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The Adventures of Traction Control
A short story by Dave Hackett
It
had been raining for two days straight before that
frigid November twilight. The roads were covered
with fallen leaves, tired from serving their trees
all summer. Days like these are not uncommon
around New England parts.
Joy, the pink mid-80’s Thunderbird, happened to be
on her way up to Vermont that night. Suddenly, on
Route 89 north, Joy saw a pair of eyes glaring in
her headlights. A cute deer was about to become
one with her engine compartment. Instinctively,
Joy slammed on her brakes, locking her wheels, but
they had no effect on her speed. She frantically
tried to swerve left and right, but she lost her
grip on the ice-glazed road. Spinning out of
control, Joy creamed the lovable deer with her
right-rear quarter panel, and found herself on the
side of the road, now facing south. When she tried
to get back on track, her right rear wheels would
just spin in the mess of leaves, mud, and
road-pasta.
"Help! Help! Help!" she honked as the first few
flakes of a major snow storm fell onto her warm
hood.
Believe it or not, Joy’s boyfriend, Dave heard her
honks from all the way back in Boston. Dave is a
hot, expensive, late-model European car who takes
no shit from weather. He may be fictional, but his
traction control is a harmonizing blend of three
real traction control systems. Before the gas
station attendant could shut his fuel door, Dave
was on his way to rescue the fair maiden and get
laid.
It wasn’t long before he had to utilize his first
traction control aspect, ABS (Antilock Brake
System). ABS minimizes skidding during hard stops
by eliminating wheel lock. In this way, it also
makes steering the vehicle more controllable with
the brakes applied. Before we can talk [further]
about how ABS, or how any other method of traction
control works, we must understand some terms
incorporated with the subject. "Traction" is
defined as the amount of grip between the tire
tread and the road surface. Higher traction allows
greater braking and cornering force to be
generated. Traction can be thought of in terms of
"tire slip." It is defined to be the difference
between vehicle speed and the speed at which the
tire tread moves along the pavement. Tire slip is
expressed as a percentage. The greater the
percentage of tire slip, the less the amount of
traction. So, when Joy slammed on her brakes, her
wheels locked, producing 100% tire slip, and no
tire traction.
Furthermore, if a vehicle must brake and turn at
the same time, the available traction must be
divided between the stopping and the lateral
forces which make the tire prone to slip. When the
rear wheels lock, the rear end has a tendency to
swing out with the turn. If the front wheels lock,
the vehicle loses its ability to steer and tends
to move in a straight line until the wheels are
freed again. "Oversteer" and "understeer" and
discussed in greater detail later.
Although the research and technology involved in
ABS is quite hi-tech, its operation is quite
simple to understand. Wheel speed sensors monitor
the rate of rotation of each wheel in respect to
each other during braking. All four wheels have
their own sensor in the case of four-channel ABS
systems. In single-channel systems, one speed
sensor is used for both rear wheels. The
three-channel system, like the single-channel
system, not only uses one sensor for the two rear
wheels, but also, each front wheel has a speed
sensor of their own. An antilock control module
reads the information presented by the wheel speed
sensors and takes action. If an individual wheel
is slowing down faster than the other one(s), it
is about to, or is slipping. As described
previously, this is an unwanted event. The ABS
control module operates the solenoid valves in the
brake lines to the wheels. Stopping and releasing
hydraulic fluid, the valves open and close very
rapidly (up to twelve or more times per second).
This occurs during braking until the wheels are
rotating in sink with each other.
This is what happened to Dave. He sensed a sexy
white BMW Roadster pull into the gas station he
just left, and he stopped real quickly on the
slick street. Just as he was about to make a "U,"
thoughts of Joy popped-up in his memory. He
remembered when they met on that beautiful summer
day in the Auto Palace parking lot, and the night
when they got it on in the carwash after-hours.
Disgusted with his own near unfaithfulness, a tear
came to his headlight. Dave did the right thing
and continued on to rescue his love.
Now he was in a hurry, and floored it. With his
"Traction Control Active" light flashing, Dave
took off like a homo at an NRA convention in
Texas.
Dave’s ABS system was designed to not only
minimize tire slip during deceleration, but also
on acceleration. This is what "traction control,"
or "traction assist," is usually associated with.
Without this feature, Dave’s low-profile Perelli
tires would just let go of the wet asphalt.
There are different design and methods to achieve
this traction assist effect. In Dave’s case, the
ABS control module processes the information from
the wheel speed sensors constantly (not only
during braking). When a drive wheel spins faster
than a non-drive wheel, hydraulic fluid stored in
an accumulator is pumped to pressurize the line to
the problem wheel. The solenoid value pulses the
pressure very rapidly until the wheel is brought
down to speed.
Another version of traction assist utilizes a
throttle relaxer that actually pushes against the
gas pedal when the system detects the wheel spin.
The limited throttle reduces the engines power
until the slip is minimized.
Also used on some vehicles is a system where the
wheel speed sensors tell the engine management
computer to retard spark or shut off fuel
injectors. This allows non-restricted use of the
pedal, and at the same time, fuel economy is
maximized because the engine, itself, works less
hard unlike the system with the pulsing brakes.
Most vehicles, regardless of the method used, have
a switch mounted on the dashboard so the driver
can deactivate the system if desired. This way,
travel through deep snow or mud is possible, and
you can do a smoke show if you want.
A half-hour later, Dave was well on his way to
Vermont. By this time, the snow storm had
progressed to a total white-out. Traffic on the
highway was thick, very slow, and Dave was getting
impatient. Of course, cars have no vision, so the
lack to visibility didn’t bother him. Again he was
hearing Joy’s honks of distress, so he started
weaving in and out of traffic to make some time.
Drivers in the other cars were absolutely
flabbergasted; not because there was no one
driving Dave, but because this car was handling
the slippery highway at such speeds.
Some were saying "that jerk’s gonna get people
killed," while some were exclaiming "how is that
car staying in control?"
What nobody knew was that Dave was equipped with
the new Bosch Vehicle Dynamics Control System (VDC),
which the Germans call "ESP." Not only does it aid
traction in braking and acceleration, but also
side to side lateral forces! This system corrects
for two situations: oversteer and understeer.
Oversteer is defined as the tendency of the car to
turn more sharply than it should for the amount of
steering input The rear wheels start sliding
first. Understeer is defined as the tendency of
the car that requires a greater steering motion to
make a given turn and the car turns less than it
should. The front wheels begin to slide first.
VDC is made up with the same components as the
conventional ABS systems, but the control module
is "beefed-up" or "smarter" to handle more types
of inputs and calculations. In addition to wheel
speed sensors, VDC also uses a steering angle
sensor and something unheard of in automobile
applications: a yaw sensor. Yaw is defined as the
rotary motion of the sprung mass of a car around a
vertical axis that is encountered in a spin
For example, the control module monitors what the
yaw and the steering angle sensors are saying. If
the steering angle sensor says the car should only
be making a ten degree turn, and the yaw sensor
says the car is actually turning more sharply, the
car is being oversteered. (see figure A) The
front-outside brake is pulsed. The steering sensor
says the car should be making a thirty degree
turn, and the yaw sensor says the car is actually
only making a two degree heading, the car is being
understeered. (see figure B) The rear-outside
brake is pulsated. The action of one side of the
vehicle being slowed-down is very much like that
of a Bobcat or a tank. The vehicle rotates on its
vertical axis in the direction of the slower side.
When Dave finally arrived at the spot where Joy
left the road, he was totally unprepared for what
he discovered. Biff, a big tow truck from the
local towing service, had already used his big rig
to give Joy a lift. Dave was just in time to see
her begging Biff for more service. He was kicking
himself the whole way back to Boston for not
hooking-up with that Beamer when he had the
chance. Poor, poor Dave. But what else could
possibly go wrong?
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