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Brake Pedal Pulsation - How and Why it Happens

We have all experienced the feeling of applying the brakes at 50mph only to feel the brake pedal "dancing" under our foot. Most mechanics have also experienced a "pulsation" related comeback after performing a first class brake job on their customer's car. Why did it happen? What are the causes?In order to explain this we need to know some terms.

Rotor Thickness Variation.

Thickness variation on a new rotor should be practically undetectable. To measure for RTV, mark a rotor at 15 different but equally spaced intervals. Measure rotor thickness at these intervals and record. There should be no more than .001 variation on a new (or resurfaced) rotor.

Lateral Runout

Lateral runout is measured using a dial indicator mounted on the spindle or adjacent area with the pointer resting in the middle of the rotor surface. The rotor should be held on by at least three lug nuts torqued to specifications. While turning the rotor, note runout. It should not be more than .002.

Rotor Indexing

This is the practice of marking the lug stud and rotor before rotor removal in order to return it to its previous position. This can be very important because the vehicle manufacturer may have installed the rotor in a certain position from the factory to minimize runout. It is not necessary when you are replacing a rotor.

Non-Directional Finish

This is the practice of using sandpaper or other power tools to put a non-directional swirl type finish on all rotors, new or reground. It s now recommended by all vehicle manufacturers and helps to properly break in friction and rotor surfaces.

Rotor Installation

To properly install a rotor during your brake job, the following steps should be adhered to in order to insure a trouble free installation.

  • Index the rotor if you are going to reuse it, returning the rotor to its original position helps to insure minimal runout. Machine the rotor in a brake lathe and finish with a non-directional finish. Complete this with a wash in hot soap and water, drying with paper towels. Brake Cleaner does not remove all the metal particles from the rotor, shop air and rags usually contain oil that can contaminate rotor surfaces and friction material.
  • It is extremely important to remove all traces of rust and corrosion from inside the rotor hat and the hub surface that the rotor rides on. Many techs use a wire wheel on a drill for this. This method does not do a good enough job in cleaning around the wheel studs.
  • Checking the endplay of the hub bearing. Front wheel drive hub bearings should have .002 or less endplay. More than that will result in eventual RTV.
  • Proper tightening of lug nuts. You must follow this procedure to make sure you do not induce runout while installing the wheels. Do not lubricate the lug studs. All vehicle manufacturers say that you should never lubricate the studs. False torque readings will result. Hand tighten all lug nuts in a star pattern. Tighten to one half of maximum specified torque value. Tighten to the full torque spec as specified by the vehicle manufacturer. You can induce lateral runout by using an impact wrench to tighten lug nuts. This runout will lead to RTV and pedal pulsation.
  • Checking installed runout. If the installed runout is above specs, try moving the rotor one lug at a time to see if it can be corrected. You can also use a "Brake Align" shim system to accomplish this. As a last resort (the one recommended by all manufacturers) is to machine the rotor with an on car brake lathe. This will match the rotor to the hub and done properly, with result in no runout.
  • Why runout causes pedal pulsation 1-4000miles down the road. When a rotor is installed with over spec runout, it may not be felt in the brake pedal immediately. Over time, the high spot on the rotor wipes the friction material every time it goes around. This can cause friction material to be deposited on the rotor making the high spot even higher. When it gets high enough, pedal pulsation and an unhappy customer is the result.

Many technicians will say that this is not the way I have performed rotor service in the past. Brake systems and rotors are much different that they used to be. Rotors are much lighter and some do not have integral hubs. Sealed roller bearings are replacing tapered bearings. Performing your rotor servicing as outlined above insures a quality brake job as well as a satisfied customer.

This documentation was provided by Bendix®.

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