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During my ride yesterday, I've hit a really weird mechanical and would love to understand. So here's the situation - I'm riding along, start on a hill, stand up and push some power, suddenly hear noise from my chain and I can't pedal anymore. I stopped and here's what I saw:

chain out of derailleur and wrapped around itself

After taking the chain off there is a clear damage to the lower sprocket:

Close-up of the tension (lower) pulley of the rear derailleur showing a deep scratch with a raised burr between two of the teeth.

After putting on a new chain it's skipping because of the tooth is bent out of alignment.

So my questions:

  1. how it's even possible
  2. did I do anything wrong that caused it, or am I just super unlucky?
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    Isn’t there a tap at the rear of the cage which should prevent the chain from coming out? Did it break off? Did you install the chain incorrectly, running over the tap instead of under it? – Michael Jun 14 at 5:29
  • We’ve had the exact same question before somewhere. Chain magically made its way outside of the cage. I blame quantum effects :) – MaplePanda Jun 14 at 8:23
  • My kids were seemingly able to do this almost at will in their younger days :/ – Alan B Jun 14 at 10:49
  • @Michael : Indeed, something looks wrong with the routing of the chain. Unless it bent (spread) the cage, it should never have come out at the rear of the cage between both jockey wheels. The tab in the middle of the cage precisely prevents that. It would be interesting to know whether it is still there. – Carel Jun 14 at 15:22
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    The tab was still there, but the cage was visibly spread out towards the bottom. – MarcinJuraszek Jun 14 at 15:59
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Looks fixable - take the lower jockey wheel off (3mm allen) and clean it thoroughly by scratchng the dirt off (most satisfying) and then with something liquid (hot water+dishwash or some solvent)

Use a file or sharp knife to cut away the raised burr. Don't worry about any recess/scratch. Force the bent tooth back into line - if it snaps off then that should still work OK (jockey wheel teeth don't do a lot). Then lube both sides of the bushing and under the side plates, and refit.

Or you can buy a replacement jockey wheel - they're not too expensive, or salvage one off a spare/parts derailleur.

  1. How? Looks like something got stuck between chain and lower jockey wheel, and was "eaten" so it forced the chain out. Possibly a stick, maybe a stone.
    Another possibility is the derailleur caught the wheel, though this tends to be a lot more destructive. The lower jockey wheel can't reach the cassette to collide there.

  2. Super unlucky. This kind of thing is not impossible, but rare.


The question should also be "Is the derailleur cage damaged/bent" and "has the frame's hanger been bent?" either of which will make indexing harder. But you likely won't be able to tell until the jockey wheel is cleaned up.


Coincidentally I had an almost identical problem yesterday. In my case the top jockey wheel bolt had come unthreaded, dropping the left side plate. This allowed the jockey wheel to fall off and become lost. The bolt was still there - (held purely by gravity!)

The bike still shifted, except it was clattery and kept jamming the chain.

Did you loose your top jockey wheel too? I can't see it in the photo.

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    Unlucky, but also lucky. This sort of thing can often put the derailleur into the wheel which is much more expensive. Also check the cage for damage. I'd be a little surprised if nothing was bent, possibly just the tab that should stop the chain coming out – Chris H Jun 13 at 20:41
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    This is super interesting. Thanks for your answer! I decided to replace the entire derailleur to get the bike back to a working condition quickly, and the shop will try to fix up the broken one later (everybody around here seems to be super backup up and schedules these kind of things weeks/months out, I'm thankful they managed to squeeze the replacement in today). If they manage to fix it up I'll keep it as backup or at least a source of spare parts for the future. – MarcinJuraszek Jun 14 at 1:16
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    Also, there was no damage to the wheel. So I guess based on your answer (and Chris' comment) I should be thankful a broken derailleur and chain (which was closing on 1.2k miles and I was going to replace it in a couple weeks anyway) are the only issues I have to deal with. – MarcinJuraszek Jun 14 at 1:17
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I would not be surprised if the skipping problem is partly due to the derailleur hanger being out of alignment. See the use of the Park DAG-1 derailleur alignment tool.

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