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:

https://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:

https://sheldonbrown.com/recessed

Sheldon "Mongrel Fan" Brown

yeah, I guess I got lazy using 650c as an abbreviation for 26".

I'll look into your suggestions!

Thanks Sheldon!

say I wanted to keep the cantis--somebody mentioned these:



good idea or bad idea?

Can't be bad, might be good. Looks like Mavic makes them, which is a plus.

max. tire clearance for the mavic bridges (according to speedgoat.com) is 25mm... and $25 each...

I'm aslo working on this project and am compiling a list of long-reach canti's / v-brakes, thanks to cross-posts in MTB and Cyclocross.
So far I have:

Onza H.O.'s (Onza's out of business but some of their stuff is still around - check E-bay there's one up there now and bidding's $25)

Avid Tri-align at $75

and
paulcomp motoBMX v-brakes at $125



If anyone else knows of good alternatives, please don't be shy...

also an interesting quote about the paulcomp motoBMX brakes...
"When you convert from a 26" to a 700c, drop levers will work fine,
but if you go back to 26", drop levers are absolutely not recommended
for use with these brakes; in this case you must use a long-pull lever."

Sounds like an interesting project. I gotta ask if the frame is rigged up for disks, this would be a superb set-up. 700c wheels, nice cross or touring tires and some disk brakes... but...it's probably not is it? Another thing you can do is find some super skinny 26" mtn. bike rims and run some conti 1.5 cross country's (those are sweet tires!). Years ago both Sun and Mavic made 26" versions of some road rims. Mavic's version was the venerable MA-40 while I don't remember which Sun model was miniaturized. I haven't tried to find a pair of these since somewhere around 1990, but maybe a little searching on the net would be fruitfull. Anyways....you planning on running drop bars or straight bars...? I'm curious where your going with this.

you can get a few 26" tires in widths from 1 - 1.25 inches that are skinny and roll fast you would have to change the gearing on the cranks though as you will spin out before you top out speed wise.

A super sweet deal isn't super sweet if it isn't the right product or size. I love those commercials with women who go to Mervyns because they had a sale and tell you how much money they saved. They didn't save anything if they weren't going to buy the clothes in the first place... they just spent less!

I see great deals on Sidi shoes all the time, but very few men wear a size 46 or 37. Until I join the circus or have my toes amputated, I pass on those deals too.