This article was published by AutomotiveArticles.com Staff Writer - M. Sanew - Dezoris
Jul 18, 2004, 22:49
Many people ask frequently about braking systems, and what is better and what is usable.
Everyone
wants to go faster, handle better, braking is often ignored, mainly do
to the price of major upgrades. After research and some experience most
people will find that, big brake kits, slotted, cross-drilled rotors
have not made a huge dent in 60-0 or 70-0 stopping distances.
In
this article we will look at what is important in stopping a vehicle
and what factors are involved. The reader should remember the concepts
behind braking in this article. These concepts are universal, no matter
what vehicle, we talk of, improving stopping distance is a matter of
applying applications based on driving habits and driving conditions.
Read on and find what you need to start stopping faster.
The following are terms you should know and have been supplied by brake system experts.
Clamping force:
The clamping force of a caliper in
pounds is the brake line pressure multiplied by the total piston area
of the caliper in a fixed caliper and two times the total piston area
in a floating design. To increase the clamping force it is necessary to
either increase the line pressure or the piston area. Increasing the pad area or the coefficient of friction will not increase clamping force.
What
does this mean? That your clamping force of your caliper and brake
subsystems has nothing to do with pad design or makeup nor does it have
anything to do with the type of disc used.
This next bit of
information is really the basis for stopping the vehicle. Improving
this area will get your vehicle stopping faster
Coefficient of friction:
A dimensionless indication of the
friction qualities of one material vs. another. A coefficient of 1.0
would be equal to 1g. The higher the coefficient, the greater the
friction. Typical passenger car pad coefficients are in the
neighborhood of 0.3 to 0.4. Racing pads are in the 0.5 to 0.6 range.
With most pads the coefficient is temperature sensitive so claims that
do not specify a temperature range should be viewed with some
suspicion. The optimum is to select a pad with a virtually constant
but decreasing coefficient over the expected operating range of
temperatures. As a result, the driver does not have to wait for the pad
to heat up before it bites, and the pad fade will not be a factor so
that modulation will be easy
Now that we have a foundation
we can see that finding the pad of the right material and heat range
affects your braking efficiency. You don't want a pad race pad for the
street, because you have to heat it up to its approriate heat range
before it bites. Not to far of from racing tires where their operating
range is higher, so getting them to stick requires more heat.
The difference is here you pick a pad for your car based on driving habits, much like you would tires.
If
you are on the brakes non-stop and generating excessive amounts of heat
then you want a pad and rotor combo designed to bite or grip at higher
temps.
On the street we want bite right away thus a pad with a
lower operating temp, and the trade off is fade at higher temps,
(excessive braking or high speed braking) or reduced bite.
Here is a run down of some common types of materials.
Carbon/carbon brake:
A braking system in which both discs
and pads are manufactured from carbon composite material. Utilized in
every form of racing where they are not outlawed, carbon/carbon brakes
offer significant reduction in rotating mass and inertia along with
much greater thermal capacity and dimensional stability in use. The
disadvantages include cost, a certain amount of lag time while heat
builds up (especially in the wet) and some difficulty in modulation.
Contrary to popular belief, the coefficient of friction is no better
than that of state of the art carbon metallic pads and cast iron discs.
A major advantage on super speedways is the reduction of gyroscopic
precession on corner entry.
Carbon metallic:
This is a trademark of the Performance
Friction Corporation. Pad friction compounds containing large
percentages of pure carbon along with various metallic elements.
Pioneered by Performance Friction Corporation these compounds offer
very constant coefficients of friction vs. temperature characteristics
along with increased thermal capacity. The disadvantage is that,
since they both operate at higher temperatures and their temperature
rises to operating temperature faster than other compounds, they
increase thermal shock to the disc and increase thermal conduction to
the caliper pistons and brake fluid. As a result, it is recommended to
not use drilled discs with carbon metallic pads.
Thermal
shock or disc exposure to extreme temps causes a break in the bonds of
the metals used in the discs which can cause cracking.
Thermal
shock is different from bluing which is a result of thermal stress on
cast iron rotors, but is normal for brakes that are exposed to hard
braking, it will however lead to shorter rotor life
Now on to Brake System Break in.
Bedding in: There are two types of "bedding in" with regard to brakes:
- Bedding of the friction material. All
friction materials contain volatile elements used as binders. In the
initial thermal cycling of the material these volatiles boil off,
forming a gaseous layer between the friction material and the brake. A
bedded pad will exhibit a layer of discolored material from 1.5 to 3 mm
thick.
- Bedding of the disc. Before using a new disk, all machining
and preservative oils must be completely removed following the disc
manufacturer's recommendations. Usually it involves washing with soap
and water or using one of the proprietary "brake clean" compounds. The
disc should then be mounted and checked for run out. It should be
bedded in with a number of moderate stops with lots of cool off time
between stops, gradually increasing the severity of the stops until the
entire surface of the disc is evenly discolored. This will prevent
thermal shock, distortion and the formation of "hot spots" (regional
deposition of pad material which results in a permanent transformation
of the cast iron underneath the deposit) and ensure maximum disk life.
To properly break in your pads and rotors we now know the importance of bedding in the materials.
When new pads are installed to
bed them in run them through one complete heat cycle. This may vary
based on the type of pad and its heat range. After one cycle has been
completed allow them to fully cool before subjecting them to race
conditions.
It is best to bed new pads on bedded rotors, that are
smooth and flat (Re-surfaced on a lathe) with no hot spots or bluing.
When using new pads and rotors it requires a more delicate break in
process. Heat cycling the pads once means more time for cooling the pad
and rotor.
Here is a practical way to bed pads. (Assuming you just installed the new pads)
You
should start by finding a safe location, uncrowded area preferably a
track. Warm up the motor, and vehicle. Start with a series of low speed
light braking manuvers to check brake system intgrity, post pad
install. In a closed area or safe spot start with harder stops from
lower speeds 25-40 MPH. Slowly increase the speed and stopping power
until they reach there specified heat range. This may be difficult to
tell, a true sport pad's manufacturor will provide a heat range. After
a series of 3-6 60-0MPH stops the pads should be cycled, you can always
check the pad with a prod type thermometer or thermo pyrometer.
When
the higher speed hard stops are applied if fade is evident begine the
cool down process. Slow down, avoid using the brakes, and drive until
you can park the vehicle to allow the pads and rotors to reach
atmospheric temperatures, before doing any racing or hard driving.
Following this procedure will maximize pad life, maximize the co-efficient of friction bewteen the materials.
Now
that you have that down lets get to the pedal effort you were talking
about. This is called modulation. The stiffer and tighter the pedal is
the less force is transfered to the clamping system via the booster,
and master cyl. The higher the pressure in the brake lines and more
pedal travel you have equals more force.
More pedal travel=Higher system pressure/clamping force harder to modulate
Stiffer pedal/tighter=Less pressure easier to modulate
Modulation:
The term given by the process by which
the skilled driver controls the braking torque to maintain maximum
retardation without locking wheels. Because the human being modulates
most efficiently by force rather than displacement, effective brake
modulation requires minimum pedal travel and maximum pedal firmness
The pads and rotors do not change this. They may increase your systems ability to increase hydrolic pressure though.
Mechanical pedal ratio:
The brake pedal is designed to
multiply the driver's effort. The mechanical pedal ratio is the
distance from the pedal pivot point to the effective center of the
footpad divided by the distance from the pivot point to the master
cylinder push rod. Typical ratios range from 4:1 to 9:1.The larger the
ratio, the greater the force multiplication (and the longer the pedal
travel)
This ratio can be adjusted by the installer.
So
now you can see that brake modulation is key for controlling your
braking to keep teh car stable, and has nothing to do with actual with
braking efficiency or braking tq.
Many
asked about the cross drilled rotors and how they help cool the brake
system. Well here is the short answer, air cools them, and without
ducting running to your brake discs they are hardly more effective than
your standard ventilated disc. The slotted rotors are for gassing, when
the pads reach their operating temps or higher they start to gas or
breakdown and the slots are there to allow the gas to escape.
So
what cross drilled rotors do is reduce suface area decrease
co-efficient of friction and are more prone to thermal shock, and
cracking unless, you have a specialized braking system using, special
brake fluid, stainless steel brake lines, venting all the necessary items to aid in convection
Convection:
One of only three heat transfer
mechanisms. Conduction and radiation are the other two. Convection is
the transfer of heat by fluid flow. Air can be considered to be a fluid
in a thermal model of a brake system when it is moving and is contact
with the heated surfaces of the disc or drum. In the case of a solid
disc the air moving over the surface of the disc is very random and
turbulent, but still functions to provide some cooling. In the case of
a ventilated disc, by the pressure of a forced air duct or by induced
flow that is a result of the centrifugal acceleration of the air
already in the vent of a rotating disc, air flows through the vents.
The air absorbs thermal energy along the vent path. In this way, the
heat generated by the braking system of an automobile is transferred to
the moving air stream and away from the brake disc.
As far as
cross drilled rotors go, the advent of carbon metallic friction
materials with their increased temperatures and thermal shock
characteristics ended the day of the drilled disc in professional
racing. They are still seen (mainly as cosmetic items) on motorbikes
and some road going sports cars. Typically in original equipment road
car applications these holes are cast then finished machined to provide
the best possible conditions by which to resist cracking in use. But
they will crack eventually under the circumstances described in another
section. Properly designed, drilled discs tend to operate cooler than
non-drilled ventilated discs of the same design due the higher flow
rates through the vents from the supplemental inlets and increased
surface area in the hole. That's right, inlets, the flow is into the
hole and out through the vent to the OD of the disc. If discs are to be
drilled, the external edges of the holes must be chamfered (or, better
yet, radiused) and should also be peened.
Thanks to By Stephen Ruiz, Engineering Manager and
Carroll Smith, Consulting Engineer at STOPTECH LLC for technical definitions.
It's
true, there are many pads available, for the tuner, knowing the a
person's driving style will help the average Joe make a educated choice
when upgrading brake components.
"Big Brake" kits look great on paper,
and on the car, most people would be more excited about doing a kit
with big calipers and rotors because of those reasons. However,
upgrading calipers and rotors does not mean that the car will stop
faster. Chances are if the manufacturer has a brake system designed
with one piston calipers clamping 7" disks, that probably means the
other brake system components won't be taking full advantage of the. A
tuner can't always upgrade one system, if there is a weak link
elsewhere. The systems have to be looked at as a whole. Most brake
articles on the net talk about this, treating the brake systems as a
whole. What they don't mention typically, is that OEM makers don't
produce perfectly designed brake systems. The pad and rotor combos used
on most cars are to get them stopping quick and lasting long. Which
brings this article to the main point: throwing on a sport pad is the
easiest way to increase your COF, and will increase efficiency. If the
tuner drives hard the OEM pads on most cars will gas, due to their
lower operating temperatures. Sport pads will be better for those who
like to stop fast at higher speeds or those who are on the brakes long
periods of time, without the need for big brake upgrades.
Increasing rotor size will also help,
but, not without sport calipers. If both were adde the system
may require true stainless steel teflon brakelines, and a possible
upgraded booster and master cyl. to accomodate the increased pressure
needed to keep clamping force up, along with race/syn brake fluid.
People looking
for a little more bite for hard braking might find using sport pads and
slotted rotors for gasing a real treat. This combo is best realized for
hard driving (IE: autox, roadrace). I suggest replacing or bleeding the
brake fluid to also help pedal feel after an install of this nature.
In conclusion, I have found people who
do brake upgrades are disappointed, by the longevity of them. Pads and
rotors are a wear and tear item, in most cases performance brake parts
last 1/2 as long as OEM solutions. Knowing how to break them in and how
they work is the driver's key to stopping faster.
