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Driving Safely and Wind Direction
Name: Katie H.
Status: student
Age: 18
Location: N/A
Country: N/A
Date: 2000-2001
Question:
Does the speed a car is going affect how much the wind
pushes it off course? Is it safer to drive slowly if the wind is blowing
perpendicular to your car? Why or why not? It is driving me nuts! No
one can decide on an answer or give me a good reason for either one.
Replies:
Katie,
Let us begin by assuming you mean pushed sideways off the road and/or
tipped over as
well.
The factors that keep the car on the road are the weight of the car, the
friction
between the tires and the road surface, and the shape of the car. Shape
(streamlining) is a factor because it relates to the area of the car
against which
the wind is pushing. The larger the surface area facing the wind, the
greater the
pushing force on the car. And, too, small car of a given weight would be
harder to
tip over than a taller vehicle of similar shape and equal same weight. The
height
of the car is a factor because the taller it is, (assuming the same
wheel-base),
the easier it will be to tip it over.
How the frictional factor is affected by the wind is difficult to pin down. Is
there anything being borne in the wind stream that could affect friction
-- debris,
sand etc.? How fast is the car moving down the road? If barely creeping
along, you
would have more time to react and possibly correct deviations from your
intended
course. Flying along at full throttle would not offer much time to correct a
deviation and would, thus, be foolhardy.
Assuming we may be talking about a Wizard of Oz wind, it would probably be
best to
not drive at all when the wind speed is dangerous.
Regards,
ProfHoff
Katie -
I hope a mathematician will give you an answer in numbers, but let me try
to do it
conceptually.
Both the wind speed and your car speed are vector quantities. Do you know
how to
add vectors? The total of two vector quantities can be done graphically
by drawing
two arrows - one representing each vector - tail to tail. These will form two
sides of a parallelogram. Complete the parallelogram and draw a diagonal
from your
first corner. If you drew each of the first vectors to scale, the
diagonal will be
the total of the two forces... in the same scale.
You will see that the faster you are moving the less the change in the
angle, but
your total speed will increase. I would guess that there is a speed a
which the
effect of the wind would be least. That is, the speed at which you would
go the
least distance off the original path of the car in a given time. If I
were trying
to make sense of this, I would draw a graph of the crosswind speed vs the
distance
off course per unit time.
Hope that makes sense. It is not really an answer, but perhaps a way to
find the
answer. Yours is an intriguing question. Have fun working it out.
Larry Krengel
It is safer to drive more slowly in a high wind, but not because of being
blown off course. The reliability of the tires (skidding) is the major
concern. Whether you are blown off course depends on how well your tires
grip the ground. Moving faster just means you will go a greater distance
before the wind takes its toll. If you keep your wheels from skidding, you
won't slide sideways. Keep control of your steering wheel.
The wind provides the greatest danger when your tires do the most work:
turning corners and stopping. Turning into the wind requires greater push,
greater force between tires and road, than the same turn without wind.
Turning at a greater speed requires greater force. The two increases
combined could push your tires beyond their limit, resulting in a skid.
When stopping, the tires push against the road forward. Pushing sideways to
resist the wind as well can push the tires beyond their limit, resulting in
a skid. When your car skids, you lose control. the car is sliding rather
than rolling. If the wind is strong enough to blow you into a skid when you
are hardly moving, it can do the same at a high speed.
Kenneth Mellendorf
Sometimes it helps to consider the extreme cases in order to answer
questions like
this. If the car is sitting still (i.e., driving VERY slowly) the wind
will not
blow it off course at all. If it is moving at highway speeds you know
that it can
push it off course. So the answer to your question is that the car's
speed does
affect how much the wind pushes it off course. A safe driving speed will
depend on
the wind speed and road conditions.
The reason this is true is that the wind applies a force to the side of
the car
which must be compensated for by adjusting the direction of the wheels
slightly
into the wind. It takes some time for a driver to react to a gust of wind and
adjust the steering wheel to compensate for the additional force of the wind.
During this reaction time the car is being blown off-course. If the car
is going
fast it goes further off course than if it is moving slowly.
Note that in my response I have assumed that the wind is gusting. If it is a
steady wind the driver can compensate for it when he/she starts driving
and does
not have to continually adjust for the wind forces as the windspeed changes.
Greg Bradburn
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