Chain slipping off derailleur jockey Last week, I had to abandon a training ride due to my chain slipping off my rear derailleur jockey wheel. This would only happen when shifting, and most often when either shifting chainrings or shifting multiple cogs in one go. I took the bike in to my LBS, and they diagnosed it as a bent derailleur hanger — a reasonable guess, given I took a spill in a crit the previous week (although, curiously, I did not have any problems in the race between my wreck and the training ride). They bent the hanger back into place and sent me on my way.

Tonight, on my first ride after the fix, the problem was happening again. Same deal, but less frequent. Shifting multiple cogs, especially right before/after a chainring shift would reproduce the problem with a high probability. After getting home, I put my bike in my stand and checked the alignment with a derailleur alignment gauge. It's absolutely perfect. And yet a little shifting while in the stand and I was able to easily reproduce the problem.

To recap:


chain slips off bottom derailleur jockey wheel
perfect derailleur hanger alignment
only during "significant" shifts
not limited to shifting under load
SRAM Force components


Pics are below. Any ideas? If more pictures are necessary in order to diagnose the problem, I'll be happy to provide.

Edit: After discussion in the comments, the problem is almost certainly that the derailleur cage is twisted and misaligned. Time for a new derailleur.

Edit 2: New derailleur purchased. Problem solved.
This type of problem has 5 likely causes, listed in order of elimination. 


Bent dérailleur hanger, or Bent derailleur cage. (Your derailleur hanger looks straight, but the cage appears slightly twisted in the upper photo. Could be the angle of the shot, though.) Edit: This turned out to be the correct answer, after all.
Bent, twisted, or sticky chain link. 
Badly worn dérailleur pulley wheels. This means there is not enough tooth left to hold the chain on the pulley wheel. (In my opinion this is most likely, but I'd need a photo of the derailleur side on to confirm.)
Worn chain. A badly worn chain will slip off the pulleys in this manner, sometimes. Usually, you would also experience slow and noisy shifting in the rear gears. 
Worn cassette. Usually, you would also experience slow and noisy shifting in the rear gears, not just the pulley derailment, and usually it would be preceded by a worn chain. But it is possible, especially if the chain has recently been changed. 


You've checked the derailleur hanger, so #1 is less likely. Although I would look at the derailleur cage or body. It really does appear bent in the second photo, unless the derailment is putting outside pressure on it.

You would see and feel #2 on the repair stand, so I assume that is unlikely, although it's easy to check and eliminate.

'#3 is my guess, without a side photo to confirm it. This is a photo of the difference between worn and unworn pulleys. Keep in mind, this is an extreme difference, and yours don't need to look this bad to cause your problem. Edit: After new photos, there is some wear, but not enough to cause the issue.



A worn chain is easy to check. Use the Park tool gauge or a similar tool to measure the wear. On that gauge, to see this issue, I would expect close to 1mm of wear.

A worn cassette is usually preceded by a worn chain, so if your chain is badly worn, you may need to change the cassette as well. Worst case, you may need new chain rings as well, but that is less likely.

Edited to include troubleshooting steps and conclusion:

After talking through the issue with Stephen, and looking at new and more direct photos, it is clear that the derailleur cage or body is misaligned, as discussed in option #1. 

There is pulley wear as discussed in option #3, but not to a major degree. 

You can follow the logic in the comments below:


Derailleur hanger is confirmed straight by my own DAG. Cage might be twisted. I'll check, and upload a pic with a dead-on shot. It's carbon fiber, though — will CF bend? No sticky chain links. I'll put a new chain on to rule out #2 and #4, though. How would a worn cassette cause the issue, given that this is happening before the chain enters the cassette? – Stephen Touset 42 mins ago
A worn cassette can cause the chain to be out of line with the derailleur slightly. It's less likely, but possible. That's why it's at the end of the list. – zenbike♦ 34 mins ago
A CF derailleur cage isn't likely bent, but the derailleur body might be. In the photo, it looks twisted to the outside, so check whether the pulleys run parallel to the cogs when there is no chain on it. Only your outer plate in carbon.If the inner plate is bent or twisted, the CF plate has enough give to move with it. – zenbike♦ 32 mins ago
More pics uploaded. In the rear pic, the chain on the bottom derailleur looks a bit to the right, but this seems to be that the chain is just pushed all the way to one side of the teeth. There is about +/- 3˚–5˚ of play in the jockeys. Normal? – Stephen Touset 21 mins ago
Actually, the play is normal, especially in th upper pulley. But that derailleur cage is not aligned correctly. The cog of the gear it's in, and both pulleys should make a straight line, as perfectly vertical as possible. In that photo, ignoring the chain, the upper pulley is twisted slightly toward the frame, and the lower is even farther out of line. It is still possible that it is the angle of the photo, but it doesn't appear to be. You're also showing wear on the derailleur pulleys, if not nearly as bad as in the photo I posted. I'd say it's time for a new rear dérailleur. The cage itself should be parallel to the cogs, and that isn't the case either. – zenbike♦ 13 mins ago
One question: Have you recently disassembled the derailleur? Is there more float side to side in the lower pulley than in the upper? – zenbike♦ 8 mins ago
No to disassembly. Much more float in the upper pulley than the lower. Looks like you're right with the alignment. New derailleur time. Anything I can do in the meantime to minimize the chance of problems this weekend? As in, what should I try and bend to correct the misalignment as best I can? – Stephen Touset 8 mins ago
Ok. Well, you can try to bend the cage back in to alignment. If you take the photo you posted last, and lay a ruler through the cogs, it should pass through the points of the teeth of both pulleys. That's the goal. If its the derailleur body, trying to bend it back may make it worse, but given the problems you have, I'd try. Worst case, hit your LBS for a new one before the race. – zenbike♦ 3 mins ago
Thanks a bunch. If you'll edit your answer to include the latest conclusions, I'll be glad to accept. – Stephen Touset 1 min ago