Pad and Rotor Bed-In Theory, Definitions and Procedures
Removing the Mystery from Brake Pad Bed-In
by Matt Weiss of StopTech and
James Walker, Jr.
of scR motorsports
In order for any brake system to work optimally, the rotors and pads must be
properly bedded-in, period. This process can also be called break-in,
conditioning, or burnishing, but whatever terminology you choose, getting the
brakes properly bedded-in and keeping them that way is critical to the peak
performance of the entire brake system.
However, understanding why the rotors and pads need to be bedded-in is just
as important as the actual process. If one understands what is happening during
the bed-in process, they can tailor the process to specific pads, rotors, and/or
driving conditions. For this reason, we present this generic bed-in overview
pertaining to all brake systems, but follow with links to application-specific
bed-in procedures to fit most every set of circumstances.
What is brake pad “bed-in” anyway?
Simply stated, bed-in is the process of depositing an even
layer of brake pad material, or transfer layer, on the rubbing surface of the
rotor disc. That's it. End of discussion. Ok, not really, but although bed-in is
quite basic in definition, achieving this condition in practice can be quite a
challenge, and the ramifications of improper or incomplete bed-in can be quite
annoying.
Abrasive friction and adherent friction
There are two basic types of brake pad friction mechanisms:
abrasive friction
and adherent friction
. In general, all pads display a bit of each, with abrasive mechanisms
dominating the lower temperature ranges while adherent mechanisms come more into
play as pad temperature increases. Both mechanisms allow for friction or the
conversion of Kinetic energy to Thermal energy, which is the function of a brake
system, by the breaking of molecular bonds in vastly different ways.
The abrasive mechanism generates friction or energy conversion by the
mechanical rubbing of the brake pad material directly on the rotor disc. In a
crystalline sense, the weaker of the bonds in the two different materials is
broken. This obviously results in mechanical wear of both the pad and the rotor.
Consequently, both pads and rotors are replaced when they are physically worn to
their limit and are too thin to endure further service.
The adherent mechanism is altogether different. In an adherent system, a thin
layer of brake pad material actually transfers and sticks (adheres) on to the
rotor face. The layer of pad material, once evenly established on the rotor, is
what actually rubs on the brake pad. The bonds that are broken, for the
conversion of Kinetic to Thermal energy, are formed instantaneously before being
broken again. It is this brake pad-on-transferred brake pad material interaction
on a molecular level that yields the conversion process.
With the adherent mechanism there is much reduced rotor wear as compared to
abrasive mechanism, but it's not a free lunch – pads now become the primary wear
element in the braking system. And even though rotors are not mechanically worn
down with adherent systems, they still will need to be replaced on a regular
basis due to cracking reaching a point of failure if they are exposed to
intense, repetitive thermal cycling. This is why race teams throw out rotors
that are actually as thick or thicker than when they were brand new. It's due to
the an adherent brake pad transfer layer!
The all-important transfer layer
As stated above, the objective of the bed-in process is to deposit an even
layer of brake pad material, or
transfer layer ,
on the rubbing surface of the rotor disc. Note the emphasis on the word even, as
uneven pad deposits on the rotor face are the number one, and almost exclusive
cause of brake judder or vibration.
Let's say that again, just so there is no misunderstanding. Uneven pad
deposits on the rotor face are the number one, and almost exclusive cause of
brake judder or vibration.
It only takes a small amount of thickness variation, or TV, in the transfer
layer (we're only talking a few ten thousandths of an inch here) to initiate
brake vibration. While the impact of an uneven transfer layer is almost
imperceptible at first, as the pad starts riding the high and low spots, more
and more TV will be naturally generated until the vibration is much more
evident. With prolonged exposure, the high spots can become hot spots and can
actually change the metallurgy of the rotor in those areas, creating “hard”
spots in the rotor face that are virtually impossible to remove.
Bedding fundamentals
In general, bed-in consists of heating a brake system to its adherent
temperature to allow the formation of a transfer layer. The brake system is then
allowed to cool without coming to rest, resulting in an even transfer layer
deposition around the rotor circumference. This procedure is typically repeated
two or three times in order to ensure that the entire rotor face is evenly
covered with brake pad material. Sounds easy, right? Well, it can be if you have
the proper information.
Because the adherent temperature range for brake pads varies widely
(typically 100°F-600°F for street pads and 600°F-1400°F for race pads), each
bed-in needs to be application-specific. One could try to generate a
one-size-fits-all procedure, but too little heat during bed-in keeps the
material from transferring to the rotor face while overheating the system can
generate uneven pad deposits due to the material breaking down and splotching
(that's a technical term) on to the rotor face.
In summary, the key to a successful bed-in is to bring the pads up to their
adherent operating temperature in a controlled manner and keep them there long
enough to start the pad material transfer process. Different brake system
designs, pad types, and driving conditions require different procedures to
successfully accomplish the bed-in. The recommended procedures below should
provide you with the information you need to select the bed-in procedure
appropriate for your application.
StopTech's Recommended Procedure for Bedding-in Stock-Sized Brake Systems
by Matt Weiss of StopTech and
James Walker, Jr.
of scR motorsports
When a system has both new rotors and pads, there are two different objectives
for bedding-in a performance brake system: heating up the brake rotors and pads
in a prescribed manner, so as to transfer pad material evenly onto the rotors;
and maturing the pad material, so that resins which are used to bind and form it
are ‘cooked' out of the pad.
The first objective is achieved by performing a series of stops, so that the
brake rotor and pad material are heated steadily to a temperature that promotes
the transfer of pad material onto the brake rotor friction surface. There is one
pitfall in this process, however, which must be avoided. The rotor and,
therefore, the vehicle should not be brought to a complete stop, with the brakes
still applied, as this risks the non-uniform transfer of pad material onto the
friction surface.
The second objective of the bedding-in process is achieved by performing
another set of stops, in order to mature the pad itself. This ensures that
resins which are used to bind and form the pad material are ‘cooked' out of the
pad, at the point where the pad meets the rotor's friction surface.
The bed-in process is not complete until both sets of stops have been
performed.
Bedding-in Street Performance Pads
For a typical performance brake system using street-performance pads, a
series of ten partial braking events, from 60mph down to 10mph, will typically
raise the temperature of the brake components sufficiently to be considered one
bed-in set. Each of the ten partial braking events should achieve
moderate-to-high deceleration (about 80 to 90% of the deceleration required to
lock up the brakes and/or to engage the ABS), and they should be made one after
the other, without allowing the brakes to cool in between.
Depending on the make-up of the pad material, the brake friction will seem to
gain slightly in performance, and will then lose or fade somewhat by around the
fifth stop (also about the time that a friction smell will be detectable in the
passenger compartment). This does not indicate that the brakes are bedded-in.
This phenomenon is known as a green fade, as it is characteristic of immature or
‘green' pads, in which the resins still need to be driven out of the pad
material, at the point where the pads meet the rotors. In this circumstance, the
upper temperature limit of the friction material will not yet have been reached.
As when bedding-in any set of brakes, care should be taken regarding the
longer stopping distance necessary with incompletely bedded pads. This first set
of stops in the bed-in process is only complete when all ten stops have been
performed - not before. The system should then be allowed to cool, by driving
the vehicle at the highest safe speed for the circumstances, without bringing it
to a complete stop with the brakes still applied. After cooling the vehicle, a
second set of ten partial braking events should be performed, followed by
another cooling exercise. In some situations, a third set is beneficial, but two
are normally sufficient.
Bedding-in Club Race or Full Race Pads
For a typical performance brake system using race pads, the bed-in procedure
must be somewhat more aggressive, as higher temperatures need to be reached, in
order to bring certain brands of pad material up to their full race potential.
We typically recommend a set of ten partial braking events, from 60mph down
to 10mph, followed immediately by three or four partial braking events, from
80mph down to 10mph. Alternately, a set of eleven stops, from 80mph to 40mph, or
a set of seven stops, from 100mph to 50mph, would be approximately the same. As
with street pads, each of the partial braking events should achieve
moderate-to-high deceleration (about 80% of the deceleration required to lock up
the brakes and/or to engage the ABS), and they should be made one after the
other, without allowing the brakes to cool in between.
Again, depending on the make-up of the pad material, the brake friction will
seem to gain slightly in performance, and will then lose or fade somewhat about
halfway through the first set of stops. This does not indicate that the brakes
are bedded-in, except where race-ready pads are being used. This phenomenon is
the same as that which occurs with high-performance or street pads (except that,
when race-ready pads are used, they do not exhibit green fade, and they will be
bedded-in after just one complete set of stops).
As when bedding-in any set of brakes, care should be taken regarding the
longer stopping distance necessary with incompletely bedded pads. This first set
of stops in the bed-in process is only complete when the recommended number of
stops has been performed - not before. As a general rule, it would be better to
perform additional stops, than not enough. The system should then be allowed to
cool, by driving the vehicle at the highest safe speed for the circumstances,
without bringing it to a complete stop with the brakes still applied.
After cooling the vehicle, a second set of the recommended number of stops
should be performed, followed by another cooling exercise. In some situations, a
third set is beneficial, but two are normally sufficient.
Racers will note that, when a pad is bedded-in properly, there will be
approximately 2mm (0.1 inch) of the pad edge near the rotor, on which the paint
will have turned to ash, or the color of the pad will have changed to look as
though it has been overheated.
In summary, the key to successfully bedding-in performance brakes is to bring
the pads up to their operating temperature range, in a controlled manner, and to
keep them there long enough to start the pad material transfer process.
Different brake system designs, pad types, and driving conditions require
different procedures to achieve a successful bed-in. The procedures recommended
above should provide a useful starting point for developing bed-in procedures
appropriate to individual applications.
