Originally Posted by
msamsen
I may be wrong, but I don’t think that’s correct. Unless the 105 FD is different from current Shimano offerings, the pull-ratio of the Shimano STI road and mtn shifters are different. So, as I understand it, you can’t mix and match road and mtn components on the front. (No issues on the rear.)
I’d be happy to be proven wrong!
Then I’d go with GRX components on the bars and XT on the driveline.
The shifters & the derailleurs need to have matching pull ratios. Having a "group set" is just a convenient means of catagorizing use cases.
Shimano 8,9,10 speed road/mountain rear derailleurs are interchangeable. Most people make the switch to mountain rear derailleurs for the additional capacity & larger mountain cogs. So the "9" of the 3x9 shifters I suggested is a "Go."
The front derailleurs are not always interchangeable. There are definite road fronts & mountain fronts. It depends on the vintage & group set. Others here would have more authority on the specifics than me. What I do know is it's easiest to match the front derailleur to the front shifter & save the compatibility trouble.
In this case, you specify you want a road brifter. So: road Claris, Sora, 105, or Ultegra, "triple" derailleurs are your options. All ought to work with the "3" of the 3x9 shifters I suggested above. So you are a "Go" there as well.
The crankset is built around a particular bottom bracket width. The spacing between the rings & chainline is the same. The primary difference is mountain has a smaller tooth counts across the board. But a tooth or 2 difference in gear diameter is insignificant as far as derailleur height/cage curvature is concerned. So buy the triple crankset that suits your fancy.
Where you often run into fitment issues is puting doubles on (68mm) mountain frames...They exist, & there is a lot out there. The larger diameter rings or narrower crank arm q-factor sometimes causes issues hitting the chain stays. It's seldom an issue going the other way.
If you have an issue with the triple road crank arms from a triple road crankset hitting the chainstays, you may be forced into a mountain triple crankset to move the pedals/crankarms wider...but I don't think that'll be an issue.