Driving on badly worn rotors leads to a host of brake problems. Well-maintained rotors keep your automobile safe. The key to being safe lies in paying attention to signs of worn rotors. If your brake system makes any squealing sound, vibrates, pulsates, wobbles or shakes, the entire system needs to be checked. Checking the system means inspecting the rotors and pads for wear and the fluid level of the system. Worn rotors can lead to dire consequences.
Worn rotors have less mass and therefore heat up rapidly, because the rotors no longer can absorb and dissipate the heat evenly. The rapid heat absorption leads to brake fade and accelerated pad wear. In brake fade, the rotors and pads get so hot that it boils the brake fluid in the calipers, which results in spongy brakes and less brake force. In the case of accelerated pad wear, the heated rotor develops rough spots, and these in turn eat away at the pad faster. If the rotor overheats, in most cases there isn't enough rotor to resurface, and the rotor needs to be replaced.
According to Carroll Smith, a leading brake expert, brake rotor warp does not exist. Brake rotors do not become so heated that the brake rotor material warps. He explains that brake rotors become uneven due to pad material being transferred to the rotor, and not the rotor itself warping. The pads become so hot that the heat melts the pad material onto the rotor, forming a new material called cementite. Cementite is a rough material and a poor heat sink. Brake rotor unevenness leads to pad wear, and if unchecked, pulsation with high-speed braking. This means the tires wobble and vibrate, leading to a jerky steering column and to anti-lock brake system failure. Replacement of the rotor is critical in this case.
Corroded Brake Rotors
Badly worn rotors also result from rust, corrosion and lack of use. Rust or corrosion causes unevenness in a rotor. If you drive in extreme winter conditions where they throw salt on the road, corrosion of the rotor occurs, especially if you leave the car parked for long periods of time. If you drive with a partially corroded rotor, the corroded parts wears at a different rate then the non-corroded areas of the rotor. This uneven wear results in excessive thickness variation and a badly worn rotor. Worn rotors make the caliper piston travel farther when the brakes are applied. If the distance between the caliper piston and the rotor exceeds specifications, brake fluid leaks or the piston sticks, causing brake failure. Resurfacing of the brake rotor is warranted in this case.