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Braking Theory |
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Ask any of your physicist pals, and they'll tell you
that brakes convert the kinetic energy of vehicle motion into heat.
Translation: Brakes stop the car or more accurately, brakes stop the wheels.
There's a big difference, because the most powerful brakes in the world will
not stop your vehicle effectively if the road surface has little or no
traction. Mash the brake pedal and the wheels will stop turning sure enough,
but the vehicle will skid along happily. You, on the other hand, will be a lot
less happy. Many drivers tend to think of a skid as "brake failure" when in
fact the situation is really a failure of the driver to understand the traction
conditions and to drive accordingly. |
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Brake Basics |
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The typical passenger-vehicle brake system is relatively simple. When you step
on the brake pedal, the force your leg exerts is applied to a device called a
master cylinder. The master cylinder contains a piston that pressurizes a
network of hydraulic brake lines that lead to each of the vehicle's wheels. At
each wheel, that brake fluid pressure operates the brakes by driving pistons
that force replaceable linings against a rotating drum or disc. Friction is
what slows the wheel, and in turn, the entire vehicle.
When the friction material (a.k.a. pads, linings, shoes) is almost worn out,
metallic tabs are designed to create a squealing noise when the brakes are
applied to (hopefully) alert the driver that the brake linings are due for
replacement. Heed the warning. Worn linings have less fade-resistance than new
linings. Plus, if you ignore the warnings long enough, you can do costly damage
to the rotors, drums and other components. Even with regular replacement of the
linings, some additional service is typically required over the long haul. The
surfaces of drums and discs wear unevenly in normal use and eventually need to
be re-machined to work properly.
All modern braking systems are many times more powerful than the vehicle's
engine, so at full throttle, even a very powerful vehicle can be easily stopped
with the brakes. All vehicles also have a parking brake (sometimes called the
emergency brake) that works independently of the regular brake system. The
parking brake typically acts on only the rear wheels and is mechanically
operated to work in case of a hydraulic problem with the regular service
brakes. |
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Better Braking |
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Many engineering refinements over the history of the automobile have
spectacularly improved the function and reliability of braking systems. Power
brakes are standard on virtually all modern passenger vehicles, and they use
energy supplied by the engine to help power the brakes so that the strength of
your right leg doesn't have to do all the work. To eliminate the possibility of
sudden, complete brake failure, modern vehicles actually have two parallel
brake systems, with each system controlling two of the vehicle's wheels. This
way, even if one system has a major failure, the other system can still stop
the vehicle (albeit less effectively).
Brakes themselves have dramatically improved over the years, too. A few decades
ago, drum brakes were in wide usage, and they're still used on the rear wheels
of many vehicles. This type of brake employs a drum-shaped assembly that spins
with the wheel. Inside the drum, stationary "shoes" faced with replaceable
friction material are forced against the drum when you push the brake pedal.
Drum brakes work well, but they have a hard time shedding heat well enough to
prevent fade when used really hard. Brake fade occurs when the brake overheats
dramatically; braking power is vastly reduced, and the brake components and
linings can be damaged.
A significant advancement came in the form of disc brakes, which today are used
almost universally on front wheels (which do most of the work under braking)
and on many rear wheels. Disc brake systems have a metal (or exotic material in
some racing applications) disc (or rotor) that spins along with the wheel, and
a stationary "caliper" that squeezes the disc with replaceable friction
material when the brakes are applied. With plenty of airflow on the exposed
discs, these types of brakes are much less fade-prone.
Additionally, the discs are often internally vented to allow even greater
airflow. Back when brake fade was a common problem on long mountainous
descents, drivers would shift the transmission into a lower gear to allow
engine braking to take some of the load off of the brakes. With modern brakes,
this is usually no longer required, except in situations such as towing a heavy
load downhill. |
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Anti-Lock Braking Systems |
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A vehicle's tires generate the maximum deceleration when braking power is
brought right to the brink of wheel lock-up—but not beyond. Once the brakes
lock and the wheels skid, actual deceleration is reduced and directional
control via the steering is lost. Electronically controlled anti-lock braking
systems (ABS) have netted great advances in vehicle controllability and reduced
stopping distances in most real-world situations, particularly in rain or when
cornering. ABS uses a combination of electronics and hydraulic controls to
allow normal braking right up to the point of wheel lock-up, then the system
intervenes to reduce fluid pressure to the brakes to keep vehicle deceleration
at its maximum given the road conditions.
Typical ABS systems have speed sensors at each wheel that continuously feed
information to a centrally located ABS computer. The computer uses this data to
determine overall vehicle speed, and to detect when a wheel begins to lock.
Since each wheel is independently controlled (in a four-channel ABS system),
pressure is automatically limited or reduced to only the wheel that is locking.
Three-channel ABS is a slightly less complex system used on some vehicles; it
allows for independent control of each of the front wheels, but applies the
same braking pressure to both rear wheels. Measurable performance differences
between these two types of ABS are slight, and both types of ABS have a
significant advantage over non-ABS brakes. When one wheel locks on a non-ABS
car, the only way to allow it to spin again and regain full directional control
is by the driver reducing the brake pedal pressure, which reduces the braking
force at all four wheels at once. ABS is capable of providing shorter stopping
distances in difficult situations than would a conventional system, even with
an expert doing the driving.
Driving with ABS requires no special training, though you might need to
un-learn a technique that makes some sense with non-ABS brakes. With old-style
brakes, drivers were commonly told to "pump" the brakes when they were
approaching wheel lockup. This rule of thumb was meant to help the average
driver avoid fully locking the brakes and skidding straight ahead without
steering control. With ABS, you simply push on the brake pedal as hard as
necessary to make the stop. If traction is marginal, you may feel a pulsing
sensation through the brake pedal, which is completely normal. Throughout the
stop, you have steering control, so you can swerve or turn if required to avoid
an obstacle. |
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