What is going on with my derailleur?

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.

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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.

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.

@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.

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