from the sheldon brown glossary
A form of cantilever brake that works like a centerpull caliper. The "L"-shaped arms cross over above the tire, so the left brake shoe is operated by the right side of the transverse cable. A U-brake uses studs that are above the rim, rather than below the it, as with conventional cantilevers. They use the same type and placement of studs as rollercam brakes do.
In 1986-88 there was a fad for equipping mountain bikes with U-brakes mounted underneath the chain stays. This provided a nice clean look to the seat stay area of the bicycle, and provided a somewhat simpler cable routing. In addition, since the chain stays are larger and more rigid than typical seat stays, the "problem" of flexing of the studs under load was reduced. Conventional cantileves cannot be mounted on the chainstays, because the cantilevers would get in the way of the cranks.
Although U-brakes were cool looking and powerful, the fad died quite abruptly when people actually started using the bikes that were sold with chainstay-mounted U-brakes. They had several serious drawbacks:
The inaccessible location made it very difficult to service or adjust the brakes.
They complicated the process of wheel removal.
They tended to get clogged with mud.
Due to the high-mounted studs, if you didn't monitor the brake shoe wear carefully, as they would wear, they would hit higher and higher on the rim. Eventually, they would overshoot the rim and start rubbing on the tire sidewall. This is one of the fastest known ways to destroy a tire.
Stabled = 91 Browning Test unit - Trimble Double U - kestrel MXZ Black - Kestrel MXZ White - 87 Chas Roberts Snow Leopard - 88 Overbury's pioneer - Proflex RF-1 team replica - Mk1 Thorn Nomad