Another thing I like about the converted old mtn bike setup is the versatility of gearing and how easy it is to change. You can run an old mtn bike w/ 1, 2, or 3 rings up front and from 11-36 teeth in the rear. Major gearing choices for speed, hills, dirt, loads, trailers, whatever.

Currently running a triple (28, 38, 48) and sram 990 11-32 cassette in the rear because I pull a trailer so often on our hills, bike paths, and dirt, here.

I also like a double up front w/ about 34 and 50 teeth with around 11-25 road type spacing in the rear. This gives a little more elegant shifting/gearing when used mainly on city streets.