- In
order to calculate the amount of clamping force generated in
the caliper, the incoming pressure is multiplied by the area
of the caliper piston. In our example, the 558 psi that had been
generated at the master cylinder has traveled through the brake
pipes and lines and is pushing against two 1.5-inch pistons per
caliper. Therefore, the effective area of the caliper will be
equal to two times the area of a single 1.5-inch piston. Working
the numbers reveals that 558 psi will generate 2068 pounds of
clamp load
- [558 psi x (1.84 in. x 1.84 in.) x 2].
As you have probably
already guessed, increasing the caliper piston diameter increases
the clamp load for a given input pressure--but again, this does
not stop the car. Putting on bigger calipers might seem like
a good idea at first, but the tradeoffs might make you think
twice. Increasing the diameter , will increase
the compliance in the system. (Bad news for pedal feel!)
-
- Increasing
the diameter will increase the size and weight of the caliper.
(Bad news for unsprung weight!)
- Increasing the diameter will increase
the fluid volume requirement of the system. (Bad news for master
cylinder sizing!)
-
- So,
when thinking about that big six-piston caliper convervsion keep
in mind that the size and number of caliper pistons on your car
were originally matched to the brake pedal and master cylinder
to generate an appropriate clamp load for a given brake pedal
input force. Changing any one of the components will shift the
balance one way (increased pressure required) or the other (higher
pedal forces required) to generate the same clamp load. Remember:
Bigger calipers don't create any more "stopping power"
and they do not "decrease stopping distance"--they
just generate higher clamp loads for a given pressure input.
One final caliper note
of interest: You may have heard the terms "fixed caliper"
(indicating that the caliper body is bolted directly to the suspension
upright) and "floating caliper" (indicating that the caliper
body is free to float on sliding guide pins). Although there
are pros and cons associated with each type, there is not enough
room in this article to dig into the details of their design
differences. For now, let it suffice to say that the above math
works out the same for either design.
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- So,
to this point, our example brake pedal, master cylinder and caliper
have amplified the original 90 pounds of driver input to over
2000 pounds--an increase of more than 22 times, but we still
haven't stopped the car.
-
- The Brake Pads
- Ths part might surprise some and offend
others, but it is a big misconception that changing brake pad
material will magically decrease your stopping distances. In
fact, you may have even seen published "data" which
attempts to correlate stopping distance to friction coefficient.
Although it may appear that there is a relationship between the
two, there really isn't, and here's why.
The brake pads have
the responsibility of squeezing on the rotor (a big steel disc
which is mechanically attached to the road wheel) with the clamping
force generated by the caliper. There is a lot of black magic
surrounding the material composition and formulation of the friction
puck, but what really matters is the effective coefficient of
friction between the brake pad and the rotor face.
By knowing the clamp
load generated by the caliper and the coefficient of friction
between the pad and rotor, one can calculate the force acting
upon the rotor. In this particular example, let's assume the
brake pads have a coefficient of friction of 0.45 when pressed
against the rotor face. The rotor output force is equal to the
clamp force multiplied by the coefficient of friction (which
is then doubled because of the "floating" design of
the caliper), or in this case 2068 pounds x 0.45 x 2 = 1861 pounds.
Nothing magical about it.
By increasing the coefficient
of friction of the brake pads, the results are the same as increasing
the caliper piston diameter--higher forces will be generated
for the same input. But as before, this force is not what stops
the car.
-
- So
why change brake pad materials in the first place? Because increasing
the coefficient of friction can allow for the use of smaller/fewer
caliper pistons and/or will reduce the amount of pedal force
that the driver needs to apply in order to generate a given rotor
output force.
-
- That's
about it from a design standpoint, but the racer has another
point to consider: heat. In the example above, the rotor output
force was calculated assuming that the coefficient of friction
between the brake pad and the rotor was constant, but in the
real world, this is not the case. As the temperature of the components
change, the physical properties of those components change, and
in the case of the brake pads, the coefficient of friction can
change dramatically.
-
- While
street pads might have a coefficient of 0.30 around town, after
a few laps on the track, the coefficient can drop to below 0.10,
a condition commonly known as "brake fade." (Note:
This should not be confused with brake fluid fade, which results
from water in the brake fluid turning to vapor at high temperatures.)
On the race track, this means that the brake pedal force required
to stop changes from lap to lap. And as racers, we know this
can be kind of, well, unsettling, to say the least.
 
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