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I have a road bike equipped with full Shimano 105 11-speed groupset from top to bottom. The current rear derailleur is RD-5800. Is it ok to replace it with a Deore M5100 Rear derailleur, which is also 11-speed? I only plan to use it as a road bike, maybe some easy gravel ride too. I have a 11-28 teeth cassette. Maybe I will also swap that with 11-30 cassette. Is there anything I need to look out for?

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It's not compatible: MTB and road products above 10 speeds have different pull-ratios (ratio between cable movement derailleur displacement), and also because the Deore is designed to be paired with cassettes much larger than cassettes typically found on road bikes.

If your goal is to fit a larger cassette: the medium cage version of the RD-R5800 is compatible with 11/32. The short cage version is however limited to 11/28 cassettes.

The largest cassette that would be compatible is 11/34, with a GRX derailleur (RD-RX810) or a 105 RD-R7000-GS (that is now discontinued, so availability is less guaranteed), but I would only go in that direction if you have the short cage version and need this capability, or if your your rear derailleur is broken. The gain compared to what a medium cage can support is marginal.

You'll also need a slightly longer chain if you install a larger cassette (2 links longer from 11/28 to 11/32).

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    The RD-R7000 105 derailleur also handles 34t in the GS version, no need to use GRX.
    – Noise
    Jul 22 at 9:55
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    Actually, the GS version of the 5800 Rd is likely to handle an 11-34. I am doing this with a 6800 GS RD.
    – Weiwen Ng
    Jul 22 at 10:01
  • @Noise Good point. I actually hesitated to mention it because it has been recently discontinued. But it's true it can be still found, and on the second hand market.
    – Renaud
    Jul 22 at 10:09
  • @Renaud I expect it will be available for a while as current model bikes are shipping with it and Shimano will support existing systems for some years. The electronic/di2 105 has been announced but is not yet actually available to order, meaning R7000 is the current product.
    – Noise
    Jul 22 at 11:18
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    @Noise Agree, and that's why I edited the answer. Eternal problem of "shopping questions"...
    – Renaud
    Jul 22 at 11:21

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