On sliding or floating calipers, the piston(s) extends out of the caliper when the brake pedal is applied, causing one brake pad to make contact with the rotor (the action). As soon as contact is made, the caliper slides the other way (the reaction), which causes the other pad to make contact with the rotor.
This action and reaction clamps the pads onto the spinning rotor to slow or stop the vehicle. This design is called a sliding or floating caliper because that is exactly what it does. Sliding calipers ride on metal tracks, and floating calipers ride on pins or bolts with special seals and lubricants. It is easy to understand the concept if you visualize the C-clamp’s operation.
After you remove your foot from the brake pedal, a square-cut seal around the piston flexes back and acts as a return spring. It slightly retracts the piston so that the pads move away from the rotor just enough to allow free rotation. This ring sits in a groove in the bore around the piston; so, it is not actually mounted on the piston like an engine piston ring. The piston can be made of steel, aluminum, or a phenolic material. Phenolic is a synthetic material with superior heat resistance.
A caliper consists of a housing with a cylinder for hydraulic pressure to push the piston into the brake pad and into contact with the rotor. A seal in the bore of the cylinder allows the piston to build pressure and force the pads into the rotor. A dust seal (arrow) keeps out contaminants.
This C-clamp represents the way that the brake caliper squeezes the brake pads on both sides of the rotor equally to bring the vehicle to a stop. A piston takes the place of the threaded shaft, but the theory of operation is the same. As the piston pushes the pad into one side of the rotor, the caliper slides the other way and applies the other pad.
This caliper piston is made of steel but can also be made of aluminum or phenolic material. It rides in a seal in the cylinder bore. Because it has a larger diameter than the master cylinder piston, a hydraulic advantage exists, which means that a greater force is applied to the brake pads to help stop the vehicle.
The sealing ring (arrow) in a caliper piston is different than that of a ring on a piston in an engine. It is in a groove in the bore of the cylinder rather than on the piston itself. The groove it rides in is chamfered so that the seal flexes when the brakes are applied.
The sealing ring around the caliper piston is known as a square-cut seal. After the piston moves out to apply the pads during braking, it is retracted as the seal relaxes to its normal shape after being deflected. This reduces brake drag and improves fuel economy.
The brake caliper holds the pads in position and is bolted to the steering knuckle. A separate bracket may be used between the caliper and the steering knuckle on some applications. Stamped steel metal slides or guides and anti-vibration hardware or clips sit between the pads and caliper or bracket. This hardware often puts a small amount of spring tension on the pads to hold them in position to reduce vibration and prevent brake squeals, squeaks, and rattles.
Most disc brake systems use shims on the back of the pads, sheet metal guides, and spring clips to reduce brake pad rattle and squeal. Without these components, the pads would likely cause unwanted noise.
A brake pad contains a specific formulation of components to provide adequate stopping with a minimum of noise, dusting, and wear. Various formulations are used to handle the demands of heavy loads and heat or the extremes of emergency vehicles and race vehicles.
This single-piston floating caliper is a common design. It is lightweight and easy to service while still able to supply good stopping power and heat dissipation. A growing number of calipers are made of aluminum to reduce weight.
Some vehicles use a dual-piston floating caliper. The pistons are on one side of the rotor, so the caliper must still slide freely to engage the outboard pads during braking just as the single-piston caliper does. The dual-piston design can apply more force evenly across the pad for greater stopping power.
Most vehicles use a sliding or floating caliper design with either one or two pistons on one side of the rotor. It is imperative that the caliper slides freely for the brakes to work properly and the pads to wear evenly. The advantages to this design are that it is lightweight, easy to manufacture, low cost, and not prone to leakage. The disadvantages are that flexing can occur because the piston(s) is only on one side of the rotor. This can cause tapered pad wear. The slides can become contaminated and drag, leading to noise and one pad wearing more than the other.
A few vehicles use a fixed caliper, meaning that it does not slide. It has one or more pistons on each side of the pads that apply at the same time when the brake pedal is pressed. These calipers are usually heavier and have more potential for leaks, but they are known for excellent stopping and extremely even pad wear. Their weight dampens noise, and this generally results in minimal brake squeak or squeal. The greater weight is also a factor in vehicle handling. Because the brake caliper is “unsprung,” its weight has a greater negative effect on suspension operation versus “sprung” weight. This can be mitigated through the use of aluminum and more expensive alloys.
This is a fixed caliper, meaning that it is rigidly mounted and does not slide like a floating caliper. It has pistons on both sides of the rotor and a passage or transfer tube for fluid to flow between the two halves. Fixed calipers are generally found on older vehicles and performance cars.
Fixed calipers were popular on performance cars in the 1970s but are not as common today. They are found on older vehicles, some modern exotics, and high-performance racing applications. Some of the latest GMC and Chevrolet light trucks are equipped with fixed calipers.
Low-Drag Calipers
Because of government Corporate Average Fuel Economy (CAFE) and emission control standards, low-drag calipers were designed some years ago. Cars of the 1970s with disc brakes did not have calipers that retracted much, so they dragged the pads on the rotors until flex in the components caused enough “knock back” to free the rotors. If you jacked up a car in those days and attempted to spin the wheel, you probably had to pry the caliper piston back a bit to allow the wheel to spin. This wasn’t considered to be a problem, however, until fuel prices increased in 1973. Engineers knew that a wheel that was easier to spin improved fuel economy, so low-drag calipers entered the scene.
A special square-cut seal around the caliper piston in the bore flexes when the brakes are applied. When the brakes are released, the seal flexes back, which pulls the piston back away from the pads like a spring. This results in free-wheel rotation much quicker and, as a result, better fuel economy. It also likely improves pad life due to less dragging on the rotors. When you use less fuel, the vehicle has lower emissions.
Quick Take-up Master Cylinders
Some manufacturers developed master cylinders that supplied more fluid volume