I get asked this question at least 3-4 times a week….”why does my car shake when I step on the brakes?”
The main reason why this is happening is because the brake rotor (the large metal round disc that the brake pads grab to stop the vehicle) has become warped, out of round, and not smooth and straight any more. This usually happens because of the massive amount of friction heat that is built up when you apply the brakes causes the metal in the brake rotor to expand.
If water comes in contact with the rotor at this time it will cause the rotor to contract. If you have slots or holes in your wheels you should be able to see the brake rotor just behind the wheel.
It’s this expansion and contraction due to normal wear and tear which cause the rotor to warp and wobble. There is now a high spot on this spinning disc rotor. So when you step on the brakes, the brake pads grab the rotor as it is spinning around and when the brake pads come to the high spot in the rotor the pads are forced back and forth as they come in contact with the high spot on the rotor.
Think of the disc rotor as a normal old fashioned clock on the wall, and the high spot is located on the 12’o clock position and the rest of the clock face is nice and smooth. You apply your brakes, and as the rotor turns around the 12’o clock position the high spot pushes back against the brake pads.
This push back is what you feel in the brake pedal, as the brake pedal inside the vehicle pulsates up and down when the high spot is hit. This high spot also causes the steering wheel to shake back and forth. The faster the vehicle is traveling (and rotor is spinning faster) the more severe the pulsation and shaking will be.
When the car comes to a slow and final stop you will really feel the pulsation in the vehicle, and the brake pedal but not so much in the steering wheel. Sometimes you will feel a slow forward and backwards rocking in your seat as the brake pads come into contact with the high spot in the rotor.
Most vehicles these days are equipped with 4 wheel disc brakes, and the rear brake rotors can also be susceptible to warping as well as the front, although the front rotors usually warp more easily than the rear.
If you can feel or see the steering wheel shake when you apply the brakes, the front rotors are the culprit not the rear. The rear brakes will not affect the steering wheel shaking issue but will cause the brake pedal to pulsate and shake the entire vehicle if the rear rotors are warped.
Resurface Brake Rotors
The majority of the time when the brake rotors are warped it means you are due for a brake job, replacing the brake pads and resurfacing (truing) the brake rotors. The brake rotor can be machined to remove the high spot and make the rotor surface nice and smooth again.
In some cases the high spot is too big and resurfacing the rotor is not an option. Each vehicle will have different specifications as to how thick the rotor must be to maintain safe braking and integrity of the rotor. In some cases the rotors will have to be replaced.
If you just had your brake pads replaced and the rotors were not machined and now you have a pulsating brake pedal, well you will need to do the job again and resurface the rotors this time.
If you had a recent brake job and they did resurface the rotors but you now have pulsation in the brake pedal I would suspect the rotors were trued to thin and the metal is not able to withstand the expansion and contraction process, thus warping easily.
I would say on average that a brake rotor can be machined twice before it needs to be replaced. This depends though on how bad they were originally warped, how much metal is left to be machined and are there cracks or other damage to the rotor that machining can not remove.
Video showing you the “truing” process- take a watch