Grivooga

I have a MTB that I'm building up with dirt drops and STI levers. I need to make the road triple levers work with an XT front derailleuer. Apparently shimano uses a different cable pull length between their road and mountain triples. Does anyone know where to find the cable pull lengths for these two shifting systems so I can calculate the ratio and make myself an adapter pulley. JTek makes a shiftmate straight pulley for doing this but the cable routing on this bike leaves me with nowhere to mount it so I have to go custom (and custom will be financially cheaper since I have access to the tooling to do it).
I've had a bunch of gotcha moments with this build and am really wishing I'd just gone with barcons and Tektro RL520 levers but at this point I'm too far into it scrap what I've got and rebuild. I may do it anyways if these STI levers are too problematic with the dirt but not today.

Mudu93

The simple and most economical fix is to get a road front derailure. They are cheap and easy to find. If you must have the XT fd then scrap the brifters and go with barcons like you said.

Retro Grouch 

Been there, done that, got the t-shirt.

Try to look at it this way: You're a lot smarter now than you were when you started. To me a big portion of the benefit of these kinds of projects is the "stuff" that I learn about through my mistakes. Some people pay thousands of dollars tuition to go to school. Our education comes from from the bike parts we buy that don't work together.

Grivooga

I already tried a road derailleur that was meant for 50-42-30 rings. Using that part fixes the indexing issue but then the chain drags across the bottom of the derailleur cage when I'm in my smallest ring, a 22, and in the middle of the rear cogs. Doing the math (napkin math so I'm ignoring the seat tube angle to make it simpler) the difference in diameter between my largest (42t) and smallest (22t) ring is 3.503 inches. The difference on the road rings would be 3.184 in. That means the mountain derailleur needs a cage that is .318 inches taller than the one designed for the road rings; though in reality the road cage is taller than it strictly needs to be and an additional .2 would probably be enough to get by here. I guess I could probably get away with using the road derailleur if I used a 24t bottom ring as then the diameter difference would be the same, though the curve of the derailleur cage still wouldn't match the curve of the big ring (but that could be dremel adjusted).

Still even if it worked without extra persuading I don't see how that's cheaper than my proposed method. I can find the ratio and turn myself an adapter pulley on the lathe. I'm not billing my time since this is a personal project so my only cost is a small piece of scrap aluminum, a bolt, and two nylon washers. Hell if I'd stop writing this I probably could have just gone and borrowed a MTB, measured the cable pull and been half way to done in the time I've been responding so far.

Any opinions on whether It would be alright to drill and tap the underside of the downtube about 2-3 inches in front of the bottom bracket? I'm guessing I'd have to helicoil it if I want the threads to be sturdy. This'll be on a 2005 Specialized Hardrock which has a pretty beefy feeling Aluminum frame so I'm thinking it might be ok so long as the pulley is well supported to minimize side-loads since I don't have any metric heli-coils. I really wanted to get it tucked up out of the way but upon further inspection placing it in my first choice location, the back of the seat tube right below the derailleur clamp, won't work very well, there's just not enough space and the angles aren't very good. Underside of the downtube seems a bit more damage prone but the geometry works better.

Shimagnolo

I'm using an Ultegra triple FD with an XT drivetrain and STI's w/o problem.
The chainrings are 22/32/48.
Initially, I had the stock 44T large ring, but the cage was so deep it hit the chainstay unless I mounted the FD 3/8" above the large ring.
So I swapped the 44 for a 48, and got better shifting, and a taller top gears.

Grivooga

Nice to know. I guess the Ultegra cage must be significantly deeper than this old one I have here. I don't even know what it is, there are no markings on it but it feels pretty good otherwise.

stedalus

I had an Ultegra double FD on my 46/33/22 touring bike and had no clearance issues. Tiagra FDs are standard on most touring bikes with STI shifters, so the high zoot stuff is not necessary unless you want it.

I would be worried about stress cracks from a hole drilled into the frame, and it seems like an extreme measure when a used Tiagra FD can be had for less than $10.

LarDasse74

+1

SHimano road triple derailleurs should work with a 42-32-22 setup - I have seen much weirder arrangements working fine.