Forty years ago, if you had what passed for a sportbike back then you had to put up with some pretty basic brakes. The cutting-edge technology of the time used drums: semi-circular brake shoes mounted inside an oversized wheel hub. When you pulled the brake lever, a cable turned a cam inside the hub, pushing the shoes out onto a drum inside the hub, thus slowing the bike down. They look archaic now, but offered good stopping power, albeit with little in the way of feel or control.
But in 1969, Honda's CB750 introduced brand new braking technology. It was the first ever road bike to come with a single front brake disc, and the drum brake was relegated to the rear wheel.
THE BASIC LAYOUTS
Disc brake design is pretty simple in principle. Small pads of friction material are mounted inside a hydraulic caliper. When the lever is pulled, hydraulic pressure is transferred to the caliper and it pushes a piston out, forcing the friction pads against a disc. The caliper is mounted on a sliding bracket, so as the piston moves out, it first pushes the piston-side pad against the disc surface. Then, the whole caliper moves back on the sliding bracket, pulling the pad on the other side against the other side of the disc. These sliding calipers are often used on rear brakes now-they're simple, light and cheap to make.
But sliding the caliper on a bracket wastes some braking effort. So, caliper designers began working on opposed-piston designs. In this design, the caliper is bolted solidly to the fork leg, and there's a piston on each side of the caliper, facing inwards (hence 'opposed'), and the hydraulic pressure from the lever is fed to each piston. When you pull the lever, both pistons move out together, clamping a brake pad onto each face of the disc and applying the braking force.
MORE PISTONS = MORE POWER?
The braking force is produced by friction pads pushing onto the disc. But the effect of this force varies according to the size of the disc. If you use a larger disc, the force is applied at a further point from the wheel center, giving a larger leverage effect. You can feel this yourself with a cordless drill: if you grip the large part of the chuck, you can stop it from turning quite easily, but if you try to grip the narrow part it's much harder to stop.
So, applying the braking force to a larger disc gives more braking force at the wheel. But larger discs are heavier, which adversely affects steering inertia, suspension movement and acceleration.
Honda's CB750 brought big...
Honda's CB750 brought big changes back in the summer of '69.
The trick is to make the braking area narrower (in the radial dimension), so the heavy, steel part of the disc is as narrow as possible. But then, with only one piston on each side of the caliper, the area the pad is working on ends up very small, making it hard to create enough frictional force.
The answer is to have more than one piston on each side of the caliper, making the pad wider and narrower. This gives more area for the pads to work on, increasing the friction without needing a deep, heavy disc. Adding more pistons gives a wider working area for the pad, and six, eight, or even ten piston calipers have all been made.