Major causes of pulsating brakes/Runout

Technical Bulletin – Major causes of pulsating brakes/Runout

You're driving, and you press on the brakes, only to feel that pulsating sensation. The first diagnosis that comes to mind could be a warped brake rotor. Before going any further, generally speaking, a warped brake rotor means any brake rotor that isn't perfectly flat and parallel with the plane rotation. There are many causes that can lead to a pulsating brake.

One of the most common causes of a warped brake rotor is the improper installation of the wheel. Modern brake rotors are thinner at the friction surface and overall there is less material everywhere. That fact alone makes them vulnerable to warping thanks to both heat and improper lug nut torque. Your service technician should be very well aware of this, and therefore tightens the lug nuts in the correct sequence with a torque wrench.

Take out of the equation that everything is assembled correctly, and then the most common cause of brake pulsation is from the uneven transfer of friction material from the pad to the rotor.

Now, there is a situation when you'll feel normal brake pulsation. If your vehicle is equipped with an anti-lock brake system (ABS), and you have to hit the brakes quickly, you will feel a pulsation, but that is completely normal.

Keep in mind that brakes work best when within a specific temperature range, and the friction material that makes up the brake pad is designed with those temperature ranges in mind. A thin layer of friction material from the brake pad adheres to the rotor. This bedding-in process is what makes up this initial layer. The problem arises if the brakes are bedded in when new, or if the pads are severely overheated at any point, then that friction material becomes uneven and you can then have that pulsating problem. Should your brake rotors overheat, they can develop heat checks, and unfortunately they can't be resurfaced and your only option is to have them replaced.  Maybe use a pad that is scorched in for better mating with rotor or a more robust pad and ensure proper break in period of pads & rotors.

***References:  SSGM, General Motors, Dr. Johnny’s Car Clinic