Originally Posted by
TimothyH
The only difference between the 8000 series rear derailleur and the GRX are cosmetic. There is zero doubt in my mind that GRX 2x systems will also work with a much larger cassette than the officially supported 11/34.
The GRX 2x inner chainring is 80 BCD and so a 26 tooth little ring is possible. The outer ring is still 110 BCD and I don't know if 42 tooth rings are available to make a 2x 42/26 setup.
-Tim-
You're right that you can get away with a larger cassette to get the gearing low enough. I'd still never need a 46T while touring.
It appears that GRX is following the rest of Shimano's 11s line by using an offset bolt hole pattern that's not compatible with standard 4-bolt chainrings. So even though you might be able to find an 80 and 110 BCD chainring in lower tooth counts, unless they're specifically made for the Shimano offset pattern, they're not going to fit.
It's for these kind of reasons that I long ago abandoned drop bars on touring bikes. It's so much easier to use mountain bike drivetrains that are already optimized for low gearing. Throw on some Jones bars or the VO Crazy bars and you can move around and stay comfortable all day.