Originally Posted by
grinningfool
Addressing your original post, you say you are using an XT 9 speed cassette with 105 hubs. The 105 hubs are 130mm, and you will need a 135mm mountain rear hub with the XT cassette. Otherwise, you'll need to use a road cassette with your 105 hubs, which won't give you the low gearing you desire.
No, that's not true. A MTB cassette will work fine on a 130 mm road hub.
As to the OP's question about disc brakes. Road brake levers (STI's among them) pull less cable than brake levers intended for V-brakes and some mechanical disc brakes. Thefore you need disc brakes that are compatible with the shorter cable pull of road levers and Avid's BB-7's are compatible. Their other disc brake models are not.