The function of the rear brake disc of the car
The rear brake disc is the core rotating component of the disc brake system, mainly generating braking force through friction with the brake pads, assisting in vehicle deceleration, stopping, and maintaining stable parking.
Core Function
Provide rear wheel braking force: When the brake pedal is depressed, the brake caliper clamps the rear brake disc that rotates with the wheel. Through friction, kinetic energy is converted into heat energy, directly reducing the rotational speed of the rear wheel, achieving vehicle deceleration or stopping.
Maintain braking stability: During braking, the rear brake disc cooperates with electronic systems (such as ABS, EBD) to reasonably distribute braking force between the front and rear wheels, preventing the decrease in rear wheel adhesion caused by the forward shift of the center of gravity, and ensuring the stability of the vehicle's posture.
Assist parking function: In conjunction with electronic parking brake (EPB) or mechanical parking brake, it clamps the rear brake disc to lock the rear wheel, preventing the vehicle from sliding or skidding when on a slope or stationary.
Working Characteristics
Moderate braking force ratio: Due to physical inertia, when braking, the vehicle's center of gravity shifts forward, and the rear wheel bears approximately 30%-40% of the braking force (the front wheel accounts for about 60%-70%). Therefore, the rear brake disc mainly plays an auxiliary balancing role rather than a dominant emergency stop function.
Heat dissipation and durability: It needs to have good heat resistance and wear resistance. Some models adopt a ventilated disc design to enhance heat dissipation, avoiding heat attenuation caused by continuous braking, which may lead to brake failure. In short, the rear brake disc is not only the component that makes the vehicle stop, but also the crucial stabilizer that ensures a smooth, safe and uncontrollable braking process.
The marks on the rear brake discs of a car are mostly caused by worn brake pads or foreign objects. Light marks are normal, but deep marks accompanied by abnormal noises require repair.
Most of the time, this is a normal physical phenomenon during driving and there is no need to be overly anxious. However, it is necessary to determine whether to handle it based on the depth of the marks.
Why do they get marks?
Brake pad material is hard: If there are hard points or metal particles in the brake pad, during friction, it will act like sandpaper, leaving grooves on the disc.
Small stones are caught: During driving, sand or stones get stuck between the brake pad and the disc, and the hard objects scraping will form scratches.
Part or habit problems: The brake booster does not return properly, causing continuous wear, or frequent emergency braking causing overheating and deformation, which will also leave marks.
When should it be repaired?
No need to handle: If the marks are very light and uniform, driving without abnormal noises and no shaking of the brakes, it is a normal wear and can continue driving.
Must repair: If the grooves are very deep (such as over 0.6 millimeters), or there is a sharp sound from the brakes, a shaking sensation in the foot, or an increased braking distance, it is necessary to check immediately.
How to handle for better results
Replace components: If the brake pad is severely worn or has hard points, replace the new brake pad directly; if the disc is worn beyond the limit, it also needs to be replaced.
Repair and clean: If the marks are not deep, they can be polished and repaired (with a disc), remove the foreign objects that are caught, and check if the booster is returning normally.
The rear brake discs of a car are not replaced according to a fixed mileage, but are determined based on the thickness limit, surface shape, and braking performance. As long as any of the following conditions are met, it must be replaced immediately:
Core replacement criteria
Thickness lower than the minimum limit value: Check the "MIN TH" mark on the side of the brake disc (usually the rear disc is 22-24mm, specific to the model). When the measured thickness is ≤ the marked minimum thickness, or the total wear depth exceeds 2mm, it must be replaced.
Irreparable cracks: Radial or circumferential cracks appear on the disc surface, regardless of length, as they are prone to fracture due to high temperature and cause brake failure, need to be replaced immediately.
Severe surface damage: The groove depth exceeds 1.5mm, or the edges form obvious stepped protrusions (height difference > 1mm), resulting in insufficient contact area of the brake pad and affecting braking efficiency.
Severe deformation and shaking: The steering wheel or vehicle shakes significantly during high-speed braking, and the measured disc surface wobble exceeds 0.1mm, and the disc thickness cannot be restored by polishing (or the thickness will be lower than the safety value).
Abnormal signal indications
Abnormal noise: When lightly pressing the brakes, a sharp "metal scraping against metal" sound is heard. After excluding the period of new pad wear-in, it is mostly due to the brake pad wearing out the metal backing plate and directly scratching the disc surface, which requires simultaneous replacement of the disc and pad.
Brake performance decline: Feeling that the brakes have become softer, the travel distance has increased, or the braking distance has significantly increased under the same force, it may be caused by overheating and attenuation of the disc or insufficient thickness. Notes for Attention
Pairwise replacement: The left and right brake discs on the same axle must be replaced simultaneously to avoid uneven braking force and steering deviation.
Based on actual measurement: Do not judge solely based on mileage (such as 100,000 kilometers). In urban traffic or when driving in mountainous areas, the discs wear out faster. It is recommended to measure the thickness each time you replace the brake pads.
Rust is not a fault: Surface rust caused by rain or parking is a normal phenomenon. It can be removed by a few brake applications. There is no need to replace them.
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