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For some niche cases, gravel riders would appreciate to use wider range cassettes (11-40, 11-42 or 11-46) on their double chainring gravel setup, which would allow have ranges close to 600% (vs 507% for the latest GRX 2x12 or 510% for the latest 1x12).

The GRX RD-RX810 (or RD-RX815 for the Di2 version) is the official 11-speed double chainring GRX rear derailleur and is derived from road derailleurs (its specs are very close to the 105/Ultegra rear derailleurs), with similar max sprockets size and capacities. When searching how to fit a larger cassette, the answer that can be found is to replace the derailleur attachment by a goat link/tan pan, that would move the derailleur away from the cassette and give the necessary clearance to use such cassette. But that is done at the detriment of shifting performance on small sprockets, as the jockey wheel is too far from the cassette to do its work properly. From my experience on another bike, the decrease in shifting performance is significant, and I would not repeat the experience.

I recently came across another "hack", which is to start from the 1x version of the GRX rear derailleur (RD-RX812 or RD-RX817 for Di2), and swap the cage for the one of a XT RD-M8000-SGS rear derailleur. The body of the RD-RX812 seems to be indeed closer to the one of the RD-M8000, both are designed around 11-42 11-speed cassettes ...and it looks like the cages of the RD-RX812 has the same part reference as the one of RD-M8000-GS (short cage). Considering that the RD-M8000-SGS (long cage) has a capacity of 47, that is enough for a 46-30/11-42 setup, it seems like a more elegant solution: the parallelogram works within their design specs, only genuine Shimano parts are used. The drawbacks are: the rear derailleur must be replaced, and the cage replacement is not as easy as fitting a goat link. This video shows this hack, but pushed further with an 11-46 cassette and an Ultegra 50/34 crankset. This other video (thanks DoNut) applies the same hack to a 1x drivetrain, it shows how to replace the cage but the shifting should be seriously compromised as the parallelogram is not designed for this kind of cassette.

Given the second hack seems much more elegant, I'm surprised to not see more occurrences of it online — the price doesn't seem to a valid argument for me, as people able to afford RX810 equipped bikes can afford swapping a rear derailleur and cage plates are easy to find online (for around 25€). So is there a catch I didn't think about?

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  • Funny, I've just watched another video which is related or the same mod. Going 11-51t there.
    – DoNuT
    Commented Feb 7 at 13:24
  • @DoNuT yep I saw it as well, but that would lead to the same problem than using the RX810 with a 11-42 cassette: very poor shifting performance on the smaller sprockets. Using the 12-speed derailleur he mentioned in another video seems a much better solution, just a pity that pull ratios and cable pulls are not published to confirm it. But the mod proposed is "your" video is indeed the same as the one I mentioned in the question, but this use is out of spec, while doing it on a 2x is much closer to original specs, so I'm surprised of the small attention it gets.
    – Rеnаud
    Commented Feb 7 at 13:38
  • I’ve got my gravel bike set up with a 2x Di2 GRX drivetrain and a wide-range cassette, using the software to make sure I don’t end up crosschained. Is that in scope, or are you only interested in mechanical setups?
    – RLH
    Commented Feb 7 at 14:34
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    I'll save answering for someone who has lived with this setup, which I haven't. It seems cool and pretty much no downsides other than the niche application and the relatively advanced nature of it. People do sometimes overestimate how much more useful having such a wide range actually is, though there's no doubt it has its place. Commented Feb 7 at 18:59
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    I’ve got an RX817 on an 11-42 with a GRX Di2 front derailleur. We certainly didn’t have to take apart the derailleur, and I don’t remember having to do any programming shenanigans to get the 817 to work as part of a double system.
    – RLH
    Commented Feb 8 at 5:07

1 Answer 1

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I have a bike set up with

  • CS-M8000 11-42 11-speed cassette
  • RD-RX817 rear derailleur (GRX Di2, 42tooth max)
  • FD-RX815 front derailleur (GRX Di2)
  • FC-RX600 crankset (46/30)

This set of equipment has a 37-tooth difference between small-small and big-big, which exceeds the 31-tooth capacity of the RD-RX817. The Di2 programming, however, allows me to lock out problematic combinations, keeping the gears I can actually access within the bounds of the derailleur capacity.

The specific software setup I use is “front derailleur auto-shifts to track position within the cassette”, with the right auxiliary button set to downshift the front if I need more torque in a hurry. I’ve got the non-Di2 brake/dropper lever on the left (and thus the only way I could end up in a big-big combination would be a software fault that bypassed the software bad-ratio lockouts and a second fault that (software, mechanical or a low battery shift-lock) that kept the bike on the big chain ring.

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  • Thanks for the detail, interesting configuration and nice illustration of a benefit of an electronic transmission — in combination with a dropper. Also good to know that the 1x derailleur can handle more than a simple 11-42 cassette (but as I wrote in another comment, given the cage length is identical to the one of the M8000-GS — same part number for the inner plate, I would not be surprised that this rated capacity of 31T is largely underestimated). I'll leave the question open though, because it doesn't address the question of the cage swap.
    – Rеnаud
    Commented Feb 9 at 9:06
  • @Renaud: Certainly, and completely understood on keeping the question open.
    – RLH
    Commented Feb 9 at 10:54

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