Anatomy and Function of a Disc Brake

Last month a friend of mine asked my advice about replacing the disc brakes on his full suspension mountain bike with Shimano V-brakes. Of course I asked, “why?” He responded that he had been having quite a bit of trouble with the pads rubbing on the rotors. “I’ve tried everything,” he said. Finally his frustration boiled over on a White Rim Trip. He decided then he would switch back to cable brakes.
“What was everything?” I wondered. “Does he understand how bicycle hydraulic brakes work? Did he know all the possible adjustments?” despite appearing very complicated, the bicycle hydraulic disc brake is very simple. It just takes a little time to get to know each of the separate parts and their function.
The hydraulic brake system consists of four main parts: the brake lever and master cylinder, the caliper, the rotor and the brake line.
The brake lever is the one component that the rider interacts with. It consists of the body, upon which a master cylinder is attached, and the part we squeeze called the lever blade. The master cylinder is the reservoir for the brake fluid. It also contains a piston, which moves fluid into the brake line. The brake lever has two, sometimes three adjustments to be made. First, the horizontal angle of the lever can be changed on the handlebar. The most comfortable angle is adjusted to place the hand, wrist and forearm in a straight line. Next, a small screw in the lever body adjusts the resting position of the lever blade from the handlebar grip, called reach. Generally the reach is set closer for small hands and further for large. On some upper end models, it is also possible to adjust how much the lever blade moves when squeezed, called throw. Throw and reach are not purely independent of each other. For example, it might be challenging to have short reach and long throw.
On the action end of the system are the rotor and caliper. The rotor is a piece of machined steel or aluminum, or combination of both mounted to the wheel. They are not as fragile as they appear. However, they will bend under extreme force. Brake rotors come in different sizes; a larger diameter will provide more braking power. Changing the rotor size requires an adapter, specific for that brand, size and position (front or rear).
The brake caliper is mounted to the frame or fork; depending on the brand and size it may or may not have an adapter. The internal parts of the brake caliper include the brake pads and one, two or four moving pistons. These pistons, along with fluid behind them, are what squeeze the brake pads against the rotor. The brake pads and pistons are self-adjusting (more on this later). The caliper is adjustable laterally to provide clearance for the rotor to pass through. This can be done a couple different ways. Some manufacturers use thin washers between the caliper and mounting adapter to change the position of the caliper. The second method has the caliper manufactured with oval bolt holes.
Connecting the system together is a brake line consisting of a plastic inner liner and either plastic or metal on the outside. The fluid that moves through this line may be DOT automotive brake fluid or a specially designed mineral oil.
The way this all works is pretty simple. When the brakes are actuated, fluid moves from the master cylinder, through the brake line and into the caliper. Voila! We stop. It is easy, but there is more.
When the lever is squeezed, a piston pushes fluid from the reservoir through the brake line, into the caliper, forcing the pistons to clamp the brake pads against the rotor. In a two (or four) piston caliper, the fluid is routed behind each piston. When the brake is released, fluid returns back to the master cylinder. However, as the brake pads wear the pistons remain extended to keep a constant distance from the rotor. This is the self-adjustment mechanism. Therefore an increasing amount of fluid will remain in the caliper instead of returning to the reservoir.
Sometimes the self-adjustment needs a little help. Maybe the brake lever got bumped when the wheel was out. More often, heat from use may cause the rotors to change shape. That same heat will also affect the brake fluid and any air in the brake line in the form of expansion. The hydraulic brake is a closed system. Therefore, any expansion has nowhere to go, and it presses the pistons out. Usually when the brakes cool all returns to normal. Occasionally it may not. One or both pistons might not retract to its proper position in the caliper and the brake will continue to rub.
In any case brake pad clearance will need to be reset. If the brake caliper is positioned correctly, this can be accomplished a couple ways. One way is to gently press the rotor against the brake pad that is rubbing. Another method is to remove the wheel, and insert a plastic (!) tire lever or brake pad spacer between the pads to move the pistons. However, when the brake pads are new, the pistons are fully retracted and don’t have much room to move. Remember that closed system? With each of these methods, the pistons are reset back into the caliper. The brake pad clearance will be reset the next time the brakes are actuated.
“Aren’t disc brakes complicated?” is a question often asked by new bike shoppers. Yes, and no is my answer. In detail and design, the hydraulic bicycle brake is very complex. On the other hand, with an understanding of the basic principles, there is no reason to be fearful of them. This understanding, combined with good maintenance habits will keep your bike friction free and stopping smoothly. As for my friend’s brakes, he called soon after with a question about his new brakes. “How do I stop squealing brakes”, he asked. “I have a solution, but it’s expensive”, I replied.