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.

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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    We need a photo - ideally of the chain in the bad position. I know the little stub in the derailleur, its dead-simple to install a chain on the wrong side by accident, bu that's not your problem. – Criggie Jan 17 at 1:09
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    @Criggie Sorry for the delay. I uploaded some photos for you to check out. – M Dabbs Jan 31 at 19:44
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    Your chain is improperly routed. No way it got that way while riding. You could force it that way by removing the screw holding the bottom idler and prying the cage open, but then the screw and idler would almost certainly go walkabout. – Daniel R Hicks Jan 31 at 21:17
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    @Criggie No. I use a park tools chain clean machine thing. – M Dabbs Feb 1 at 16:10
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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 at 16:16

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 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 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 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 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 at 21:37

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