Originally Posted by
zen_
You should be able to run an 11-40 cassette w/o an extension on the GRX derailleur. Here is a video of the setup with an 11 speed GRX system using the 48/31 crank. With that setup the only caveat is not using big-big (48-40).
https://www.youtube.com/watch?v=wNeYYM9_aeM
Not trying to sway you either way (I like to fiddle with things too), but the GRX 600 46/30 crank, 400 derailleurs, and a Sunrace 11-40 cassette would most likely work well.
That does look convincing. However, after some more digging, I learned that my ST-405 left shifter pulls more cable than MTB left shifters, meaning that with proper limit screw settings, I should be able to use an MTB double front derailleur, e.g. 10 speed XT FD-M786 (if it pulled less cable, I'd be screwed). Therefore I can use an MTB crankset, e.g. 10 speed XT FC-M785 (or SLX, etc from same era) which came as 40/28, 38/26, and 38/24. Chainrings are conveniently available for these cranks still. Although I was kind of hoping for a "road" build, I've pretty much only ridden MTB cranks for the past 15 years or so, and it's fine. Better yet, I have the 40/28 Shimano crankset and FD sitting here on another bike, as well as a SRAM X7 39/26, so I can swap over the parts and try these out with 11-36 cassette. Later I can decide whether to try for the 11-40 (would be Sunrace CSMX3 1140t), which would be good especially if I went 42/28 with the SRAM crankset.