Parking Brake for Rear Discs
If disc brakes are used on the rear of a vehicle, they operate just like the front brakes but they are likely smaller and have the parking brake system built into them. Some applications use a corkscrew-like shaft that is actuated by a cable or electric motor drive to apply the parking brake. Others may have a small drum system inside the rotor for parking brake application, which is more effective due to self-energizing brake shoe action.
This rear caliper uses an integral parking brake that moves the piston into the pad when the cable is applied. It uses a corkscrew design inside the piston that is prone to rust and binding. The piston must be rotated as it is pressed back into the caliper with a special tool prior to pad replacement.
Some rear disc parking brakes use a small set of brake shoes inside of a drum cast inside the rotor. This is referred to as the rotor hat section. These shoes should never wear out because the rotor should be stationary when the parking brake is applied.
Brake Pads
Brake pads are relatively small pieces of friction material attached to a steel backing plate that press against the rotor under hydraulic force. A process known as sintering is used to make brake pads. Most pads are composed of several elements compressed together and heated in a mold.
The composition of the formula used varies from vehicle to vehicle and among different manufacturers. It can possess high percentages of different compounds such as organic material, semimetallic particles, ceramic, carbon metallic, Kevlar, or silicone carbide. Some pads excel in heat control, braking ability, and even reduce wheel dusting. Some wear faster than others but may offer more aggressive braking for racing or heavy loads. Other components include binders (glue) and fillers. Sometimes even ground-up walnut shells are used in the mix as a filler!
Brake engineers design tailored systems for the needs of each specific vehicle platform, often with a special patented mix of ingredients in the brake pad formula. Vehicle engineers must meet strict Department of Transportation (DOT) standards for stopping distance, and they often stamp the heat-handling ability onto to the edge of the pad or shoe. This code gives an indication of how much heat the friction material can handle when both hot and cold. This is called an edge code and is classified using letters of the alphabet, with higher letters meaning better heat handling (GG could handle more heat than EE, for example).
Most pads today are bonded, meaning that the friction surface is attached to a steel backing plate using high pressure and heat in a mold during manufacturing. Pad backing plates may have a rough surface with fish-hook-like teeth where friction is bonded to help the pad material remain on the plate. Older designs used rivets to attach the friction “puck” to the backing plate. Some newer designs have holes in the plate; friction material is pressed into it during manufacturing to help hold it to the plate during operation. Some even have a thin layer of friction material on the backside of the pad to act as a shim for noise control.
Some older disc brake designs used a brake pad with the friction material riveted to the steel backing plate. This type of pad cannot wear as much as a bonded pad without the rivets causing damage to the brake rotor. Some pad manufacturers placed a thin insulator between the friction material and backing plate for quieter operation.
Most brake pads in use today have the friction material bonded to the steel backing plate. During manufacturing, friction material is placed under high pressure and heat to form it and bond it to the backing plate.
Some manufacturers have holes through the steel backing plate so that friction material can flow through during manufacturing. This helps keep the friction material in place. One well-known manufacturer has a thin friction surface on the other side that acts as a very effective shim for noise reduction.
Pad Materials and Applications
A pickup truck designed to haul a 36-foot fifth-wheel camper that weighs 12,000 pounds would likely be equipped with semimetallic brake pads from the factory. This composition includes a high level of metallic particles in the formulation that are extremely good at drawing heat away from the rotors and conducting it into the suspension system and wheels.
For severe-duty applications such as police, ambulance, or racing, a pad containing even more metallic particles is available, often called severe-duty or carbon-metallic. These pads work well on extremely hot surfaces but do not grip well on cold rotors, so this formulation is not a good choice for a small lightweight sedan. Severe-duty pads also tend to be noisier and cause more extreme rotor wear due to their high metal content and aggressive nature.
German vehicles often use silicone-carbide formulations, which are also aggressive and can cause rotor wear and high wheel dusting, but the benefit is excellent stopping power. It is not uncommon for some German vehicles to require new rotors during every brake job because the aggressive friction wears the rotors.
Lighter weight vehicle applications used organic and asbestos formulations years ago. Asbestos was found to cause cancer, so it has been replaced with non-asbestos formulations. Some light-duty pads also contain copper particles. Brake pads containing copper have come under scrutiny because of the copper dust potentially being washed into drains on the street and entering the water supply.
Ceramic formulations are extremely popular on lighter weight applications. They are generally quieter and shed a lighter color dust, which improves wheel appearance. However, they tend to be less able to conduct heat away from the rotor. They act more like a barrier or insulator. This can cause the rotor to run hotter, so they are not a good choice for the truck pulling the camper mentioned earlier, even though less dust may be seen on that vehicle’s wheels. Some pads deposit a darker color dust and wear away some metal from the rotor for them to work properly. The bottom line is that it is not wise to sacrifice braking ability when pulling a heavy load to have cleaner wheels!
Brake pad manufacturers often use chamfered edges and slots in the friction surface to reduce vibration and wear particle accumulation. Both can cause brake noise.
There are no standards for the percentage of compounds required to call a pad a certain name, such as Kevlar or ceramic. Advertisers often use these as a way to help their products sell, even though the percentage may be extremely low.
There is no perfect pad for all applications just as there is no tire that can do everything for every demand. If a tire can last 80,000 miles, it will not be able to give good traction in rain and snow. If it is aggressive for off-road use, it will probably be extremely noisy on the highway. If it has amazing grip around turns, it probably won’t last long. Nearly everything on a vehicle is a compromise. That also applies to fuel efficiency versus speed. Fast sports cars typically use a lot of fuel. Little fuel-efficient cars aren’t fast.
In the same way, each brake pad has its own strengths and weaknesses, and the choice of pad material should not be based solely on price or how clean the wheels remain. On the internet, you can find pads for the same vehicle from $15 to $115, so it might be confusing to determine which pad is right for your application. Most quality pad manufacturers have catalog listings