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Page Nomenclature |
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c.g. = center of gravity
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SUV = sport utility vehicle |
Introduction
The first cars were less concerned with
performance than with negotiating rough, pot-holed horse-and-buggy roads.
Naturally they were styled after wagons of the day with oversized wooden-spoke
wheels and had ground clearance measured in feet, not inches. As roads became
paved and thus smooth, car performance quickly improved. By the 1920's, it
became clear to designers that high center of gravity, (c.g.), high speeds and quick cornering
weren't compatible. One
notable design of the 10's to correct this difficulty was the American Underslung, a lowered chassis design
advertised for its road-holding capabilities, especially around corners. Rational
design to keep the c.g. as low as practical thus appeared early
on. During the 1930's and 40's car height was lowered, streamlining appeared and
really powerful engines were installed. An interesting car of the 30's was the
Cord, which featured front-wheel drive. Engineers could prove on paper, using
vector geometry, that front-wheel drive reduced corning forces on the front
steering wheels as compared to the increased forces caused by rear drive.
Nothing on earth at the time could out-corner this early front-wheel drive car.
By the 1950's, the body of applied mathematics and physics available to
automotive engineers was complete. Of course, then, as now, stylists had a lot
to say about what went out the factory door; so some old cars really appear
strange today, like those with the giant tail fins, originally touted for
aiding directional stability, sort of like the vertical fin on a jet plane. As we entered and passed through the
1950's and '60's, front wheel drive was largely ignored (a notable exception
being the Toronado) in favor of cheaper-to-produce and easier-to-service rear
drive designs, but cars were made
"longer and lower", then "wider". Some cars made in the
50's thru the present are about as wide as allowed by state laws; any wider
and they would need truck navigation lights installed. As we will see later,
lower and wider is better for stability. Through
the 70's to the present, manufacturers have offered the public a galaxy of
choices. There have been front-wheel drive and rear-wheel drive cars. Sedans
grew very large and then compact. Pickup trucks are favored by millions, not
for cargo carrying, but for ordinary commuting. From the roadster to the SUV,
today there are a myriad of designs for our selection.
Engineers vs. stylists
We will focus in now on one type of vehicle,
in particular, the sport utility vehicle, SUV, and whether or not it is apparent engineers are in
fact the last word, as it should be, or if instead, upper management is caving
in to fads. It is clear from an ethical standpoint that no auto factory manager should
let sound engineering fly in the wind in favor of creating cars that
just look right to the buyers, but violate some basic laws of physics. Not to
say styling isn't useful; it is, if used in the name of art and comfort. So
long as stylists affect only cosmetics and not compromise utility and safety, they have their
rightful place in the design department.
Whether or not the engineers of the
major auto manufacturers have been allowed to control final product design
released to the driving public is debatable. The most recent trend is not lower
vehicles, but the opposite! The fad now is high-profile,
"king-of-the-road" styles that make the driver feel he is sitting on
top of the world, looking down at those driving those old-fashioned
ground-huggers. SUVs in particular are popularly bought for everyday passenger
car use. On visual inspection, they intuitively look very robust and very
heavily built; not a vehicle likely to be unstable at all. Are these
impressions valid from an engineering standpoint, or have they given
uninformed buyers a false sense of security? Lately, SUVs have suffered
from bad press in regards to their frequency of upset, or rollover. Are they
really intrinsically prone to rollover? It is the purpose of these following
web pages to investigate the SUV phenomenon to seek and answer to these all-important
questions.
Fundamental rule of car
design
A fundamental rule in
passenger car design for decades has been that a when a vehicle is mishandled,
say, when a driver has to brake heavily, swerve around an unexpected obstacle,
or if he enters
a decreasing-radius curve too fast, it should behave as predictably and benignly as the
state-of-the art permits. In particular, when the limits of traction (determined
by the coefficient of friction) are exceeded, it is important that a passenger car
slide, not tip over. In the case of a curve, an overly fast car is supposed to
slide out to and take a larger-radius course; it is not supposed to upset.
An upset at high speed means destruction of the vehicle and up to a 30%
fatality rate to the occupants. Likewise, when an emergency occurs, any driver
is liable to "overcontrol" in an effort to steer or brake out of
trouble. An extreme steering input sets the car into curvilinear motion and
has the same effect as a car in a curve at too high a speed; if the friction
limits are exceeded, the car should slide to a greater curve, not roll over. Sometimes, an extreme braking action can lock up an axle, especially the rear
axle. If this happens, a car can go into a "spinout" situation.
Again, a properly designed passenger car vehicle should slide to a stop
upright. This desirable behavior, that of sliding, vs. tipping will be shown both graphically and
mathematically as doable. Even if one does not own
an SUV, there exists a real potential for injury or death from one, if that
SUV is inadequately designed: If a driver of an SUV feels his car is
"tipsy", then he might be reluctant to make any radical maneuver with it, for fear of tipping it, even if means ramming an innocent victim's
car. Thus a potential life-saving accident avoidance action might be lost at
the cost of of an innocent third party. It therefore absolutely mandatory that
manufacturers pay heed to the laws of physics. But since it appears they don't
always do that, every user of SUVs would be advised to command at least a
working knowledge of vehicle stability and safety. This web will hopefully
help the reader meet that objective.
As stated above, a buyer of an SUV should not
have to be involved in math, physics, or automotive engineering before making
a purchase. But, he really needs to be for his own protection and the protection of
others. Anyone connected to the legal aspects of tort law also needs to
be versed in this matter. Several different configurations of vehicles, in
particular, a conventional compact station wagon and its cousin, an SUV of
similar size are investigated. Sixbullets has done most of the math for
the reader and it is not overly complicated. The all-important term of c.g. is
discussed. A pair of equations and an accompanying chart
for do-it-yourself sliding vs. tipping evaluation is shown. The equations
and chart are very easy to use, requiring perhaps an eighth grade knowledge of
algebra. Examples are given of actual data and illustrations showing
schematically what happens in a curve to a safely designed vehicle as well as
an unsafe one. Finally, a practical driving test will be described which can
be used by qualified personnel to actually road test a car for cornering
stability.
SUV
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