Today I read an interesting article on the Gravel Cyclist website. I'm currently running on a Shimano GRX groupset with the RX810 derailleurs and 48-31T cranks. According to Shimano, the highest cassette you can run on an RX810 derailleur is 34T. But, as the Gravel Cyclist learned, you can actually run as high as a 40T cassette.
Strange timing in that I was on my way to the bike shop today to pickup a rear wheel for training. I was gonna pickup another 11-34T cassette but, after reading this article, I bought a Shimano XT CS-M8000 11-40T cassette instead.
I can confirm that, yes, this indeed does work. Like the Gravel Cyclist video explains, you have to adjust the tension using the "B screw" (as Shimano calls it). I nearly screwed up and didn't give it enough tension. Just have to be careful with it.
One tip when adjusting is to switch to the second biggest cog just before the big 40T one. That one is close to a 34T so you shouldn't have any problems. From there, adjust the tension so that there is enough room to allow for the switch to the 40T cog. Once that's done you should be able to switch with no problems.
On mine, I had to adjust the tension on the shifting cable a bit as well as a bit of adjusting the L-limit screw. I might have to tweak some things but going back and forth between wheel sets should be just a matter of adjusting the tension a bit. Even then, I might be able to get away with going back and forth between 11-34T and 11-40T cassettes without adjusting anything. Just have to dial it in a bit I think.
Very pleased to hear this! Means I can run my 700c gravel wheelset with better low-end gears. Nice!