Lighter Cars Not Necessarily Riskier, Physicists Say

However, SUVs and Cars Cannot Co-Exist Safely With Current Designs

Jan. 26, 2006--Are smaller cars fundamentally more dangerous than SUVs? In the January issue of Physics Today, a group of physicists and analysts goes beyond simple physics to state that the answer is "no" -- while recommending changes in SUVs and passenger cars alike that could reduce the number of fatal accidents on roadways.

While weight is the simplest variable to consider in a collision, such an analysis ignores other factors that contribute to automobile safety. These other factors include vehicle structure (e.g., the amount of space between the driver and the side or front end of the vehicle), the design of seatbelts and airbags (not to mention other safety features such as electronic stability control which tend to appear first in more expensive vehicles), and incompatibilities in the designs of two colliding vehicles, write Marc Ross and Deena Patel (both of the University of Michigan in Ann Arbor) and Tom Wenzel (Lawrence Berkeley National Laboratory in California).

For example, the current disparity in design between passenger cars and light trucks is a major safety issue, the authors write. For one thing, typical light trucks have two high horizontal rails that extend far to the front, making them especially dangerous to passenger cars in a collision. The front of these light trucks tends to be too stiff and too high, and the stiffness is very different between different parts of a light truck's front end.

"The evidence is compelling that body-on-frame light trucks cannot safely coexist with passenger cars under existing conditions. That problem is critical because so many light trucks are used nowadays as car substitutes," Ross, Patel, and Wenzel write. In fact, they write, SUVs are more than twice as risky as a car to occupants of a car struck on the side.

If one vehicle's front is unnecessarily stiff, the other vehicle may have to absorb so much of the energy that its passenger compartment is compromised, according to the researchers.

However, redesigning the front ends of light trucks-so that they distribute the force of a crash more evenly-and employing technologically available composite materials that absorb more crush energy per kilogram in both kinds of vehicles could reduce the fatalities from such crashes.

The authors also point out that a light vehicle does not have to be small. Lighter, and more fuel-efficient, vehicles could potentially be made without reducing their size, especially with lightweight composite materials that are under development. A critical advantage of lighter vehicles is that they cause fewer deaths and injuries when there is a two-vehicle crash.

The researchers call for other design improvements that could go a long way towards improving safety: strengthen passenger compartments so that they protect occupants better during a crash; improve safety belts and airbags to restrict sideways movements in passengers and nudge occupants into the best possible positions when a crash is imminent; redesign front ends of cars to increase the energy that is absorbed in the early stages of a crash.

Physics Today is the monthly magazine from the American Institute of Physics, based in College Park, Maryland.

For more information:

Article available at Physics Today Online
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The PDF of the full article is also available.

Contact:

Ben Stein
American Institute of Physics
Bstein@aip.org
301-209-3091