I've been looking at a number of vintage mountain bikes with the thought of building a touring bike. Many of the 80's mountain bikes have a u-brake on the chainstays. All my current shopping is being done on CL and ebay, so it's hard to see these bikes in person. Do you think or know if u-brakes would interfere with mounting fenders? Thanks!
Back in 1988, I bought a new Trek 830 MTB. It had the U-brake and was a very solid bike that lasted until just last year, when I finally broke a drop-out. I strongly suspect it was the result of clamping the bike into a trainer the previous winter, and not the result of riding. I bough it for road use and so it was very tall, with the horizontal top tube.
Anyway, I had several types of fenders on it over the years and had no problems. My tire sizes ranged from 2" down to my favorite 1-1/4" slicks. I miss that bike. It had all the attachment points of a true touring machine and was extremely comfortable for long road trips when fitted with different bars and saddle.
I recommend finding an MTB that has the Shimano BioPace or Sakae OvalTech chainrings. Sheldon Brown has a good write-up on this on his site.
The only issue I could imagine would be if the brake bridge was not drilled, missing or too far from the wheel for the fender to reach. U-brakes usually have very long reach so tire clearance, etc shouldn't be an issue. Chain stay mounted U-brakes had a reputation for collecting mud so if you're planning on riding on a lot of muddy roads you might want to pass on them.
I had a mid '80s Specialized Stumpjumper with U-brakes that I toured, commuted and went off road with. Fitting fenders was no problem, but the U-brakes were the reason I dumped the frame and got a Bridgestone MB-3 frame off of e-Bay. They collect mud and slop so that the pivots seize up and the brakes won't work. Because the reach is so long, the pads contact the rims further and further up on the rim as the pads wear, until the brake pads are rubbing the sidewalls of the tire when you apply the rear brakes. I finally stopped using the rear brake and just used the front brake, then I gave the frame away and used the parts to build up the MB-3 frame. My suggestion is to find a steel framed mountain bike with mounting studs for cantilver or V-brakes.