kdog said:
U brakes! Do you mean those horrendously useless calliper style rim brakes that sit down near your bottom bracket bolted onto the chain stays?
Urrgh!! I breifly tried one in the early 90s- there's a reason they quietly died.
U-brakes worked great if you bothered to set them up worth a damn. They're not really an improvement over V-brakes, but they are much superior to most traditional cantilevers that were the main alternative at the time.
The main drawback to U-brakes was their tendency to clog up with mud when ridden in mud, which is really no different from caliper brakes. And there is the matter of them needing special frame mounted studs to attach them, like cantilever studs but differently located and larger in diameter. They're still used commonly on BMX bikes, though in that world they're more often called "990" brakes, after the Dia Compe model that was industry standard for a long time.
U-brakes have the situational advantage of using traditional short throw levers, unlike V-brakes which needs special long throw levers. That makes U-brakes compatible with most e-levers.
Both cantilever/V-brakes and U-brakes have adapter plates available for mounting them on the brake bridges of frames intended to use caliper brakes. As Amberwolf pointed out, adapter mounted versions lack the stiffness of frame stud mounted versions, but they're still generally more powerful than calipers of similar reach.
Apropos the OP's question: You can probably find dual pivot caliper brakes of appropriate reach to mount on the center bolt hole above your wheel. That's the simplest way to go. Measure the vertical distance between the bolt hole and the braking surfaces of the rim; that's the reach number you'll use to choose a brake. Something like the Dia Compe DL800 might work (up to 79mm reach), or your frame might require something longer.
