Wine Bottles and SUVs

The Steering Column

Most people instinctively understand that a wine bottle will tip over more readily than a mayonnaise jar. But the same people are flabbergasted when told that sport-utility vehicles roll over more frequently than four-door sedans. To help stamp out this inexplicable ignorance, the National Highway Traffic Safety Administration introduced its new Rollover Resistance Ratings in January.

If you log onto the safety agency's Web site,, then click on "Crash Tests" and "Model Year 2001 Front, side and rollover resistance ratings," you will find a chart listing certain vehicles on a basis of one to five stars. As in the front and side crash ratings, which are often touted in advertising, five stars is the best rating (the least likely to roll) and one star is the worst.

To generate this rollover resistance rating, NHTSA begins with a simple calculation that determines a vehicle's Static Stability Factor (SSF). The formula is:

SSF = Track/(2 x Hcg)

Track is the distance between the midpoints of the left and right tire treads where the rubber meets the road. The front track and rear track dimensions are averaged.

The center of gravity height (Hcg) is a measure of how high the vehicle carries its weight. Although the height of the center of gravity is generally related to overall vehicle height, it is determined by the elevation of a vehicle's heaviest components. For example, Britney Spears and Roseanne both stand five feet four inches tall, but Roseanne carries her weight a bit lower than does Spears.

Since the automakers do not publish the center of gravity (CG) height of their vehicles, NHTSA had to determine those dimensions through measurement. This task was farmed out to the specialists at Transportation Research Center (TRC) near Columbus, Ohio. (TRC also has a 7.5-mile oval track where we have frequently conducted top-speed tests on very fast cars.)

Technicians at TRC used something called the Vehicle Inertia Measurement Facility to measure the CG height. This device, which costs roughly a million dollars, is essentially a stout platform onto which the test vehicle (with a full tank of fuel and a 175-pound dummy in the driver's seat) is driven and then clamped into place. The platform and vehicle balance on a pivot that is several feet above the tire contact patch. Hanging some weights on the end of the platform tilts the entire assembly, and the CG height can be calculated by the degree of tilt produced by the known weight.

With the CG height and the vehicle's track dimensions measured, the SSF can be calculated. The results range from about 1.00 for something like a two-wheel-drive Chevrolet Blazer to roughly 1.65 for a Dodge Viper or Chevrolet Corvette. Since not even a government agency would expect our arithmetically addled population to make heads or tails of a string of numbers with two decimal places, NHTSA converted these numerical SSF factors to a simpler five-star rating system.

Not only does NHTSA's reckoning of a vehicle's rollover resistance increase with the number of stars, but the safety agency has also performed a statistical analysis to align its star rating with the real-world likelihood of a rollover accident. For example, NHTSA's one star, assigned to vehicles with an SSF of 1.03 or less, corresponds to a rollover risk of more than 40 percent in a single-vehicle crash, according to the safety agency's statistical regression. The stars break down as follows:


1 1.03 or less more than 40%

2 1.04-1.12 30-40%

3 1.13-1.24 20-29%

4 1.25-1.44 10-19%

5 1.45 or more less than 10%

No sports cars were in the initial batch of 75 vehicle rollover ratings released in January, but the 22 sedans all received four- or five-star ratings. So did seven of the eight vans. Seven of the 10 pickups got three stars, and to no one's surprise, SUVs held down all but one of the one- and two-star ratings (a four-wheel-drive pickup truck also got two stars).

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