Originally Posted by
HTupolev
RD-R812 is not "long cage." Its cage is actually shorter than the one on RD-RX810, since due to being a "1x" derailleur, it does not need to be able to wrap as much chain.
Thanks for the clarification. I wasn't considering the 1X factor.
Originally Posted by
HTupolev
Personally I'd probably err toward the RD-RX810 for what you're trying to do. It can tolerate a larger cassette than spec: I know multiple people running the RX400 or RX810 on their gravel bikes with an 11-40 or 11-42 cassette and a 16-tooth front difference, with pretty reasonable results.
I think that the RX-810 comes with the 2X kit that will be going on the new bike. Wonder why Shimano doesn't mention the max cog size of 40-42 in the spec for the RX-810? Their "consumer" literature only mentions the 11-34/36 cassette with the 2X group.