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I was on a ride a few months ago and while shifting, my chain came out of my derailleur. So this is a little confusing. Let me explain. The chain was resting on the outside of the bottom part of the rear derailleur cage. There is a little silver cylinder behind the lower jockey wheel that touches but doesn't actually connect the two sides of the cage. Somehow my chain go pushed to the outside of that. When it happened it felt like the chain had gotten shifted off and my cranks jammed up. I stopped pedaling as quick as I could.

Anyways, to get home I had to take out the bolt that secures the bottom jockey wheel in the cage, and open the cage up to get the chain back on the correct side of the silver cylinder. I put the jockey wheel back in with the chain in place and limped home.

I took the bike to the LBS and explained the problem. They didn't know what might have caused it. They checked the hanger, and it was a little out of alignment, but they said it wasn't that bad and fixed it. I also asked them to check that the derailleur itself wasn't bent, but who knows if they actually did that.

Flash forward to yesterday and the same thing happened. I fixed it the same way and limped home again. I've never seen this before and definitely don't want it to happen when I am sprinting.

A few things:

  • My derailleur is an 11 speed Dura-Ace.
  • Chain is not worn/bent/no frozen links
  • Cassette is near new
  • I was shifting both times, but not putting a lot of power down.
  • Hanger should have been OK.
  • The bike has less than 1,000 miles on it
  • I didn't notice any wear on the bolt or threads and the bolt was tight as could be with locktite in the threads both times

Any idea what might be going on/how to fix it?

Photo of hanger and derailleur

Photo one of what happened.

Photo two of what happened.

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    I know the little stub in the derailleur, its dead-simple to install a chain on the wrong side by accident, but that's not your problem.
    – Criggie ♦
    Jan 17, 2020 at 1:09
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    @Maarten-Monicaforpresident The bike has less than 1,000 miles on it. I didn't notice any wear on the bolt or threads and the bolt was tight as could be with locktite in the threads both times.
    – M Dabbs
    Jan 31, 2020 at 19:45
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    @DanielRHicks Yeah. It got that way while riding. Twice. I am an experienced rider who used to wrench on bikes at a shop. This is the first time I have seen this and why I am on the internet instead of my bike. I was unable to get the chain back into proper routing without taking the bolt out of the idler. By doing this I was able to pry the cage apart enough to get the chain past that silver cylinder. The cylinder is not actually connected to the front of the cage.
    – M Dabbs
    Feb 1, 2020 at 16:16
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    Are you getting chain suck (due to worn cassette/chainring), causing excess slack in your chain at odd times? Is the bolt holding your lower jockey wheel on tight and not slipping? Can you force the bottom open where the guard (cylinder) is at all by hand or is there a bit of play?
    – Benzo
    Feb 6, 2020 at 15:40
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    Do you have a GoPro or something? You could try making a mount that allows you to record the derailleur cage to see what’s going on.
    – MaplePanda
    Aug 26, 2020 at 19:34

3 Answers 3

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I see no way for the pictured setup to occur while riding.

  • The silver bushing is bolted or rivetted to the cage plates
  • Cage plates are contiguous plates of steel with no gaps
  • Lower jockey wheel is also bolted to chain plates.

These three items form a box through which the chain runs. No amount of folding or flipping the chain would get it out of that box.

The only options I can think of are

  1. Someone misassembled the chain in the first place - Plausible because the chain is low tension at that point, and you may not notice the clattering. I've personally ridden with a group ride up a gravel track and someone else only noticed the same problem halfway up (hi Brendon!)
    Notice the scarring on that bushing - the chain has been running on it for a while.
  2. Someone has fiddled with your chain while the bike is out of your sight. Unlikely because its a dirty business messing with chains and that's a long way to go for a joke

If your chain has a quicklink then fixing is easy and can be done roadsize with pliers, or some people can split them by hand.

Opinion If the bushing is only attached on one side. then its not doing you a lot of good. Consider removing it completely and see how things go. Store the small part, in case it gets worse and you need to refit it later.

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    Could also be corrected by removing the screw holding the bottom idler and loosening the one holding the top idler. This would produce enough slack to pry open the cage and slip the chain inside.
    – Daniel R Hicks
    Jan 31, 2020 at 21:18
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    @DanielRHicks True, but I'd hate to have those screws not get torqued properly and work their way loose after a while - that would be a nasty surprise, likely to result in the dreaded call of shame. Or worse. Better to break and rejoin the chain - I'm pretty sure even Shimano went to quick links for 11 speed, so it's easy.
    – Andrew Henle
    Feb 1, 2020 at 1:58
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    On a DuraAce 11 speed derailleur (at least this one) the silver bushing is NOT connected to the front of the cage. It is riveted to the back and only touches the front. Like I said, I had to take the bolt out of the bottom idler to get the chain back in place, both times. I am a freak bout keeping my bike silent, and I am an experienced rider. So, it wasn't like this and I didn't notice. And unless my wife is out in the garage wrenching on my bike, no one messed with it, as that is the only time I leave my bike alone.
    – M Dabbs
    Feb 1, 2020 at 16:40
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    I have done a cursory check for frozen links, but didn't know that Shimano moved to quick links on 11 speed, so I will check that out. Really the only way I could imagine it might have happened was a rock getting between the chain and idler and forcing the chain outside the silver bushing. But this happening twice seems pretty unlikely.
    – M Dabbs
    Feb 1, 2020 at 16:42
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    If The silver bushing is bolted or rivetted to the cage plates is incorrect, then that could explain a path for the chain to get out, but it would need to flex the whole cage, and apply a force to the chain.
    – Criggie ♦
    Feb 1, 2020 at 21:37
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@mdabbs Since this happened to me several times now, and the only answer I got was 'you have probably mounted it wrong' :-) I decided to figure out how it could happen. The reason was a lot simpler than expected. My chain was a bit too short, so when I am on the big chainring and switching gears down to the next-biggest or biggest cog on the cassette, the derailleur gets stretched out so much that the chain is pushed out of the cage when pushing the pedals.

The solution is simply to have the chain correctly sized.

Hope this helps you as well. :-)

To explain how this happens, I have attached two images.

First one is from the side when the chain is too short, and the chain is about to climb up on the largest cog on the cassette. Notice how the chain strains the pulley wheels and if the derailleur will be stretched more by the chain, the chain will start to push downward to the pin at the bottom of the cage. enter image description here

And second image, on the derailleur when the chain is streched even more, so it starts to push itself out of the cage. enter image description here

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    Really the chain should be correctly sized so that the big chainring and largest sprocket can be selected without over-extending the derailleur, but without creating too much slack. There are guides available that show proper chain length sizing
    – Argenti Apparatus
    Sep 15, 2020 at 22:04
  • Good idea - Sounds plausible, but the photos suggest OP's chain is perhaps long, not short. Not sure how this could happen in this case. Also, OP's little tang is right on the corner which shouldn't be tensioned that direction.
    – Criggie ♦
    Sep 15, 2020 at 22:12
  • @ArgentiApparatus yes, ofc that's what I meant. (editing the answer now). A correctly sized chain will avoid this problem... but it was an aha-moment when I realized that this mysterious event was caused by the chain being too short.
    – tiptoptoppen
    Sep 17, 2020 at 10:54
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    Good attempt to answer a very difficult question. However, it’s still not clear to me how the chain could jump outside the tang (or whatever we call it, there’s no formal name) on the RD cage because of its length or the RD position.
    – Weiwen Ng
    Sep 17, 2020 at 11:09
  • That edit is a good improvement.
    – Criggie ♦
    Sep 17, 2020 at 11:49
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I had one of those bolts that hold the jockey strip out before. The bolt screws into a tapped hole in the inner cage. If the lower bolt's threads were stripped, then the bolt could work loose - the cage would no longer be attached at that lower point - but maybe not so loose that the bolt and jockey fall out.

During shifting, the cages could become misaligned (think of the outer one twisting and the inner one not), the jockey staying intact, riding on that loose bolt. If the misalignment was enough (say 1/4" or so), the chain could escape the cage and find its way outside that retaining nub (particularly if there were some stiff links).

Then, the cages (somehow) re-align themselves before the rider has a chance to see how they've accomplished this magic trick.

I'd closely inspect that jockey bolt. Maybe there's enough grit and loc-tite in there to make it appear tight but its not. Removing the retaining nub altogether would solve the problem if it happens again, at least until the nut completely works its way out and the jockey escapes, then you're walking home.

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