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Chain mis-tracking at rear derailleur idler pulley -- what's the problem?

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Chain mis-tracking at rear derailleur idler pulley -- what's the problem?

Old 03-10-15, 06:25 PM
  #1  
FlashBazbo
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Chain mis-tracking at rear derailleur idler pulley -- what's the problem?

I've been doing all my own mechanic work for about ten years. There's never been anything I couldn't diagnose and fix. Also, I've been riding various versions of Shimano's Di2 for about five years without any problems (other than the occasional bent chain). But now I'm stumped:

I've got Shimano 6870 Ultegra Di2 11-speed on my gravel bike. Occasionally, with the rear derailleur about halfway across the cassette and the front on the small ring, the chain gets misaligned (to the side) on the rear idler pulley and it makes a noise much like the chain lifts on a roller coaster. The derailleurs are properly adjusted on the cogs -- no noise normally. And no noise or problems when either derailleur is in any other position. When the chain gets misaligned on the idler pulley, it clears itself if I pedal backwards for a revolution or two. The chain problem doesn't always happen, but it happens about half the time and it happens regardless of whether I'm on a smooth or rough surface.

The idler pulley looks new and the bearing has no play. I even replaced the rear derailleur with another to see if that fixed it . . . it didn't.

Does anyone have an idea of what the problem is . . . and how to fix it?
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Old 03-10-15, 06:59 PM
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OK, your chain is trying to tell you something, but you don't speak Chainese..

Before I translate, I need to confirm the details. Either the chain is coming off to the outside when you shift toward the inside, or it's coming off to the inside as you move the RD to the outside.

If neither are right, stop reading here and we'll try again.

But if either is true, then you have a bent or twisted hanger or derailleur cage. Let's start with one, and the other is the same but opposite. If the chain comes off the outside, either the hanger is bent so the pulley is too far inboard, or twisted to point inboard.

You can do a rough eyeball confirmation bu putting the RD to the middle sprocket and noting the angle between the chain and pulley. In a mid position the chain should be coming just about straight on. If you have to shift to either side to find the best straight on alignment, that confirms the issue, but you have to use other methods, starting with checking the hanger, to separate a simple bend (most common) from twist.

BTW- while I say check the hanger, the problem could rside within the RD cage, so if the hanger checks out, you need to check the cage next.
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Old 03-10-15, 11:32 PM
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I recently had this problem. Mine was worn rivets on the the d'er cage causing tension in the opposite direction in higher gears to "pull" the chain off of the pulley. I have since purchased a new d'er, but sorta fixed this one by offsetting the cage slightly to the inside, and will use it as a backup should the need arise.
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Old 03-11-15, 12:08 AM
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Well worn pulleys can make this chain depulleying worse. BITD when chains had side plates that extended past the rollers by a bunch this problem was a lot less an issue. As chains got smaller in plate dimensions the problem became more common. These days it's only the pulley teeth that engage the chain, the side plates are pretty much all flush with the rollers. Andy.
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Old 03-11-15, 05:23 AM
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Thanks for the replies. FBinNY, you caused me to take a closer look at what's actually happening -- at least on the stand. To my surprise, the mis-tracking ISN'T happening when I shift the rear derailleur. It happens, under certain conditions, when I shift the front.

With the chain on the 4th or 5th sprocket (counting from the 11T) and the big ring, if I shift to the little ring, the chain tracks off the idler to the inside (small chainring side). It's off track at initial contact with the idler, but is on the idler within 1/4 of the way around. If I back-pedal even an inch at the crankarm, the chain gets back on track.

The jump from 50 to 34 up front is apparently allowing enough chain slack to let the chain pop off the idler teeth. In that nano-second of slack, the fd is pulling the chain off the idler to the inside. At least that's my theory. Anyone know how to fix it?

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Old 03-11-15, 09:41 AM
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Originally Posted by FlashBazbo View Post
Thanks for the replies. FBinNY, you caused me to take a closer look at what's actually happening -- at least on the stand. To my surprise, the mis-tracking ISN'T happening when I shift the rear derailleur. It happens, under certain conditions, when I shift the front.

With the chain on the 4th or 5th sprocket (counting from the 11T) and the big ring, if I shift to the little ring, the chain tracks off the idler to the inside (small chainring side). It's off track at initial contact with the idler, but is on the idler within 1/4 of the way around. If I back-pedal even an inch at the crankarm, the chain gets back on track.

The jump from 50 to 34 up front is apparently allowing enough chain slack to let the chain pop off the idler teeth. In that nano-second of slack, the fd is pulling the chain off the idler to the inside. At least that's my theory. Anyone know how to fix it?
I believe it's still an alignment related problem. The wave that moves back when you shift the front dislodges it, but if the chain is coming straight on it shouldn't go over the edge. If you can find pulleys with deeper teeth, they'll engage the chain better and prevent or reduce derailing, but when the chain comes from an angle derailing is still an issue. Note also that they sell thousands upon thousands of these derailleurs and if the pulleys couldn't retain the chain, there would have been a modification.

As I posted earlier, shift to the middle of your cassette, and see if the chain appears to come to the pulley straight on as it should. If not, then you have to align things so it does, with it coming from angles only when the RD is inside or outside of the chainline plane.
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Old 03-11-15, 11:42 AM
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Originally Posted by FBinNY View Post
As I posted earlier, shift to the middle of your cassette, and see if the chain appears to come to the pulley straight on as it should. If not, then you have to align things so it does, with it coming from angles only when the RD is inside or outside of the chainline plane.
With the chain on the middle cog and the small ring, the chain appears to come to the pulley straight -- parallel top and bottom. With the chain on the middle cog and the large ring, the chain appears to come slightly from the outside -- and the idler is then slightly outboard of the jockey wheel.
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Old 03-11-15, 11:59 AM
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Originally Posted by FlashBazbo View Post
With the chain on the middle cog and the small ring, the chain appears to come to the pulley straight -- parallel top and bottom. With the chain on the middle cog and the large ring, the chain appears to come slightly from the outside -- and the idler is then slightly outboard of the jockey wheel.
I wish I could see it, and maybe help more. Some of these things are subtle and we can only guess at possibilities with info over the net.
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Old 03-23-15, 06:42 PM
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I think I finally found the problem and fixed it!

Since it didn't cost much, I replaced the derailleur hanger -- but the problem persisted. I checked the alignment of everything, but everything looked okay. So I started messing with the rear derailleur adjustment again (Shimano 6800 GS Di2). I've been using Di2 for five years and it's always worked perfectly if I adjusted it according to the book, but why not give it a try?

Since the chain was mis-tracking toward the inside, I decided first to adjust the derailleur closer to the bigger cog. (Adjust it toward the bike until it makes noise, then come out just TWO taps.) No help. Then, I decided to do what made no sense to me -- I adjusted the derailleur farther from the bigger cog. (SIX taps.) Voila! The noise went away. At least on the stand, no more mis-tracking. I rode the bike down the block and back, no more mis-tracking.

Tomorrow, I have a 90-minute hilly Tempo ride scheduled. That should give me plenty of opportunities to test the issue. If anything is left of the issue, I will try adjusting seven taps from the "noise point." I think I'm close to having this problem behind me.

What concerns me, though, is that I don't know the real cause of the problem. Maybe it's a function of the chainline geometry on Niner's RLT? Maybe of the geometry of a wider rear wheel spacing? I don't know. I just hope it's cured and doesn't come back.
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Old 03-23-15, 11:20 PM
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Please continue feedback and good luck.
Cheers,
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