By Jim Langley
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.”
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.
Why do you dismiss checking the front derailleur alignment? This was my problem and has been fixed.
Operator error also, failing to trim when on big ring and near large cog lets the FD take a full swing at an enhanced angle can cause the chain to off
A bent derailleur is another common cause in my experience.
Sorry for the confusion, Frank. I didn’t actually “dismiss” front derailleur alignment. I wrote that I was providing tips based on the assumption that the front derailleur is properly adjusted. In other words, if your front derailleur is incorrectly adjusted and throwing the chain, it should be obvious to you what’s causing the chain to come off. I was focusing on things that are less obvious and need to be figured out. Hope that explains better. Thanks! Jim
Jim, how does checking your chainline translate to those of use with a triple up front?
Oops, that was an oversight on my part, Gregory. I meant to explain that in the story. To check chainline on a triple, you ballpark it by putting the straightedge between the small/medium chainrings followed by between the medium/large chainrings. On each straightedge position you note where the straightedge rests on the cassette and find the middle point. That middle point should be on the middle point of the cassette with a perfect chainline. It’s not as easy to see as with a double since you have to eyeball it, but it still is pretty easy to see if your chainline is off and by how much. Thanks for letting me know I forgot to explain that. Jim
Thanks Jim! – Greg
Largest chain flexes under load, causing chain to skip or fall off. Both sides ie when pedaling on left side, chain ring flexes left. Same with right. There does not seem to be any play in bottom bracket bearings.
Roy, have you checking your chainring bolts to make sure they’re tight. They loosen over time and when they’re loose, the chainrings will move the way yours do. Just go around and put a wrench in every bolt and see if any are loose. They should all be quite tight. The wrench won’t turn if the chainring bolts are correctly tightened (about 10Nm of torque). Hope that fixes it for you,
Jim
Great article. Thank you. My chain often fell off after I stopped the bike, after any backward movement of the pedals. In my experience, this can happen even if the drivetrain is set up perfectly with new parts. My mistake was that I was stopping the bike in the big/big (crosschained) gear combo. This is a combo that my particular bike can’t handle when the crank is turned backwards when stopped. Now, to prevent parking lot drops, I shift the cassette to a middle gear, before stopping the bike.
That’s a great tip, Ricky. Thanks for sharing!
Jim
On one of my bikes, the frame is too flexible relative to my weight/leg strength. I weigh 250 lbs. The frame twists enough to cause the chainring to be misaligned with the chain if I put too much force on the pedals.
Yes, I’ve seen this with forceful riders and pedaling. This is one of the reasons that custom framebuilders go to oversize designs to stiffen the frame and reduce the flex, Rich.
Thanks!
Jim
I have experienced this and after investigating all of the aforementioned possibilities finally zeroed in on the rear derailleur pulley spring being worn out. My rear derailleur wasn’t taking up the chain slack quickly enough.
I’m glad you identified the problem with your rear derailleur, Roger. Thanks for explaining how it caused the chain drop, too.
This could be the source of my problem. The chain drops once a ride when hitting bumps or changing gears. The system is a DI2 system that has a lot of rides and is 5 years old. I have checked for chain stretch as well.
On big bumps my chain sometimes drops to the small chainring.
Is this indicative of a too long chain?
Its a 34/43 x 32-11 8-speed setup.
Ps-no FD on bike. I just ride in the 43T ring
Jim, another reason for the rear derailleur pulley misalignment is the derailleur hanger being loose. I had this problem on a mtn. bike recently. The derailleur wasn’t wanting to shift correctly and the pulley (jockey wheel) wasn’t running true. As I was looking at the derailleur from behind I noticed the pulley was out of alignment. I thought the derailleur was bent. As I was starting to try and bend the rear derailleur I noticed the hanger moving. The screw holding the hanger in place was loose. I tightened the hanger screw and solved the shifting problem. While I didn’t have a chain drop problem at the time it defiantly was the cause of the rear derailleur hanger misalignment. which could cause chain drop problems. Even worse get into the spokes if it gets too loose.
One of several things I love about a 1X drive (single chain ring) is dropping a chain is a thing of the past.
I rarely drop a chain on my road bikes, but my gravel bike is another story. What I’ve learned is that on either type of bike, on rough surfaces, the chain comes off the BOTTOM of the inner chainring and any amount of backpedaling will cause it to come off completely and jam at the chain watcher in front of the front derailleur. IMO, chain watchers mounted in that location are largely unnecessary, as the front derailleur cage ensures that the chain stays on there, as long as it doesn’t come off the bottom and you don’t backpedal. What is actually needed is a chain watcher below the bottom bracket, so the chain cannot come off the top or bottom. These have been available for MTBs for years and I’m surprised that no one in the industry has recognized the need for them on road/gravel bikes.
I recently installed a rear derailleur with a clutch that limits chain bounce and prevents chain slap, and so far, that seems to have made a big difference. However, it also seems like overkill for a road bike.
Thank for this, just saved me a few quid. I had just fitted a new 11 – 40 cassette, and the chasin (new) kept coming off front chainring, when I shifted on to largest rear sprocket. Suspected I need longer cage on rear mehc, but then read you list, noticed worn front chainring, swapped out and smooth as silk. Thanks! Been building bikes for 32 years, just shows we are always learning.
I am 49 years old cyclist from Maryland. I had an accident last summer due to chain falling of the front chain ring. The chain got jammed in the front derailleur, and I flew over my handlebar broke 6 ribs, broke my right clavicle, got a hemothorax on my right lung, and scraped every joint on my right of body, and was hospitalized for almost a week. luckily I was only going 25 miles an hour a downhill.
After a few weeks of recovery I took my bike to a LBS, and I was told the rear derailleur spring was not working, and reason for chain slack and cause of accident. I replaced the rear derailleur, cassette, and chain to get back on my recovery rides at home. Fast forward 9 months and I am having similar issues, not sure if the front chain rings are worned out, When I stand on my pedals the chain keeps jumping on the rear derailleur or the front. It might be the front derailleur not properly aligned.
Due to this coronavirus horse chit I may have to wait a while and take it to a LBS certified by Shimano to make sure that this is the issue. I checked the bottom bracket, and everything is tight. I have been building bikes for many years, and I never had this issue with prior Shimano components, but I am not sure if Shimano Dura Ace 9000 have manufacturing defects.
Peace!
Ouch. I think for a confidence point of you view you should replace:
-cassette
-chain
-chain rings
Make sure they are all comptable, and the same brand components.
I find that a chain and cassette should be replaced together. Every time. And a front chain ring every other time, you replace chain and cassette.
If you are running dura-ace then you must have loads of money (!) so replace:
-cassette
-chain
-chain rings
and hopefully that will cure it.
I have found unexplained chain jumping worn chainrings, Some times it can just be luck, in that you heel kocks chain off, although I guess with clip pedals unlikely . . . .
Stiff links.
Claggy lube.
Double check all indexing.
Double check limiter screws, especially at max stretch.
Tricky one.
Hi,
I just tore up a front derailleur because my chain kept throwing off the outside. I thought it was the derailleur’s fault but riding it without (new one is ordered) it still happens. Only while shifting down cassette–into smaller sprockets–and only in the smaller half of the cassette. I checked all your suggestions, all ok. And I checked the der-hanger alignment and chain wear. Chain is almost abnormally un-worn–it still reads zero stretch after about a year’s riding. I corrected some looseness in the chain tensioning, the pivot thing. And chain length is correct, 110 links on short chain-stays. It’s Campy’s 12-speed Chorus and had been working flawlessly. It still seems to work fine now that I’ve gone through a complete tweaking. BUT it still throws–about every 10th time I run it up and down the cassette. Chain seems to hop cogs and ties itself in a knot.
Only odd thing I see is a lot of side-to-side flex in the chain. Seems like a lot but beats me. I hate to put a new front derailleur on if it’s only going to be ripped up again.
Sorry for such long wind….
Sorry, to hear.
After further investigation, my front chain ring was worned out and was causing the chain to jump off. So not only did I had to replace my rear derailleur but my front chain ring. Make sure your rear cassette is not worned out, this will cause misalignment issues. Make sure your front derailleur is installed between 1 – 3mm from top of chain ring, and proper tension is applied. Always set the front derailleur tension on the small chain ring.
Hi. I am NOT a cyclist. I don’t know about bicycles. I have a 20 year-old 6-speed beach cruiser. The chain never fell off in 20 years. I had the bicycle overhauled by a little old guy in my town and now the chain falls off every time I want to ride it. It’s a mess because he just greased everything. Taking it back to the guy is a pita & I already had to call him because he lowered the seat and ‘wove’ my bike lock into the seat springs, apparently on purpose. He’s in his 80s and Because of Covid he can’t come out when I’m there at his house. Is there something simple I can check? Thank you.
Did he replace the chain? If yes, then perhaps the chain wheel (cog near pedals) and freewheel (cogs at back) need replacing aswell.
If he didn’t replace the chain. Then perhaps he adjusted the rear mech?
Where does the chain fall off, at the front or the back?
Hi
I am a cyclist, but a mechanical dunce!
I have started to experience the situation that when I’m pedalling on the large chainring, in a middling gear, the chain suddenly drops to the smaller chainring. I can then adjust the gear and carry on, and within a mile or two can switch back to the large chainring in the normal way.
However, something is clearly out of alignment, stretched or bent. Any advice would be gratefully received!
Regards
Chris
Am running ultegra di 2 and changed the rear cassette from 11 28 to an 11 34. Is a long arm rear mech and doesnt catch on largest sprocket but am experiencing drop on front chain rings when shifting into 2nd largest or largest rear sproket. Have reset front changer twice and still getting the issue any suggestions?
2021 Diverge comp E5 1x drive with 500 miles on it.
Bike rode flawlessly for the first 500 miles then this morning my chain kept jumping off the front 1x drive to the inside when shifting to the bigger gears in the rear cassette.
Problem when cleaning chain and sprockets night before, chain came off I mismatched the chain links and chainring on a narrow wide narrow chainring. I have been road biking since April and did not know narrow wide narrow chainring was a thing…
Posted this just in case it helps someone.