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?
Thanks.
I think you would use the GRX RD-RX815. It calls for a min. large cog of 30T and a maximum of 34T.
I’d be pretty tempted to go with the RD-RX817 (SHIMANO GRX DI2 Rear Derailleur 11-speed (42T max low sprocket) | SHIMANO BIKE-US) instead and run an XT (or XTR if you’re feeling saucy) 11-42 or 11-40 cassette for a little more range.
I have an RD-RX812 (cable driven version of the RX817) and an XT 11-42 cassette on my road bike with a 44T 'ring and it’s great.
Thanks.
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.
While I am not speaking from personal experience, I have heard that Shimano’s derailleurs are quite good at accepting larger cassettes, and that an extra 2 teeth is not a problem.
If that doesn’t work, you can still use a rear derailleur extender like e. g. from Wolftooth.
I can speak from personal experience that this is true for mechaincal shimano derailleurs. But I have no clue how a di2-version behaves in this case.
The GRX-815 will comfortably go up to a 11-40. That was my set up (with 46/30 up front) on my prior gravel bike and it worked great.
Top info. Thanks.
I thought that RD was 1x specific. But if it works for you, either I’m misremembering or it works anyway. 
Edit: I meant the larger cage RD, I just got confused by the model numbers. But good to know the RD will comfortably take 36 teeth. 
I just checked the Shimano website - I was looking for cassette options - and it says:
I can confirm the RX815 is great for this. I went 2x during the summer with 11-36, then when fall hit pulled the FD and went 1x 11-36 for CX racing. It was really perfect.
Thanks for the confirmation. I guess the RX817 is only stated as 1x by Shimano, because of the higher max sprocket. Besides that, nothing to worry about I guess.
Just a data point for your reference: I am running a Shimano Dura-Ace R9150 Di2 (specified 11-30 max) with a 11-36 cassette without any other modifications. Runs smoothly.
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.
That’s the cassette I also intend to use. Thanks for the info.
I’ve also used an Ultegra 8050 medium cage (rated for 11x34) with an 11x36 cassette with no issues or problems.
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.
Same with the di2 stuff. +2 over large cog spec is no problem.