The Complete Guide to Brake Discs: Structure, Principles, Replacement, and Common Issues

I. Brake Structure and Principle

During braking, the kinetic energy of the vehicle is entirely converted into thermal energy generated by friction between the brake pads and the brake disc. The vast majority of this heat is absorbed by the brake disc, causing its temperature to rise. In extreme cases, the surface temperature of the brake disc can exceed 600°C.

II. Brake Disc Replacement Interval

1. Is there a standard for brake disc replacement?

Brake discs are marked with their minimum allowable thickness, indicated as MIN THK or MIN TH (minimum thickness for the material). It is recommended to inspect the brake discs for abnormal wear and measure the thickness of the braking surface every 50,000 to 60,000 kilometers.

When measuring the minimum thickness, it is advised to take measurements at 9 points across three circles around the disc. If any measurement reaches the minimum thickness, the brake disc must be replaced.

2. Conditions requiring brake disc replacement

• Immediate Replacement Required:
  The brake disc has reached its minimum service thickness → Must replace.

• Replacement Recommended:
  The brake disc surface shows uneven grooves → Replacement recommended.

• Mandatory Replacement:
  Through-cracks appear on the brake disc surface (easily occurs after resurfacing when the disc has become too thin) → Must replace.

• Mandatory Replacement:
  The braking surface turns blue, indicating disc quenching (caused by high surface temperature followed by sudden cooling) → Must replace.

• Mandatory Replacement:
  Abnormal wear of the brake disc → Must replace.

III. Common Brake Disc Issues

1. Grooves on the brake disc

Some scratches on the brake disc are normal. Shallow lines are common, and as long as the scratch depth does not exceed 0.6 mm, there is generally no cause for concern.

What causes deep scratches? Consider two scenarios:

• Only front or only rear discs have grooves:
  This is usually caused by driving habits or uneven braking force distribution between front and rear axles.

• Only one brake disc has grooves (others are normal):
  This is typically due to material issues or external factors, such as:

  • Foreign matter trapped between the brake disc and pad, especially when hard braking occurs.

  • Uneven force from the brake caliper piston.

  • Manufacturing defects in the brake pad (e.g., uneven composite material, excessively hard pads, or large metal particles within the pad).

Recommended treatment:
Remove the grooved brake pad. Place low-density sandpaper on a flat surface and rub the pad horizontally and vertically against the sandpaper until smooth.

Consequences of ignoring the issue:
The grooves will deepen over time, potentially leading to weak braking, uneven friction, brake judder, scraping noises, and reduced braking performance.

2. Brake disc resurfacing ("disc polishing")

Resurfacing uses a professional brake disc lathe to smooth out grooves, restoring a flat surface. While relatively inexpensive, resurfacing reduces disc thickness and strength. For deeply grooved discs, resurfacing may introduce safety hazards.

Repeated resurfacing permanently leads to brake force imbalance. This may go unnoticed during normal driving but can be critical during high-speed emergency braking.

Brake judder occurs because prolonged friction between the disc and pad creates grooves, reducing braking efficiency and causing vehicle vibration when braking.

3. Brake disc rust

Brake discs are made of cast iron (except for advanced ceramic discs). Uncoated discs require removal of anti-rust oil before installation; otherwise, rust will form in the presence of water and oxygen. This is especially common after rainy days, when shiny discs become covered in rust, resembling those of an abandoned vehicle.

How to handle brake disc rust:

• Rust on small ends and outer circumference:
  This does not affect braking safety. If appearance matters, it can be polished or cleaned with rust remover.

• Light rust on the braking surface:
  Light rust can be removed through normal braking. The friction between the pad and disc will wear away the rust. Several braking actions will restore a shiny surface.

• Severe rust on the braking surface:
  If braking does not effectively remove severe rust, use fine sandpaper. Prolonged neglect of severe rust (especially on vehicles parked for long periods) can cause an uneven disc surface, leading to vibrations (e.g., steering wheel or body shake) during braking.

4. Proper brake bedding-in process

• After installing new brake pads, depress the brake pedal several times before driving off.

• Ensure any excessive clearance caused by installation is fully eliminated.

• Drive approximately 200 km to achieve optimal braking performance.

• Avoid continuous braking and emergency braking during the break-in period.