Brakes, Rotors, Pads & Procedures etc.

nice read about brakes, rotors, pads and procedures (found on
http://forums.nasioc.com/forums/showthread.php?s=&threadid=368072
written by a guy from tire rack) - I attached the text; pictures are in the original source)

----------------------------------------
ROTORS
----------------------------------------

Rotors ...
Slotted or drilled ????
slotted rotors maintain approx. 96% of the friction surface
drilled rotors maintain approx. 85-93% of the friction surface
drilled and slotted only maintain 80-91% of the friction surface

For many years most racing rotors were drilled. There were two reasons -
the holes gave the "fireband" boundary layer of gasses and particulate
matter someplace to go and the edges of the holes gave the pad a better
"bite".

Unfortunately the drilled holes also reduced the thermal capacity of the
discs and served as very effective "stress raisers" significantly
decreasing disc life. Improvements in friction materials have pretty
much made the drilled rotor a thing of the past in racing. Most racing
rotors currently feature a series of tangential slots or channels that
serve the same purpose without the attendant disadvantages.

the process of drilling rotors and slotting rotors was done for 1 reason
and 1 reason only it is to disipate the gases that build up between the
pad and the rotor which occurs under extreme heat ( when braking very
aggressively like on a road course) and it has absolutely nothing to do
with heat disipation. the only way to transfer more heat away is by
using a larger heat sink which means use of a larger rotor whether in
diameter or thickness. Since the caliper will only allow for a certain
rotor thickness that solution is not very applicable because, if you are
changing tha caliper opening width you might as well get a larger rotor
diameter at that time

1) The brakes don't stop the vehicle - the tires do. The brakes slow the
rotation of the wheels and tires. This means that braking distance
measured on a single stop from a highway legal speed or higher is almost
totally dependent upon the stopping ability of the tires in use - which,
in the case of aftermarket advertising, may or may not be the ones
originally fitted to the car by the OE manufacturer.

2) The brakes function by converting the kinetic energy of the car into
thermal energy during deceleration - producing heat, lots of heat -
which must then be transferred into the surroundings and into the air
stream.

The amount of heat produced in context with a brake system needs to be
considered with reference to time meaning rate of work done or power.
Looking at only one side of a front brake assembly, the rate of work
done by stopping a 3500-pound car traveling at 100 Mph in eight seconds
is 30,600 calories/sec or 437,100 BTU/hr or is equivalent to 128 kW or
172 Hp. The disc dissipates approximately 80% of this energy. The ratio
of heat transfer among the three mechanisms is dependent on the
operating temperature of the system. The primary difference being the
increasing contribution of radiation as the temperature of the disc
rises. The contribution of the conductive mechanism is also dependent on
the mass of the disc and the attachment designs, with disc used for
racecars being typically lower in mass and fixed by mechanism that are
restrictive to conduction. At 1000oF the ratios on a racing 2-piece
annular disc design are 10% conductive, 45% convective, 45% radiation.
Similarly on a high performance street one-piece design, the ratios are
25% conductive, 25% convective, 50% radiation.

3) Repeated hard stops require both effective heat transfer and adequate
thermal storage capacity within the disc. The more disc surface area per
unit mass and the greater and more efficient the mass flow of air over
and through the disc, the faster the heat will be dissipated and the
more efficient the entire system will be. At the same time, the brake
discs must have enough thermal storage capacity to prevent distortion
and/or cracking from thermal stress until the heat can be dissipated.
This is not particularly important in a single stop but it is crucial in
the case of repeated stops from high speed - whether racing, touring or
towing.

4) Control and balance are at least as important as ultimate stopping
power. The objective of the braking system is to utilize the tractive
capacity of all of the tires to the maximum practical extent without
locking a tire. In order to achieve this, the braking force between the
front and rear tires must be nearly optimally proportioned even with ABS
equipped vehicles. At the same time, the required pedal pressure, pedal
travel and pedal firmness must allow efficient modulation by the driver.

5) Braking performance is about more than just brakes. In order for even
the best braking systems to function effectively, tires, suspension and
driving techniques must be optimized

-------------------------------------------
BRAKE FADE
-------------------------------------------

Brake fade ... what is happening to my stopping power ???

there are a couple of causes of brake fade

#1.)
When the fluid boils in the calipers, gas bubbles are formed. Since
gasses are compressible, the brake pedal becomes soft and "mushy" and
pedal travel increases. You can probably still stop the car by pumping
the pedal but efficient modulation is gone.

#2.)
Pad fade is when the temperature at the interface between the pad and
the disc exceeds the thermal capacity of the pad, the pad loses friction
capability due partly to out gassing of the binding agents in the pad
compound. Pad fade is also due to one of the mechanism of energy
conversion that takes place in the pad. In most cases it involves the
instantaneous solidification of the pad and disc materials together -
followed immediately by the breaking of bonds that releases energy in
the form of heat. This cycle has a relatively wide operating temperature
range. If the operating temperature exceeds this range, the mechanism
begins to fail. The brake pedal remains firm and solid but the car won't
stop. The first indication is a distinctive and unpleasant smell that
should serve as a warning to back off.

In either case temporary relief can be achieved by heeding the warning
signs and letting things cool down by not using the brakes so hard. In
fact, a desirable feature of a good pad material formula is fast fade
recovery. Overheated fluid should be replaced at the first opportunity.
Pads that have faded severely should be checked to make sure that they
have not glazed and the discs should be checked for material transfer.
The easy permanent cures, in order of cost, are to upgrade the brake
fluid, to upgrade the pads, or to increase airflow to the system
(including the calipers). In marginal cases one of these or some
combination is often all that is required.

------------------------------------------
WARPED ROTORS - pictures are on the link
------------------------------------------

Directly from Stoptech ...

The term "warped brake disc" has been in common use in motor racing for
decades. When a driver reports a vibration under hard braking,
inexperienced crews, after checking for (and not finding) cracks often
attribute the vibration to "warped discs". They then measure the disc
thickness in various places, find significant variation and the
diagnosis is cast in stone.

When disc brakes for high performance cars arrived on the scene we began
to hear of "warped brake discs" on road going cars, with the same
analyses and diagnoses. Typically, the discs are resurfaced to cure the
problem and, equally typically, after a relatively short time the
roughness or vibration comes back. Brake roughness has caused a
significant number of cars to be bought back by their manufacturers
under the "lemon laws". This has been going on for decades now - and,
like most things that we have cast in stone, the diagnoses are wrong.

With one qualifier, presuming that the hub and wheel flange are flat and
in good condition and that the wheel bolts or hat mounting hardware is
in good condition, installed correctly and tightened uniformly and in
the correct order to the recommended torque specification,

Here are more common problems found with rotors
cracked discs, (FIGURE 1)

discs that had turned into shallow cones at operating temperature
because they were mounted rigidly to their attachment bells or top hats,
(FIGURE 2)

a few where the friction surface had collapsed in the area between
straight radial interior vanes, (FIGURE 3)

and an untold number of discs with pad material unevenly deposited on
the friction surfaces - sometimes visible and more often not. (FIGURE 4)

------------------------------------------
THE NATURE OF BRAKING FRICTION
------------------------------------------

Friction is the mechanism that converts dynamic energy into heat. Just
as there are two sorts of friction between the tire and the road surface
(mechanical gripping of road surface irregularities by the elastic tire
compound and transient molecular adhesion between the rubber and the
road in which rubber is transferred to the road surface), so there are
two very different sorts of braking friction - abrasive friction and
adherent friction. Abrasive friction involves the breaking of the
crystalline bonds of both the pad material and the cast iron of the
disc. The breaking of these bonds generates the heat of friction. In
abrasive friction, the bonds between crystals of the pad material (and,
to a lesser extent, the disc material) are permanently broken. The
harder material wears the softer away (hopefully the disc wears the
pad). Pads that function primarily by abrasion have a high wear rate and
tend to fade at high temperatures. When these pads reach their effective
temperature limit, they will transfer pad material onto the disc face in
a random and uneven pattern. It is this "pick up" on the disc face that
both causes the thickness variation measured by the technicians and the
roughness or vibration under the brakes reported by the drivers.

With adherent friction, some of the pad material diffuses across the
interface between the pad and the disc and forms a very thin, uniform
layer of pad material on the surface of the disc. As the friction
surfaces of both disc and pad then comprise basically the same material,
material can now cross the interface in both directions and the bonds
break and reform. In fact, with adherent friction between pad and disc,
the bonds between pad material and the deposits on the disc are
transient in nature - they are continually being broken and some of them
are continually reforming.

There is no such thing as pure abrasive or pure adherent friction in
braking. With many contemporary pad formulas, the pad material must be
abrasive enough to keep the disc surface smooth and clean. As the
material can cross the interface, the layer on the disc is constantly
renewed and kept uniform - again until the temperature limit of the pad
has been exceeded or if the pad and the disc have not been bedded-in
completely or properly. In the latter case, if a uniform layer of pad
material transferred onto the disc face has not been established during
bedding or break-in, spot or uncontrolled transfer of the material can
occur when operating at high temperatures. The organic and semi-metallic
pads of the past were more abrasive than adherent and were severely
temperature limited. All of the current generation of "metallic carbon",
racing pads utilize mainly adherent technology as do many of the high
end street car pads and they are temperature stable over a much higher
range. Unfortunately, there is no free lunch and the ultra high
temperature racing pads are ineffective at the low temperatures
typically experienced in street use.

Therefore - there is no such thing as an ideal "all around" brake pad.
The friction material that is quiet and functions well at relatively low
temperatures around town will not stop the car that is driven hard. If
you attempt to drive many cars hard with the OEM pads, you will
experience pad fade, friction material transfer and fluid boiling - end
of discussion. The true racing pad, used under normal conditions will be
noisy and will not work well at low temperatures around town.

Ideally, in order to avoid either putting up with squealing brakes that
will not stop the car well around town or with pad fade on the track or
coming down the mountain at speed, we should change pads before
indulging in vigorous automotive exercise. No one does. The question
remains, what pads should be used in high performance street cars -
relatively low temperature street pads or high temperature race pads?
Strangely enough, in my opinion, the answer is a high performance street
pad with good low temperature characteristics. The reason is simple: If
we are driving really hard and begin to run into trouble, either with
pad fade or boiling fluid (or both), the condition(s) comes on gradually
enough to allow us to simply modify our driving style to compensate. On
the other hand, should an emergency occur when the brakes are

cold, the high temperature pad is simply not going to stop the car. As
an example, during the mid 1960s, those of us at Shelby American did not
drive GT 350 or GT 500 Mustangs as company cars simply because they were
equipped with Raybestos M-19 racing pads and none of our wives could
push on the brake pedal hard enough to stop the car in normal driving.

Regardless of pad composition, if both disc and pad are not properly
broken in, material transfer between the two materials can take place in
a random fashion - resulting is uneven deposits and vibration under
braking. Similarly, even if the brakes are properly broken, if, when
they are very hot or following a single long stop from high speed, the
brakes are kept applied after the vehicle comes to a complete stop it is
possible to leave a telltale deposit behind that looks like the outline
of a pad. This kind of deposit is called pad imprinting and looks like
the pad was inked for printing like a stamp and then set on the disc
face. It is possible to see the perfect outline of the pad on the disc.
FIGURE 5

It gets worse. Cast iron is an alloy of iron and silicon in solution
interspersed with particles of carbon. At elevated temperatures,
inclusions of carbides begin to form in the matrix. In the case of the
brake disk, any uneven deposits - standing proud of the disc surface -
become hotter than the surrounding metal. Every time that the leading
edge of one of the deposits rotates into contact with the pad, the local
temperature increases. When this local temperature reaches around 1200
or 1300 degrees F. the cast iron under the deposit begins to transform
into cementite (an iron carbide in which three atoms of iron combine
with one atom of carbon). Cementite is very hard, very abrasive and is a
poor heat sink. If severe use continues the system will enter a
self-defeating spiral - the amount and depth of the cementite increases
with increasing temperature and so does the brake roughness. Drat!

------------------------------------------
PREVENTION
------------------------------------------

PREVENTION

There is only one way to prevent this sort of thing - following proper
break in procedures for both pad and disc and use the correct pad for
your driving style and conditions. All high performance after market
discs and pads should come with both installation and break in
instructions. The procedures are very similar between manufacturers.
With respect to the pads, the bonding resins must be burned off
relatively slowly to avoid both fade and uneven deposits. The procedure
is several stops of increasing severity with a brief cooling period
between them. After the last stop, the system should be allowed to cool
to ambient temperature. Typically, a series of ten increasingly hard
stops from 60mph to 5 mph with normal acceleration in between should get
the job done for a high performance street pad. During pad or disc
break-in, do not come to a complete stop, so plan where and when you do
this procedure with care and concern for yourself and the safety of
others. If you come to a complete stop before the break-in process is
completed there is the chance for non-uniform pad material transfer or
pad imprinting to take place and the results will be what the whole
process is trying to avoid. Game over.

In terms of stop severity, an ABS active stop would typically be around
0.9 G's and above, depending on the vehicle. What you want to do is stop
at a rate around 0.7

to 0.9 G's. That is a deceleration rate near but below lock up or ABS
intervention. You should begin to smell pads at the 5th to 7th stop and
the smell should diminish before the last stop. A powdery gray area will
become visible on the edge of the pad (actually the edge of the friction
material in contact with the disc - not the backing plate) where the
paint and resins of the pad are burning off. When the gray area on the
edges of the pads are about 1/8" deep, the pad is bedded.

For a race pad, typically four 80mph to 5 and two 100mph to 5, depending
on the pad, will also be necessary to raise the system temperatures
during break-in to the range that the pad material was designed to
operate at. Hence, the higher temperature material can establish its
layer completely and uniformly on the disc surface.

Fortunately the procedure is also good for the discs and will relieve
any residual thermal stresses left over from the casting process (all
discs should be thermally stress relieved as one of the last
manufacturing processes) and will transfer the smooth layer of pad
material onto the disc. If possible, new discs should be bedded with
used pads of the same compound that will be used going forward. Again,
heat should be put into the system gradually - increasingly hard stops
with cool off time in between. Part of the idea is to avoid prolonged
contact between pad and disc. With abrasive pads (which should not be
used on high performance cars) the disc can be considered bedded when
the friction surfaces have attained an even blue color. With the carbon
metallic type pads, bedding is complete when the friction surfaces of
the disc are a consistent gray or black. In any case, the discoloration
of a completely broken in disc will be complete and uniform.

Depending upon the friction compound, easy use of the brakes for an
extended period may lead to the removal of the transfer layer on the
discs by the abrasive action of the pads. When we are going to exercise
a car that has seen easy brake use for a while, a partial re-bedding
process will prevent uneven pick up.

The driver can feel a 0.0004" deposit or TV on the disc. 0.001" is
annoying. More than that becomes a real pain. When deposit are present,
by having isolated regions that are proud of the surface and running
much hotter than their neighbors, cementite inevitably forms and the
local wear characteristics change which results in ever increasing TV
and roughness.

Other than proper break in, as mentioned above, never leave your foot on
the brake pedal after you have used the brakes hard. This is not usually
a problem on public roads simply because, under normal conditions, the
brakes have time to cool before you bring the car to a stop (unless,
like me, you live at the bottom of a long steep hill). In any kind of
racing, including autocross and "driving days" it is crucial. Regardless
of friction material, clamping the pads to a hot stationary disc will
result in material transfer and discernible "brake roughness". What is
worse, the pad will leave the telltale imprint or outline on the disc
and your sin will be visible to all and sundry.

The obvious question now is "is there a "cure" for discs with uneven
friction material deposits?" The answer is a conditional yes. If the
vibration has just started, the chances are that the temperature has
never reached the point where cementite begins to form. In this case,
simply fitting a set of good "semi-metallic" pads and using them hard
(after bedding) may well remove the deposits and restore the system to
normal operation but with upgraded pads. If only a small amount of
material has been transferred i.e. if the vibration is just starting,
vigorous scrubbing with garnet paper may remove the deposit. As many
deposits are not visible, scrub the entire friction surfaces thoroughly.
Do not use regular sand paper or emery cloth as the aluminum oxide
abrasive material will permeate the cast iron surface and make the
condition worse. Do not bead blast or sand blast the discs for the same
reason.

The only fix for extensive uneven deposits involves dismounting the
discs and having them Blanchard ground - not expensive, but inconvenient
at best. A newly ground disc will require the same sort of bedding in
process as a new disc. The trouble with this procedure is that if the
grinding does not remove all of the cementite inclusions, as the disc
wears the hard cementite will stand proud of the relatively soft disc
and the thermal spiral starts over again. Unfortunately, the cementite
is invisible to the naked eye.

Taking time to properly bed your braking system pays big dividends but,
as with most sins, a repeat of the behavior that caused the trouble will
bring it right back