Shimano list that the GRX rear derailleur minimum low sprocket is 40T. If the biggest sprocket on your trainer’s cassette is a lot smaller than that you’ll experience sub-optimal shifting performance because the guide pully will be too far from the larger sprockets. It’ll work it just won’t have that super smooth Shimano shifting.
So, if he happens to have the RX810 2x version, it will be great. Even with the 1x version, it should be OK but may lack a bit of snap is all. Worst case he can make an easy B-Tension screw adjustment and reverse it when heading back outside.
Yup, good to go. Do keep in mind the potential need to tweak your barrel adjuster a tad if there is any noise and misalignment when you install the new bike. Tolerances exist and sometimes we have the need to adjust the tension a bit when swapping between a trainer and wheel.
If you do make any adjustment, remember how much and which direction you changed, or jot it on a sticky note. You will need to revert back when heading back out onto the road.
Grx cassettes are a hair narrower than Ultegra. Can fit a 11-34 on a 10 speed freehub, 11-32 Ultegra doesn’t fit.
You will probably need some barrel adjustment but after that, it’ll work just fine. If you’re going for erg mode, I’d find the quietest gear with the straightest chain line and send it. Doing a barrel adjustment every time you swap from indoors to out will get old after about the 2nd time. If it were me, I’d find the cheapest cassette of the same size, bc I know I’ll tell myself it’s fine, but the reality is that’s just one more step that’ll keep me from doing it, esp this time o year when motivation is lacking!
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