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Burnishing, Conditioning, Breaking-In, or Bedding-In
Any idea what I'm talking about? The lingo can vary, but the goal is the same - preventing brake judder / pedal vibration. Put as simply as possible, bedding in a set of pads is the process of depositing an even layer of brake pad material across the face of the rotor. This layer of brake pad material is commonly called a transfer layer, and the key here is to get an "even" layer.
It only takes a few thousandths of an inch in variation to cause brake vibration. Initially you may have a hard time feeling it, but as the pad starts riding over the high and low spots, the variation will increase. As the thickness of the transfer layer increases in spots, the pedal vibration becomes more obvious. Eventually, the high spots can become hot spots. Hot spots can affect the metal on the rotor, creating hard spots on the rotor face. At that point, if the car is going to see track time, I think it's best to throw the rotor away and start fresh. Some people will suggest turning the rotors and using new pads. That is a cheaper alternative, but it only works for rotors that are designed to be turned. Mercedes-Benz and Porsche both advise against turning rotors.
Bedding in the pads is pretty simple, as long as you don't get arrested while doing it. The basic principal for bedding in pads is to (1) Heat the rotor to a temperature where the transfer layer will form. (2)Transfer material from the pad to the rotor. (3) Allow the rotor is to cool without coming to a stop. This allows an even transfer layer to be deposited on the face of the rotor. Repeat the procedure a few times, and the bed in process is complete. The concept sounds easy, but some may be wondering, "how?"
The only complicated part of this procedure is determining how much heat is required. The temperature where the pad material can form a transfer layer is "pad specific". The stock pads on a Mercedes-Benz will require a lot less heat than those of a race car. A street car will need 100°F to 600°F, whereas race pads will require 600°F to 1400°F. If the rotors don't get hot enough, the transfer layer won't adhere to the rotor. Too much heat causes an uneven transfer layer to set up.
In case anyone has strange ideas, everything here is based on the premise that a full brake job has just been performed. To ensure that we're all on the same page - by "just been performed", I mean no miles have been put on the car since the brake job was done. Feel free to drive the car to wherever the bed-in process will occur, but that's about it. Also, a full brake job consists of new pads, new rotors, and preferably a fluid flush. In some cases turned rotors can be substituted for new rotors (this will depend on the vehicle), but new pads are a must. Anyone who's doing a budget "reuse old pads" brake job should stop reading this. Those folks may want to Google "how to do a budget brake job that doesn't result in bent bodywork, airbag deployment, and a dramatic increase in insurance premiums".
With the new pads and rotors, the car will be accelerated and braked, so that the brake rotor and pad material are heated. There is one thing that should not be done during this procedure - the car should not be brought to a complete stop with the brakes still applied. Doing so risks putting an uneven transfer layer on the rotor.
The procedure for a street car is as follows:
- Accelerate to about 65 MPH. Lightly apply the brakes, and slow the car down to about 45mph. Accelerate back up to 65
MPH. Repeat this procedure four times. What this does is slowly raise the temperature of the pads and rotors. If this
step is skipped, it's possible to thermally shock the rotors and pads in the next steps.
- This is the part that will get you arrested if done in public. With the car at 65 MPH, and without engaging ABS (and
obviously NOT locking the wheels), dive on the brakes and bring the car down to 10 MPH or so. Now jump on the gas pedal
and bring the car back up to 65 MPH as fast as possible. Repeat this insanity seven times. Unless red lights appear in
the review mirror, do not come to a complete stop. If you come to a complete stop, especially after to fifth slowdown,
you may have problems.
You may experience some brake fade on the last slowdown or two. Some fade is to be expected. As is the smell of heated brake pads. If the brake fluid wasn't flushed, moderate fade may occur. After the brakes have cooled, the brakes will function normally and the smell will go away.
- Speaking of cooled, the rotors and pads will need about five minutes of "brake free" driving to cool down. This can be done after the eighth slowdown. After the brakes have cooled, this job is complete.
Some final thoughts
For semi-race pads, add another half dozen near stops from 80 MPH down to 10 MPH.
Depending on the pads, a second bedding in cycle may be required. This is usually the case when new pads are used with old rotors or a big brake kit is installed.
Pedal firmness will often improve after the second cycle and bleeding the brakes when the second cycle is complete is a good idea for track use.
Lastly, don't bring your wife or girlfriend along for company while performing this procedure. Someone has to be able to bail you out of jail for reckless driving.