Originally Posted by
paramount3
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.
OK, that doesn't work. SRAM X7 Crankset with 42/28 almost works with XT FD-M785, but the derailleur cage jams against the big ring sometimes. I can make it shift on the workstand, but I doubt it would work in the real world. I think the problem is that this derailleur is for chainstay angles 66-69 degrees. I calculated the chainstay angle of my frame as 63.8 degrees. I don't have a braze-on so there's no way to shim the derailleur mount at an angle (no easy way, anyway, that I know of).
I then tried the Tiagra 4700 derailleur with the SRAM X7 42/28. Here, the chainstay angle is right, and I think there's a chance it would shift the 42T ring (specified min 46T for the big ring). However, the chainline for this derailleur is 43.5 mm, and it need another 4-5 mm to reach the big ring. I could try a GRX FD-RX400, which has a 46mm chainline, but that might not be far enough, and I can't find an FD-RX400 anywhere. I am considering chainring spacers.
unterhausen it sounds as if you have done this with a SRAM MTB crankset. Do you have the details for how to make this work?
The other thing I will try is a Shimano trekking front derailleur. I have another bike with a FD-T610, which I believe is the FD-T610-3, designed for 63-66 degree chainstay angle. I will try this as it should match my chainstay angle of 63.8 degrees. However, this is a triple mech, so I'm not sure it will handle the 14T drop from 42 to 28.
I really thought all of this crap would be solved with the GRX groups, yet here I am hacking again. 42/26 crankset with 11-36 cassette is a wonderful range for all sorts of road/gravel/touring cyclists, there is no engineering reason for this not to exist, yet it doesn't exist.