They're Just Aren't Compatible

Sept. 11, 1998
Passenger cars often fare badly in crashes with sport-utes, but new design strategies are addressing the problem

Sarah Lewis
Contributing Editor

AChevrolet Suburban slammed into Albert and Marianne Hassan’s Chrysler Concorde at a Merrick, N.Y., intersection while they were en route to pick up their 3-week-old son. According to published reports, the 5,000-lb sport-ute hit the driver’s side door, shoving the family sedan sideways for 50 ft and smashing it against a tree. Mrs. Hassan was blinded and critically injured. Mr. Hassan died almost instantly from internal bleeding, while the driver of the Suburban walked away without a scratch.

There is a growing feeling that accidents like this one are becoming all too common. Sport-utes and light trucks are getting a big share of the blame. One reason: the decline in annual traffic fatalities stopped in 1992, the same year sport-ute and light-truck sales exploded, growing by 12.3%. Indeed, this sales increase was the first in four years. Mere coincidence? Many think not.

In collisions between light trucks and cars, car occupants make up 80% of the fatalities, a U.S. Department of Transportation (USDOT) study concludes. Furthermore, says USDOT, reducing the weight of light trucks by 100 lb would cut road fatalities by 40 a year and decrease critical injuries by 601 a year.

An explanation of these figures could have a lot to do with how sport-utes are designed. Most are derivatives of trucks, using a body-on-frame design. With this comes higher ground clearances for off-road driving and a geometric mismatch with passenger cars. In collisions, the taller sport-ute misses the passenger car’s crash-energy-absorbing bumper altogether, letting the passenger car absorb most of the impact. And in some crashes, the larger vehicle climbs over the top of passenger cars, further increasing the danger for small-car occupants.

All of this was not a big concern until the popularity of sport-utes and trucks soared. Last year, 44% of new vehicles sold were light trucks and sport-utes, according to a recent National Highway and Traffic Safety Administration (NHTSA) report. After looking at sales and accident figures, NHTSA began conducting a new crash test. The organization now runs sport-utes into the sides of Honda Accords to determine how cars and trucks can be modified to reduce traffic deaths. One suggested modification is to standardize bumper heights and put sport-utes, light trucks, and other passenger vehicles on equal footing.

“Standardizing bumper height becomes a challenge when you have trucks and SUVs whose mission is to carry heavy loads and go off road,” explains a Lexus spokesperson. Yet, as the Insurance Institute for Highway Safety (IIHS) reports, the vast majority of sport utes are never driven off road. They are more frequently used as “suburban commuter vehicles.”

“I would be hesitant to say adjusting frame height is the one magic silver bullet that will make SUVs crash compatible with other cars,” the Lexus spokesperson says. “Before we race off and redesign or reregulate SUVs, we have to look at the big picture.”

Mike Love, product planning manager at Lexus, says there are really three factors that come into play in collisions: vehicle mass, geometry (where the vehicles hit each other), and stiffness (how the vehicles absorb energy). “When designing a SUV, it is difficult to satisfy the need for car compatibility without giving up some of the attributes trucks are supposed to have, such as ruggedness, the ability to tow heavy weight, and go off road,” he says.

“Even if everyone on the road drove exactly the same car, we would still have a compatibility issue,” Love explains, “because of side impacts and cars being run into by semis and trains.”

The Lexus spokesperson says the company’s priority is to protect the occupants of the primary vehicle (in this case the sport ute occupants). He believes this is where the industry focus should be.

But most sport ute designers have started to design vehicles with “crash safety” in mind. The Mercedes-Benz ML320, for example, was developed with an eye toward vehicle incompatibility. “Everything boils down to the location of the crash structure and the height of the frame,” says Roland Utenwoldt, M-Class product development specialist. “We basically incorporated passenger- car technology in a light truck.”

The M-Class is built with body-on frame construction and independent suspension. Front and rear suspensions mount on the side of the frame, letting engineers lower the crash structure to 19.5 in. (The crash structure of a passenger car is usually 16 to 19 in.) The ML 320’s ground clearance is 8.4 in., giving it off-road flexibility.

Softer crumple zones in the vehicle’s front and rear let them absorb crash energy, rather than transferring it to the passenger car. While there is some debate over whether this puts the ML 320’s occupants more at risk, Utenwoldt contends it is the passenger compartment, not the crumple zones, that must remain intact in accidents.

“The idea is to dissipate as much energy as possible,” he says. “In a very stiff vehicle, you have a foot or two of deformation possible before intrusion into the passenger compartment. With softer crumple zones, you are going to have three or four feet of space. By sacrificing the front structure of the vehicle, you are able to maintain the integrity of the passenger compartment.”

Julie Rochman, IIHS director of communications, agrees. “To do well in the government’s offset crash tests, you need a stiff safety cage, but the front end shouldn’t be stiff,” she says. “A stiff front end often transmits crash energy into the occupant compartment, and so we see a lot of lower leg and foot injuries.”

The Lexus RX 300 is another sport-ute designed to “wreak less havoc” on passenger cars. Built on a unibody similar to passenger cars, its ground clearance measures only 7.7 in.

As with most innovations, new designs are migrating into the market in the moreexpensive models first but are working their way down to popularly priced sport utes and trucks. The Chevrolet Blazer, for instance, now carries front and rear crumple zones.

Yet some experts say design changes to sport-utes are not the complete solution. A Ford spokesperson emphasizes that frame rails — not bumpers — absorb crash energy, and says there is little difference between the frame rail height of the Mercedes-Benz ML 320 and many other SUVs. Still other experts believe the industry’s attention should also be focused on passenger cars.

“Changes should be made to the construction of passenger vehicles as well as SUVs,” says Love. “After all, there are a lot of light trucks and SUVs on the road today, and they aren’t going to go away.”

Side impacts cause about 30% of all traffic fatalities, according to an IIHS report. Rochman says that although sport-utes often have adequate side-impact protection, many passenger cars’ side-impact standards need a good once-over. It seems manufacturers are trying to anticipate the regulatory curve. Engineers at Saab, Volvo, BMW, and Ford are designing deployable head and side-impact protection systems for their passenger cars.

Pulling Knowledge From the Wreckage

Safety engineering research may be returning to its roots. When the field was just beginning, crash dummies had not been invented, and by necessity engineers studied real-life crashes. The National Highway and Traffic Safety Administration (NHTSA) is returning to this research method, hoping to decrease highway death rates. This means members of NHTSA’s Crash Injury Research & Engineering Network (Ciren) spend time in hospitals, junk yards, and laboratories.

Ciren connects automotive engineers with seven U.S. trauma centers to study severe traffic accidents. Work begins when a victim arrives in the emergency room. Ciren staff photograph the injuries and vehicles, take X-rays, and interview the victim. Sometimes the vehicles are examined. The Ciren team reconstructs the crash to determine what impacts and vehicle parts produced the injuries. This data goes to automotive engineers so they can design safer vehicles.

A recent accident studied in this way is of particular interest to Ciren. It involved two 1992 Volkswagen Jettas colliding at a T-bone intersection. One had smashed headlong into the driver side of the other. The male drivers of both vehicles were approximately the same age and weight. “The male that was struck in the side, where there was only 14 in. of crush space, suffered an injury to his aorta,” says Lou Brown, the NHTSA official who heads Ciren. “But they were able to patch him up with surgery.”

“The fellow in the frontal crash had almost no intrusion into his occupant compartment,” he says. “But the two-point belt cut into his abdomen and lacerated his liver. He died about 61⁄2 hours later.”

After studying hundreds of highly documented case studies like these, Ciren team members have determined that front and side-impact crashes are the deadliest. Among the vehicle parts that cause the most serious injuries are two-point safety belts, gear shifts, parking brakes, and even steering wheels and foot pedals. Ciren is now concentrating on sport-ute and passenger-car crashes, as well as accidents involving the elderly.

“Ciren brings information to the design and automotive engineers directly,” Brown says. “Automotive engineers say they’ve never really seen this data because it has never before been available. That is, until the case is in court and they are trying to defend their product rather then use it as a research tool.”

Medical professionals are also learning about car crashes and injuries. “The body is unlike crash dummies in that it is almost a recording instrument in and of itself,” Brown says. “It reacts to the laws of physics and leaves tell-tale signs in the form of injury.” He says instead of treating the victim from head to toe, emergency room staff now know that direction of impact makes a major difference in the injuries they need to look for.

Ciren also suggests design changes to automakers. It recommends that companies add load-limiting devices to their vehicle’s safety systems. Safety-belt webbing that stretches to release forces in excess of what a liver could stand (about a 4,000-lb tension load) could reduce fatal injuries. Safety-belt pretensioners might also help. They work by firing a pyrotechnic device at the same time as the air bag deploys. It pulls the seat belt snug against the chest to eliminate slack and provide maximum protection.

What is most distinctive to Ciren, according to Brown, is the immediacy of the program. “It may take 15 years before vehicles are built with a new device that meets new performance standards,” he says. “As an engineer, it is rewarding to think we are already having an impact on the streets and in the trauma centers just by sharing this information.”

© 2010 Penton Media, Inc.

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