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Automotive
Disc Brakes

Disc brakes look very much like the brakes on a bicycle. Pads are used as fiction elements. They are squeezed at the edge of a rotating wheel. Automotive disc brakes, the wheel is a separate unit called a rotor. Hydraulic pressure, generated by the driver pressing on the cars brake pedal, is transferred to a unit call a caliper which in turn applies pressure on two brake pads that slows and brings to a stop the turning rotor.

Disc brakes have four major advantages over drum brakes.

1. Disc brakes are more resistant to heat fade during high-speed stops or braking over a long period of time (down long hills). They dissipate heat more efficiently.

2. They are also resist water fade because the rotation of the rotor tends to throw off moisture.

3. Disc brakes perform more straight-line stops. Due to their clamping action, disc brakes are less apt to pull.

4. Disc brakes automatically adjust as the pads wear.

Up until the mid-1960s all motor vehicles had drum brake on all for wheel. Ever since cars have had disk brake on the front wheel and today we see more and more cars with disk brakes on all four wheels.

Disc Brake Components

• The disk brake, regardless of its design, consists of a hub and rotor assembly, a caliper assembly, and a brake pad assembly.

Hub and rotor assembly:

– Rotor:

• The typical rotor can be solid or ventilated and is made of cast iron that has a high coefficient of friction and withstands wear exceptionally well.

– Hub:

• The final drive axle is attached to hub.

• The rotor is then held on to the hub by the lug nuts.

Caliper Assembly

• The disc brakes used today are typically of two basic designs.

– Fixed caliper

– Floating caliper

• There is also a sliding caliper, but its design is very similar to the floating

caliper.

• Fixed caliper disc brakes:

– Fixed caliper disc brakes have a caliper assembly that is bolted in a fixed position and does not move when the brakes are applied.

– The pistons in both sides of the caliper come inward to force the pads against the rotor.

• Floating Caliper:

– A typical floating caliper disk brake is a one-piece casting that has one hydraulic cylinder and a single piston.

– The caliper is attached to the spindle anchor plate with two threaded location pins.

Brake Pad Assembly

• Disc brake pads:

– Brake pads are metal plates with linings either riveted or bonded to them.

– The linings are made of semi-metallic or other non-asbestos material.

• Disc pad wear sensors:

– The most common design wear sensor is the audible. When the pad wears down to a preset level, the wear sensor rubs on the rotor producing a high pitch squeal that can be heard inside the vehicle.

Wear Indicators

• Other disc pad wear sensors

– Visual sensor

• Informs the driver of the need for new linings. This method employs electrical contacts recessed in the pads that touch the rotor when the lining are worn out, completing a circuit and tuning on a dashboard warning light.

– Tactile

• Tactile sensors create pedal pulsation as the sensor on the rotor face contracts the contact the sensor attached to the lower portion of the disc pad.

Rear Disc/Drum (Auxiliary Drum) Parking Brake

• The rear disc drum or auxiliary drum parking brake arrangement is found on some vehicles.

• On these brakes, the inside of each real wheel hub and rotor assembly is used as the parking brake drum.

• The auxiliary drum brake is a smaller version of a drum brake and is serviced like any other drum brake.

Rear Disc Parking Brakes

• Instead of using an auxiliary drum and shoes to hold the vehicle when parked, these brakes have a mechanism that forces the pads against the rotor mechanically.

Disc Brake Diagnosis

• Brake Warning Lights

• Red warning light:

• Indicates there is a problem in the regular brake system, such as low brake fluid levels or that the parking brake in on.

• Yellow or amber warning light.

• This light turns on for two reasons:

• The ABS system is performing a self-test or there is a fault in the ABS system.

• Blue or yellow warning light:

• Lets the driver know the wheels are slipping because of poor road conditions.

• Pulsating Pedal

• A vibration of pulsation in the brake pedal when the brakes are applied if a brake rotor is warped.

• If this symptom exists, check the rotors for runout and parallelism.

• A warped rotor should be replaced and is often caused by improper tightening of the wheel lug nuts.

• Dragging Brakes

• Check the parking brake first.

• Make sure it is off.

• Check the rear wheels to make sure the parking brakes are released when they should be.

• If the problem is not in the parking brakes, check for sticky or seized pistons at the calipers and wheel cylinders.

• Spongy Pedal

• This problem is caused by air in the hydraulic system.

• Although bleeding the system may remove the air, you should always question how the air got in there.

• Check for leaks and for proper master cylinder operation.

• Hard Pedal

• A hard pedal normally indicates a problem with the power brake booster.

• However it can also be caused by a restricted brake line or hose.

• Restrictions can also be caused by frozen caliper or wheel cylinder pistons.

• Grabbing Brakes

• When the brakes seem to be overly sensitive to the pedal pressure, they are grabbing.

• Normally this problem is caused by contaminated brake linings.

• If the linings are coved or situated with oil, fin the source of the oil and repair it.

• Then replace the pads and refinish or replace the rotors.

• Pulling

• When a vehicle drifts or pulls to one side while cursing or when braking, the cause could be in the brake system or in the steering and suspension system.

• Check the inflation of the tires and verify that the tires on each axle are the same size.

• Check the operation of the brakes.

• If only one front wheel is actually doing the braking, the vehicle will seem to stumble or pivot on that one wheel.

• If no problems are found in the brake system suspect an alignment of suspension problem.

Automotive Disc Brakes