Brake pads are the most critical safety components in a car's braking system, playing a decisive role in the overall braking performance. Typically, brake pads consist of steel plates, adhesive insulation layers, and friction blocks. The insulation layer is made of non-thermal conductive materials, serving as a heat insulator. The friction blocks are composed of friction materials and adhesives.
The function of brake pads is to squeeze the brake disc and brake drum, generating friction to slow down or stop the vehicle. In drum brakes, the diameter of the brake drum can increase when heated, leading to an increase in brake pedal travel, which can cause the braking response to be less effective than expected.
Therefore, when driving a vehicle with drum brakes, it's important to avoid thermal degradation of the brake pads due to high temperatures caused by continuous braking. The brake system can respond slowly, making it difficult to control the brake pedal force, which is not conducive to frequent braking. Drum brakes have a more complex structure with many components, and they require adjustment of the brake clearance, making maintenance more challenging.
The front brake pads are usually slightly larger than the rear brake pads. Rear wheels may also use drum brakes. The wear rate of brake pads is related to how often they are used and the amount of force applied to them. Car design fully takes into account braking inertia and the forward rush that occurs during braking. When braking, the rear brake pads engage first, effectively pulling the car, and the front brake pads engage a few milliseconds later to avoid a jerky forward motion. During low-speed or slow braking, the rear brake pads handle most of the braking, with the front brake pads briefly engaging to bring the car to a complete stop.
In high-speed emergency braking, the car may pitch forward due to inertia, so the front brake pads tend to last longer than the rear brake pads. In other words, while all four brakes are technically working simultaneously, the rear brakes do the majority of the work. The rear brakes are responsible for most of the braking force and engage first. Under normal driving conditions, the braking force of all four wheels increases simultaneously, but due to the flexible connection between the car body and the wheels, the car's center of gravity shifts forward during braking. This phenomenon, known as braking mass transfer, results in the front wheels bearing more than 70% of the braking force, causing the front brake pads to wear out faster than the rear ones.
To accommodate this, the front brake pads are typically thicker than the rear pads. It is generally recommended to replace brake pads when they are down to 4mm in thickness. The thickness of the brake pads can be visually inspected on the vehicle. The wear rate of brake pads is mainly affected by the material of the pads and driving habits. Some drivers have better driving habits, which result in slower wear.
Additionally, brake pads are equipped with "wear indicators" (alarm pads), which are metal strips placed next to the brake pads. When the brake pads wear down to a certain thickness, the alarm pads come into contact with the brake disc and emit a sharp sound to alert the driver that the brake pads need to be replaced. However, since the alarm pads are usually only on one side, uneven wear might prevent the warning from occurring. Therefore, it's recommended to check the brake pads during every vehicle service.
Performance degradation, or "brake fade," occurs when the braking ability deteriorates, and the stopping distance increases due to high temperatures, such as when driving on downhill mountain roads. The temperature of the brake discs can reach 450 to even 700 degrees Celsius, causing the brake pads to lose effectiveness. High-quality brake pads experience minimal fade, while counterfeit or inferior products can experience severe fade, almost losing braking ability at high temperatures. Restoring the original braking performance is a crucial aspect of brake pad quality.
In summary, the wear rate of brake pads is related to their frequency of use and the force applied to them. Car design takes into account braking inertia and forward thrust, causing the rear brake pads to engage first during braking. This helps avoid a forward jolt, as the front brake pads lag by a few milliseconds. During slow, low-speed braking, the rear brake pads handle most of the braking, with the front pads engaging briefly to bring the vehicle to a complete stop.