We’re always on the lookout for interesting problems to share with you in the hopes we can head off trouble before it happens – and/or provide tips for diagnosing and dealing with glitches. The issue for you this week is a drivetrain miscue that came up on my club’s last Saturday ride.
We had returned to town and stopped to say our goodbyes, and while still standing over his Trek, one of the guys backpedaled and his chain promptly came off the chainring and jammed in the front derailleur. He said something like, “that always happens and it drives me nuts.”
I told him I understood, and that it’s not just frustrating, a chain falling off and jamming can result in component or even frame damage. Here are a few tips about chains falling off and then the advice I gave my clubmate for figuring out and fixing his problem.
Basic things to know about chains falling off
For anyone who has never had a chain come off the chainring, you should know that it can happen on any bicycle whether you’re pedaling normally or backwards. In fact, it’s so likely – now that we’ve gone to wider and wider frame spacing and more and more gears – that many new bikes come with built-in chain-drop stoppers, called “chain catchers.” (If you want to add one to your bike, but don't want to spend much money, consider John Marsh's eArticle on how to make one yourself: How to Make a Chain Catcher.)
So, the most important thing to know is what to do if it happens, which is to stop pedaling immediately. As long as you stop spinning, you shouldn’t be able to damage your frame or components. Sometimes, such as when it happens on an uphill, in order to stop pedaling, you need to react quickly, get out of your pedals and jump off the bike or else you may fall over or crash.
If you don’t realize the chain has come off and you keep pedaling, especially with force, the chain can get badly jammed between the crankset and the frame or, worse, it can saw into the frame. On a steel frame, this might only trash the paint. On aluminum or carbon frames it can saw into and damage the frame. Depending on how the chain jams, keeping pedaling can also bend or even break the chain and/or bend the derailleurs and chainring(s).
Pedaling a dropped chain back on
In a best case, you won’t have to stop or get off your bike. You’ll only need to stop pedaling long enough to look down (careful – don’t run into anything) and make sure the chain is not jammed. If it’s not stuck, you can usually very gently pedal and shift the chain back onto the chainring.
Or, if you had to dismount to stop pedaling, find a small stick so you don’t get greasy and use it to lift the chain back onto the bottom of the chainring as you backpedal by hand. This is easier if someone lifts the bike off the ground so you can use both hands. Or find a tree and hang your bike on a low branch by the tip of the saddle.
Getting the drop on chain drop
Now, here’s how to go about fixing a recurring chain drop. These steps are in order of the most common things that can cause chain drop. Note that I’m assuming the front derailleur is adjusted correctly and it’s not throwing the chain.
Check the crankset and bottom bracket
If the crankset is loose, it can move sideways during pedaling, which can allow the chain to come off. So make sure the crankarms are securely attached and that there’s no side-to-side play in the crankarm or bottom bracket. Some bottom brackets can be tightened. Worn out ones will need to be replaced, but they’re not overly expensive.
Check the chainrings
If the chainrings wobble side to side, that can throw the chain off, too. Loose chainring bolts can cause this, so check all of them with the appropriate wrench to ensure they’re tight. Once you know the bolts are tight, check the chainrings to make sure they spin straight and true. Just sight from above as you turn the crank by hand. If there are wobbles, you should true the chainrings or have it done (you don’t need new chainrings).
Check the lower derailleur pulley
Sometimes the lower pulley (called the idler pulley) will be misaligned with the chain. Since its job is steering the chain correctly, when it gets bent, it can cause the chain to drop. The easiest way to see this problem is to stand behind the bicycle as someone else shifts through all the gears. The pulley should remain perfectly aligned with the chain. If the chain is trying to come off the side of the pulley in some gears (usually toward the outside shifting limits), gently bending the pulley back in line could solve your chain drop.
Check the chain length
If the chain is even slightly too short, it will be more likely to fall off because there’s not enough slack. Drivetrains need a little chain slack to operate smoothly and efficiently and provide a margin of error. You can tell if a chain is undersized for a bike by shifting onto the largest chainring and the largest cog. All bikes should be able to make this shift and in this extreme gear the derailleur should still appear “comfortable,” not stretched out to the max with the chain super taut.
Check the chainline
As one last drivetrain check, you can evaluate your bike’s chainline, which is the relationship of the crankset to the cassette. Just know that it’s not always essential for a bike to have a perfect drivetrain. And also, that it’s not always possible to achieve a perfect chainline on some bikes without changing out components you might not want to change.
All it takes to check the chainline is a straightedge that’s long enough (about 20 inches/50 cm) to reach from the crankset to the cassette. Ideally, you’ll find one that’s also thick enough to fit snugly between the chainrings because that makes the check easy. If your straightedge is a loose fit, you’ll need to ballpark the measurement or shim the straightedge on both sides.
To do the chainline check is as simple as putting the straightedge between the chainrings and letting the end rest on the cassette (see photo). On a perfect chainline, a straightedge will exactly bisect the cassette. If the straightedge is off by more than a cog in either direction, you can decide whether or not to look into solutions based on your shifting performance and any chain drop issues.
Jim Langley is RBR's Technical Editor. He has been a pro mechanic and cycling writer for more than 40 years. He's the author of Your Home Bicycle Workshop in the RBR eBookstore. Check out his "cycling aficionado" website at http://www.jimlangley.net, his Q&A blog and updates at Twitter. Jim's streak of consecutive cycling days has reached more than 8,000. Click to read Jim's full bio.