The function of the rear brake disc of the car
The core function of the rear brake disc is to work in conjunction with the brake pads, converting the kinetic energy of the wheels into heat energy through friction, allowing the vehicle to slow down or stop. In simple terms: when you step on the brake, the caliper clamps the brake disc that rotates along with the wheel, relying on friction to bring the vehicle to a stop.
Specifically for the rear wheels, there are a few features to note:
Share braking force: During braking, the vehicle's center of gravity shifts forward, and the rear wheels typically only bear 20%-30% of the braking force. Therefore, the rear brake disc is usually smaller than the front wheel disc. Many family cars use solid discs for the rear wheels and ventilated discs with better heat dissipation for the front wheels.
Assist in stabilizing the vehicle: The braking force of the rear wheels is specially matched to prevent the vehicle from skidding when braking too hard, helping to maintain vehicle stability.
Consider parking function: Some cars' handbrakes (parking brakes) act on the rear wheel brake discs, locking the rear wheels through calipers or internal drum structures.
Although the rear brake disc has a smaller load than the front wheel disc, it is still a safety component. If you hear abnormal braking sounds, the pedal shakes, or see deep grooves on the disc surface or severely rusted edges, you should check it promptly. If necessary, replace it.
The "failure" of the rear brake disc is not a single situation. The specific handling depends on the type of damage and the degree of wear: Minor wear or rust can be observed or ground and repaired. If there are cracks, deep grooves, or the thickness is below the limit, it must be replaced immediately. Do not delay.
First, determine which situation you are in, and then decide whether to repair or replace.
First, determine if it is really broken
Brake disc damage is usually classified into several types, and the handling methods are completely different:
After rain or on cold damp days when starting the vehicle, the brake has a harsh sound: It is likely just surface rust on the disc surface. Lightly step on the brake a few times to remove the rust, and the sound will disappear. This is a normal phenomenon and does not require repair.
There are slight scratches or shallow grooves on the disc surface: If the groove is not deep (such as not exceeding 2mm), and it does not affect the braking effect, you can first use sandpaper to polish or treat it to the smooth surface and observe.
There are obvious deep grooves, cracks, or the brake disc shakes the steering wheel or the vehicle body: These are problems that must be addressed. Cracks will expand and even cause the brake disc to break, which is very risky.
The braking effect has deteriorated, the braking distance has increased, or the braking has become softer: This is a warning sign, indicating that the disc or pad has been severely worn and requires immediate inspection at the dealership.
Must replace the hard indicators
If any of the following occurs, do not hesitate and replace immediately:
Thickness below the limit value: Generally, the original factory thickness for the front disc is 28-32mm, and for the rear disc, it is 24-28mm. If it has worn off more than 3mm or is below the minimum thickness marked on the disc, it must be replaced. For example, if the original 30mm disc wears down to less than 27mm, replace it.
Disc surface has cracks: It cannot be driven any longer and must be replaced immediately, regardless of thickness.
The groove depth exceeds 1.5-2mm: If you can feel the groove with your fingernail, it is time to replace it.
The brake pad has worn out, and it is like "metal on metal": In this case, the brake disc is likely also damaged, and the disc and pad must be replaced together.
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