Shifting Big Gears with Shimano GRX: 2x 48/31 with 11-40 XTR Cassette

Shimano’s gravel-bike specific groupset, GRX, available for Di2, mechanical, 1x and 2x drivetrains, is a milestone in the evolution of the gravel bike. Shimano didn’t slap together a road / cyclocross group and call it “gravel”, rather they spent close to five years developing a groupset for what is arguably the fastest-growing genre of cycling, circa 2019.

shifting big gears with shimano grx 11 - 40

* Apologies for some camera auto-focus issues due to low-light conditions *

Shimano may arguably have the best shifting systems on the market for road, mountain and gravel, but the company has always been a little conservative. One example is the recommendations of maximum cog sizes with their derailleurs.

shifting big gears with shimano grx 11 - 40

A perfect example is the maximum cog recommended for the company’s 11-speed Dura-Ace 9100 mechanical / 9150 Di2 rear derailleurs. 30T was the stated max cog, but we debunked that myth and went all the way to 36T in another video.

shifting big gears with shimano grx 11 - 40

The recommended maximum cog size for Shimano GRX Di2 2x or mechanical 2x is 34 teeth. However, the GRX 2x derailleur cage looks capable of handling bigger cogs. Thus, JOM of the Gravel Cyclist crew got tinkering and set about debunking Shimano’s gear limits on GRX. Will GRX Di2 2x shift a 48/31 chainring combo paired to an 11-40 XTR cassette?

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  1. Avatar Stephen Poole

    Looks fine to me, not surprisingly. FWIW, earlier this year I had 42×27 rings with an 11-40 cassette and Ultegra RX RD and all the gears could be made to work. The chain was long enough for 42×40, and there was still tension in 27×11, not that I used either of these gears. It was also difficult to avoid rub in 42×35, possibly on the top of the cage. The 40T cog ended up being overkill, and I would have been much happier with closer ratios on the flat in Northern Europe; 11-34 would have been plenty.

    Last year out of curiosity I tried a variety of cassettes on another bike with 52×36 rings an a 5800SS RD. This was originally set up with an 11-28 cassette, but the chain was deliberately left a bit long just in case. Cassettes up to 11-36 were fine, 11-40 *would not* shift into bottom gear but 35T was okay, and with 11-42 it would shift onto the 37T cog but not off again; I had to remove the RD to free the chain. This was all in the stand, not on the road. Still, given Shimano rate that RD for 30T maximum I was quite impressed that 36T was okay,

    The 5800GS on another bike handles 36T with no problems and 40T (with a chain sized for 36T) was okay on the small ring. YMMV with all of this stuff depending on chain length, hanger geometry, etc.

    • JOM JOM

      Excellent, thanks for the intel Stephen… love it when someone else goes tinkering and breaking the rules.

      • Avatar skipper

        What is the practical difference between the 1x GRX815 RD and 2x GRX817 RD that prevents the longer cage 817 from working with a 2x setup? I dont understand why the “1x” RD would not work with a 2x setup.

  2. Avatar Dale Bartlett

    I have the same Di2 2X GRX setup, but with the RD-RX805 rear derailleur. I also tried the 11S 11-40 XTR cassette and I got good shifting with the rear derailleur clutch off. With the rear derailleur clutch on, shifting up and down the cassette was very slow. Not good. In your video, was the rear derailleur clutch on or off? Maybe the RD-RX815 rear derailleur works better with the 11-40 cassette than the RD-RX805 rear derailleur?

    • JOM JOM

      I neglected to mention the clutch was off. The extra resistance would make a difference. Generally, I always leave the clutch off unless I am expecting bad weather, etc.

      • Avatar Stephen Poole

        With the RX mechanical RD shifting was fine with 11-40 with the clutch on. One thing might be that with the clutch on I suspect the B tension needs to be a little higher to keep the pulley from contacting the 40T cog, but this only affected noise, not shifting. YMMV.

  3. Avatar Vitaliy

    Is there a noticeable difference between RX805 and RX815 derailleurs? Especially when it comes to cage length

  4. Avatar Tom craney

    Thanks Jom, no one attempts the real world work arounds like you. Do you think a hanger extender could be installed to jump to 42?

    • JOM JOM

      Hi Tom,

      I expect it would work, but like anything, you’ll have to give it a whirl and see how it goes. Sorry for not giving you a 100% answer.

  5. Avatar Simon Race

    The article and your narrative in the video make reference to XTR, but looking at the video when you pause it before demonstrating the Big Ring at the front, the cassette you have fitted looks like the XT. I’m not bothered about the weight saving of the XTR, so can you confirm that the cheaper cassette was the one in the video please ?

    • JOM JOM

      It wasn’t my cassette, it was a loaner, and it may very well be XT. Sorry for any confusion! Regardless of the model, it should still shift no problems.

    • Avatar Vitaliy

      For what it worth.. Have rotor 46/30 oval rings with R8050 derailleur on the front and 11-42 M8000 XT cassette on the back. GRX 815 with wolftooth roadlink DM works as a dream, even big/big combo.
      Was able to make it work OK with 11-40 cassette without roadlink

  6. Avatar John

    What derailleur model is that? I have been using GRX 817 with 34/50 chainrings and 11-40 XTR cassette.
    The longer cage seems to take up more chain that your setup-

    • Avatar skipper

      To be clear I understand… You are you using the GRX 817 RD (“1x”) with a 2x by setup (34/50 chainrings)

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