Auto experts measure sport-utility stability

Height and track width are key when gauging the likelihood of a SUV

By , Staff writer of The Christian Science Monitor

Wider is better - that's the message from auto-safety experts to consumers concerned about whether their sport-utility vehicle (SUV) will roll over on them.

Several recent reports show that sport utes are more prone to rollover accidents than passenger cars, but neither carmakers nor the Federal government offers any clues about which models are more tipsy than others.

In the absence of government ratings, however, experts say consumers can look for some simple things to judge the relative stability of SUVs.

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Just bring along a tape measure or else take a detailed look at the size specifications in the vehicle's brochure.

Two key measurements are height and track width.

Height covers the distance between the ground and the top of the roof. Track width is the gap between the left and right tires, measured at the center of the tread.

Divide the height by the track width. "Anything over a 1-to-1 ratio [of height to track] and you start to get into trouble," says Joe Kimmel, president of the Transportation Analysis Institute, who devised an SUV rollover index of height, weight, and track width, called the K index (see chart below).

Vehicles with more stability are less likely to roll over if they go around a curve too fast. Instead, they are more inclined to lose traction and spin out - not a desirable situation but one with less potential for damage.

Of course, the actual center of gravity is more significant than overall height. But since there's no easy way to measure the center of gravity, it's safe enough to generalize that taller vehicles have higher centers of gravity, says Mr. Kimmel.

Loading up an SUV with passengers and luggage also raises the center of gravity and thus makes it more tippy. So beware of carrying big loads.

Yet there is a good side to heavier SUVs. "Think about getting a bunch of guys and trying to tip a VW Beetle on its side in the high school parking lot," says David Cole, director of the Center for the Study of Automotive Transportation at the University of Michigan in Ann Arbor. "Now think about trying to do the same thing to a big tank. You can't even lift it."

The same goes for obstacles that can trip up a car.

Most rollovers don't happen during emergency avoidance maneuvers. According to federal statistics, they happen when a sliding vehicle is "tripped" by hitting something, such as a curb, a different road surface, or another car.

"The thing to remember is that any vehicle can roll over, even a sports car," says Dr. Cole.

It's just harder to roll a sleek sports car than an SUV, most of which are 20 percent to 30 percent taller than they are wide.

Other factors also influence a vehicle's tendency to roll over. For example, tires. The more traction they have, the more likely the car is to roll. Tires with less traction will slide instead.

Many SUVs, shod with tires designed to increase traction off road, don't necessarily have the best traction on pavement. And that's good for avoiding rollovers. Tires that jack up a car for off-road clearance raise the center of gravity and greatly decrease stability, says Cole.

Another factor receiving a lot of attention, particularly in Consumer Reports magazine, is wheelbase, the distance between the front and rear axles. SUVs with wheelbases shorter than 110 inches show a greater propensity to roll over.

Experts aren't sure why shorter SUVs are more likely to roll. It may be that they are simply smaller and lighter, Kimmel says.

Safety advocates want the National Highway Traffic Safety Administration to publish ratings and set vehicle stability standards that would reduce the number of trucks susceptible to rollovers.

"Every SUV should be able to pass a simple avoidance test," says Clarence Ditlow, head of the Center for Auto Safety in Washington.

The issue is crucial, says Kimmel, because rollovers are the "most dangerous accidents by far."

Rollovers have much higher fatality rates than other accidents in SUVs.

For a safer SUV, experts say, look for one that's lower, wider, longer, and heavier.

*Send comments to evarts@csmonitor.com

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