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Take a little time to bed your new rotors and pads in correctly and you will enjoy maximum brake performance and product life.

 

Correct Bedding-in of New Brake Rotors and Pads

by Clive Murphy

March 23, 2014

 

Far too many people make the mistake of fitting new rotors and pads and then going out immediately
and driving hard in order to test the performance their newly fitted brakes.  Do not make this mistake!

Using your brakes hard immediately after fitting new rotors and pads can result in permanent damage
to the rotors, poor brake performance, brake vibration and reduced product life. Take a little time to bed
your new rotors and pads in correctly and you will enjoy maximum brake performance and product life.

 

Why is it important to bed-in your new rotors and pads?

 

1. To avoid rotor distortion and cracking.

 

During the rotor casting process molten iron is poured into moulds at temperatures in excess of 2450°F
and allowed to cool rapidly into solid form. This rapid cooling causes internal stresses to be ‘stored’
within the rotor casting. Gradual and moderate thermal cycling (as is experienced when bedding-in
rotors properly after fitment) can effectively relieve these stresses but sudden, dramatic temperature
changes can lead to distortion or cracking of the rotor. If you’ve ever watched a block of ice in a glass
crack suddenly when a warmer liquid is poured into the glass, then you have witnessed a perfect
demonstration of stored internal stresses being relieved suddenly through cracking. The stresses in the
ice are formed as a result of the rapid cooling of the water, in a freezer, into solid form. If the ice block
were warmed slowly it would not crack. It’s the sudden change of temperature (or thermal shock),
caused by the ice coming into contact with the warmer liquid, that causes the internal stresses to
relieve themselves by cracking.
Following a moderate and progressive bed-in process is an effective way to stress-relieve new brake
rotors and avoid ‘thermal shock’ that leads to rotor distortion and/or cracking. Although all
Powerbrake™ rotors undergo multiple stress-relieving processes during manufacture that dramatically
increase their resistance to distortion and cracking, they will still benefit from correct bedding-in.

 

2. To achieve maximum contact area between the pad and rotor surfaces.

 

The friction surface of a new set of brake pads is far from being perfectly flat. To a lesser extent the
braking surface of a new rotor also has minute ‘peaks and valleys’ left by machining or grinding during
manufacture. The result is that a new set of rotors and pads start off by braking on only part of the
maximum surface area that is intended to do the work. The parts of the friction material that are initially
in contact with the rotor will overheat very quickly. Similarly, the areas of the rotor surface that are
initially making contact will heat quickly, while the non-contact areas remain cooler. This differential
heating of the rotor surface can result in rotor distortion. Proper pre bed-in allows the new rotor and
pads to achieve maximum contact before being subjected to intense heat.

 

3. To allow a layer of friction material to be evenly transferred to the rotor surface.

 

Correct transfer of pad friction material to the rotor surface is essential in order to avoid brake vibration
and maximize brake performance. Following the correct bed-in procedure allows a thin layer of pad
friction material to be evenly transferred to the rotor surface. Running the brakes too hot before this
layer has been established will result in random, uneven transferring of fiction material to the rotor
surface. This results in DTV, which in turn causes a downward spiral in which the brake rotors can be
permanently damaged. (To learn more about DTV and brake vibration click here).

 

4. To ‘burnish’ the brake pads.

 

Many pad compounds use organic resins in conjunction with binding fibres to bind the friction material
together. At high temperatures these resins start to evaporate. The gases that are formed as a result of
the evaporating resins can form a film or ‘cushion’ between the pad and rotor surface causing ‘pad lift’
and reducing friction. This phenomenon is called “outgassing” and is one of the causes of brake fade.
Outgassing is far more prevalent with newly installed pads in which case it is often referred to as “green
fade”. Bedding-in a new set of pads correctly will burn off the resins near to the contact surface of the
friction material resulting in stable friction co-efficient. The above process is referred to as “pad
burnishing”.

 

Bedding-in procedure for new rotors and pads

 

IMPORTANT: Brake function and performance will always be reduced during the period between the
fitting of new brake rotors and/or pads and the completion of the manufacturers recommended bed-in
procedure. Caution should be exercised during this period until the brakes are operating at full
efficiency. If at all possible cycle 1 and 2 (below) should be conducted in a quiet area, away from traffic
and other obstacles. Allow plenty of safe run-off space in case of the onset of “green fade”. The amount
of “green fade” experienced will differ from one friction material to another.

 

Pre Bed-in Cycle:

 

- Drive moderately for 1 week (approx 200 miles) after fitting new rotors and pads. Use medium brake
pedal pressures during this time and try to avoid long brake applications from speeds in excess of 70
mph. Normal urban commuting, using moderate brake applications about once per mile is ideal. Long
stretches of open freeway driving do not count, as the brakes are not being used regularly. It is a fairly
good idea to increase following distances during this time making provision for possible “green fade” in
an emergency stop. Never left foot brake or drag the brakes during this time. None of the MTR
temperature recording paints on PowerbrakeÔ rotors should have changed colour during this period.
- At the end of the first week it should be clearly visible that the pad friction material has achieved
maximum contact area with the rotor surface. The ‘pad track area’ of the rotor should be an even lightgrey
colour.

 

Bed-in Cycle:

 

- Drive for a few miles, using the brakes moderately to bring them up to operating temperature.
- Perform 6-8 consecutive brake applications of increasing intensity from 60mph down to 20mph. If
wheel lock-up (or ABS activation) is referred to as a 100% brake effort then you should be aiming to
build up to a 70% brake effort for the last 2 applications. The applications should be performed
consecutively, accelerating normally back up to 60mph after each application, before braking again.
- Do not come to a complete stop during the above brake application. Doing so can result in ‘pad
etching’ during which friction material is unevenly deposited onto the brake rotor causing DTV and
brake vibration.
- It is normal to experience a strong smell and even see a small amount of smoke rising from the
brakes from the 4th brake application onwards. This is not a problem. Be prepared to experience some
“green fade” from the 4th application onwards.

- Drive on, using the brake as little as possible, for a number of miles allowing the brakes to cool using
the airflow through the rotors, before parking the vehicle and allowing the brakes to cool completely to
ambient temperature. This cooling cycle is important.

 

NOTE: In some cases, particularly when using high-performance road or crossover street/track pads, it
may be necessary to repeat the above bedding-cycle occasionally. This may be necessary after long
periods of very gentle brake use. Pads transfer material to the rotor surface optimally when operating
within their recommended temperature range. Operating the pads below this temperature range my
lead to a scrubbing off of the friction material layer on the rotor surface. Repeating the bedding cycle
will re-establish the friction transfer layer and re-burnish the pads resulting in optimum performance.

Clive Murphy of Powerbrake USA graciously allowed us to reprint this article on the important issue of bedding-in new brake pads.  Check out his website at www.powerbrakeusa.com.

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