put the chain onto the larger chainring and the smallest rear sprocket. Then turn the H screw clockwise until you see the rear derailleur cage start to move towards the centre of the bike. You need to move it to the point where the chain can move freely into the smallest sprocket, but can go no further than that.
Again, though, if the chain wasn’t coming off before but has recently developed the habit, the setup has somehow changed. Before you do anything, check that the rear wheel is sitting correctly in the dropouts with the rim running centrally in the chainstays, and make sure that the rear derailleur and its hanger (the piece to which the rear derailleur is bolted) aren’t bent. In most cases, the rear derailleur pulleys should be lined up directly underneath one another.
What if your chain is still coming off?
If you do all this and your chain is still coming off, there are several other things to look at:
• Your front derailleur could have moved. Check that it is positioned correctly by taking a look at our article on how to index front gears.
• Is your drivetrain worn and in need of replacement? Shifting performance can start to become less smooth and more erratic as components near the end of their useful lives.
• It might be that the chain is worn, has a stiff or bent link, or has become clogged up with dirt. A visual inspection while turning the cranks should reveal if there’s a problem here.
• The chainring, or a chainring tooth, could be bent. Again, a visual check will tell you what you need to know.
• It could be that you’re running the chain at too extreme an angle – in the larger chainring and the largest sprocket (as in the picture above), or the smaller chainring and the smallest sprocket. Some systems can handle this (even though it's not a particularly efficient way to ride), some can’t.
• Your chain could be too long or too short. Before putting a chain on, thread it onto the larger chainring and the largest sprocket but don’t run it though the rear derailleur. Pull the two ends together and add one complete link (one inner and one outer half link) to get the correct chain length.
• You’re running a chain that’s not compatible with the rest of your drivetrain, in which case you need to change it.
There are other reasons why your chain could come off, but we think these are the main ones. If you think there’s an important cause we’ve missed, tell us about it down below.
Mat has worked for loads of bike magazines over 20+ years, and been editor of 220 Triathlon and Cycling Plus. He's been road.cc technical editor for eight years, testing bikes, fettling the latest kit, and trying out the most up-to-the-minute clothing. We send him off around the world to get all the news from launches and shows too. He has won his category in Ironman UK 70.3 and finished on the podium in both marathons he has run. Mat is a Cambridge graduate who did a post-grad in magazine journalism, and he is a past winner of the Cycling Media Award for Specialist Online Writer.