Why Would a Car Shake When You Brake?by Dan Ferrell
Vibration is a rather noticeable problem of the brake system that you can feel on the brake pedal or steering wheel every time you apply the brakes. Most of the time, the pulsation comes from an out-of-round or warped brake disc. This is a serious issue that can lead to failure of the brake system and a dangerous road accident. Do not overlook the problem; inspect the system immediately when excessive shaking occurs.
Rotors are part of the brake disc assembly and provide a friction surface for the brake pads, which makes it possible for the vehicle to slow or stop the wheels. The rotor is usually made of cast iron and, on most front-wheel drive models, it is separate from the wheel hub.
You can check for disc runout and warping using a couple of tools. Mount a dial caliper near the disc rotor to check for disc runout, resting the tip near the edge of the rotor. Rotate the rotor by hand and read the dial measurement. Compare this number against the specification on your vehicle service manual. To check for disc warp or thickness variation, use an outside micrometer to take disc measurements at various places around the rotor. Compare these numbers against the specifications on your service manual.
The brake rotor is a metal disc mounted on the wheel assembly. Depending on year and model, some vehicles may have disc rotors on the front or both front and rear wheel assemblies. The friction walls, on both sides of the rotor, should be perfectly parallel and of a specified thickness. Wear and brake system problems may cause disc run-out or variations in thickness around several points of the rotor.
If an out-of-round or warped brake disc is still within the manufacturer specifications, its friction area can be machined to correct surface imperfections. However, manufacturers place a limit on the amount of material that can be removed from a brake disc, which make it necessary to replace in some cases.
When an out-of-round or warped disc becomes a serious problem, the whole body of the car may vibrate every time you apply the brakes when driving. If the disc is thin enough, it will accumulate too much heat during operation and affect other brake components, which may lead to brake system failure.
- Modern Automotive Technology; James E. Duffy; 2003
Since 2003 Dan Ferrell has contributed general and consumer-oriented news to television and the Web. His work has appeared in Texas, New Mexico and Miami and on various websites. Ferrell is a certified automation and control technician from the Advanced Technology Center in El Paso, Texas.