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Chain skipping on new chainrings

Old 03-30-21, 09:56 AM
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Chain skipping on new chainrings

I recently replaced my Rotor 52-36 chainrings to Rotor 50-34 and replaced the chain while I was at it. What I'm finding is that when I shift from the smaller inner chainring to the outer larger chainring the chain doesn't bite into the large ring and grinds for a 1/4 turn or so before it settles in. I've got Ultegra Di2 6800 and Rotor 3D+ cranks. The chainrings are round, not the oval ones.

After I installed the chainrings I adjusted the front derailleur down to compensate for the smaller diameter and used a 2mm hex wrench as a feeler gauge at the smallest point between a tooth and the FD cage. I'm pretty sure the FD is otherwise configured correctly because the chain doesn't drop and there's no rubbing as I move up and down the cassette in both rings. The larger ring is installed correctly as I have the pin seated behind the crank arm. I did not remove a link from the chain as compared to the old chain but I find that the chain has tension throughout the shifting range.

Any ideas? Is 2mm too much of a gap? Are there ramps on the inner chainring that need need to be synced with the outer (I didn't see any)? The shifting was as smooth as butter before I touched it so I know the Rotor setup works with my bike. Anything else I'm missing?

--Brad
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Old 03-30-21, 10:03 AM
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10 sp? 9 Sp? 8sp? 11sp? Who would know? What about the cassette? What chain exactly did you put on? Chain ring spacing the same as before? Small chainring flipped?
Small chainring might need to be shifted to sync with the big one.
I don't know
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Old 03-30-21, 10:08 AM
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Ultegra Di2 6800 is 11 speed as per the OP
I agree that the inner ring could be out of sync with the big ring
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Old 03-30-21, 10:10 AM
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I'd go ahead and shorten the chain..... After making sure that I felt good enough about Big/Big combo being able to handle a shorter chain.

And if you want to do a more proper chain sizing, then run it over the Big/Big combo without going through the DR and see what that looks like with the current chain length. Normally two links longer will be the suggested.

In any case it's always good to know what the manufacturer of your rear DR recommends for chain length whether you go by that or trust your own judgement.
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Old 03-30-21, 11:12 AM
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More details: Ultegra Di2 6800 full drive drivetrain, except for the Rotor cranks and rings, with Shimano 11-28 cassette. New chain is Shimano HG701. The inner ring has printed on it, paraphrasing, "install this side towards the frame" so I know it's not flipped. Chain is properly installed with the printing on the outboard side. I used the same ring bolts so I'm hoping that the spacing is the same.

I've got an e-mail into Rotor asking if there is any syncing I need to keep in mind for the inner ring. I'll wait to hear back from them to see if they have anything and then I'll look at the size of the chain.
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Old 03-30-21, 11:27 AM
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Originally Posted by MidTNBrad
I've got an e-mail into Rotor asking if there is any syncing I need to keep in mind for the inner ring. I'll wait to hear back from them to see if they have anything and then I'll look at the size of the chain.
Usually the inner ring will have a small tab on its inner edge halfway between two chainring bolt holes that has to be lined up with the crank arm to match it to the outer ring
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Old 03-30-21, 11:36 AM
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Originally Posted by alcjphil
Usually the inner ring will have a small tab on its inner edge halfway between two chainring bolt holes that has to be lined up with the crank arm to match it to the outer ring

Yep, I bet that's it. I've got the old ring and it does have a tab that I didn't notice. I had a 20% chance of blindly getting it right and I guess I wasn't lucky.

Thanks for the help!
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Old 03-30-21, 01:17 PM
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Do you have the rings installed with the proper side facing out (if counterbored, it should be obvious, but you never know)?

If indexed shifting, do you have the pins in the proper orientation with each ring and the crank arm?

If friction shifting, bend the very end (forward most 5mm or so) of the inner plate of the front derailleur cage inward (toward the chain), making the opening slightly narrower. That often helps with chain pick-up.
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Old 03-30-21, 06:55 PM
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Originally Posted by JohnDThompson
Do you have the rings installed with the proper side facing out (if counterbored, it should be obvious, but you never know)?

If indexed shifting, do you have the pins in the proper orientation with each ring and the crank arm?

If friction shifting, bend the very end (forward most 5mm or so) of the inner plate of the front derailleur cage inward (toward the chain), making the opening slightly narrower. That often helps with chain pick-up.
All of these questions have been answered previously. None of them will help. Positioning of the small ring really doesn't matter, there are only pins/ramps on the large ring. Rotor rings are not the best shifting chainrings by a long shot. Without seeing the bike the only thing I can offer is worn big ring...improper setting of high limit...
The OP says his chain is skipping, is it that or is just balky shifting? They are NOT the same thing.

Last edited by cxwrench; 03-30-21 at 07:01 PM.
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Old 03-31-21, 01:11 PM
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Rookie Mistake

Originally Posted by alcjphil
Usually the inner ring will have a small tab on its inner edge halfway between two chainring bolt holes that has to be lined up with the crank arm to match it to the outer ring

Sure enough, I didn't have the inner ring installed properly. There is a little tab indicating that it must be installed in-line with the crank arm. Much, much better. Thanks for the help.

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Old 03-31-21, 02:34 PM
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There is nothing about a small ring that needs to be positioned correctly unless you have one of the non-round elliptical rings. There are no ramps, pins, nor any other 'shape' that will aid shifting. I have no idea what you did when you repositioned it, but that should have absolutely no affect on your shifting.
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Old 03-31-21, 02:46 PM
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Originally Posted by cxwrench
There is nothing about a small ring that needs to be positioned correctly unless you have one of the non-round elliptical rings. There are no ramps, pins, nor any other 'shape' that will aid shifting. I have no idea what you did when you repositioned it, but that should have absolutely no affect on your shifting.
Not so. OP figured it out for himself and shared his lesson.

The teeth of the inner ring need to be synced with the ramps of the outer ring .

Here's a related anecdote. In the early days of 9-speed road, Shimano made two tooth-ramp patterns of chainrings, A and B. When replacing just one ring, you had to match it to the old one. And sync it right.
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Old 03-31-21, 02:55 PM
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Originally Posted by oldbobcat
Not so. OP figured it out for himself and shared his lesson.

The teeth of the inner ring need to be synced with the ramps of the outer ring .

Here's a related anecdote. In the early days of 9-speed road, Shimano made two tooth-ramp patterns of chainrings, A and B. When replacing just one ring, you had to match it to the old one. And sync it right.
Maybe you should read this:

The truth about Shimano chainrings.

The big ring has to be in the correct position, the small ring doesn't matter. The A and B (and apparently E) were only on the big rings.
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Old 03-31-21, 09:08 PM
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Originally Posted by cxwrench
Maybe you should read this:

The truth about Shimano chainrings.

The big ring has to be in the correct position, the small ring doesn't matter. The A and B (and apparently E) were only on the big rings.
You read the narrative but did not look very closely at the orientation of the ramps, pins, gates, and teeth to the bolt holes. And if you read down through the comments, on May 16, 2016, Andre Bondeson explains that Tall Cyclist neglected to mention what he considers the major difference between A & B, the different alignment of those ramps, pins, gates, and teeth to the crank bolts. And Tall Cyclist did not address Bondeson's comment. Note that in his entire thread Tall Cyclist does not acknowledge the difference in orientation obviously apparent in his photos. Tall Cyclist's explanation does not explain why OP's front shifts started working after he realigned the small chainring. Was OP just imagining all this?

Nikola (the tall guy) starts his presentation with a narrative about missed front shifts, up and down. He says that about 40 percent of the time his chain lands somewhere between the two rings, and says that he is shifting at full power and sometimes out of the saddle. Apparently this guy has never heard of soft-pedaling, backing off the pressure on the cranks so the chain can loosen slightly and allow the derailleur to do its job better. Nikola has made an astute observation on chainring profiles that may or may not be relevant, but seems too incurious to learn why his chain is ending up in no-man's land on 40% of his shifts.

Last edited by oldbobcat; 03-31-21 at 09:47 PM.
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Old 03-31-21, 10:14 PM
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That is interesting. Depending on the orientation of the ring the teeth align differently with the big ring and it's ramps & pins. I'm gonna have to mess around with this and see what happens.
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Old 04-01-21, 08:57 AM
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Originally Posted by cxwrench
That is interesting. Depending on the orientation of the ring the teeth align differently with the big ring and it's ramps & pins. I'm gonna have to mess around with this and see what happens.
Let us know what you find. Most of my chainring replacements were done on the clock, so I never had time to experiment. I just followed the manufacturer's instructions, made sure it worked, and gave it back to the customer. And if it didn't work, I can't tell you how many hundreds of hours I worked staring at the thing on the workstand, tweaking this and that, and studying it in slow motion (especially front derailleurs), trying to understand what I missed. You'd say that my experimentation was always directed at making something work rather than knowledge for its own sake. If solving a problem made me smarter, so much the better.

Last edited by oldbobcat; 04-01-21 at 09:04 AM.
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Old 04-01-21, 10:23 AM
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Take a look at the picture I posted above. Right above the bolt hole in the 12:00 position there's a tooth pretty much centered but not so at the 5:00 and 7:00 holes. I'm betting that if the small ring isn't installed correctly then, as discussed above, the pins are oriented with a rivet on the chain rather than a space to grab onto thus bad shifting.
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Old 04-01-21, 11:54 AM
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Originally Posted by cxwrench
There is nothing about a small ring that needs to be positioned correctly unless you have one of the non-round elliptical rings. There are no ramps, pins, nor any other 'shape' that will aid shifting. I have no idea what you did when you repositioned it, but that should have absolutely no affect on your shifting.
Look carefully at the photo, the tab is lined up perfectly with a tooth....and that tab happens to sit exactly halfway between two bolt holes. Note that on every other Bol hole pair, there is no tooth sitting perfectly halfway between them. You may be right, but I've found doing all of my own maintenance, that installing everything exactly as spec'd by the engineers alleviates a lot of issues...as in lets me focus on my weak "motor", instead of worry about balking shifts, etc.
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Old 04-01-21, 02:12 PM
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Back in the days of friction, you had nothing to push and lift the chain onto a larger cog than the strength in your thumb, the stiffness of the derailleur cage, and the cross-section of the teeth on the chainring. Which is probably why I saw so many bent chainrings in those days. And why a PX-10 with Stronglight 93 cranks shifted better than a UO-8.

With ratcheted shifting the pull on the cable comes from the shifter, which isn't necessarily as strong as your thumb. That means you can't force the shift if it isn't happening, but it also means less chance of damaging the chainring. The pins, ramps, gates, tooth cross section, and shapes of the derailleur and outer plates on the chain work together to push and lift the chain onto the big ring. The derailleur shoves the chain over and the landscaping on the chainring lifts it over the top. I've spent hours watching this in slow motion, looking for exactly where the chainring got bent, the ramp got worn off, the pin got nicked or loosened or knocked out, the tooth got damaged, or the chain got kinked or a stiff link. It's all about doing what you have to to get the bike off the stand so it won't come back.

The cool thing about working with a good bunch of mechanics is, you can say, "Can I borrow your eyes for a minute? Watch this and tell me what I'm not seeing." Some customers will tell you you're **** if you can't figure it out right away, but if you're thorough and you help, share, and ask questions, the guys (and girls) you work with have your back.
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