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SUV incompatibility in three-car crash
This three car pile-up illustrates the problem of SUV incompatibility. Note the top of the SUV hood is level with the window of the smaller sedans.

High & Mighty: Incompatibly Aggressive

Part 3 of synopic review of Keith Bradsher's SUV expose, 'High and Might.'

By EVWorld.Com

Here's an easy question for you car buffs.

True or false. It is because SUVs usually weigh more that they inflict greater damage on an opposing vehicle [and its occupants] in a two-car crash?

If you answered, true then you'd be wrong. It turns out, as automakers now know, that it isn't the disproportionate weight of a sport utility vehicle that makes them so destructive, it's their height.

Technically the term is called "vehicle incompatibility" and its one of the rawer nerves that causes the auto industry to cringe when its brought up, even in technical gatherings like SAE Congresses.

According to Keith Bradsher, author of "High & Mighty," the issue of vehicle incompatibility was first raised by the French transportation ministry in a paper published in 1971. The French contended back then that, "too much attention was being given protecting the vehicle occupant," at the expense of better understanding and addressing "vehicle aggressiveness."


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It would be the "aggressiveness" of sport utility vehicles and their full-sized light duty pickup cousins that would lead to as many as 2000 unnecessary deaths a year in North America.

To understand what the French meant by "aggressiveness" it is necessary to understand something of the dynamics of an automobile crash. We commonly think of the energy in a gallon of gasoline or diesel fuel as propelling the car forward. In fact, it is actually imparting a significant portion of that energy to the vehicle and its occupants in the form of momentum. In a crash, that energy is suddenly dissipated, often with horrible results.

One important engineering strategy for dealing with this energy is to let the vehicle's structure absorb as much as possible in the form of crush or crumple zones, usually in the front of the car. As the front of the car accordions together, much of the energy can be dissipated, reducing to a degree injury to the occupants, who hopefully are also restrained by seatbelts and now airbags.

Bradsher points out in his book that the trouble with nearly all SUVs is that they are extremely aggressive in their design. By this he doesn't mean how they look. It is how they are engineered and built that is the problem.

To better understand, next time you walk through a parking lot, notice how high the top of the hood is on your typical SUV compared to that of a common mid-size car. The problem is very apparent. There is a complete mismatch between sedan and the SUV. In most cases, the top hood of the SUV will be about the level of the window sill on the sedan.

While the height mismatch is obvious to even a casual observer, that is less apparent but of far more critical importance is what lies behind that snarling radiator, the frame of the SUV.

Unlike modern cars, which are now built using a unibody construction method that retains only the vestigial remains of a frame, the vast majority of SUVs today are still built on top of a keel of twin steel beams joined by equally strong cross-members. While this makes for an extremely rugged and durable structure on which to mount the vehicle chassis, it also poses a serious safety issue because it just doesn't crush all that well.

It is the combination of height and ridged structure that makes the SUV such an aggressive killer. Surprisingly, this fact came out in a study commissioned by the Motor Vehicle Manufacturer's Association in 1984 to counter a study by the AAA that looked at the issue of weight disparity between heavy trucks and cars.

While the later study argued that "weight was the dominate factor in injury rates…."

"... the study had another conclusion which, with hindsight should have attracted more attention. Pickup trucks and vans were punching deeper into car bodies during collisions than weight differences alone would have predicted. ‘In summary, the vehicle damage analysis generally supported the hypothesis that something other than vehicle weight was involved in light truck aggressiveness,' the report said. ‘That ‘something else' presumably includes structural factors such as frontal stiffness.'"

This conclusion was supported just two years later by NHTSA in January 1986. This federal agency concluded after dozens of crash tests smashing standard crash barriers using bumpers at different heights into the side of VW Rabbits that, "there appeared to be more potential benefit in altering the front end characteristics of the striking vehicle than in modifying the side of the struck vehicle."

They reached this conclusion after discovering that by lowering the hood height of the striking vehicle, they could significantly reduce the level of injury to potential occupants in the VW Rabbit. In fact, the change was so dramatic that it "dropped the risk of serious chest injuries to a mere 11 percent, compared with 97 percent for the standard car hood. "

Bradsher observed, "This was an extraordinary accomplishment, as safety engineers go to huge lengths in vehicle design to achieve much smaller improvements than this."

"But no one was paying attention," he added.

It wouldn't be until the late 1990s that the car companies would secretly begin studying the incompatibility issue, Bradsher contends, with the aid of high-power computer modeling and multi-million dollar crash tests.

But before that, two researchers, Hollowell and Gabler, who had actually pioneered the original work on computer crash modeling had begun to reanalyze NHTSA crash data and reached some chilling conclusions.

"The new calculations showed that SUVs were nearly three times as likely as cars to kill the other driver in a crash. Full-size pickup trucks and full-size vans were even deadlier, but the small pickups on which most SUVs were based were less deadly. Hollowell and Gabler also calculated that when a car strikes another car in the side, the driver of the struck car is 6.6 times as likely to die as the driver of the striking car. But when an SUV hits a car in the side, the death ratio rises to 30 to 1."

Again the reason is aggressive incompatibility.

"SUVs ride almost 200 mm (8 inches) higher than mid-sized cars - - a geometric incompatibility that would readily permit the SUV to override any side structure in a car and directly strike the car occupant."

Ford Motor Company would initiate a lengthy series of secret crash tests and computer simulations in 1997 that looked at the incompatibility issue. In 2001 Ford's safety engineers would conclude, "Preliminary results indicated that geometric incompatibility was the dominating factor in the studied vehicle design characteristics."

In other words, the way SUVs were built, and not just their weight , was killing other motorists. Ford had one a total reversal of its position in 1997…"

The industry has argued that all SUVs have done is shift the deaths from one type of vehicle to another with fewer people dying in SUVs and more dying in smaller vehicles.

To this, Bradsher responds, "So what do the safety numbers show?"

"Explorer drivers die slightly less often in two-vehicle crashes as large car drivers, but take many more drivers of other vehicles to the grave with them. For each Explorer driver whose life is saved in a two-vehicle collision by choosing an Explorer instead of a large car, an extra five drivers are killed in vehicles struck by Explorers."

Ford's on head of safety engineering, Priya Prasad stated in a SAE conference in April 1998, that design of light trucks, and not their weight was causing about 1,000 unnecessary deaths a year in other vehicles they struck. It would be at a 1998 Windsor, Ontario auto conference that Dr. Martinez of NHTSA would raise this estimate to 2,000 deaths annually.

What steps have carmaker's taken to address this problem?

They've quietly lowered their vehicles slightly and drilled holes in the steel frames to promote some crumple and reduce its penetration into the struck vehicle, but all told, such actions are estimated to reduce unnecessary deaths by just one-third. But as more and more SUVs and light trucks find their way from the show room to the highway each year, there is little prospect of the death rate actually declining, especially, Bradsher contends, as used SUVs get into the hands of younger, more reckless drivers.

Times Article Viewed: 5366
Published: 09-Nov-2002

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