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I have a full 105 5800 group for my current drivetrain on my bike. I would like to replace the right crankset (chainring + crankarm) with an Ultegra R8000 right chainring/crankarm. Is there full compatibility? Or do I need to replace/adjust chain, front derailleur, left arm, BB, anything in the rear?

Of course I'd keep the same number of teeth and crank length.

I've checked Shimano's compatibility chart and it looks fine to me, but it's very complex, I'm no expert and this is a big purchase that I don't want to regret.

Thank you!

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    Honestly, why? It's not going to shift any better, and it won't make you any faster. If you want to get faster, the price of an Ultegra crankset is a decent fraction of the cost of a power meter, which you already seem willing to pay. And a power meter can help you get a lot faster.
    – Andrew Henle
    Jul 2, 2022 at 17:42
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    Precisely, the reason I want to change is that the Ultegra crankset is a power meter (the stages one). They don't sell that in 105 version.
    – emiazz
    Jul 2, 2022 at 18:57
  • I would keep both crank arms the same brand and type, if only for them to have the same weight. I made the same change you want to make, a few years ago, I replaced a 105 crank set by a Stages Shimano Ultegra two-sided power meter crank set.
    – Christine
    Jul 5, 2022 at 20:11

1 Answer 1

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You are good to go with what you are proposing. You may also need a FD-7000 (or FD-8000) front derailleur. There is a very minor difference in the FC-8000 vs. FC-5800, but it is truly minor.

Both are the same width, and use a Hollowtech II axle (which I believe is 24mm).

The only measurable specification difference (besides the weight) is that the smaller chainring is 0.4 mm closer to the large chainring (it is the same difference that the Ultegra 6800 --> 8000 has, as your 5800 measures precisely to a 6800). That is a small difference and will likely not be an issue with your 5800 front derailleur. But be prepared to invest in a new FD, just in case.

If the current front derailleur is working fine and adjusted fine, you likely will not even need to adjust that as well. If you do find that you cannot fine tune the FD-5800 to your satisfaction, you can upgrade to the FD-7000 or FD-8000 (105 and Ultegra, respectively). Your ST-5800 shifters/brifters are fully compatible with any of these newer FDs (per the Shimano cross-reference table).

Shimano compatibility crosswalk

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    Thanks a lot for the super thorough answer, and the good news! Looks like I can try and if it doesn't shift well, I'll have to take it to a bike shop for adjustment and maybe replacing the derailleur. There's already a little bit of chafing noise at the front derailleur, so I might need to have it checked anyway.
    – emiazz
    Jul 2, 2022 at 18:59
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    @emiazz - happy to help and welcome to Bike Exchange. If you feel this is the best answer, you can click the check mark next to the answer that you "accept" it as the best. If another answer comes along that is better, you can change it and check that one instead later. It is the way to communicate a good answer and part of how Bike Exchange (and other Stack Exchange run areas) work.
    – Ted Hohl
    Jul 2, 2022 at 19:06
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    Ah! Done. First time here.
    – emiazz
    Jul 3, 2022 at 8:04
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    @emiazz thanks. We all had a first time at one time.
    – Ted Hohl
    Jul 3, 2022 at 14:34

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