(Oh, those offal, earful, oreful, puns! Cæsar and desist!)
photo by Jeff Archer, First Flight bikes
Roller-cam brakes are a type ofrim brake. In connection with this article, please read the lead article about rim brakes. It covers, among other things,
Most brake problems result from excessive friction or poor installation of the cables, not poor setup, or poor-quality brakes. Also see the article on cables for information on cable selection and adjustment of brake cables and brake levers.
A roller-cam brake works like a centerpull caliper, but its pivots are attached directly to the frame or fork. Instead of having a transverse cable, a roller-cam brake 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 traditional cantilever brakes, 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 cantilevers do not work on cantilever studs made for conventional cantilevers, because conventional 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.
Roller-cam brakes were developed by Charlie Cunningham, one of the Marin County, California mountain-bike pioneer bicycle builders. Roller-cam brakes were first sold by Wilderness Trail bikes, and were made in quantity by SunTour.
Cunningham has continued to refine his brake designs with "toggle link" and "lever link" versions replacing the cam and rollers, using the same brake arm and pivot locations.
Install the arms with them at their maximum spread and tighten the bolts that hold them to the frame. This is how you set the springs.
A roller-cam brake uses a conventional brake lever, not the special lever with longer cable pull used with direct-pull brakes (V brakes). Adjust the cable so the cam sits in the low-mechanical-advantage part of its curve when the brake is not applied. (See the article about cables). Adjust the spacing between the rollers or the extension of the brake shoes so the transition to higher mechanical advantage occurs just as they come into contact with the rim. (See information on brake shoes in the lead article on rim brakes.)
Check the brake shoe adjustment frequently due to their migrating higher and higher up on the rims. If you don't keep on top of the adjustment, they will eventually start rubbing on the tire sidewall. Many thousands of tires have been ruined by this. Also, as the shoes wear, the transition point from low to high mechanical advantage gets farther from the rim, and eventually the cam may run out of travel, and so the brake shoe travel also must be adjusted..
|Articles by Sheldon Brown and others|
Last Updated: by Harriet Fell