The most prominent symptom of warped brake rotors may be the pulsations felt when the brakes are applied. Slight deformities can result in light up and down motions felt in the brake pedal. More severe cases can produce a loping or lurching sensation as the vehicle stops. Lateral wobble, or "run out" of brake rotor surfaces can be caused by a number of faults. Blame may lie with associated brake parts, or seemingly innocent driving habits. Even the actions of a neglectful parts clerk might result in warped brake rotors.
The Skinny on Brake Pads
Excessive heat is the major cause of brake rotor distortion. New brake pads are often made with slots in the friction material, to help dispel the heat generated in normal operation. Ventilated shims are sometimes added to the metal backing plates of pads to improve pad cooling and to quiet operation. Brake pads can lose the ability to absorb and deflect heat if they are worn too thin. Worn pads often display no significant loss of stopping power, which is why routine inspections may prevent warped rotors.
Adding weight to a vehicle increases the distance needed to bring it to a stop. A car occupied by the driver alone can stop in a much shorter distance than the same car loaded with passengers and luggage. The longer braking force is applied, the more the brakes are heated. Towing a boat or trailer further aggravates this situation, and vehicles that routinely travel with heavy loads stand a much better chance of warping their rotors.
Many vehicles are equipped with drum brakes on the rear axle. They seldom draw much attention, and often seem to require no maintenance. Rear brakes normally supply a lesser portion of a vehicle's stopping power, and shortfalls might go unnoticed. However, extra stress is placed on the front disc brakes when the rear brakes provide reduced assistance. Full function of drum brakes can often be restored by a simple cleaning and adjustment process that most reliable repair shops perform for a minimal fee.
The Blame Game
Constantly pushing a car to its limits, or grossly overloading a vehicle, place the blame for warped rotors on driving habits. Defects in the brake system that went undetected during an inspection can be attributed to the mechanic. The clerk at an auto parts store may also sell you brand new rotors that are already warped. Contrary to the warnings emblazoned on new rotor cartons, inattentive auto parts clerks sometimes shelve new rotors standing on edge. The relatively soft cast iron rotor can bow under its own weight and will need to be trued on a lathe. This can occur despite the claim of "no machining needed" emblazoned on the rotor's packaging.
- Automotive Suspension Steering Alignment and Brakes; Walter E. Billiet and Walter Alley