Wheelsmart

Hello Good Friends,

My chain has started jamming next to the derailleur jockey wheel. This happens to the jockey wheel closest to the ground and occurs with increasing frequency when I shift in the middle of the rear cassette. The chain jams to the left, the side closest to the rear wheel.

I took it to the LBS, and they shortened the chain and replaced the jockey wheel to the tune of $60. It didn't solve the problem. I don't have it in the budget to go back to the shop, but I need to fix this before I race this Sunday.

Any idea what could be causing this?

Thank you!

FBinNY 

Generally when I see a chain coming off the side off the lower pulley, my first is always a bent or twisted hanger. Of course it could be a bent cage.

Either puts the idler pulley out of line with the chain coming to it, and increases the chance of the chain running over one side or the other. If it's a hanger issue, it could be that the lower pulley is inboard or outboard of the one above so the chain angle is more than normal. A twisted hanger causes the same problem in a slightly different way. Here the chain angle is OK, bit the pulley is not aligned to receive it.

I'm disappointed that a shop took $60.00 from you without properly diagnosing and solving the problem. Shift to a gear combination that tends to derail. Then bend down behind the bike and sight up the chain, and see if you can confirm the mis-alignment. If the RD looks OK, then have a different shop check the hanger (a $10-15 job).

Also you might consider a pulley with larger teeth. These maintain a firmer hold on chains by virtue of engaging them deeper. The other thing that helps is your choice of your chain. Some chains have plates wider than the roller between them, while others have plates just about the same size as the rollers. The larger plates help keep chains on by mimicking the action of a V-pulley, catching the teeth of the idler wheel and guiding it in.

gforeman 

After paying $60 and not fixing it, I would certainly go back to said LBS and give them a chance to correct it. Tell them you feel you should not have to pay again.

At least give them a chance. You already paid them.

FBinNY 

I agree, but if someone came in with a specific problem, and all the mechanic could come up with was to replace a part and charge serious dough to do so, why would you expect him to suddenly have an epiphany and now know how to fix it?

I tend to forgive mechanics who miss something, but not if it's what they were specifically asked to fix. I'd feel differently if it were a subtle problem, and the mechanic said, "I think I've got it, but let me know if not and I'll try something else"

cny-bikeman

Agreed the shop did a poor job of diagnosis, more likely a result of guessing rather than logic. In addition it should have been test ridden, especially in the problem combos, before being released to you.

Shortening chain - The assumption is that the chain is loose, allowing it to drift off the pulley. But then the problem should be much worse in the smallest rear cogs, and on the small chainwheel no matter what rear cog. This is also not a common problem, in spite of the fact that many people still ride in the small/small combo (the worst chain deflection and lowest tension) and on worn and low quality chains.

Replace pulley - Because the pulley is worn, or wobbles too much? Why would it unship only on one side?

I agree that the problem lies in the hanger or the derailleur pulley being misaligned. Actually, in many cases a good mechanic can spot an off-kilter pulley assembly by simply pulling back on the derailleur and watching for the pulley to go non-parallel as it rotates. The shop should have checked hanger alignment as a matter of course with this type of problem, Then, if still not fixed put on a different derailleur to see if it fixes the problem.

OP: Three pieces of info would be helpful - questions that the mechanic should have asked as well:

1. What front/rear combos does this most frequently occur? "...increasing frequency when I shift in the middle of the rear cassette." is not clear to me. Does that mean towards the middle or from the middle, and in what direction?
2. Does it occur during riding in one gear or near a the time of a shift in gears?
3. When it started was there any equipment change, accident or other event that immediately preceeded it?

p.s - To other posters: the final question is one that provides critical information no matter what problem you are having. I've seen a few instances recently where only after several exchanges the OP mentions - "Well, I did have a fall just before all this started."

Wheelsmart

These are all extremely helpful responses. Thank you very much. Here is some further information.

1. I experimented on the trainer, and can say that the chain comes off the jockey wheel most readily when shifting from the big ring to the little ring.

2. After reading your posts, I have determined that there is some misalignment with the two jockey wheels. When I turn the wheel backward, you can see the chain shift over to the right every couple of links or so. Just by eye you can see that the lower pulley is off to the right by one or two mm. This is what must make the chain pull off to the left.

3. I can't think of anything that caused this misalignment. The problem first presented when I was learning to adjust the derailleur on a different wheel, and was changing the tension kind of wildly. I wonder if that somehow forced the chain off once, and the first incidence caused the pulley to be skewed, resulting in the problem repeating? The cage does not appear crooked.

5. What to do next. I do not see an easy way to realign the pulleys with the cage. Bending the cage by hand yields little result, and I am a little afraid of breaking it where it connects to the rest of the derailleur. The shop told that the cage cannot be replaced separately, and I simply cannot afford a new derailleur right now! If this snags during the race it will be bad news. Any ideas?!

5. The interaction with the bike shop. The lower pulley was $25. She replaced it because it looked a little worn, thinking that might be a contributor. Looks like that was not the issue at all. She also tried to realign the cage in case that was an issue, and while we were there, the problem seemed to be solved. It was the shifting from the big to little ring that was not tested at the shop. I was charged $40 for the labor, which included a derailleur readjustment. That seemed like a steep price, but she spent the better part of 45 minutes troubleshooting. A more experienced mechanic like you guys would not have needed to take all that time. At least I received a 20% discount because I race on their team.

I appreciate the sage advice that I am receiving from you all.

cny-bikeman

Although it was considerate of you to post a picture there is no substitute for a trained eye, in person. The wrong perspective or camera angle can make things seem different than what they are. I can't tell anything for sure from the picture, especially as the camera's pointed downward. If looking at it in person I could determine the problem within minutes.

In my experience it is a matter of luck to be able to correct a pulley misalignment if it is indeed in the derailleur. But again, the FIRST step is to have the hanger alignment checked. Only then will you know if derailleur replacement is even necessary. In fact one may make the situation worse if you try to "straighten" the derailluer when in fact it is the hanger that is causing the problem.

As to cause, it is possible for the cage to be pulled out of line by the chain, but it would have had to have been a pretty bad tangle - one you would remember.

Wheelsmart

I went back to the shop today with a lot more confidence after the advice that I received here and very generous help that I received from an offline reply. I told them that they should have checked the hanger alignment first, and that the pulley probably never had to be changed. When I showed them the misaligned pulley, I was first told that I might need to replace the derailleur. I explained that I was committed to fixing it and that a replacement was not an option at this time. Sure enough, the hanger was misaligned. But that was not all--the pulley was still off to the right--having been damaged by the chain jam. At this point, a more experienced mechanic overheard my passionate and--thanks to you all--intelligent explanation of what had to be done. He stepped in and replaced the derailleur cage and pulley with no additional charge. Finally I feel like the shop did me right. I have learned a lot about repairs from this experience.

FBinNY 

As Mr. Shakespeare said "All's well that ends well"

gforeman 

Perfect, that is why I suggested the return to the shop. Most (Good) shops want to keep a good reputation. Glad it all worked out.

zonatandem

Customer satisfgaction . . .

GreasyBici

I just fixed my friend's bike, who was having issues with the chain falling off the jockey wheel while rear shifting. His problem was related to the rear derailleur not maintaining proper chain tension, and it was fixed by an adjustment of the B-screw, also known as body gap screw, which is the third limit screw that pushes on the derailleur hanger and adjusts the gap between the guide wheel of the derailleur and the teeth of the cassette. On my friend's bike, this screw came out too far resulting in lack of consistent chain tension. If you unscrew the b-screw all the way you can actually make the chain go totally slack, hard to demonstrate without a video. When there is enough slack in the chain, it doesn't get held on the guide/jockey wheel tightly enough to stay put and the chain falls off. His bike is a 1x, but I bet the problem could also occur with shifting a standard 2x or 3x front crank. Anyway, we tightened the b-screw just by 1 turn and you could see the tension returning to the chain and derailleur hanger and voila the bike was fixed! It would make sense that you would run into problems on the small chainring in the front since this would cause the rear derailleur hanger to be in its most "loose" position.

I've seen numerous comments that the above issue could be due to a bent rear derailleur hanger or cage. Speaking from experience, I have ridden many bikes with significantly bent derailleur hangers and cages and never had an issue with the chain falling off the guide/jockey wheels (albeit the shifting was garbage), presumably because when the chain tension is appropriate, the chain is not given the opportunity to jump off. Not saying the cause of your woes cannot be due to a bent hanger/cage, but based on my experience I think the rear derailleur cage/hanger would have to be so far bent that the defect would be obvious on visual inspection. The reason a lot of people think this problem is because of a broken rear derailleur is because the problem will go away when you install a new derailleur and adjust the new derailleur's b-screw to the proper position.

Other possibilities to explain why a chain might be falling off guide/jockey wheels:

1. Chain line issues- I've worked on some bikes with chain line issues that were caused by incorrect bottom bracket/crankset combinations. This manifested primarily as sloppy shifting, and the chain would also fall off the crankset when pedaling backwards quickly. Theoretically this could also cause the chain to fall off the derailleur wheels. If this were your problem, your bike would have been performing poorly ever since owning it, not an acquired problem.
2. Bent, twisted, or sticky chain link.
3. Badly worn dérailleur pulley wheels. This means there is not enough tooth left to hold the chain on the pulley wheel.
4. Worn chain. A badly worn chain will slip off the pulleys in this manner, sometimes. I imagine the chain would be so badly worn it would be jumping off the cassette under heavy pedaling. A mildly worn chain/cassette should not cause this problem.

Hope this helps!

FBinNY 

It's been 10 years.

I think it's safe to assume the OP's bike is fixed by now.

maddog34

oh cool.. a ZOMBIE Thread, just in time for HALLOWEEN........ :-D