Roll Over, Newton

The design of sport utility vehicles is enough to make the father of physics turn in his grave

By Curtis RistApr 1, 2001 6:00 AM


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Seymour Cray, the thomas edison of the supercomputer, liked to stay on the cutting edge of technology. So successful was his quest to create an ever-faster computer that during the cold war, the U.S. Department of Defense regarded him as a national security resource.

But one September afternoon in 1996, at the age of 70, Cray fell victim to a far clunkier machine. He was driving a sport utility vehicle in Colorado Springs when a car swerved and rammed his left rear door. Cray's SUV spun counterclockwise and rolled over three times. Although he was wearing a seat belt, the crash broke his neck and he died of severe head injuries two weeks later. "The irony of Seymour Cray's death was that supercomputers were used early on in the auto industry to simulate the crush characteristics of vehicles," says Carl E. Nash, an adjunct professor of engineering at George Washington University and a former chief of the accident investigation division of the National Highway Traffic Safety Administration (NHTSA). "Through his work, Cray contributed hugely to roadway safety, yet he got himself killed in a type of vehicle that seems to defy that."

Rollover deaths are startlingly common events, resulting in nearly a third of the 35,806 passenger fatalities from traffic accidents in 1999. Any car can turn over, but SUVs are much more likely to do so, a fact given national attention recently by the reported failures of Firestone tires on Ford Explorers. That controversy brought to light disturbing statistics. According to the NHTSA, more than 60 percent of SUV occupants who died in traffic accidents in 1999 were involved in rollovers. By comparison, only 23 percent of all car occupants who died were involved in rollovers. "Fundamentally, most SUVs are working trucks with station wagon bodies grafted onto them," says Nash, who has worked as a consultant in lawsuits concerning SUV stability. "From the aspect of physics, they're simply the wrong type of vehicle to have on the road."

Rollovers are a leading cause of auto-related deaths, accounting for 10,857 fatalities in 1999, and SUVs are three times as likely to roll over as other cars. The problem is part physics— SUVs have high centers of gravity— and part perception: SUV drivers don't realize how fast they're going or how vulnerable they are.

At the same time, of course, SUVs are hugely popular. A decade ago, light trucks and vans accounted for just 20 percent of all vehicles. Now they account for a third and are expected to number half within a decade. "We're talking about the vehicle that will soon be in the majority," says Clay Gabler, an associate professor of mechanical engineering at Rowan University in Glassboro, New Jersey. "Any problems we see now will only magnify, and everyone— automakers, safety testers, and researchers like myself— is struggling to catch up with that fact."

The underlying problem is one that Seymour Cray could easily have grasped. But like most drivers, he probably didn't realize that every time he stepped into his SUV he was taking part in an ongoing demonstration of Newtonian physics. Driving an SUV takes more skill and attention than driving a regular car, yet many drivers seem to think just the opposite. They "listen to the radio or talk on the cell phone without regard to the underlying forces of motion that affect a vehicle," says Carl Lopez, author of Going Faster! and an instructor at the Skip Barber Racing School in Connecticut. "Yet these laws remain constant no matter what kind of car you drive, from a Formula One race car to a giant SUV. And they absolutely affect how the vehicle will handle."

At the heart of the laws of motion lies a vehicle's center of gravity, the point at which an object's mass is in equilibrium. "You could literally attach a hook to the center of gravity and pick a car up, and it would be perfectly balanced front to rear, top to bottom, and from side to side," Lopez says. The center of gravity is the single point through which all of the forces affecting a vehicle— from braking and accelerating to turning— act. And its location, especially its height off the ground, is crucial to understanding a vehicle's stability on the road.

A car, like any moving object, has to obey Newton's first law of motion. Once it's moving, inertia will keep the car's center of gravity traveling in a straight line with a constant velocity until a force such as friction makes it change speed or direction. Yet even when a car heads straight, the load on its four tires— which cling to the pavement with postcard-sized patches of rubber— can change radically.

Accelerating, for example, shifts the bulk of the load to the two rear tires. This presses the driver back into the seat and reduces the weight on the front tires, thus diminishing their ability to change the direction of the vehicle. "In drag racing, the acceleration is so extreme and the load transfer so great that you'll sometimes see the front ends of the cars lift off the roadway," Lopez says. Braking has the opposite effect: The balance suddenly shifts to the front, taking the weight off the rear tires and occasionally making them lose contact with the pavement. A car with a short wheelbase (the distance between the front and back tires) and a high center of gravity can be made to lurch forward and backward so violently that it does a somersault end over end.

Front somersaults in modern vehicles are rare, but lateral somersaults— rollovers— are not. Vehicles most often roll over when drivers attempt to execute a turn. Because the car's center of gravity keeps moving in a straight line, the only way to change direc-tion is to turn the front wheels. (In all but a few exotic vehicles with four-wheel steering, only the front tires are steerable.) As the tires turn sideways against the direction of travel, they create a lateral force that is opposed by an equal force— sometimes known as centrifugal force— in the opposite direction, which propels the car toward the outside of the turn.

As with acceleration and braking on a straightaway, turning causes the load of the car to shift toward the two tires on the outside of the turn. In a left-hand turn, for example, the load increases on the passenger-side tires, especially the one in front. As long as some weight remains on the inside tires, the car will stay upright. "But if you end up with no weight on the inside tires, they'll lift into the air," Lopez says, "and you're essentially riding a bicycle." Drivers are rarely aware when the inside tires become weightless in a tight, fast turn because the tires may be less than half an inch off the ground. But at that point, anything at all— a gust of wind, an outside tire hitting a pothole or a curb or the soft shoulder of a road— can flip the car over.

A vehicle's springs, shock absorbers, and tires can help control these forces, but in general, the tendency to roll over can be quantified by a simple ratio. That ratio is found by dividing the height of the vehicle's center of gravity into half the distance between the centers of the two front tires (called track width). The higher the ratio, known as the static stability factor, the more likely a vehicle is to stay on its feet. This makes perfect sense: A wide, flat piece of sheet metal is harder to flip over than a tall, thin metal cylinder. But the implications for car designers aren't always obvious.

Vehicles with the lowest centers of gravity— less than a foot off the ground in some race cars— are very stable. But they are useless on anything other than a smooth racetrack. If most cars were designed like race cars, Lopez says, "every time you went to the supermarket, you'd have to call a tow truck to pull you off the speed bump." So, over the course of a century of car manufacturing, a compromise has emerged: Most cars are built just high enough to clear road obstacles yet with a center of gravity low enough— about 20 inches off the ground— to prevent most rollovers.

SUVs, unfortunately, tend to have a center of gravity five or six inches higher than that of passenger cars and a track width that's about the same. According to figures compiled by the NHTSA, one popular 2001 model SUV has a track width of 58.6 inches and a center of gravity 27.53 inches off the ground. The best-selling passenger car by the same manufacturer has a track width of 61.9 inches and a center of gravity 21.7 inches off the ground. The numbers may seem similar, but they combine to give a static stability factor of 1.06 for the SUV and 1.43 for the passenger car. Statistically, that means that the SUV has a 37 percent chance of rolling over in a single-vehicle crash, whereas the passenger car has only a 10.6 percent chance of rolling over. For the SUV to be as stable as the car, its track width would have to be 20 inches wider than it now is.

The static stability factor "treats a complex situation far too simplistically," Robert Strassburger, a vice president at the Alliance of Automobile Manufacturers, believes. "Driver behavior and weather are widely recognized as the dominant causes of rollover crashes. Even among vehicle factors, suspension characteristics, tires, inertial properties, advanced handling systems, and other factors all directly affect a vehicle's likelihood of rolling over." It's true that numbers alone don't roll vehicles over, and even the tipsiest SUVs will stay upright if they are driven correctly. Yet speed may be "the most important nonvehicle variable" in a rollover, according to the NHTSA, and an SUV's design seems to encourage some drivers to go faster than usual.

The problem lies in one of an SUV's most likable traits: its high driving position, which allows drivers to see over traffic. People judge motion by what's called optic flow, says Ron Noel, an assistant professor of psychology at Rensselaer Polytechnic Institute in Troy, New York. From an airplane, for example, the ground appears to be crawling along, although the plane is moving at hundreds of miles an hour. By contrast, a race car that hugs the ground feels as if it's going extremely quickly, even at speeds of only 30 or 40 miles per hour.

In experiments using a video camera placed in different vehicles, Noel developed a formula that relates one's perception of speed to one's height above the ground. The seat in a typical SUV is 20 inches higher than in a car, Noel says. "By our model, that would mean that a person who is doing 60 mph in an SUV would be perceiving speed the same as someone doing around 40 in a regular car." As a result, he says, SUV drivers tend to take turns too quickly. As their tires lose their grip on the roadway, they can slide out of control, strike a curb or soft road shoulder, and trigger a rollover.

Antilock brakes— standard equipment in many SUVs and cars— can aggravate the problem. Sensors by each tire make the brakes pulse on and off when a tire starts to skid. That works fine when a vehicle is traveling straight ahead. But if a driver slams on the brakes while going around a curve or just before steering to avoid something, the laws of physics can turn against him. "When the brakes pulse on, the force vector of the friction generated by the tires is in whatever direction the vehicle is moving," Nash says. When they pulse off, the tires suddenly grab onto the pavement and the car's vector of force shifts laterally. "That on-and-off lateral force can actually be enough to flip a vehicle that is already leaning over, particularly if it has a high center of gravity."

Despite all these forces working against SUVs, a rollover shouldn't be the disaster it often is. A 35-mph frontal collision, to which cars are subjected in federal safety tests, is the equivalent of dropping a vehicle on its nose from a height of about 40 feet. The impact from a rollover should not be anywhere near that severe. "In a rollover, a vehicle rarely gets more than about a foot off the ground as it's rolling— and if the roof were strong enough, a person inside would be shaken but otherwise uninjured," Nash says. "But if the roof crushes over you, it's curtains." Unfortunately, in an SUV the roof is much more likely to collapse than it would be in an ordinary car.Once again, simple physics works against SUV owners. That's because when any object rolls, it turns on a longitudinal axis that passes through its center of gravity. In a passenger car, the corners of the roof and the outer edges of the tires lie roughly along the perimeter of this tube. "So when a passenger car rolls over, it goes somewhat smoothly," Nash says. In a typical SUV, the edges of the roof rise five or six inches beyond the tube, so the roof hits the ground harder, and passengers are more likely to sustain fatal head injuries.

Auto manufacturers are trying to make SUVs safer. The 2002 Ford Explorer, for instance, has a track width two and a half inches wider than its predecessor's. That is far short of the 20 inches by which it would have to be widened to match the stability of the current Ford Taurus, but Ford believes the change will raise the vehicle's rollover resistance rating from two stars to three out of a possible five. (The Taurus, by comparison, earns four stars.) The company is also replacing the Explorer's antiquated rear suspension system, in which the rear axle is bolted to two front-to-rear leaf springs. A new independent coil-spring suspension will allow each wheel to react to individual loads, giving the car a smoother ride and offering better control. Optional side-curtain air bags will protect drivers and passengers during a rollover, and special sensors will keep the bags inflated for up to six seconds, rather than the standard fraction of a second, just in case the car rolls over a number of times. The result, according to Ray Nicosia, the truck engineering manager at Ford, will be "the safest Explorer yet," as proven by extensive company testing— including crash simulations using Cray computers.Carl Nash, for one, thinks that SUVs still have a long, long way to go. "We have the means to make SUVs safer through greater expenditures on design— which would include wider track widths, lower centers of gravity, and stronger roofs," he says. "Until we see those sorts of changes, simple physics will prevail: Vehicles with higher centers of gravity will tend to roll over more than those with lower ones, and kill more people."

SUVs at the Crossroads

Two years ago, University of Texas engineer Kara Kockelman and her student Raheel Shabih videotaped and timed cars moving through two intersections in Austin. On average, passenger cars took 1.73 seconds from the moment they entered the intersection until the next vehicle entered; SUVs took 2.44 seconds. Of the extra 0.71 second, 0.4 was needed just to move the longer, more sluggish SUVs along. But the other 0.31 second was lost behind the SUVs, as vehicles hung back— presumably because their drivers' views were obstructed. Seven tenths of a second may not seem like much, but it adds up. If a driver's time is worth $10 an hour, Kockelman calculates, an SUV that's driven one hour a day in a congested city will cost an additional $4,000 to $7,000 in delays over the life of the vehicle compared with a passenger car's delay costs.— C.R.

A Weighty Issue

When an SUV crashes into a passenger car, the SUV usually comes out ahead. It's not just a matter of weight— an SUV weighs 900 pounds more, on average, than a typical car— but of design. SUVs tend to be built in two separate pieces: a sheet-metal body atop a ladder frame, which consists of two steel beams that run the length of the underbody and curve up in front like the runners of a sleigh. In a collision, that ladder frame "acts something like a battering ram," says Rowan University's Clay Gabler, who has studied SUV "aggressivity" with his former National Highway Traffic Safety Administration colleague Tom Hollowell. Instead of crumpling and absorbing shock, an SUV's beams tend to slide over car bumpers and doorsills, punching into the other vehicle's passenger compartment.

The most dangerous crashes are those in which one vehicle hits a second in the side, where there's little structure or protection for the passengers. SUVs are particularly dangerous in such cases because their bumpers ride 10 inches higher than those of regular cars. Rather than stopping at the door of a passenger car, they can ride over the doorsill and strike the occupant's head. Gabler and Hollowell calculated that when a typical passenger car hits another car in the side, people in the hitting car are five or six times less likely to die than those in the struck car. But when an SUV does the crashing, the people inside it are 20 times less likely to die than those in a passenger car that's been hit. "That's an amazing and really frightening statistic," Gabler says.

Even when people aren't crushed in a crash, their bodies often slam up against the seat belt, dashboard, steering column, or window, and the violent deceleration can cause internal hemorrhaging or worse. When two cars of the same mass collide head-on at the same speed, their momentum cancels out and the cars come to a dead stop. But when an SUV crashes headlong into a passenger car, its momentum forces the lighter car backward. That can produce a severe change of velocity in the smaller car, and a greater risk of injury.

Manufacturers have begun to make some safety changes, such as lowering bumpers by a couple of inches. Nonetheless, Gabler says: "You can put in all the safety features you want, but you still won't change the basic problem. They're heavier, they have a stiffer frame— and we've just begun to see the problems that this will cause as this population of vehicles grows. It's just not enough socially to look at how people are surviving in SUVs if they're killing everybody they hit."— C.R.

Tread Reckoning

An SUV's two-ton weight rides on only around 60 square inches of tire tread, so it can skid easily— and flip over if the tires fold or hit a curb. Wider tires don't help, surprisingly, since the crucial factor is the distance between tires. Underinflated tires grip the road better but are more likely to buckle.

Dead Man's Curve

When a car takes a corner, its momentum carries it forward while its front wheels force it sideways. In a right-hand turn, that puts most of the pressure on the left front wheel. If the car is going fast enough, the right rear wheel will lift off the pavement first, followed by the right front wheel. If the car is an SUV, with a high center of gravity, it's liable to keep on going over— pivoting on its left front wheel and rolling.

A Question of Guardrails

Hundreds of thousands of miles of guardrails line curves along the nation's roads and highways. But according to Malcolm Ray, an associate professor of civil engineering at Worcester Polytechnic Institute in Massachusetts, most of them won't help anyone driving a pickup truck or an SUV. Ray has used computers to model various crash scenarios in which a pickup truck, barreling down a highway at 65 mph, veers into a guardrail at a 25-degree angle. "Typically, these vehicles go up and over guardrails," he says. "And if they happen to be contained on the roadway, they'll sometimes flip over— which is not a good thing at all." Ray has since confirmed the modeling by studying actual crashes. Not all guardrails are a problem, he says: Both the stiffest and most flexible ones seem to do an adequate job of keeping trucks on the road without flip-ping them. But the majority of metal rails are just stiff enough to cause trouble. "These railings exist in every state," Ray says. "I haven't even begun to estimate the cost of replacing them."— C.R.

The Web site of the Alliance of Automobile Manufacturers:

More about the National Highway Traffic Safety Administration's rollover resistance ratings can be found at For NHTSA ratings of 2001 models, see

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