I suspect that because GRX only comes with flat mount calipers, and there's no Shimano standard for combining FM calipers with 180mm rotors.
Yeah, weird thing was that they didn’t list the br-rx810 as incompatible with the 180mm xt rotor.
Worst case scenario is me getting a pair of ugly ultegra rotors, but rather not.
Is the frame flat Mount?
@nogk started
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I suspect that because GRX only comes with flat mount calipers, and there's no Shimano standard for combining FM calipers with 180mm rotors.