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

  1. 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.

  2. 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?

    1. 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.

      1. 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.

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