by Jim Kerr
Brake pulsation has become a common concern on many newer vehicles. The vehicle stops smoothly when new, but after a few thousand kilometres the brake pedal starts to pulsate when stopping, the brakes feel like they are grabbing and releasing, and smooth stops are impossible. Sometimes the problem starts after the wheels have been removed. This was seldom a concern on vehicles in the 70’s and 80’s, so what has changed to make it a problem today?
The answer is weight. To increase fuel efficiency of new cars and reduce manufacturing costs, the accounting department (often referred to as the “bean counters”) ensures every part of the vehicle has its weight reduced as much as possible. This works fine for most parts but ask a brake system engineer about brake rotors and you will hear that heavier is better. Heavier brake rotors can absorb more heat, so they fade less and give better braking performance. Heavier rotors also tend to be more stable, so they seldom warp, which is one of the causes of brake pulsation.
Sometimes the brake engineers get their way; other times the bean counters win and the brake rotors are built lighter. That is when problems may occur. Sure, they could build brake rotors that are light, powerful, and stable. Carbon fibre and ceramic rotors are available and used on racing cars but the costs would be prohibitive for most passenger vehicles. Porsche does offer a high performance brake option on their 911 Turbo model. This car stops in amazingly short distances with their stock brake system. I can’t imagine how much better the $5000 brake option could be!
So most of us are stuck with conventional cast brake rotors. If they are not handled correctly, they can warp and that is the beginning of brake pulsation. A warped rotor wears unevenly from side to side as it rubs against the brake pads, and soon the sides of the rotor are no longer parallel to each other. Tolerances for parallelism are often less than .0005 inch maximum, or about 1/4 the thickness of a hair. When the rotor sides are not parallel, the pistons in the brake calipers that apply the brake pads are pushed in and out rapidly. This rapid motion is transmitted through the brake system and into the pedal. We feel it as a pulsation.
Changing a tire is one common cause of rotor warpage. If there is any dirt, rust, or corrosion between the wheel and the brake rotor, it will be clamped unevenly when the wheel is installed and the rotor will warp.
Incorrect tightening of the wheel nuts also can warp a rotor. When installing a wheel, snug up the wheel nuts and then tighten them in two stages using an alternating criss-cross pattern. Using a torque wrench is critical on modern vehicles. Some shops tighten wheel nuts with air impacts. Others use “torque sticks” designed to limit the torque on the nuts. Neither is accurate enough for today’s cars. Make sure they use a torque wrench. If you experience brake pulsations after changing a wheel, loosen the wheel nuts and retorque them. If this is done as soon as possible, the rotor will usually correct itself. Leave it too long (more than 1000 km) and it remains warped.
Another cause of brake pulsation is brake rotor run-out. Many manufacturers allow the rotor and hub to wobble up to .003 inch because of machining tolerances. More than that and the rotor wobbles too much – it acts like it is warped. Some cars are especially sensitive to rotor run-out. If it is more than .001 inch, then brake pulsations can occur.
The answer is to correct the run-out to less than .001 inch. One way to do this is to machine the rotors in place on the car instead of using off-car brake lathes as most shops do. This will correct for run-out in both the rotor and the hub. The trouble with this is on-car brake lathe machines tend to be more fragile and cost three to four times an off-car brake lathe so very few repair shops own one.
Recently, another method of correcting rotor run-out has been introduced to the market. Brake Align(r) has designed special tapered shims that are placed between a removable rotor and its hub to compensate for rotor run-out. Run-out is measured on the rotor and one of three different size shims is installed. Run-out can be corrected to within .001 inch.
Brake Align(r) shims have been tested for hundreds of thousands of kilometres and research by the company shows that if rotor run-out can be kept under .001 inch, then brake pulsations are no longer a problem.
Correcting brake pulsations or preventing them is just a matter of details. Keep mounting surfaces clean, machine rotors so the sides are parallel, reduce rotor run-out to a minimum, use the proper procedure to torque wheel nuts, and brake pulsations should no longer be a problem on your vehicle.