Page Nomenclature

c.g. = center of gravity

SUV = sport utility vehicle


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. 


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