STI shifters with XT crank?

[bikermitch]

I'm considering building out a Surly Long Haul Trucker frame and was trying to work out the best component combinations.

I strongly prefer STI shifters compared to barcons.  My current touring bike has an XT rear derailleur with Ultegra STI shifters and works fine so I'm not worried about that.  I'm not sure what combination would work best for the front though.  I'd prefer to have 43/32/22 chainrings.  Can I put on an XT crankset and front derailleur or an XT crankset / Ultegra front derailleur combination and have that work with STI shifters?

[cyclesafe]

The XT front deraileur will work fine.  Your issue with Ultegra STI will be with the brakes.  You'll need Travel Agents if you use anything other than road disks or calipers.

[DaveB]

No, the XTR front derailleur is not likely to work with road STI shifters,  The geometry and cable pull for MTB derailleurs and shifters are different from those for road derailleurs andn shifters.  You will need a road fd to work with  STI's.  You can use the XTR crank with a road fd.  

[edmilkman]

Hi bikermitch, I was told each front derailleur works best with only the size big chainring it was designed for. An Ultegra designed for a 52 tooth ring will have problems with a smaller mtn 44 big ring and vice versa. This is because the "cage" radius is the same as the radius of the chainring it is designed to push the chain onto. Also there are problems of spindle length with different bottom brackets that may make the rings not line up well??  I have no first hand experience myself - keep asking

[DaveB]

I was told each front derailleur works best with only the size big chainring it was designed for. An Ultegra designed for a 52 tooth ring will have problems with a smaller mtn 44 big ring and vice versa.

Theoretically this is true but in reality a road fd intended for a 52 or 53T large ring will work OK with a 44/46 big ring.  The shifting won't be the very best but you probably won't notice the minor difference.  

Years ago Shimano had the RSX triple group that came with 46/36/26 chainrings and an RX-100 road fd meant for a 52T chainring.  It shifted fine with that "mismatched" combination.  

You probably can't use an MTB front derailleur satisfactorily with a road crank since the cage radius is too tight and the fd would have to sit too far above the chainring to clear it properly.