I am planning top build a cross bike with an 1x Shimano Di2 Setup (11 speed). For my riding the optimal cassette would probably be a 11-36 cassette with a 40t chainring.
If I want to go with Di2, which rear derailleur will be best suited? For the long cage grx (RX817) and XT Di2, Shimano says the minimum for the largest cog is 40t. For the mid cage version (RX815) max is 34t. So, what to do if in between those? I guess both would work? Hast anyone tried this setup?
The purpose of the bike shall be a race-oriented cross bike, but I will also use it for some fast-paced single track gravel adventures. In between the off-road segments there is stretches of pavement. The largest chainring possible is 42 or even 40 at max. I am afraid that with an 11-42 cassette I will be spinning in one of the highest cogs constantly when riding pavement. With an 11-36 cassette I would probably use one cog more towards the middle.
Perhaps I will try with the longer cage, so I can switch between casettes if necessary. 11-36 for cross, 11 (10)-42 for everything else.
Another data point: Been running 11-36t SRAM PG-1170 with a GRX Di2 RX815 RD for the last 1000 miles or so with no issues. This is all with a 46/34t chainring in front. Just needed a slight tweak of the B-screw during initial set up and no issues since.
Yes, exact same cassette and only B-screw adjustment needed. I did have to go back to original Shimano pulley wheels, they seem to have a bit more play than other after-market brands and improved shifting.