How Does a Brake Caliper Work?

In the disc brake system, the brake caliper fits over the rotor like a clamp. Inside each caliper is a pair of metal plates bonded with friction material — these are called brake pads. The outboard brake pads are on the outside of the rotors (toward the curb) and the inboard brake pads on the inside (toward the vehicle).

When you step on the brake, brake fluid from the master cylinder creates hydraulic pressure on one or more pistons in the brake caliper, forcing the pads against the rotor. The brake pads have high-friction surfaces and serve to slow the rotor down or even bring it to a complete halt. When the rotor slows or stops, so does the wheel, because they're attached to one another.

­Older cars and trucks used drum brakes, where a rotating drum and brake shoes mounted inside the drum creates friction with the motion of the wheels. This friction caused heat and gases to build up inside the drum, which often resulted in a loss of braking power known as brake fade.

Because the brake pads in disc brake systems are external to the disc rather than contained within a drum, they are more easily ventilated and heat doesn't tend to build up quite as fast. For this reason, drum brakes have been largely replaced in modern vehicles by disc brakes; however, some less expensive cars still use drum brakes for the rear wheels, where less stopping power is required.

There are two main types of calipers: floating (or sliding) calipers and fixed calipers. Floating calipers move in and out relative to the rotor and have one or two pistons only on the inboard side of the rotor. This piston pushes the entire caliper when the brakes are applied, creating friction from the brake pads on both sides of the rotor.

Fixed calipers, as the name implies, don't move, but rather have pistons arranged on opposing sides of the rotor. Fixed calipers are generally preferred for their performance, but are more expensive than the floating kind. Some high-performance fixed calipers have two or more pairs of pistons (or "pots") arranged on each side of the rotor — some have as many as six pairs total.

Special tools are useful when working with brake calipers, especially when replacing the brake pads. We'll talk about that in the next section, and then discuss the different types of brake calipers available for different types of vehicles.

Brake pads don't last forever. Every time the pads in a disc brake system come in contact with the spinning rotor, they wear down a little. Gradually, these brake parts (the pads) become thinner and thinner. To compensate for this, the piston in the caliper emerges from the hollow cylinder where it resides inside the caliper. As it does so, it pushes the worn-down brake pads further and further inward toward the rotor.

Eventually, the brake pads will need to be replaced with fresh, unworn pads. Unfortunately, the caliper piston (which is now nearly fully extended) makes it difficult to remove and replace the pads. The piston needs to be pushed back into the caliper.

How to Push Back Fixed Calipers

This is where special brake pad tools come in. The job of a brake caliper tool is to retract the piston or pistons back into the caliper so that the brake pads can be easily removed and replaced. The piston can't simply be pushed back into the caliper because it's threaded, like a screw, and needs to be wound back in.

While it is possible to use, say, a pair of pliers to do this, it isn't recommended. You can damage the piston, the caliper and your hands, too. The brake caliper tool typically fits over the piston at one end and has a handle at the opposite end that allows it to be rotated. As it rotates, the piston is wound back into the caliper.

How to Push Back Floating Calipers

Floating calipers also need to be serviced if the pins that they slide on begin to stick. This is usually caused by dirt or rust. When this happens, the caliper cannot fully retract the brake pad from the rotor and friction continues, even when the brake pedal isn't being pushed. This can cause excessive wear on the pad, inefficiency in fuel use, and even warping of the rotor if enough heat builds up.

Up to this point, the description we've given of a brake caliper doesn't fully describe all models. In the following pages we'll look at more specialized types of calipers and see how they differ from the more common types.

Motorcycles are smaller than automobiles and therefore need less stopping power. Nonetheless, the ability to slow down or stop is in some ways even more important on a motorcycle, since the driver is largely unprotected. So what kind of brake calipers does a motorcycle need?

­The answer is relatively simple — small and light. Unlike the larger calipers used on some cars and trucks, motorcycle brake calipers must be kept small to avoid weighing down the bike and getting in the way of the rider. Motorcycle brake calipers are commonly made of lightweight materials such as aluminum, which also has the added advantage of being rustproof. Some motorcycles are bigger and more powerful than others; obviously, those bikes need more stopping power.

Many smaller, less-powerful bikes still use drum brakes, but most of the larger bikes now have disc brakes — especially on the front wheel. To increase the stopping power, calipers on more powerful motorcycles typically have multiple pistons. Some have two or even four pistons, while others may have as many as twelve pistons in a single caliper.

On a motorcycle, the front caliper attaches to the fork — the metal assembly that holds the front wheels and suspension in place and supports the handlebars. Until recently, the calipers have been attached to the forks using bolts that run at right angles to the rotor. In recent years, with the growing popularity of radial disc brakes (a technology that began on racing bikes), calipers have been attached farther away from the fork using bolts that run parallel to the rotor's surface. These radially mounted calipers reduce the amount of vibration in the fork caused by traditionally mounted calipers.

Next, let's find out what characteristics make for a good truck brake caliper.

There's no denying it: Trucks and SUVs can be excessively big. With that increased size comes increased momentum. This means that trucks and SUVs need more stopping power than a car. So where can they get the stopping power they require? Truck brake calipers.

The stopping power of a caliper is determined by its clamping force — the amount of force it can apply to the surface of a rotor. This is primarily a function of the number of pistons and the surface area of the brake pad (where it actually contacts the rotor). Obviously, a brake caliper with a greater clamping force is capable of slowing or stopping a vehicle easier and faster than a caliper that has a lower clamping force.

More than most brake parts, truck brake calipers have to deal with a lot of heat. Heat is bad for brakes because it can lead to brake fade and reduced stopping distances. Good ventilation in the caliper is essential for continuous, consistent brake performance. In addition, a larger brake rotor (or brake disc) surface can also help spread the heat over a larger area.

Trucks and SUVs aren't the only vehicles that require special brake equipment; however, the calipers in these heavy-weight vehicles do have a difficult job to perform. Some might even say that truck brake calipers should be considered high-performance brake calipers. In the next section we'll look at some of the features that high-performance brake calipers can offer other vehicles.

Disc brakes were originally developed for race cars. Everyone knows that race cars move along at high speeds — but they also need to decelerate rapidly. In the early days of racing, when most cars were equipped with drum brake systems, brake fade led to a number of on-track mishaps. Disc brake systems were better ventilated, and as a result, reduced some of the stress that racing and excessive heat build-up can cause — in turn, reducing (but not eliminating) brake fade.

In time, these powerful brakes trickled down to less performance-oriented vehicles. Now, they're even found on most economy cars. Nonetheless, high-performance cars are still a major market for better and more powerful brakes, and variations on the basic brake caliper design help these brakes provide superior stopping power.

There are certain limitations to just how much the vehicle's brakes and its calipers can do to stop a vehicle; while they may be able to bring the wheels to a stop, it's up to the gripping power of the tires to do the rest, and improved brake parts can't help beyond a certain point. However, there are several ways in which brake calipers can be (and have been) improved. Some common features found in performance brake calipers include: