System: Brakes

ABS: Antilock
Brake Rotors and Pads
Brake System Basics
Disc Brakes
Drum Brakes
Hydraulics
Master Cylinder, Brake fluid, Bleeding
Parking Brakes
Power-assisted Brakes

ABS: Antilock Brake Systems

If the brakes are applied too hard when driving on slippery road surfaces, they may lock up or stop the wheel. The wheel then loses frictional contact with the road and skids and the vehicle is no longer under control. Experienced drivers know that the way to prevent lock-up is to pump the brake pedal up and down rapidly.

Many late-model cars are now equipped with an antilock brake system (ABS). The antilock brake system does the same thing as an experienced driver. It senses that a wheel is about to lock-up or skid and it rapidly interrupts the braking pressure to the brake system at that wheel.

The basic parts of a typical antilock braking system are shown below. The brains behind the antilock brake system is the computer, which monitors system operation at all times. It processes information from the wheel sensors and determines wheel speed. From this information, the electronic controller can determine whether one wheel is turning slower than the other wheels.

The computer gets its information on wheel speeds from wheel sensors located on each wheel. Each sensor assembly consists of a magnetic pickup sensor and a toothed sensor ring. The front sensor rings are attached to the back side of the rotor assembly. The rear sensor rings are attached to the axle shaft. The pickup assemblies are bolted to brackets at each wheel.

The wheel sensors are essentially magnetic pickup assemblies. Each pickup assembly consists of a permanent magnet with a coil of wire wound around it. The sensor is positioned extremely close to the sensor ring, which rotates as the wheel turns. As the teeth pass the pickup assembly, the signal is induced in the coil by electromagnetic induction as the magnetic field goes from strong to weak and back to strong. This signal change is used by the computer to determine wheel speed.

The antilock brake system uses a hydraulic control unit in place of the standard master cylinder. The hydraulic control unit consists of a master cylinder, a vacuum or hydraulic booster, electric pump, accumulator, a solenoid valve body assembly, and pressure control and warning switches.

The electric pump is a high pressure pump designed to run at frequent intervals for short periods of time. The pump fills the hydraulic accumulator and supplies high pressure brake fluid to the brake system.

The accumulator is a nitrogen gas-filled assembly used to store and supply pressure to the brake system. The accumulator is attached to the pump housing. The top chamber of the accumulator is filled with nitrogen gas. The bottom chamber contains brake fluid, which is supplied from the hydraulic electric pump. A diaphragm is used to separate the two chambers.

CAUTION: Do not disassemble any accumulator. The nitrogen gas contained in the accumulator is pressurized to 1,200 psi (8,274 kPa). Antilock brakes use extremely high pressure, so always follow service manual procedures when working on one of these systems.

During operation, the electric pump supplies brake fluid to the lower chamber of the accumulator, and the diaphragm moves upward, compressing the nitrogen gas in the upper chamber. The nitrogen gas, which is under pressure in the top chamber, then pushes down on the diaphragm, causing the brake fluid in the bottom chamber to be maintained at a very high pressure. During normal braking conditions (no antilock control), the accumulator supplies pressurized brake fluid to the booster and the rear brakes. During antilock braking conditions, the accumulator also supplies pressurized brake fluid to the front brakes. The accumulator can provide pressure required for a number of stops if the electric pump should fail.

The solenoid valve assembly is a set of electrically operated solenoid switching valves. The main solenoid valve opens a connection between the boost pressure chamber of the brake power booster and the internal master cylinder reservoir and closes the flow to the reservoir during antilock control. This provides a continuous supply of high pressure brake fluid during antilock control to replace the fluid being allowed back to the reservoir. When antilock control stops, the main valve closes and the return to the reservoir is reopened. By closing the main valve, accumulator pressure is removed from the front brake circuits within the master cylinder.

A set of smaller solenoid valves is located in a solenoid valve body. The valve body contains three pairs of electrically operated solenoid valves: a pair for each of the front brakes and a pair that controls both back brakes together. Each pair contains a normally open inlet solenoid valve and a normally closed outlet solenoid valve. During normal braking conditions (no antilock control), brake pressure is supplied to the brakes through the inlet solenoid valves upon brake application.

The computer determines the rotation speed of each wheel. If it senses a possible wheel lock-up, it goes into the antilock function and then applies voltage to the appropriate solenoid valves. When the system goes into antilock control, the computer will open and close the appropriate inlet and outlet solenoid valves, which control the operation of any of the brakes on any of the four wheels and prevent wheel lock-up. When the system is in antilock brake operation, the brake pedal will pulsate at an extremely fast rate. Pressure control and warning switches warn the driver of any malfunction in the system.

Brake Rotors and Pads

Disc brakes are used on the front wheels of most cars and on all four wheels on many cars. A disc rotor is attached to the wheel hub and rotates with the tire and wheel. When the driver applies the brakes, hydraulic pressure from the master cylinder is used to push friction linings against the rotor to stop it. A simplified illustration of a disk brake is shown below.

A disc brake rotor assembly is shown below. The rotor is usually made of cast iron. The hub may be manufactured as one piece with the rotor or in two parts. The rotor has a machined braking surface on each face. A splash shield, mounted to the steering knuckle, protects the rotor from road splash.

A rotor may be solid or ventilated. Operation of the master cylinder if there is a rear system failure. Ventilated designs have cooling fins cast between the braking surfaces. This construction considerably increases the cooling area of the rotor casting. Also, when the wheel is in motion, the rotation of these fan-type fins in the rotor provides increased air circulation and more efficient cooling of the brake. Disc brakes do not fade even after rapid, hard brake applications because of the rapid cooling of the rotor.

The hydraulic and friction components are housed in a caliper assembly. The caliper assembly straddles the outside diameter of the hub and rotor assembly as shown below. When the brakes are applied, the pressure of the pistons is exerted through the shoes in a 'clamping' action on the rotor. Because equal opposed hydraulic pressures are applied to both faces of the rotor throughout application, no distortion of the rotor occurs, regardless of the severity or duration of application. There are many variations of caliper designs, but they can all be grouped into two main categories: moving and stationary caliper. The caliper is fixed in one position on the stationary design. In the moving design, the caliper moves in relation to the rotor.

The caliper cylinder bore contains a piston and seal. The seal has a rectangular cross section. It is located in a groove that is machined in the cylinder bore. The seal fits around the outside diameter of the piston and provides a hydraulic seal between the piston and the cylinder wall. The rectangular seal provides automatic adjustment of clearance between the rotor and shoe and linings following each application. When the brakes are applied, the caliper seal is deflected by the hydraulic pressure and its inside diameter rides with the piston within the limits of its retention in the cylinder groove. When hydraulic pressure is released, the seal relaxes and returns to its original rectangular shape, retracting the piston into the cylinder enough to provide proper running clearance. As brake linings wear, piston travel tends to exceed the limit of deflection of the seal; the piston therefore slides in the seal to the precise extent necessary to compensate for lining wear.

The top of the piston bore is machined to accept a sealing dust boot. The piston in many calipers is steel, precision ground, and nickel chrome plated, giving it a very hard and durable surface. Some manufacturers are using a plastic piston. This is much lighter than steel and provides for a much lighter brake system. The plastic piston insulates well and prevents heat from transferring to the brake fluid. Each caliper contains two shoe and lining assemblies. They are constructed of a stamped metal shoe with the lining riveted or bonded to the shoe and are mounted in the caliper on either side of the rotor. One shoe and lining assembly is called the inboard lining because it fits nearest to the center line of the car. The other is called the outboard shoe and lining assembly.

As already mentioned, the caliper is free to float on its two mounting pins or bolts. Typical mounting pins are shown in the exploded view of the floating caliper. Teflon sleeves in the caliper allow it to move easily on the pins. During application of the brakes, the fluid pressure behind the piston increases. Pressure is exerted equally against the bottom of the piston and the bottom of the cylinder bore. The pressure applied to the piston is transmitted to the inboard shoe and lining, forcing the lining against the inboard rotor surface. The pressure applied to the bottom of the cylinder bore forces the caliper to move on the mounting bolts toward the inboard side, or toward the car. Because the caliper is one piece, this movement causes the outboard section of the caliper to apply pressure against the back of the outboard shoe and lining assembly, forcing the lining against the outboard rotor surface. As the line pressure builds up, the shoe and lining assemblies are pressed against the rotor surfaces with increased force, bringing the car to a stop.

The application and release of the brake pressure actually causes a very slight movement of the piston and caliper. Upon release of the braking effort, the piston and caliper merely relax into a released position. In the released position, the shoes do not retract very far from the rotor surfaces.

As the brake lining wears, the piston moves out of the caliper bore and the caliper repositions itself on the mounting bolts an equal distance toward the car. This way, the caliper assembly maintains the inboard and outboard shoe and lining in the same relationship with the rotor surface throughout the full length of the lining.

The stationary or fixed caliper has a hydraulic piston on each side of the rotor as shown below. Larger calipers may have two pistons on each side of the rotor. The inboard and outboard brake shoes are pushed against the rotor by their own pistons. The caliper is anchored solidly and does not move. The seals around the pistons work just like those already described. The main disadvantage of the stationary caliper is that it has more hydraulic components. This means they are more expensive and have more parts to wear out.

Brake System Basics

Braking action begins when the driver pushes on the brake pedal. The brake pedal is a lever, pivoted at one end, with the master cylinder push rod attached to the pedal near the pivot point. With this lever arrangement, the force applied to the master cylinder piston through the push rod is multiplied several times over the force applied at the brake pedal.

A pedal assembly is usually mounted to a brake support bracket as shown below. The bracket is mounted to the inside of the engine compartment cowl or firewall. The master cylinder push rod that connects the pedal linkage to the master cylinder goes through a hole in the firewall.

The master cylinder is mounted on the opposite side of the firewall in the engine compartment. . If the car has manual brakes, the cylinder will be mounted directly to the firewall. If a power booster is used, it will be mounted to the firewall and the cylinder is mounted to the booster.

Disc Brakes

Most late-model cars use the moving caliper design. This design uses a single hydraulic piston and a caliper that can float or slide during application. Floating designs 'float' or move on pins or bolts. In sliding designs, the caliper slides sideways on machined surfaces. Both designs work in basically the same way.

A cross-sectional view of a single piston floating caliper is shown below. The single-piston caliper assembly is constructed from a single casting that contains one large piston bore in the inboard section of the casting. Inboard refers to the side of the casting nearest the center line of the car when the caliper is mounted. A fluid inlet hole and bleeder valve hole are machined into the inboard section of the caliper and connect directly to the piston bore.