Ultegra 6870 Di2 FD binding on upshift after crankset swap
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  1. #1
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    Ultegra 6870 Di2 FD binding on upshift after crankset swap

    I'm pretty comfortable with the mechanical stuff on my bikes, but today I ran into something I've not encountered before (on a friends bike).

    Hoping someone here might point me at something obvious I'm missing.

    Bike is a BMC GF01 with Ultegra Di2 (6870 FD). We swapped out the front crankset from the 6800 50/34 to a 9100 50/34 (with a single sided Stages PM).

    After the swap, the FD is pinching the chain against the inside of the big ring causing it to bind slightly before continuing up to the chainring.

    I only had a few minutes to look at it, and the setup seems to be within Shimano's tolerances. I checked the angle of the FD, and the height. It was a scoche high (~3mm), so I lowered it down to 1mm to see if that made any difference. It did improve, but the shift up to the big ring still seems to bind the chain against the ring a bit before it moves up to the chainring.

    We made sure the inner ring was mounted in the correct orientation.

    I spun the crank and didn't notice anything looking warped or otherwise wonky looking down the chain line from the front.

    Just curious if anyone might have experienced this and point me at whatever obvious thing it is I'm missing.

    We didn't have a lot of time to mess with it today, but we will try going back to the old crank tomorrow to see if it improves.

  2. #2
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    Sounds odd for sure - you know the 2 cranks in question have a bit of a difference in ring spacing (0.4mm) so you may want to reset the high and low limits but that wouldn't do what you are describing. I know you didn't mention changing the chain so probably didn't mess with it but I would still make sure the chain is facing the right way, lettering on the links facing out not in (but not sure that would do anything for the front shift either) Of course you are turning the crank at a reasonable speed when shifting right?
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  3. #3
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    I did not know there was a spacing difference on the two cranks. That is definitely interesting.

    Good call on the chain.

    The bike is a New Old (2016) Stock purchase from a local shop (he got a *very* good deal due to some questionable color choices by BMC designers). The chain and crank were as they came from the shop, so it's possible the chain could be reversed. I'll have him check.

    The limits looked really good to my eye, but it will be worth checking them when we have a little more time.

    We did look at the chainrings and pondered orientation. I think we determined that the printing faces towards the outside (drive side) on the big ring, but the small ring printing/stamping faces inwards on the small ring (this is consistent across the three DuraAce and Ultegra cranks we checked).

    Regarding crank speed, it happens during riding conditions or when turning by hand. We will be a little more thorough about the diagnosis when we have more time to work on it.

  4. #4
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    Quote Originally Posted by Finx View Post
    We swapped out the front crankset from the 6800 50/34 to a 9100 50/34 (with a single sided Stages PM).
    So what did you do about the "rear crankset"?

    My guess is that there is a difference in the distance of the chain rings from the frame between the two cranksets.

  5. #5
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    Like Srode said, the gap between the chainrings are different for 6800 vs 9100 crank.
    And more specifically, the INNER ring of the 9100 crank is now closer to the frame. The outter ring of the 6800 and 9100 cranks remain the same distance to the frame.

    My guess is that you will need to micro-adjust the FD to make it shift further outward a bit so that it can push the chain up the big ring with force. Right now, it seems that the FD is not pushing hard enough, thus making the chain seemingly "stuck" against the big ring, until the big ring rotate enough to have its pickup pins pick up the chain.

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