Modern cars make use of disc brake systems that efficiently stop the car. These brakes are called disc brakes due to the presence of the metal disc or the rotor that is connected to the wheel. The rotor will spin with the wheel of the car and while you apply brakes, the brake pads will rub against the metal rotor to slow down the car.
Technically, the brake rotor will help transform the kinetic energy of the car to heat energy. The responsibility of the brake rotor is to absorb the kinetic energy transformed heat energy and dissipate it. The brake rotor will need a specific amount of mass and a smooth surface to do this properly. If these conditions are not met, the rotor will not be able to make good contact with the brake pad, eventually decreasing the effectiveness of the system. In worst cases, it could even lead to brake failure.
It is well known that a new brake rotor is a thick one and will be able to absorb much heat and dissipate it efficiently. Yet with time and usage, the metallic rotor will wear away and if it is worn too much, the vehicle will not brake properly, as there isn’t enough metal to facilitate effective heat transfer. In fact, each of the brake rotors will be stamped with the minimum thickness it should have for the brake to function properly, and you need to ensure that all the rotors have the necessary thickness while performing a brake service on your car.
After thickness, the next thing to consider is the rotor surface. This is not just about the smoothness and quality of the surface, but also about how evenly the brake pads contact the rotor surface. Many factors can affect the sweep surface of the rotor and the most common problem is the scratched or scored rotor surface.
If you allow the disc brake pads to wear beyond the point of replacement, metal parts of the brake pad can come in contact with the rotor surface when you apply the brakes and it can grind channels and grooves into it. Even if you replace the brake pad, the new one will only contact the ridges of the scored brake pad reducing the contact area and decreasing the brake performance drastically.
This is why the brake rotor of your car needs to be resurfaced on a lathe to make the surface smooth and even. If you find that the rotor is not thick enough to be smoothened, it needs to be replaced for the brake to function properly.