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I've seen this question about making the same change (50/34 to 50/39) on a Shimano Sora chainset, but I am asking specifically about Ultegra R8000.

I have a bike with Ultegra R8070 (Di2, hydraulic disc brakes, 34/50, 11-28T). I find I am often in the 34-13, and needing a slightly harder gear, but as Di2 doesn't allow the 34-12 combination, I then have to switch chainrings and run up the cassette, only to do the reverse as the gradient/wind changes.

I don't want to change the cassette (as suggested in a comment on the Sora question) as my issue is with the small-sprocket end of the cassette, and I don't want to mess with the gears when in the big ring.

I realise I will have a bigger low gear after doing this (although I could replace the 11-28T cassette with 11-30T).

Is it possible to replace the inner 110mm BCD 34T-MS with a 110mm BCD 39T-MW?

In the dealer's manual for the Ultegra FC-R8000 chainset, Shimano says

Be sure to use the specified gear tooth combination. If an unspecified gear is used, the chain may fall in between the gears, damaging them.

... and then 39T-MW is only allowed with 53T-MW, and 34T-MS with 50T-MS.

Has anyone tried this? Is it okay to replace a 34T inner ring with a 39T one or not?

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    I think this is the case of a user error. You should be using 34T when climbing steep hills (or when anticipating the hill will become steep in the future) and 50T when on flatland. Using 34/13 is cross-chaining, you shouldn't do it. 34/12 is even worse. Fix your shifting habits and the problem goes away. I cannot imagine a case where wind would make 34T the preferable chainring on flatland, but then again I might not live in the world's most windy country. You could of course swap the cassette to something that has bigger than 28T as the biggest sprocket to make the 50T more useful.
    – juhist
    Commented Apr 9, 2023 at 15:28
  • You probably have 17-19-21 available on your cassette. Did you try 50-19?
    – njzk2
    Commented Apr 10, 2023 at 15:49

3 Answers 3

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If you don't need the 34/28 combination for climbing, you can switch to a tighter cassette such as an 11-25 or even an 11-23 and get more gears in the 50/16-17-18-19 range. You could then stay on the 50 most of the time and get a bonus of smaller gaps between the gears you do use. (I don't think Shimano produces an 11-21 any more...)

And if you don't need anything bigger than 50/14, a 14-28 would give you all of 14-15-16-17-18-19-20-21 without interruption and you'd still have the 28 for climbs.

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I have not tried it myself, but I would advice against doing it for the following reasons:

  1. You have an expensive high quality groupset that is engineered to perform very well when within specs. Going outside of what the manufacturer recommends would most likely decrease what you paid for (and could even cause damage).
  2. When configuring Synchronize Shift, you are asked to select your front chainring configuration. Since what you are trying to do would yield to an unsupported one, you may not be able to select it in the software, as shown below:

enter image description here

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I would also not advise it based on the following experience:

I have seen Shimano put their shifting ramps in different places on what one would assume to be a similar chainring just because the sister chainring was different. The example was on a MTB crankset. In order to obtain a slightly lower climbing gear I chose to get a 38/24 crankset (both were XT of the same, relatively modern generation - 2x10 speed) to replace the 38/26 I currently had. I had considered purchasing just the 24-tooth small chainring and replacing the 26 (simple, right?). Well I chose to buy the full crankset instead, and on inspecting the two cranksets side-by-side, the 38-tooth large rings had the shift ramps in a significantly different places from each other. I never would have expected that and had I purchased the 24-tooth small ring, it is possible that my shifting could have suffered. Pretty surprising revelation there.

If you were considering this with older equipment that did not have the built-in shifting ramps and enhancements in modern cranksets/chainrings, I might say give it a go if the gearing is practical, but the potential for issues with modern equipment does not warrant the investment.

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