What are car brake pads?

Automobile brake pads refer to the friction material fixed on the brake drum or disc that rotates with the wheel. The friction lining and friction pad in it are subjected to external pressure to generate friction to achieve the purpose of vehicle deceleration.


The heat insulation layer is composed of materials that do not conduct heat, and the purpose is heat insulation. The friction block is composed of friction material and adhesive, which is squeezed on the brake disc or brake drum to generate friction during braking, so as to achieve the purpose of deceleration and braking of the vehicle. Due to friction, the friction block will be gradually worn, generally speaking, the lower the cost of the brake pads, the faster the wear. After the friction material is used up, the brake pads should be replaced in time, otherwise the steel plate will be in direct contact with the brake disc, and eventually the braking effect will be lost and the brake disc will be damaged.


From the type of brake can be divided into:

– Brake pads for disc brakes

Brake shoes for drum brakes

– Ribbon for large trucks

According to the material, it can be divided into:

—Resin-based brake material

—Powder metallurgy brake material

—Carbon/carbon composite brake material

—Ceramic based brake material

Braking principle

The working principle of the brake is mainly from friction, using the friction between the brake pad and the brake disc (drum) and the tire and the ground to convert the kinetic energy of the vehicle into heat energy after friction, and stop the car. A good and efficient braking system must be able to provide stable, sufficient and controllable braking force, and have good hydraulic transmission and heat dissipation capabilities, so as to ensure that the force exerted by the driver from the brake pedal can be fully and effectively transmitted to the master cylinder And each brake wheel cylinder, and avoid hydraulic failure and brake failure caused by high heat. The brake system on the car is divided into two categories: disc brakes and drum brakes, but in addition to the cost advantage, the efficiency of drum brakes is far inferior to that of disc brakes.

Transmission of braking force

The force exerted by the brake caliper piston on the brake pad is called: brake pedal force (Pedal Force). The force of the driver stepping on the brake pedal is amplified by the lever of the pedal mechanism, and then amplified by the principle of vacuum pressure difference through the vacuum power booster (power boost), which is used to push the brake master cylinder. The hydraulic pressure from the brake master cylinder utilizes the incompressible power transmission effect of the liquid, and is transmitted to each sub-cylinder through the brake oil pipe, and the pressure is amplified by the “Pascal principle”, pushing the piston of the sub-cylinder to exert force on the brake pads. “Pascal’s Law” means that the pressure of liquid at any position in a closed container is the same.

The pressure is obtained by dividing the applied force by the force receiving area. When the pressure is equal, we can achieve the power amplification effect by changing the ratio of the applied force and the force receiving area (P1=F1/A1=F2/A2=P2) . Used in the brake system, the ratio of the master cylinder to the slave cylinder pressure is the ratio of the piston area of the master cylinder to the piston area of the slave cylinder.

Equipment: ABS

ABS: Anti-lock Brake System, we all know that the greatest braking effect occurs at the moment before the tire locks, if the braking force can be kept in balance with the tire friction, then the maximum braking will be obtained Effect. When the braking force of the brakes is greater than the friction of the tires, it will cause the tires to lock up. Once the tires lock up, the friction between the tires and the ground will change from “static friction” to “dynamic friction”. tracking ability. Since the locking of the tire is the result of the comparison between the braking force and the friction between the tire and the ground, that is to say, the limit of whether the tire is locked or not during the driving of the car will depend on the characteristics of the tire itself, the condition of the road surface, the positioning angle, and the tire pressure. , The characteristics of the suspension system are “different from time to time”. ABS uses the speed sensors installed on the four wheels to judge whether the tires are locked or not, eliminates the uncertain factors of human senses, accurately controls the timely release of the hydraulic pressure of the brake cylinder, and achieves the purpose of preventing the brakes from locking . Most of the current ABS adopts a design that can continuously step on and release 12~60 times per second (12~60Hz). It can maintain the braking force as close to the limit as possible. The accuracy and reliability that ABS can achieve has exceeded the limit of human beings, so we say: ABS is the most cost-effective equipment when buying a car. Especially the relative danger of Air-Bag.

Questions about ABS

Recently, there have been many reports that the car accident rate of cars equipped with ABS is higher than that of cars without ABS, which has caused many people to question the efficacy of ABS. This is caused by the general car owners’ lack of awareness of the braking system and ABS. Many people mistakenly believe that the installation of ABS can increase the braking force or the limit of the friction between the tire and the ground. In fact, although ABS can maximize the braking force Maintained at the maximum limit, but could not raise the limit. Here to reiterate: The limit of the friction between the tire and the ground is determined by the characteristics of the tire itself, the condition of the road surface, the positioning angle, the tire pressure, and the characteristics of the suspension system, but does not include ABS. ABS can fully and effectively exert the ability of the braking system, but it does nothing to improve the braking force or friction. In addition, when using ABS to dodge at high speed in an emergency, please remember to perform the main deceleration action in a straight line before turning the steering wheel. When turning the steering wheel, do not release the brake pedal, and do not panic because of the ABS feedback action from the pedal. There are also many people who think that ABS must step on the brakes to be effective. This is another misunderstanding of ABS. Of course, the anti-lock braking system only works when the wheels are locked. If you drive on an icy road, as long as you tap the brake lightly, the ABS may stop moving; Large-size hot-melt tires are driven on flat and dry roads. If your braking system has not been strengthened, even if you step on the brake pedal with all your strength, the ABS may still not move, because your braking force is not enough. Tires lock up. If car dealers can fully and effectively inform the above two points when selling cars equipped with ABS to consumers, then ABS can truly become an “active safety” equipment, otherwise consumers will feel confident when stepping on the brakes. The probability of accidents may not decrease but increase.

Modification of brakes

Inspection before modification: An efficient braking system is a must for general road vehicles or racing cars. Before the brake modification, a comprehensive confirmation must be made on the original brake system. Check the brake master cylinder, sub-cylinder and brake oil pipe for signs of oil leakage. If there are any suspicious traces, you must investigate and replace the problematic sub-cylinder, master cylinder or brake pipe or brake pipe if necessary. The biggest factor affecting the stability of the brakes is the smoothness of the surface of the brake disc or brake drum. Abnormal noise or unbalanced brakes are often caused by this. For the disc brake system, there must be no wear grooves on the surface, and the thickness of the left and right discs must be the same, so as to obtain the same braking force distribution. In addition, it must be ensured that the discs are protected from side impacts. The balance of the disc and brake drum can also seriously affect the balance of the wheel, so if you want excellent wheel balance, sometimes you must dynamically balance the tires.

Brake Fluid

The most basic modification of the brake system is to replace it with high-performance brake fluid. When the brake fluid deteriorates due to high temperature or absorbs moisture in the air, the boiling point of the brake fluid will decrease. Boiling brake fluid will make the brake pedal empty, which will happen suddenly when the brakes are used vigorously and frequently. The boiling of brake fluid is the biggest problem faced by the brake system. Brakes must be replaced regularly, and the mouth of the bottle must be sealed tightly when storing after opening to prevent moisture in the air from coming into contact with the brake fluid. Some types of vehicles restrict the brand of brake fluid used, because some brake fluids will corrode rubber products. You must refer to the warnings in the user manual to avoid misuse, especially when using brake fluids that contain silicone components. More importantly, do not mix different brake fluids. For general road vehicles, the brake fluid should be replaced at least once a year, and for racing cars, it should be replaced after each race.

brake hose

Generally, there will be a section of the brake system that is made of soft rubber tubes to cooperate with the suspension activities, but the rubber itself is elastic, and it will deform under the hydraulic pressure of the brake system, resulting in changes in the diameter of the tubes, reducing the The transmission effect of brake oil hydraulic pressure makes it impossible for the brake cylinder to produce stable braking force. Such a situation will increase the degree of deformation with the age of use and severe operation of the brake system. Originally used in aircraft hydraulic systems, metal oil pipes that can withstand high pressure and high temperature can improve this situation. The inner is made of Teflon, and the outer layer is covered with metal coils, which is not easy to deform and provides excellent hydraulic transmission effect, so that the hydraulic pressure from the brake master cylinder can be completely used to push the sub-piston, providing a stable Braking power. In addition, the metal material is also not easy to break, which can greatly reduce the chance of brake failure caused by oil pipe damage. Brake oil pipe is a necessary modification for racing cars (especially RALLY racing cars), and it provides another kind of safety guarantee for general road vehicles.

Cooling of the brakes

Excessive temperature is the main reason for the deterioration of brake pads, so the cooling of the brakes becomes extremely important. For disc brakes the cooling air should be directed towards the clamps. Because the main reason for the decline of the brakes is the boiling of the brake oil in the fixture, if the cooling air can be introduced into the fixture during driving through an appropriate pipeline or through a specially designed rim. In addition, if the heat dissipation effect of the rim itself is good, it can also share part of the heat from the disc and fixture. The scribed, drilled or ventilated discs can maintain a stable braking effect and avoid the sliding effect caused by high-temperature iron filings between the brake pads and the disc, effectively ensuring the braking force.

coefficient of friction

The most important performance index of brake pads is the coefficient of friction. The brake friction coefficient stipulated by the national standard is between 0.35-0.40. Qualified brake pads have a moderate and stable friction coefficient. If the friction coefficient is lower than 0.35, the safe braking distance will be exceeded or even the brake will fail. If the friction coefficient is higher than 0.40, the brakes are prone to lock suddenly and a rollover accident will occur.

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