When a brake pedal pulses beneath your foot, your first thought is probably “warped brake rotors.” Your next thought might be about how it happened, because if you’re going to get paid for the repair you want to make sure it doesn’t happen again. Fortunately, most things that cause brake pedal pulsation happen in the service bay. With close attention to detail, it’s easy to avoid these particular comebacks. The term “warped brake rotor” is used pretty loosely to define any brake rotor that isn’t flat and parallel with the plane of rotation. But there are other things that can cause brake pedal pulsation and there are ways that rotors can distort that don’t always produce pedal pulsation, but do affect braking. Understanding these things is the only way to guard against them or deal with them successfully if they do come back.
If you know how a disc brake works, it’s easy to visualize why you can feel warped rotors in the brake pedal. The rotor is shaped like a potato chip, the high spots push back against the caliper pistons as they rotate between the brake pads. That push is transmitted directly to the pedal and if the suspension bushings are worn, it might even cause steering wheel shimmy. Several things can cause a rotor to warp. Uneven heating or cooling is one culprit. A frequently cited example is driving through a deep puddle on a dry day with hot brake rotors. Another example is parking the vehicle without a “cool-down lap” after heavy brake use, such as after towing in hilly terrain. By far, the single most common cause of warped brake rotors is improper installation of the wheel. There was a time this was not so, but modern brake rotors are made thinner for faster cooling and to reduce unsprung weight. They’re not just thinner at the friction surface, there’s less material everywhere, including the hat section. This makes the rotor more susceptible to warping due to a combination of heat and improper lug nut torque. Manufacturers specify lug nut torque for the same reason they specify cylinder head bolt torque: to control distortion of the parts being bolted together. Most front-wheel-drive cars have one-piece cast iron brake rotors shaped like a top hat. The center “hat section” is clamped between the axle hub and the wheel. On most rotors, before the lug nuts are tightened, the rotor doesn’t fit flat against the hub; there’s a slight gap either toward the center of the hub or toward the outer diameter of the hub flange. That gap is less than five hundredths of a millimeter, but it’s supposed to be there. As the lug nuts are tightened, the hat section is forced flush against the hub, closing the gap. This slight bending of the rotor loads the hub/rotor assembly, increasing the overall strength and stiffness of both parts. If the lug nuts are over tightened or torqued unevenly, the stress loading is uneven. After a few heat cycles, the uneven loading creates a permanent distortion, as much a tenth of a millimeter, according to General Motors (GM). Usually, the rotor face stays flat but tilts with respect to the plane of rotation. In this case, even without any thickness variation, the rotor will wobble between the pads and cause brake pedal pulsation. GM recognized this problem several years ago and approved the use of a product called Brake Align, which is a selection of shims that fit between the hub and rotor to correct lateral rotor runout. (See GM service bulletin 0105-23-001). You can avoid this kind of rotor warping by tightening the lug nuts in the proper sequence using a torque wrench – every single time. Another common cause of warped rotors is contamination. Imagine the bending forces and stress risers created by corrosion, clumps of brake dust or anything else clamped between the hub and rotor, or between the wheel and rotor. Because they are dissimilar metals, aluminum wheels often corrode where they contact the rotor hat. Before assembly, GM recommends cleaning the hub, rotor and wheel with a wire brush, and other manufacturers recommend brushing a thin coat of anti-seize on each surface. But remember, it’s a brake system, so don’t use so much anti-seize that it flings out onto the braking surfaces.
The late Carroll Smith, a racing driver and engineer, wrote of what he learned about disc brakes while working on the original Ford GT40 race car in the 1960s when automotive disc brakes were still new. Smith noted that although a brake rotor can warp in many different ways, he never saw a properly assembled rotor warp in a way that causes brake pedal pulsation. He said that, whether on a race car or a street-driven grocery getter, the rotor irregularity that most commonly causes pedal pulsation results from uneven transfer of friction material from the pad to the rotor. To understand this, we need to understand two basic concepts about brakes. First of all, brakes are designed to work within a specific temperature range and the pad friction material is designed with that range in mind. The friction material used for racing doesn’t work at all when the brakes are cold, and the materials that work best for noise reduction can be severely overheated when used on a race car. Secondly, during normal use, friction material abrades from the brake pad and adheres to the rotor, forming an extremely thin layer of friction material on the rotor. When the pads and rotor are new, the bedding-in process establishes the initial layer. With continued use, the friction material is abraded away from the rotor and replaced with new material from the pad. If the brakes are not properly bedded-in or if the pads are severely overheated at any time, the transfer of friction material from pad to rotor becomes uneven. Heavier deposits can be seen as stains or dark spots on the rotor that won’t wash off. Even if you can’t measure a thickness variation at these spots, they will have a different coefficient of friction when hot. Since that layer of material is only microns thick, it can be cut away by a brake lathe. If uneven deposits happen again, it’s time to select a brake pad that can handle higher temperatures.
A Fresh Surface
If the brake pads are worn but everything else looks OK, it’s tempting to just install new pads. It’s also a good bet that the brakes won’t grip that old rotor like the old pads did, especially if they’re a different friction material. The main reason for replacing or resurfacing otherwise flat rotors is to give the new pads a fresh surface for that thin layer of friction material. By now the main advantage of on-car brake lathes is well understood: the new friction surface they create is completely parallel to the plane of rotation. This goes a long way towards eliminating comebacks, but with close attention to the details noted earlier, older tools and techniques work just as well. Before removing the rotor, it should be checked for run-out with a dial indicator. If there is run-out, it’s a good idea to check the hub too.
Hubs warp, wheel bearings wear, CV-joints become stiff and cause the stub axle to wobble ever so slightly. Each of these things alone could produce such a slight run-out that it can’t be measured, but stacked together it can be felt in the brake pedal. By changing the position of the rotor on the hub, total run-out can be reduced or eliminated. Check it with a dial indicator. Worn caliper sliders can also cause pedal pulsation and they can cause rotor wear that doesn’t necessarily show up in the pedal. Uneven pad wear can show up with a fixed caliper that has a jammed piston on one side. As the brake pads wear, the piston that moves freely will constantly bend the rotor slightly toward the stuck piston. With enough heat cycles, the rotor can actually assume that new shape and the brake pads will wear unevenly. There may not be any measurable warp, but unless the rotor is resurfaced during a bra ke job the new pads won’t bed in properly. Is pedal pulsation dangerous? When mild, it’s merely annoying. But even mild pulsation can influence ABS performance and increase braking distance in a panic stop. In extreme cases or when traction is marginal, uneven braking caused by pedal pulsation can make the car difficult to control. Fortunately, you can control most of the causes of brake pedal pulsation, through understanding how brakes really work and by paying attention to the details.