Originally Posted by
cetmefree
U brakes are similar to single pivot road calipers
No. U-brakes were a failed attempt at a 'better' brake back in the mid 90s. They mounted on studs similarly to cantilever or V-brake studs but they were of a different diameter and a different placement than either and thus can't be replaced by either. They wrapped around the wheel like center pull brakes and collected crap about as well

Often they were mounted on the below the chainstay where they collected mud.
U-brake:
Ugly U-brake