When it comes to creating speed, most of us have a pretty good idea of what’s involved. Essentially, the faster we want to go, the more power we need to extract from the engine. What’s less obvious is that as velocity increases, so does the required braking force needed to overcome it.

In the bad old days of mechanical drum brakes, leverage and the self-energizing, or “servo,” effect of the brake were all that was needed to stop even the fastest motorcycle, but back then a fast bike had maybe 50 horsepower and rarely exceeded 400 pounds. As bikes grew heavier and more powerful, it became evident that something better was needed. That something was the hydraulic disc brake.



Hydraulics 101

Why hydraulic disc brakes? The short answer is that disc brakes provide a lot of stopping power in a compact and uncomplicated package—far more in most cases than even the most exotic drum brake. On the downside, they require a lot of effort to work properly, which is where hydraulics come in. Hydraulic actuation provides an easy, predictable way to multiply force without using a rattrap of levers, pulleys and cranks, and thus lends itself quite nicely to motorcycle application.

Bear in mind that under normal circumstances liquids are incompressible. When we connect two (or more) containers of liquid with a pipe and then put pressure on one container, that pressure is transferred directly to the liquid in the other container(s). Here’s what makes hydraulics so useful. If the containers are the same size, the pressure exerted on one container is transmitted unchanged. However, if the two containers are different sizes and pressure is exerted on the smaller of them, the pressure, when measured at the larger container, will be multiplied in direct proportion to the size differential between the two containers.


That’s a simplified explanation, but in a nutshell it’s how a hydraulically actuated disc brake works. Two cylinders are connected with a hose and filled with fluid. The smaller, or master, cylinder has a lever connected to it that pushes against a small piston. When the piston moves, hydraulic pressure is transmitted through the fluid to the brake caliper, where it acts against a much larger piston. The larger piston multiplies the force to press the brake pad against the disc, slowing the motorcycle. Everything else is just details.
As you may have guessed, varying degrees of power and “feel” can be built into the brakes simply by manipulating the size difference between the master cylinder and the caliper.