Project idea: Upgrade old MTB to road groupset for commuter? I'm thinking of building up a fat-tire commuter bike. I need fenders, racks and wide tires since I live in the northwest (potholes, rain, more rain, hills). I also need it to be fast since I want to be fast on my commute. 

I was thinking about buying an old MTB and replacing the groupset with something like the Shimano 105. There's plenty of nice old steel MTBs on Craigslist for ~$100. I would essentially only want the frame.

The dropout spacing on the old MTBs is 130mm, just like the 105 hubs. I'd have new 26" wheels built around those hubs and use the 105 crankset, rear and front derailleurs. I'd also put in a drop bar and I think I should be able to use the 105 brifters with the MTB cantilever brakes. Then I'd finish it off with tires like gatorskins or marathons.

Does that sound reasonable? Anything I forgot to think of?
V brakes and mechanical MTB discs aren't compatible with normal road levers. You'll need either V-brake specific drop bar levers or cantilever brakes.
Road cranks have narrower chainline and larger chainrings and MTB chainstays are wider. You may run into compatibility issues here, too.
You probably won't be using the highest gears anyway, since Marathons or Gatorskins have huge rolling resistance compared to proper road tires
Fast, potholes and rack don't fit together. Get a messenger bag or backpack instead.
Drop bars and road components aren't going to make you that much faster. The most difference is in ergonomics.


What I would do would be MTB groupset with road cassette, V-brake compatible levers and bar end shifters.