Two weeks ago, I fell off my bike. Actually, I was violently thrown over the bars landing heavily on my right palm and elbow. The chain locked and pulled the rear wheel out of the drops. After an initial inspection, I could not understand the cause of the problem. The quick release may not have been tightened sufficiently but I could not determine the source of the problem.
This week, I dismantled the drive train and looked more carefully. The chain was worn so I decided to replace it and the freewheel. Once reassembled, I noticed a constant chattering noise coming front the rear derailleur. At first, I thought it may have been damaged or bent from the fall but it was correctly aligned. On closer inspection, I noticed the bottom jockey wheel was badly worn. And, worn in a manner I have never seen before. Each of the lugs had been worn to a point, not flat as I’ve seen before.
I have never seen this type of wear previously but understand from local mechanics (and a Google search) that this is the normal wear pattern caused by frequent shifting, particularly on commuter bikes that are ridden in the wetter, dirtier weather.
I can’t say for certain the worn chain and jockey wheel caused the chain to lock up. However, they are the only parts that were worn. And, they wore quickly, in less than a year. This is my commuter and, it is equipped with lower end parts. I roll with Dura-Ace and Ultegra components on the road bikes and Alivio on the Kuwahara commuter. Is this the reason for the quicker wear? Quality. Generally I need to replace the chains, cassettes, brake pads and tires each season. However, I have seldom needed to replace the jockey wheels.
Could this have caused the chain to lock up?
PedalWORKS 
Sounds reasonable to me, maybe the worn chain and pulleys together… I’ll try to get a very knowledgable friend on this one…
I would appreciate that. I don’t want another fall like that.
Already done brother. He goes by “Saltyvelo”, look for his comment in the near future. If you don’t hear from him by Tuesday let me know, I’ll go another route.
The wear on that jockey wheel is undubitedly a common wear pattern, as the chain “stretches” it will wear a cassette in the same manner. The wear isn’t a function of shifting in my mind, just poor maintenance – specifically, letting the chain run too long on a bike and/or not cleaning frequently. I’m guilty of it as well when it comes to my commuter – especially when it is winter with a cold garage and short days. I don’t think quality has much to do with it, as I would bet the jockey wheels are the same across all lines for shimano (I didn’t, however, research it).
Are you riding with the axle at the end of the horizontal dropouts? The wheel can be a pain in the butt with horizontal dropouts, but the clamping force should be tremendous. If you’re at “the end” of the dropouts, trying moving off it a bit to insure you have adequate clamping force from your skewer. If the run of the mill QR skewers are a pain for you to get on and off, try a DT swiss RWS. The force required to dislodge a properly clamped wheel in a horizontal dropout + rider weight would be tremendous. And scaring should be seen on the drop out itself, which I can’t see from the pictures.
My biggest concern would be clamping force of the wheel in the horizontal dropout. Outside of that, yes it is possible if the chain gets caught in the RD you can “pedal” the wheel out. Although, again, very unlikely IF the wheel is adequately placed in the dropouts. If the wheel doesn’t come off any other problem will most likely be less catastrophic.
I must admit that I do not clean the chain on the commuter as often as the road bikes. Clearly the rapid wear on both the chain and jockey wheels was the result.
Just before the fall, I replaced the tires on the bike and perhaps did not fasten the rear wheel properly. I can’t say for sure but am usually careful about this making certain the axle is well seated in the drop outs. However, I will check all bikes to make certain this does not happen again.
Thank you.