Brake
Systems and Upgrade Selection
By
Stephen Ruiz, Engineering Manager
and
Carroll Smith, Consulting Engineer at STOPTECH LLC
While almost
every current passenger car is capable of a single stop from
maximum speed at or near the limit of tire adhesion, the
braking systems of most passenger vehicles and light trucks
and some sports cars are not adequate for hard or sport
driving or for towing. Most stock brake systems lack
sufficient thermal capacity - the system's ability to absorb
and transfer heat by conduction, convection and radiation into
the air or surrounding structure during severe driving. In
addition many stock calipers and their mountings are
structurally not stiff enough at higher line pressures and the
resultant higher clamping loads. That is why even though there
is enough front brake torque to lock the front wheels at
highway legal speeds, caliper flex at the increased system
pressure required to stop the car from high speed may prevent
wheel lock up. Needless to say, most OEM brake pads are also
not designed for severe use, since cold stopping performance
and quiet operation typically are considered more important to
new car buyers.
Several factors
should be considered in the selection of high performance
aftermarket braking systems. Some have to do with performance
and safety, some with ease of installation and some with cost.
The object is to select the system that will reliably fulfill
your long-term needs with the least trouble and the least
cost.
There
are a few basic facts that must always be kept in mind when
discussing brake systems:
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.
For maximum
brake potential, vehicles benefit from proper corner weight
balance, a lower CG, a longer wheelbase, more rear weight
bias and increased aerodynamic down force at the rear.
To
go further it is necessary to understand some of the physics
involved, and that requires some definitions.
1) Mechanical
pedal ratio: Because no one can push directly on the brake
master cylinder(s) hard enough to stop the car, 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).
2) Brake line
pressure: Brake line pressure is the hydraulic force that
actuates the braking system when the pedal is pushed.
Measured in English units as pounds per square inch (psi),
it is the force applied to the brake pedal in pounds
multiplied by the pedal ratio divided by the area of the
master cylinder in square inches. For the same amount of
force, the smaller the master cylinder, the greater the
brake line pressure. Typical brake line pressures during a
stop range from less than 800psi under "normal"
conditions, to as much as 2000psi in a maximum effort.
3) Clamping
force: The clamping force of a caliper is the force exerted
on the disc by the caliper pistons. Measured in pounds
clamping force, it is the product of brake line pressure, in
psi, multiplied by the total piston area of the caliper in
square inches. This is true whether the caliper is of fixed
or floating design. Increasing the pad area will not
increase the clamping force.
4) Braking
torque: When we are talking about results in the braking
department we are actually talking about braking torque -
not line pressure, not clamping force and certainly not
fluid displacement or fluid displacement ratio. Braking
torque in pounds-feet on a single wheel is the effective
disc radius in inches times clamping force times the
coefficient of friction of the pad against the disc all
divided by 12. The maximum braking torque on a single front
wheel normally exceeds the entire torque output of a typical
engine.
A
few things are now obvious:
1) Line
pressure can only be increased by either increasing the
mechanical pedal ratio or by decreasing the master cylinder
diameter. In either case the pedal travel will be increased.
2) Clamping
force can only be increased either by increasing the line
pressure or by increasing the diameter of the caliper
piston(s). Increasing the size of the pads will not increase
clamping force. Any increase in caliper piston area alone
will be accompanied by an increase in pedal travel. The
effectiveness of a caliper is also affected by the stiffness
of the caliper body and its mountings. It is therefore
possible to reduce piston size while increasing caliper
stiffness and realize a net increase in clamping force
applied. This would typically improve pedal feel.
3) Only
increasing the effective radius of the disc, the caliper
piston area, the line pressure, or the coefficient of
friction can increase brake torque. Increasing the pad area
will decrease pad wear and improve the fade characteristics
of the pads but it will not increase the brake torque.
FRONT
TO REAR BRAKE BIAS
Stability and
control under heavy braking is at least as important as
ultimate stopping
capability. All cars, from pickups to Formula One, are
designed with the majority of the braking torque on the front
wheels. There are two reasons for this - first, if we ignore
the effects of aerodynamic down force, the total of the forces
on each of the vehicle's four tires must remain the same under
all conditions. When the vehicle decelerates, mass or load is
transferred from the rear tires to the fronts. The amount of
load transfer is determined by the height of the vehicle's
center of gravity, the length of the wheelbase and the rate of
deceleration. Anti-dive geometry does not materially effect
the amount of load transferred - only the geometric results of
the transfer. Second, when a tire locks under braking, braking
capacity is greatly reduced but lateral capacity virtually
disappears. Therefore, when the front tires lock before the
rears, steering control is lost and the car continues straight
ahead - but this "under steer" is a stable condition
and steering control can be regained by reducing the pedal
pressure. If, however, the rear tires lock first, the result
is instantaneous "over steer" - the car wants to
spin. This is an unstable condition from which it is more
difficult to recover, especially when entering a corner.
Most mid-engine
pure racing cars are designed with 55-60% of the total static
load and 45-50% of the total braking torque on the rear tires.
These cars feature literally tons of rear aerodynamic down
force and the footprints of the rear tires are always
significantly larger than those of the front. Most passenger
cars are front engined; none of them have any appreciable
download and almost all of them have the same size front and
rear tires. In extreme cases (front wheel drive) they may have
70 % of the total static load on the front tires. They are
therefore designed with a preponderance of front brake torque.
Most current production cars feature anti-lock brake systems
(all cars should). Sophisticated ABS systems ensure that,
under heavy braking conditions - even braking with tires on
different surfaces - each tire is braking at something very
closely approaching its maximum capacity while the ABS system
prevents lock up.
THE REAR BRAKE LINE PRESSURE-LIMITING
VALVE
Since the load
transferred from the rear tires to the fronts under braking
decreases the braking capacity of the rear tires, a rear brake
line pressure-limiting valve (often referred to a
proportioning valve) is utilized to prevent rear wheel lock up
on most passenger cars that do not feature ABS. Its function
is to limit the amount of pressure transmitted to the rear
brakes under very heavy braking. Assuming a tandem master
cylinder with equal bores, front and rear line pressures are
the same until some pre-determined threshold is reached. After
this point, rear line pressure, while it still increases
linearly with pedal effort, increases at a lower rate than the
front. In a graph it appears as a distinct "knee"
point where a further rise in pressure after the valve is
noticeably diminished. The purpose is to avoid rear wheel
lockup and the attendant unstable over steer at maximum
deceleration rates when the weight transfer is greatly
reducing the dynamic load on the rear wheels. It is not a good
idea to remove the limiting valve from a road going
automobile. Remember, under steer is stable, over steer is
not. Without an effective anti-lock braking system, in any
panic braking situation we must be absolutely certain that the
unloaded rear tires cannot lock first. Therefore materially
increasing the rear braking torque is not a good idea for
highway use. If you feel that you must do so, consider
removing the OEM rear brake line pressure-limiting valve
completely and replace it with one of the adjustable units
manufactured by Tilton Engineering or Automotive Products (now
part of Brembo). Do not place a second pressure-limiting valve
in line with the OEM unit.
BRAKE
PEDAL FIRMNESS AND MODULATION
The human
brain/body system modulates most effectively by force, not by
displacement. The side control sticks on current fighter
aircraft hardly move. The feel of the brake pedal should
approach the firmness and consistency of a brick. There are
several factors at work here:
1) Brake hoses:
Optimum pedal firmness cannot be achieved with the stock
fabric reinforced rubber flexible hoses which swell under
pressure - decreasing pedal firmness while increasing both
pedal travel and brake system reaction time. The first step
in upgrading the braking system of any vehicle is to replace
the OEM flexible hoses with stainless steel braid protected
flexible hoses of extruded Teflon. Make certain that they
are designed for the specific application, are a direct
replacement for stock and are certified by the manufacturer
to meet USDOT specifications. A claim that aftermarket hose
are certified by the DOT is a caution flag. The DOT does not
certify anything. Manufacturers certify that their products
meet DOT specifications and legitimate suppliers can produce
reports from DOT approved testing laboratories. When
upgrading your brake hoses, replace both the front and rear
hoses. Due to their swelling under pressure the stock hoses
take a measurable amount of time to transmit pressure to the
calipers. Replacing the front hoses only will result in a
built in lag time to the rear brakes and may also adversely
effect the microprocessor control algorithms of the ABS
system.
2) Master
cylinders and Caliper piston diameters: While it is true
that the most effective master cylinder arrangement is the
twin cylinder with adjustable bias bar that is universal in
racing, replacing the OEM master cylinder on a road going
car is simply not practical. When selecting an aftermarket
system, make sure that the caliper bores are designed for
the specific application.
3) Disc run out
and thickness variation: Run out in excess of six
thousandths of an inch (0.006") can be felt by the
driver as can more than 0.001" of thickness variation
and any amount of material transfer from overheated pads.
Run out is caused by poor design of either vanes or the
junction between the friction surfaces and the mounting
bell, by poor machining, by thermal stress or by any
combination of the three.
4) Caliper and
caliper mounting stiffness: Clamping force tries to open the
opposing sides of the calipers - resulting in a longer than
optimum pedal travel and uneven pad wear. The only solution
is optimal mechanical design and material selection - there
is no effective development fix for "soft"
calipers. Also, the stiffest caliper will be ineffective if
its mounting lacks rigidity.
5) Out of
balance discs (or tires): The driver cannot modulate the
brake on a bouncing wheel. Compared to tires, disc diameters
are relatively small, but all discs should be balanced. As
the installation of balancing clips will interfere with
airflow the preferred method is to remove material from the
heavy side. Significant core shift in the casting (visible,
as thickness variation on individual friction surfaces will
result in incurable dynamic imbalance.
6) Pad
"bite" and release characteristics: For efficient
modulation the pads must "bite" immediately on
brake application and must release immediately when the
pedal is released. This is purely a matter of pad selection.
It is seldom a good idea to use different compound pads
front and rear and never a good idea to use a pad with more
bite or a higher coefficient of friction at the rear.
BRAKE FADE
Repeated heavy
use of the brakes may lead to "brake fade". There
are two distinct varieties of brake fade:
1) Pad fade:
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.
2) Fluid
boiling: 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. This is a
gradual process with lots of warning.
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.
TAPERED PAD WEAR
Similar to brake
fade, there is more than one distinct type of tapered pad wear
- radial taper and longitudinal taper.
1) If a caliper
lacks stiffness and tends to "open" under clamping
force, at elevated temperatures, the outboard surface (edge
with the longest radius) of the pad with respect to the disc
(axle), center will wear faster than the inboard (edge with
the shortest radius), and the pad will be tapered in its
cross section when viewed from the end. This is termed
"radial taper".
2) The trailing
area (portion) of the pad, to some extent "floats"
on the entrapped gasses and particulate matter generated
from the leading portion of the pad. The leading portion of
the pad will always be hotter than the trailing portion and
so will correspondingly, wear faster - resulting in a pad
that is tapered when viewed from the edge. This phenomenon
is termed "longitudinal taper".
The
differential in heat generated across the pad surface,
leading to trailing, is characteristic regardless of caliper
and pad design. This is why all racing calipers and most
high performance street calipers have differential piston
bores. Most high performance pads also feature a tapered
leading edge.
3) In the case
of new very thick pads like the type used for endurance
racecars, longitudinal taper will sometimes occur because
the pad literally tips inward at an angle against the disc
during "off brake" conditions. When this happens,
there is a small amount of force pushing the pad leading
edge in the direction of the disc as a result of the contact
and the friction generated. At the same time, the trailing
side of the pad is wedged back into the corner of the pad
cavity in the caliper and against the abutment plate, which
further promotes contact at the leading edge. This situation
is exaggerated with new thick pads since the increased
offset of the pad friction surface from the backing plate,
results in a relatively larger constant force vector in the
direction of the disc.
4) Taper can
also be seen where the disc is solidly fixed to the hat or
where the hat and disc are one piece. In either case, the
taper created will appears as more wear on the outer
diameter of the outside pad, and the inner diameter of the
inside pad. This is due to operating the brakes at high
temperature and the resulting thermal expansion forces on
the annular outer ring structure of the disc called the
friction plates. The center of the disc or hat limits the
expansion of the outer structure on only one side where it
is joined, typically at the outside friction plate. As a
result, the disc cones so that it is concave as viewed from
the outside (See also "Floating Discs").
Subsequently due to the coning, the pad contacts unevenly
when the brakes are applied or remains in contact with the
disc in the regions mentioned and even higher temperatures
and wear are the result.
AIR COOLING
Most of the
enormous amounts of heat generated during deceleration must be
dissipated into the free air stream.
Most high
performance (and/or heavy) cars today use some variation of
the "ventilated" brake disc in which air entering
the center or "eye" of the rotor is forced through
the interior of the rotor by the pumping action of the
rotating assembly. The most efficient practical way yet
devised to accomplish this is through the use of the
"curved vane" ventilated brake rotor originally
designed for the LeMans winning Ford GT 40s in 1966. In this
design the interior vanes are curved to form an efficient pump
impeller. They also stabilize the rotor from distortion and
serve as very effective barriers to stop the propagation of
cracks due to thermal stress. In laboratory testing STOPTECH's
innovative design developments in the 48 vane rotors have
increased air flow through the rotor by an astounding 61% over
some OEM rotors and from 10-15% over racing rotors of the same
size. This results in a cost effective but very stable direct
replacement rotor that runs typically 15% cooler than stock
and 7% cooler than racing designs.
TITANIUM CALIPER PISTONS
Caliper pistons
manufactured from Titanium do a really good job of insulating
the fluid in the caliper from conductive heat transfer from
the pads. Unfortunately it is not a simple substitution. The
design and manufacture of brake caliper pistons is a complex
engineering exercise. If the piston material is to be changed,
the designer must take into consideration the difference in
thermal coefficient of expansion between the OEM material and
the new material. The right grade and condition of Titanium
must be selected. The surface finish and treatment must be
compatible with the seals. If the seal groove is in the
piston, the groove geometry must match the OEM design. As a
point of interest, virtually all-serious racing cars use
Titanium caliper pistons with an anti-galling surface
treatment, which changes the color from a natural almost dull
silver to gold. The fact of the matter is that a simple
Titanium button placed inside the OEM piston does about 70% of
the job at a fraction of the cost with no risk of damaging
anything by disassembling the caliper.
DRILLED VS SLOTTED ROTORS
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.
PAD AREA
We have seen that
brake torque is directly proportional to Piston Area, System
Pressure, Friction Coefficient and Effective Radii and is not
affected by pad area. Pad area and geometry are however
important for several reasons:
1) Pad service life. Since pad material is consumed, an
increase in pad area results in an increase in the time
interval between pad replacements. OE designs often make
slight sacrifices in pad life by including tapered ends for
reduction of noise, vibration and pad taper. In some OE
designs the pads on the two sides of the caliper are even
shaped differently, with the inside pad being shorter in
arc-length in the direction of rotation and wider radially
than the outside pad for system design and integration
reasons.
2) Heat dispersion and dissipation over a larger surface
area and greater mass. Although in the case of a larger pad,
the pad masks a larger portion of the rotor face, absorbing
more radiant energy and shielding the area from cooling that
may cancel any actual benefit.
3) Geometry: Since rubbing speed between the disc and the
pad is greater at the periphery of the disc, the pad
geometry will sometimes be designed to reduce the area
toward the center of the disc. This is done in an effort to
produce even temperature and pressure distribution across
the face of the pad.
INCREASING DISC DIAMETER
The problem with
increasing the effective radius of the discs is that, since
the designers used the largest rotor that would fit inside the
wheel. Typically, increasing the rotor diameter means
increasing the wheel size. The expense involved is only one
objection. A major issue is the impact on of the OE suspension
geometry.
The camber curves and roll resistance characteristics of any
proper suspension system are designed for tires with a
specific sidewall height and stiffness. Increasing the wheel
diameter means decreasing the sidewall height and the
compliance of the tire. Carried to an extreme, this will hurt
cornering capability and might actually result in a loss of
braking traction due to "edging" the front tires
under heavy braking. And although technology is making
possible ultra low and stylish tire side wall heights, it does
not necessarily result in ultimate performance, just take a
look at the sidewall height of Formula One and Indy cars.
FLOATING DISCS
All metals
"grow" when heated. The diameter of cast iron brake
discs can increase as much as 2mm (0.080 inch) at elevated
braking temperatures. When the disc is radially restrained
from growing (as in all one-piece discs) the friction plates
are forced into a cone shape as temperature increases,
adversely effecting both temperature and pressure distribution
within the pads and the feel of the pedal. Racing and high
performance street discs are mounted on separate hats or
bells, usually of Aluminum. The fastening system is designed
to allow radial growth and minimal axial float resulting in a
mechanically stable system. Hats or bells should be made from
7075 or 2024 heat-treated aluminum billets that are
pre-stressed and relieved, not from 6061 or from plate stock.
SUMMARY
If the braking
system is only marginal, upgrading the pads and brake fluid
and/or getting more air to the system will probably cure the
problem at minimal cost. Replacing the stock rubber flexible
hoses with stainless braid armored Teflon hoses will improve
the ability to effectively modulate the braking force at
moderate cost. When a decision is made to upgrade the braking
system, make sure that the replacement components and system
have been properly engineered and designed for your specific
application ask technical questions and expect valid technical
answers.
1) Discs should have curved vans and both greater thermal
storage capacity and better airflow characteristics than OEM
- otherwise you will not have achieved anything worthwhile.
Depend on actual test results, not advertising claims. Discs
should be mill balanced to less than 0.75 ounce-inch (54
g-cm), run out should be less than 0.002" (0.051 mm)
and thickness variation should be less than 0.0007"
(0.018 mm). On race applications these tolerance are
typically reduced to .25 ounce-inch, 0.0005" and
0.0001" respectively.
2) Calipers should be stiff at elevated temperature. Again,
look at laboratory test results, not claims. Calipers must
be mounted true to the plane of rotation of the rotor.
3) Multi-piston calipers should have differential bores to
reduce taper wear. Piston area should be consistent with
master cylinder size.
4) Ideally no modifications to the knuckles or uprights
should be required for installation.
5) Front to rear brake torque bias should be consistent with
the dynamics of the specific vehicle.
DRIVING
CONSIDERATIONS
1) In order to
brake effectively, the tires must comply with and grip on
the road. Your braking system is no better than your tires
and suspension. The best money that you can spend is on
really good tires and really good shocks.
2) Proper
corner weight is crucial for effective straight line
braking. Optimum corner weight for braking is when the cross
corner pairs are equal. That is to say the total of the left
front and right rear equals the total of the right front and
left rear.
3) If you smell
brake lining or if the pedal starts to go soft, ease off.
4) Use at least
a 550 degree non-silicone brake fluid and make sure that
your brakes are bled properly and, when used hard, often.
Brake fluid is hygroscopic in nature - given any chance at
all it absorbs water. A fraction of one percent of entrapped
water lowers the boiling point of any brake fluid
dramatically - and causes corrosion within the system.
Replace all of the brake fluid in the system at least once a
year - more often if you constantly use the brakes hard.