- Calculating the Distance
- Okay,
last equation of the day. Given a vehicle speed of, say, 100
miles per hour, and the deceleration level from above, we can
now calculate the distance required to bring the car to a stop.
But, in order to make sure the answer comes out in feet, we first
need to juggle the numbers around a little bit:
100 miles per hour = 147 feet per second
- 0.84g = 27.0 feet per
second per second
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- Apply
the equation for stopping distance
- {distance = (initial speed x
initial speed) / (deceleration x 2)}
- and lo and behold, exactly 400 feet are
required to bring this car down to a stop from 100 miles per
hour given our original pedal input force of 90 pounds. Tah dah!
The car is now stopped.
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- Limiting Factors
- From
this example, it would appear that in order to make the car stop
in a shorter distance, there are two options:
1) Change the brake system to increase the force
between the tire and the road for a given pedal input force.
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- 2) Press on the brake pedal
harder.
- This
theory holds true, but only up to a point. Anyone who has even
driven on an icy road will get this right away. As the brake
pedal force is gradually increased, the deceleration rate will
also increase until the point at which the tires lock. Beyond
this point, additional force applied to the brake pedal does
nothing more than make the driver's leg sore. The vehicle will
continue to decelerate at the rate governed by the coefticient
of friction between the tires and the road. As you can imagine,
the coefficient of a given tire on ice is much lower than the
coefficient of that same tire on dry pavement, hence the increased
deceleration possible on the dry, paved surface.
You can take this one
to the bank. Regardless of your huge rotor diameter, brake pedal
ratio, magic brake pad material, or number of pistons in your
calipers, your maximum deceleration is limited every time by
the tire to road interface. That is the point of this whole article.
Your brakes do not stop your car. Your tires stop the car. So
while changes to different parts of the brake system may affect
certain characteristics or traits of the system's behavior, using
stickier tires is ultimately the only sure-fire method of decreasing
stopping distances.
-
- So, Why Would Anyone Want to Modify
Their Brakes?
- If
changing braking system components does not provide increased
stopping power or shorter stopping distances, why even consider
changes in the first place? Why not just leave the brakes alone
and buy new tires? Quite simply, making changes to your braking
system can have a very real, very significant impact on four
areas of brake system performance other than stopping distance:
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- 1) Driver tuning:
Modifying your brake system component sizing (brake pedal ratio,
master cylinder piston diameter, caliper piston diameter, rotor
diameter) can be performed to adjust the feel of the car to suit
the driver's tastes. Some drivers prefer a high, hard pedal,
while others prefer a longer stroke. In this regard, tuning your
brakes is a lot like tuning your shocks: every driver likes something
different, and there is no right answer within certain functional
limits. These components can be adjusted in small steps to achieve
a feel that the driver prefers.
-
- 2) Thermal control:
Modifying your brake system mass (rotor weight) can be used if
there is a thermal concern in the braking system. If your brakes
work consistently under your driving conditions, then adding
"size" to the braking system will accomplish nothing
more than increasing the weight of your vehicle. But if high
temperatures are having an adverse effect on braking system performance
or other components in general--wheel bearings, for example--then
you should consider super-sizing. Of course, brake cooling ducts
can really help out here as well.
-
- 3) Temperature sensitivity:
Modifying your brakes to address the presence of high temperatures
(brake pad material and brake fluid composition) should only
be considered if your thermal concerns cannot be resolved by
super-sizing. This is really just a Band-Aid for undersized systems,
like those found on Showroom Stock race cars that are not permitted
by their rules to upsize or cool their brakes. One might argue
that it is more cost effective to install better brake pads and
brake fluid than it would be to upsize the rotors, but all that
heat still needs to go somewhere--and more often than not it
will find the next weak link in the system.
-
- 4) Compliance:
Any changes that you can make to your braking system to reduce
compliance will increase the overall efficiency of the system--improving
pedal feel, wear, and stop-to-stop consistency. Think of it as
balancing and blueprinting your braking system. Brake system
modifications have their place to help make your ride more consistent,
predictable, and user-friendly; however, if your ultimate goal
is to decrease your stopping distance, look no further than the
four palm-sized patches of rubber connecting your ride to the
ground.
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- James Walker; JR: of scR
motorsports races a 1992 Saturn SC in the SCCAs ITA class. His
real job as an anti-lock braking systems engineer with the Robert
Bosch Corporation has him applying these very same brake system
princiales on a day-to-day basis. To find out more about his
scR motorsports race team, visit
- www.teamscR.com.
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