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Roller-cam Brake
A type of brake which uses a triangular cam. It has two see-saw like arms, pivoted in the middle on cantilever-type studs. The cam is pulled by the cable, which is attached to the narrow end. As the cam is pulled, its sloping sides push outward on rollers which are attached to the upper end of the brake arms. Roller-cam brakes permit the use of variable ratios by making the sides of the cam curve instead of being straight. Typically this is done to make the shoes travel in toward the rim fast, then more slowly as they engage...this gives more mechanical advantage in the actual braking range of travel, while allowing the shoes to back off farther from the rim than if the mechanical advantage were consistent throughout the travel range.
Compared to cantilevers, roller-cams have the advantage of not protruding past the sides of the frame, which made them popular in the late '80's when there was a fad for placing the rear brake under the chainstays...you couldn't use conventional cantilevers there, because the cranks would bump into them.
Unfortunately, roller-cams are more difficult to set up than conventional cantilevers, and they make for more-difficult wheel changes.
Roller-cam brakes do not work on studs made for cantilevers, because cantilever studs are mounted inward from the rim, while roller-cam studs are outward from the rim. "U-brakes" are interchangeable with roller-cams.
With both roller-cams and U-brakes, there is a tendency for the brake shoes to strike higher and higher on the rim as the pads wear. If the pads are not checked regularly, they eventually start to rub on the sidewall of the tire, destroying the tire in short order.
The Odyssey "Pitbull" is a caliper-type roller-cam unit, popular in BMX and recumbent applications.
For more detailed information, see our article on roller-cam brakes.
