Airburst

How far below the rim is a typical cantilever brake pivot? And how far above the rim is a U-brake pivot? I got into a discussion today at the co-op about whether a frame intended to take 26" wheels and cantilever brakes could also take 20" wheels if it had U-brakes fitted instead. We got as far as putting an old BMX wheel on the back of my Saracen MTB briefly, while it was in a workstand, but we didn't have a U-brake to check if it fitted. What with a measurement being worth a thousand opinions and all, can anyone provide figures? I have no plans to actually do it (ground clearance would be an issue, losing nearly 50 mm of height) but think of it as a thought experiment...

fietsbob

When I built my touring frame 30 years ago, I had the wheels built and the brake parts in my hand.
Mafac cantilevers, the location of the brake boss is the most important adjustment.

so rather than a number, test fit the parts, where the tubes are further apart on the rear
the swing of the caliper around the brake pivot axis is different.
Boss location is a different radius from the hub axle, than if closer together, like on the fork.

once adjustable brakes, shoe mounting slots, in Mountain bike boom,
I noted the boss location got sloppier, because they could let stuff pass ..