Originally Posted by comradehoser
Out of curiosity--Say I found a super-sweet rigid mountain bike frame (Trek 950) that is really large (22"), and not so good for me for trails due to standover, but is sized and angled pretty perfectly for a cross bike and has enough clearance for 700c wheels and knobbies, is there an inexpensive workaround that would let me swap in 700c for 650s? Like some sort of extreme-range cantilever brake or a brake bridge with bosses?
You kinda threw a joker into the deck by mentioning 650C, which has nothing to do with this bike.
Mountain bikes use 559 mm wheels, not 571 mm (650C).
It is pretty common to be able to fit 622 mm (700C) wheels into a MTB frame/fork, as long as you don't want to use seriously fat tires.
The cantilevers aren't going to work, but it is very often possible to use caliper brakes for this application.
Try installing a pair of 622 mm wheels and measure the reach as shown at:
http://sheldonbrown.com/reach
Then you can figure out what type of calipers are needed.
You may also need to deal with the recessed mount issue...See:
http://sheldonbrown.com/recessed
Sheldon "Mongrel Fan" Brown