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Chain coming off lower derailleur pulley

Old 11-22-23, 05:11 PM
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Chain coming off lower derailleur pulley

This is new, after nearly 10 years of flawless shifting. Nothing obvious has changed on the bike. Di2 Ultegra 2X FWIW.

Today while riding I suddenly felt some resistance while shifting, and immediately stopped. Eventually I figured out the chain was coming off the lower derailleur pulley while in the smallest cassette cog. This seems to be connected to shifting the front derailleur as well.

I got home and checked the derailleur hanger. It was very slightly off, but nothing like it has been in the past when I didn't have this problem. I carefully re-aligned it. I tightened the B-screw slightly, and put on a fresh chain. The derailleur pulleys both turn freely, have no crap on them, and despite being almost 10 years old, don't appear worn or damaged. The lower pulley does have some lateral play. The bolt is tight. My wife's GRX Di2 derailleur pulleys do not have that play, but my kid's mechanical Ultegra does, so I don't really know what to conclude.

Any suggestions for what else to try?
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Old 11-22-23, 05:14 PM
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Replacing the pulleys would be the obvious solution. You could take that bottom one out and see what it looks like inside, but it sounds worn out.
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Old 11-22-23, 05:22 PM
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To begin with, the bias is for chains to stay on. The pulley's teeth keep it on the same way sprocket teeth do.

So, chains generally come off when they feed from an angle bad enough to overcome the natural bias.

So, consider and check the following.

Twisted cage so pulley is out of plane, bent or twisted hanger or cage so the pulley is off plane. Worn pulley teeth not engaging deep enough to retain chain.

Stiff or frozen pulley bearing preventing smooth action.

Other than checking the hanger, there's no definitive test. You need to eyeball the alignment of chain and pulley, then work back to see the issue.
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Old 11-22-23, 05:43 PM
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There will be some lateral play in the lower pully with the road systems. Not sure why GRX doesn't have a floating pully. As long as the play is not excessive, more than a couple mm of side to side, and the teeth aren't worn badly, then the pully is ok.

How is everything with the new chain on?
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Old 11-22-23, 07:50 PM
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Originally Posted by frdfandc
How is everything with the new chain on?
Too soon to declare victory, but I did several things at once and no longer have the issue occurring on the bike stand.

(I switched out the chain -- I have two waxed ones in rotation, I straightened the very slightly bent derailleur hanger, I slightly tightened the B-screw, and I adjusted the front derailleur limits.)

Basically, nothing significant has changed in almost 10 years, so either something is worn, or something came out of adjustment. The derailleur cage by eye looks intact. Pulleys are clean and rotate smoothly, no obvious wear on their teeth, nothing obviously bent or askew, bike hasn't been crashed and has not been out of my possession.
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Old 11-22-23, 10:00 PM
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Chain tension or slack was my first thought. I'd tighten the cage pivot spring a little if possible on this derailleur. You went to the B-screw. Does the same thing but also changes the relationship between the derailleur and the cassette which may or may not be for the better.

The old SunTour derailleurs had the outboard (in relation to the bike; deep in the pivot housing) pivot spring sitting in one of several holes. To tighten, you just lifted the spring (after taking off the cage) and rotated it slightly to a tighter setting. While I was in there, I'd also clean and lube. This was about a once every 10 years operation. FB's right about the pulley but I'm not sure I've ever seen one get to the point of shedding chains.
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Old 11-23-23, 02:40 AM
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I think I will replace the pulleys since they are $20, but they look pristine.

The newer Ultegra Di2 derailleurs are set to prevent small-small cross-chaining. Maybe I just discovered why.
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Old 11-23-23, 05:47 AM
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Originally Posted by Polaris OBark
I think I will replace the pulleys since they are $20, but they look pristine.

The newer Ultegra Di2 derailleurs are set to prevent small-small cross-chaining. Maybe I just discovered why.
I was going to say that’s best avoided but not sure it would cause this problem … but maybe amplifies the effect of a knackered bearing on the pulley wheel.

Personally I never use the bottom half of the cassette with the small chainring (except those times I may have forgotten I was still in it )
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Old 11-23-23, 09:28 AM
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Originally Posted by Polaris OBark
I think I will replace the pulleys since they are $20, but they look pristine.

The newer Ultegra Di2 derailleurs are set to prevent small-small cross-chaining. Maybe I just discovered why.
Smart move. It's cheap and can't hurt anything. I have a set of pulleys on my workbench in case I ever start having similar problems with my main bike.
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Old 11-23-23, 09:36 AM
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Confession: when this started happening on a bike 30 years ago or so, I got annoyed with the unproductive troubleshooting, grabbed a pair of channel lock pliers, and gave the bottom of the cage a little twist. Not recommended, but the symptom never recurred.
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Old 11-23-23, 09:57 AM
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Originally Posted by Kontact
Replacing the pulleys would be the obvious solution.
While the OP didn't say so, I expect they were in the small-small combination. Given all the other reasons to not use this gear combination, if it is only in this combo that there is a problem, stay out of this combo. As to causes, I would point the finger at old age/worn out components.
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Old 11-23-23, 10:48 AM
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Originally Posted by KerryIrons
While the OP didn't say so, I expect they were in the small-small combination. Given all the other reasons to not use this gear combination, if it is only in this combo that there is a problem, stay out of this combo. As to causes, I would point the finger at old age/worn out components.
DI2 locks you out of small/small.
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Old 11-23-23, 10:53 AM
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Originally Posted by Trakhak
Confession: when this started happening on a bike 30 years ago or so, I got annoyed with the unproductive troubleshooting, grabbed a pair of channel lock pliers, and gave the bottom of the cage a little twist. Not recommended, but the symptom never recurred.
As the derailleur wears and the various pivots get sloppy, there is likely to be a tendency for chain and internal tension to make the parallelogram twist, which would move the bottom jockey cage to the left. I have never seen a derailleur that wanted to swing to the right.

So you may have fixed the problem by adapting the cage to the new derailleur body angle.
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Old 11-23-23, 11:39 AM
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Originally Posted by Kontact
DI2 locks you out of small/small.
Mine doesn't. It is the first generation 11 speed (hence my comment above). The battery firmware doesn't allow me to re-program it, and since the battery is only 10 years old, I hate to replace it prematurely.

But even if that is the problem, it doesn't explain why I haven't had this problem for almost 10 years. There is probably a wear contribution, or something else out of adjustment, as well. (I did have to readjust the front derailleur.)
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Old 11-23-23, 11:57 AM
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Originally Posted by KerryIrons
While the OP didn't say so, I expect they were in the small-small combination. Given all the other reasons to not use this gear combination, if it is only in this combo that there is a problem, stay out of this combo. As to causes, I would point the finger at old age/worn out components.
When it happened, I was in the large front, small rear, and down-shifting the front without downshifting the rear first (something that would happen automagically with later versions of Di2).
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Old 11-23-23, 12:32 PM
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Would there be any advantage (or disadvantage) to using the 11-speed Dura Ace instead of Ultegra? (BTD has them on sale for the list price of the Ultegra.)
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Old 11-23-23, 12:45 PM
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Originally Posted by Polaris OBark
Mine doesn't. It is the first generation 11 speed (hence my comment above). The battery firmware doesn't allow me to re-program it, and since the battery is only 10 years old, I hate to replace it prematurely.

But even if that is the problem, it doesn't explain why I haven't had this problem for almost 10 years. There is probably a wear contribution, or something else out of adjustment, as well. (I did have to readjust the front derailleur.)
Excuse me. I had misremembered that the block started with Ultegra.

Something happened to your derailleur or hanger. Either the parallelogram pivot wear, jockey wear added up to a change in geometry severe enough to become a problem, or the hanger and/or jockey cage are no longer straight because of some hickup in the chain you didn't notice while out of the saddle.

After a decade, new pulleys means a $2 a year investment in upkeep. So consider doing that, making sure the hanger is actually straight in all planes and then assess what sort of tweaking on the pulley cage will get the bottom pulley running in line. That's all you can do, short or replacing the derailleur.
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Old 11-23-23, 02:01 PM
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Yeah, the first release of the Ultegra 11-speed disc Di2 is the outlier for this behavior.

I have to confess I cross-chain small/small quite a bit more than I should.

Bike Tires Direct has the Ultegra pulleys for $15 on sale, so I have ordered those.

I probably should get a backup pair of GRX Di2 derailleurs before they stop making them.

Last edited by Polaris OBark; 11-24-23 at 09:59 AM. Reason: typo, clarity
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Old 11-25-23, 09:00 AM
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re pulling jockey wheels off and cleaning up the bearing---I know I've asked this before, but forget the answer--I've tended to put a thin layer of grease on after cleaning everything up (whatever shop grease I have, Phils or Parks).
Would a thicker or thinner chain lube be better?

thanks
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Old 11-25-23, 10:44 AM
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Originally Posted by Polaris OBark
When it happened, I was in the large front, small rear, and down-shifting the front without downshifting the rear first (something that would happen automagically with later versions of Di2).
OK I have to ask - why? I don't think I would ever do that - I never drop to the small ring until I'm maybe at the 2nd biggest cog or 3rd at most. That's a huge clunky jump in ratio, under a lot of torque potentially in the highest gear, where you could just slide it up 3 cogs instead, possibly even with a few rotations between each shift to smooth it out.
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Old 11-25-23, 12:18 PM
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Originally Posted by choddo
OK I have to ask - why? I don't think I would ever do that - I never drop to the small ring until I'm maybe at the 2nd biggest cog or 3rd at most. That's a huge clunky jump in ratio, under a lot of torque potentially in the highest gear, where you could just slide it up 3 cogs instead, possibly even with a few rotations between each shift to smooth it out.
Have you ever cut yourself with a knife or dropped something on your foot?
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Old 11-25-23, 12:21 PM
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Originally Posted by djb
re pulling jockey wheels off and cleaning up the bearing---I know I've asked this before, but forget the answer--I've tended to put a thin layer of grease on after cleaning everything up (whatever shop grease I have, Phils or Parks).
Would a thicker or thinner chain lube be better?

thanks
It looks reasonably clean. I use wax. It is possible the chain is a bit stiffer as a consequence, but this happened after a few hundred miles, and I just put a stiff, freshly waxed chain on, and am not reproducing the problem. The pulley turns freely. I have not lubed it, or the other one (which is the sealed bearing one), and both rotate freely.
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Old 11-25-23, 06:54 PM
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Originally Posted by Polaris OBark
When it happened, I was in the large front, small rear, and down-shifting the front without downshifting the rear first (something that would happen automagically with later versions of Di2).
My 2014 Di2 6850 worked okay when completely cross chained, except for the chain noise where it ticked against the big ring pickup rivets on the side of the big ring. I did this probably once every 2 or 3 rides, not keeping track of which cog I was in. The noise was annoying, and I'd soon shift away.

I probably never shifted the front with the rear still in the 11 cog -- shifting the rear is the usual thing to do.

A sudden slack chain, and a road bump or vibration at the same time might throw the chain off the pulley? Do you lube the derailleur pivots and the main rotating hanger bolt to derailleur body?

Another cyclist had the lower pulley on his Di2 derail. That drag on the chain triggered the Di2 "crash mode", and it took us a while to remember about crash mode resets -- we thought the Di2 had failed since it wouldn't shift.
He had previously crashed, and the shop mechanic had realigned the derailleur, then thought the cage was bent, and straightened the cage. No, it looks bent from the factory. The straightening made the lower pulley go out of alignment, I guess.

Last edited by rm -rf; 11-25-23 at 07:00 PM.
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Old 11-25-23, 07:08 PM
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Originally Posted by Polaris OBark
Have you ever cut yourself with a knife or dropped something on your foot?


Only once. Although I have many times mashed my toes into the small but incredibly dense wooden stool my wife leaves around the house in random places.
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Old 11-25-23, 07:57 PM
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Originally Posted by rm -rf
My 2014 Di2 6850 worked okay when completely cross chained, except for the chain noise where it ticked against the big ring pickup rivets on the side of the big ring. I did this probably once every 2 or 3 rides, not keeping track of which cog I was in. The noise was annoying, and I'd soon shift away.​​
Same with me, until a few days ago. (After my various tweaks, I can, once again, do this with impunity, although I haven't really pushed my luck).

​​​​​I probably never shifted the front with the rear still in the 11 cog -- shifting the rear is the usual thing to do.
​​​​​

You haven't really lived.

​​​​​A sudden slack chain, and a road bump or vibration at the same time might throw the chain off the pulley? Do you lube the derailleur pivots and the main rotating hanger bolt to derailleur body?
​​​​​

Never. I have only had it since 2014.

​​​​​Another cyclist had the lower pulley on his Di2 derail. That drag on the chain triggered the Di2 "crash mode", and it took us a while to remember about crash mode resets -- we thought the Di2 had failed since it wouldn't shift.
​​​​​

I noticed something was wrong immediately, so I stopped pedaling. I think I dodged a bullet.

​​​​​He had previously crashed, and the shop mechanic had realigned the derailleur, then thought the cage was bent, and straightened the cage. No, it looks bent from the factory. The straightening made the lower pulley go out of alignment, I guess.
Yeah, in my experience, I have never improved a derailleur by "straightening" the cage. I've ruined at least two 12-speed derailleurs trying to help other people (one of them my kid).
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