Help Me Diagnose Chain Drops
 

Issue: Chain drops (to the inside) when shifting from small chainring to large chainring at worse. At best, shifting from small to large is not smooth.

Equipment:
Bike - 2023 Giant Defy Advanced 1 w/ !05 di2
Chainring - Ultegra 8100 12sp 50/34 (upgraded from 105 50/34)
Crank Arms - Magene P505 Base Spider Power Meter
Chain - KMC (stock on bike)
RD and cassette - 105 di2 12 speed.

Background:
Everything was fine BEFORE I switched things over to the Ultegra chainrings and new PM. When I installed the PM, I installed it with the Ultegra chainrings (will be upgrading to Ultegra in bits and pieces in the long run). As a result, I had to adjust both the low and high limits (low via app, high via screw). I have gone on 3 rides and it has gotten better each time I turn the high limit out (towards crank arm). However, this latest ride, I am beginning to wonder if it's actually the limit of the FD or something else.

There is no problem from large to small but when going from small to large, the chain sometimes does not catch the big ring. When it is successful, it is often after struggling until the chain catches. Every once in a while, there is a smooth shift. I first assumed that I had to have the perfect load on the arm for it to shift property but that doesn't really make sense, does it? I do notice that the shift is not good when gentling spinning with very little load. When the chain does mess up, it drop immediately off on the INSIDE (not outside). This tells me that is may still be the high limit. But now I'm wondering if the KMC chain doesn't work as well w/ the Ultegra chainrings? Another potential problem is the chain is worn (unlikely since I'm just a bit over 600 miles on the chain). I have a chain checker and it doesn't seem worn.

Summary of Potential Diagnosis:
1. The high limit is set too low. I believe I have about 2 more 1/2 turns to the right (out) until it will affect the crank arm.
2. KMC chain doesn't work well w/ Ultegra and switch to genuine 105 or Ultegra chain.
3. Chain is worn and do same as #2.

Any help from the more experiences cyclists? Thanks!

4. The chainrings are not synchronised with the crank and/or each other.

Thanks. Never heard of this. How do you sync it?

Di2 front derailleur calibration is set by the amount of travel allowed between the limit screws, so just setting the high limit farther to the outside should fix it. Just keep moving it until the chain hits the big ring consistently from all rear combinations. Of course, make sure the outer plate is parallel to the rings and no more than1-2 mm clear of the tallest teeth on the big ring. I like 1-1.5 mm. Also, make sure the chain isn't worn out.

The description doesn't make sense to me.

Do you mean that the chain somehow drops to the inside of the inner ring when trying to shift it outward? That's hard to accept, though it is fairly common when shifting from outer to inner. OTOH do you mean its dropping between the two chainrings?

Getting a clear description of what, how and when, will help others offer possible solutions.

Is the small chainring on backwards?

I agree it's very odd - and what I will write won't help the OP at all. But,

I have a bike I've built up with matching ~mid nineties Campagnolo Record 8 speed parts with one exception. The shifting is 8 speed Syncro-type DT shifters with a Miche Campy 11-28 8 speed cassette and Record RD with a suitable cage for the cassette and crank (scavenged a Racing Triple cage). The only unconventional part a 10 speed era Centaur double crank, 50-34, but with the 8 speed group's FD, probably designed for larger chain wheels. Anyway, it's very odd, but when I shift from small ring to large ring if the rear is on the smallest couple of sprockets, it will often drop to the inside, like the OP is describing. Other than that, front shifting (friction, not indexed) is very satisfactory.

I haven't been able to adjust this away although I fancy myself competent enough in setting up front shifting (all my other double and triple bikes work fine). My guess is that it somehow has to do with the design of the FD, not playing well with the crankset. I haven't bothered looking for a FD that matches the 50-34 10 speed Centaur crankset since my solution has been to make that shift before I get to those smaller sprockets in back.

Like I said, this undoubtedly has nothing to do with the OP's issue, but it is similar in what happens and when.

The reason it drops on the inside is because the chain is trying to jump from small to large but having trouble catching on, and if the pedal stroke isn't smooth, or if the road buzz is causing the chain to jump around, it'll fall back down towards the small ring due to the chain tension, but it slips off the small ring for some reason. I've had this happen on mechanical FDs of the cable tension isn't high enough or the FD high limit is not set far enough, such that the throw of the front shifter isn't quite enough to help the chain make the jump onto the large ring. No experience with Di2 (eg, can you adjust the throw) but it could be just a case of needing to set the high limit correctly.

Yes, it drops to inside of inner/small when shifting from small to large. See response from other person below. That is what is happening.

This is possible. I was having a hard time figuring out which is out and which is in. I think I have it correct but now that you mention it, I can't be sure. There is an indentation. What is the right way? Can you explain the mechanics involved with the problem if installed backwards?

Yes, this is what is happening.

Thanks everyone.

One more possibility.

Since I changed chainrings AND crank arms to fit the PM, should I redo the micro adjustment to the RD? You know, the set to small chainring and 5th sprocket one?

If you used 12 speed Ultegra rings there should be no difference. Are you certain you didn't put the small ring on incorrectly?

On the 11 speed rings there is a mark that lets you know. I routinely have swapped 11 speed 105 and Ultegra rings on my 105 crank with no issue for my Di2.

Chain line doesn't change. So there is no reason why the RD settings should be changed. And that you did or intend to change them also lends credence to the ring being installed wrong or you changed something up front that affected the chain line when it should not have.


Or did you also do something at the rear of the bike at the same time you installed the rings?

Delete everything beyond the .pdf on the link above. Pages 20 and 21 are what you want.

Thank you both. I think this may be it. I do see that the "ultegra", etc markings are at the crank arm and they should be opposite. It's weird because I don't see the delta/triangle on the opposite side. But if the graphic on page 21 is correct, then I do have it turned 180 degrees.

Thanks everyone, especially the first response. I know understand. I rotated the inner crank so it all aligns and it seems to work fine on my ride this morning. I did get 1 chain drop but that's because I still need to fine tune the FD adjustments.

One thing of note. I know that changing chainrings is not too common but those chainring bolts Shimano provides are horrible. I stripped 1 and almost stripped 2 trying to, first, unscrew them and second, trying to get the right torque on them when redoing them. Luckily, I had some of the same size that the PM came with to use temporarily because those are not as long. I will get some replacements and re-do those bolts the next time I do maintenance down there.

Did you use a torx key?

Yes.

For the stripped one, I had to use the next size up torx to get it out.