Myth or fact: heat causes brake rotors to warp, creating pulsation. Trick question, right? Before you answer, think about how many of our customers have been told just that— not because we were trying to mislead them, but because we ourselves believed it. I know much has been written on the subject, but let’s consider the facts.

 

 

Reoccurring pulsation is one of the most common of all causes for comebacks. Most customers are told their rotors have “warped” due to heat and they are either sold a brake rotor resurface or new rotors to “fix” the problem. As you may have read, high-speed pulsation is caused by runout-induced disc thickness variation (DTV). DTV is seen as high and low spots on a rotor surface (Figure 1). DTV causes the caliper piston to move in and out of its bore during the brake application process, creating a rapid pressure transfer in the hydraulic system. This rapid pressure transfer is felt by the driver either in the brake pedal or steering wheel as a pulsation.

 

Let’s look at the myth that states, “Heat causes pulsation.” I recently completed some testing where the objective was to determine whether or not a vehicle at wide-open throttle (WOT) could be stopped with a discharged vacuum booster (no power assist). The protocol of the test involved reaching a speed of 55 mph, holding the accelerator pedal to the floor, tapping the brake 2 times, then applying and holding 100 lb of brake pedal force. In our test, the vehicle did not stop but actually accelerated after the transmission downshifted. The pedal force was increased to 150 lb with the same results. Eventually the brake pedal force was increased to 200 lb in an attempt to bring the vehicle to a stop.

 

 

Prior to the test, the brakes were replaced with new factory rotors and pads front and rear. The vehicle being tested had instrumentation and 4 cameras to record the results. One of the cameras was pointed at the left front caliper. During the test all 4 brakes caught fire due to the extreme temperatures and the duration of the test.

 

The temperatures were measured on all four brake assemblies immediately after the test and each were in excess of 1100°F. The temperature threshold for most semi-metallic brake pads is between 600–800°F, so we far exceeded normal operating temperatures. Guess what the brake pedal felt like during the drive back to the shop? NORMAL – no pulsation. From this test we can conclude that heat does not directly cause pulsation. In fact, brake pulsation is caused by a variation in rotor thickness, or DTV. The best way to avoid DTV is by limiting the amount of runout in a rotor during installation. Runout is checked by using a dial indicator. Rotors that exceed maximum runout specifications will allow the brake pad to contact the high side of the rotor during each revolution, therefore causing an uneven distribution of heat. This condition is the prime contributor to DTV.  Therefore, the statement “heat causes pulsation” is a myth.

 

 

In conclusion, I want to clarify something–heat can affect the intensity of the DTV induced pulsation. Think about the temperatures that would occur in the rotor pictured in Figure 1. Would it heat up evenly? No, in most cases the high surface would heat up faster. What happens when something gets hot? It expands. So the high surface will expand faster than the low area, increasing the degree of DTV and intensifying the hydraulic system rapid pressure transfer, felt as pulsation (Figure 3). That is the only way heat relates to pulsation.