2x grx Di2 with 1x RD with an 11-40 cassette

Just wanted to post this here. Got this setup to work flawlessly and wanted it documented for everyone in case you wanted to go this route.

Items needed:
RD-m8000 pulley cages
RD-RX817 pulleys
116 link 11 speed chain
Limit screw fully maxed out
48/31 grx crank
XT-CS-m8000 11-40t

This effectively allows you to run a 48/31 with an 11-40t cassette with no issues whatsoever. Including running it in a 48/40 setup (though I never would).

The 11-42t will not work(without some b-screw finesse) so please dont order it hoping it will shift idyllic.


Shimano lists the 817 as working with the 11-42. I got a 11-45 to work with this setup, but was let’s just say very creative with the B Screw. I wouldn’t make it a daily driver. 31-45 was very welcome up Lookout ridge in lastyear’s Sea Otter Gravel race.

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I’ve run the 11-40 cassette with a Di2 Ultegra RX derailleur with just a B-screw adjustment. Love the range :exploding_head:

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Right, it can “work” but the 11-40 requires nothing creative and shifts perfect. Have a buddy running a 1x with the Campagnolo 9-45 but you gotta be cognizant of that last shift lol. Like dont be hammering 300w and expecting a prototypical shift. However, the 817 with the 2x and 11-40 shifts pretty identical to the 815 with the 11-34.

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Correct. You can’t just grab 3 gears and hammer. It’s more of a one cog at a time thing with the big bailout gear. Makes me think going mechanical 12 speed with a mullet cassette would be a better idea.

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Maxing out the limit screw might not be the way to go. That causes it to no longer sit flush against the stop which over time will eat away at the stop. There’s a few vids on Youtube showing this issue.

Easiest way to get 11-40 to work with RX815 without issue is using a Goatlink 10 or 11. No other components needed. On the Goatlink the derailleur stop is angled further back meaning the derailleur screw will sit flush limiting excess forces.

Really interesting to see this! I used unmodified 2x11 RX810 mechanical derailleurs with 11-40 and 46/30 for an ultra last year and it was reliable but far from being ‘smooth’. I described it at the time as shifting like a lower tier groupset.

I hadn’t thought about swapping pulley cages though, and I do have a spare RX812 rear mech from a bike that used to be 1x. Will the 810/815 cage fit an 812/817 mech or does it need to be an m8000 cage?

(And just in case my logic is off: my understanding is that it’s the parallelogram that determines cassette range but the length of pulley cage that determines whether there’s enough total tooth capacity for a front mech, so the goal here is to use a 1x mech with a 2x cage to maximise both capacities at once)

I’ve been running the same set up but with a larger 16t guide pulley and an 11-42 cassette with no issues. Limit screw is not even maxed out. I don’t think I would be hammering away and quickly pulling across the cassette, but I am quite confident with the shifting quality

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This is the place to post photos of sexy Salsa Warbirds wearing identical outfits, right?

Been running an 11-40 cassette from the day I bought the bike. No issue and it makes life much easier when the daily TSS is above 600 and you still have hills to climb.

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I would need an ebike at that point :rofl:

Is there something different about 12 speed Di2, or would these same mods likely work with mechanical GRX 12 speed?

i though that if you have a 815 FD the system won’t work/recognize the 817 RD?

i could be wrong, but i remember reading that somewhere (which i can’t remember right now)

Not sure on app integration, but it fires from the shifters exactly the same. I had it set before to jump up/down 1 cog on the rear when changing from big/small up front. After connecting the 817 it still does the same.

nah, it’ll work OK. The only thing is you may or may not be able to setup synchronized shift.

There is some more info here: GRX Di2 - Bike Forums


did you try a bigger chainring (50/34, 52/36) or did you just stick to the grx 48/31 one?

48/31 currently. Will probably end up using a 50/34 though.

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