Watch Out for the Moose—Automakers Maneuver to Make SUVs Safe
Roopinder Tara posted on March 09, 2020 |
Simple physics dictate instability and rollovers in a hard turn.
On a flat road, the vehicle tends to roll toward the outside. About the longitudinal axis through the vehicle’s center of gravity, forces on the left front wheel produce a clockwise rollover torque mha and an opposing torque mgt, where h is the height of the center of gravity and t is the distance between the vehicle’s front wheels. If the acceleration a, in units of the gravitational acceleration g, exceeds the stability factor t/2h, the car will roll over. (Image from “Vehicle Design and the Physics of Traffic Safety,” Physics Today, Deena Patel, independent researcher, Thomas P Wenzel, et al., University of California, Berkeley, January 2005, courtesy of Research Gate.)
On a flat road, the vehicle tends to roll toward the outside. About the longitudinal axis through the vehicle’s center of gravity, forces on the left front wheel produce a clockwise rollover torque mha and an opposing torque mgt, where h is the height of the center of gravity and t is the distance between the vehicle’s front wheels. If the acceleration a, in units of the gravitational acceleration g, exceeds the stability factor t/2h, the car will roll over. (Image from “Vehicle Design and the Physics of Traffic Safety,” Physics Today, Deena Patel, independent researcher, Thomas P Wenzel, et al., University of California, Berkeley, January 2005, courtesy of Research Gate.)
The SUV you bought to keep your family safe could put your family at risk—especially if you are driving in moose country.

With their higher center of gravity, SUVs are more at risk of rollover accidents than sedans or coupes. Swinging a heavy SUV hard in one direction to avoid something that appears on the road ahead—like a moose—is easy even with the most massive SUVs. Power steering lets you do it with a finger. But you may put the vehicle’s center of gravity past its outside wheels in a tight turn. Suddenly, you are driving on two wheels. Decrease the radius or enter the turn with a high enough velocity and you will roll the vehicle over, possibly multiple times. If you hit the moose, you may set off the vehicle’s airbags, but they will deflate after a second, leaving you with only a seatbelt to keep your body from bouncing around the vehicle or flying out of it. Then it is up to the vehicle’s support pillars to keep the roof from collapsing.

It was a rollover that killed Seymour Cray, founder of Cray computers and the father of supercomputers, in a case that is often used for finite element analysis (FEA) of crashes. The 70-year-old Cray was hit while driving his SUV, which rolled over three times. He was wearing a seatbelt, but still the accident broke his neck and he died of head injuries two weeks later.

Why Moose?

A successful moose maneuver demands turning hard in the other direction after avoiding the moose to get back on the road. It was named the Moose Test (or more accurately Elchtest, or Elk Test) by Swedish journalist Robert Collin, who flipped a new Mercedes W168 in 1997. The moose/elk was hypothetical. Though the Swedish magazine Teknikens Värld conducted the tests with traffic cones, it was the same maneuver you would perform to avoid an elk, said Collin in an interview later. The rollover was quite an embarrassment for the luxury car manufacturer that had spent DM 2.5 billion ($1.4 billion) to develop the “executive subcompact” A-Class and far cheaper cars (like the ridiculed and dirt-cheap Trabant) that passed the Moose Test without a problem. Mercedes recalled every W168 sold, suspended additional sales for three months, modified the vehicle’s suspension, and installed the Electronic Stability Program. It was a DM 300 million ($170 million) fix. Other automakers followed suit, and now every new vehicle sold has an electronic stability system.

Moose like other members of the deer family have spindly legs and a high center of gravity. A full-grown moose can weigh almost as ton. Its center of gravity is eye level to a motorist. Hitting the animal head on will bring the moose over the vehicle’s hood and into the passenger compartment. There is a good chance that no one will survive the accident—including the moose.

Moose are most commonly found as you get into the Northern Latitudes, in Northern New England and Alaska in the US, Canada, the Scandinavian countries and Russia. Swedish automakers Volvo and Saab have conducted moose tests for years. While the evasive maneuver is done with cones, the Swedish National Road and Transport Research Institute has developed a moose dummy to simulate the damage caused by the real animal.

Risk of Rollovers in SUVs

Risk of rollovers in truck-based SUVs are a close second to sports cars. A study also found that SUVs were more than twice as likely to roll over in a crash. (Image courtesy of “The Relationship between Vehicle Weight, Size and Safety,” by Tom Wenzel, Lawrence Livermore National Labs, and Marc Ross, University of Michigan, March 2008.)
Risk of rollovers in truck-based SUVs are a close second to sports cars. A study also found that SUVs were more than twice as likely to roll over in a crash. (Image courtesy of “The Relationship between Vehicle Weight, Size and Safety,” by Tom Wenzel, Lawrence Livermore National Labs, and Marc Ross, University of Michigan, March 2008.)
Rollovers are not new for SUV and vehicles with high centers of gravity. The Suzuki Samurai never recovered from the 1998 Consumer Reports testing that showed the vehicle was likely to roll over even when traveling at low speeds. Still, sales of SUVs took off, as if the Samurai were the only rollover risk. It was not. In 1999, as SUVs were becoming popular, rollovers caused almost a third of the 36,000 traffic deaths that year. While more than 60 percent of SUV occupants who died in traffic accidents that year were in rollovers, far fewer (23 percent) car occupants died the same way.

The Ford Explorer, created after Ford, in a hurry to cash in on the SUV market, fashioned a family vehicle from a pickup truck. Accident statistics from Safety Research & Strategies revealed that one in every 2,700 Ford Explorers built between 1990 and 2001 rolled over and killed at least one person in the vehicle.

It was an improvement over the previous Ford SUV, the Ford Bronco II, which was killing one person for every 500 Broncos. A Frontline documentary, “Rollover,” has a Ford engineer saying that the company knew the SUV was killing people in rollovers more often than other SUVs and that the problem had been discovered in road tests prior to the release of the Bronco II. Ford engineers recommended lowering the vehicle’s center of gravity and widening its track by two inches to increase its stability, but those changes would have delayed production and the vehicle’s release date. Ford declined to make the recommended changes.

No moose in sight, but this 2017 battle of SUVs still resulted in a rollover. Uber’s Volvo went through an orange light at 38 mph, hitting a Honda CR-V and two other cars, before turning on its side. No one was killed. (Image courtesy of Fresco News.)
No moose in sight, but this 2017 battle of SUVs still resulted in a rollover. Uber’s Volvo went through an orange light at 38 mph, hitting a Honda CR-V and two other cars, before turning on its side. No one was killed. (Image courtesy of Fresco News.)
Ford would blame Firestone for the Explorer rollovers, saying it was the vehicle’s tires that were the cause, not a solid rear axle or a high center of gravity. But even after a tire recall and swap, the rollovers kept happening.

Most SUVs are sheet metal bodies bolted on to truck chassis, missing the rigidity of the monocoque or semi-monocoque construction that is common to cars. That combined with their extra mass makes them more likely to crush their occupants in rollovers. Rollovers account for only 3 percent of crashes but more than a third of occupant deaths, according to a 2005 Wall Street Journal report.

SUV drivers love the view from the top, sitting high and being able to see over cars and farther down the road. Yet that sensation may be causing them to drive faster. Being higher off the ground makes people feel like they are going slower. A person going 60 mph in an SUV would have a perceived speed of 40 mph, says Ron Noel, assistant professor of psychology at Rensselaer Polytechnic Institute. As a result, they could take turns too quickly and roll over.

Deaths and multimillion dollar lawsuits seem to little to disturb the upward trend in SUV sales. SUVs continue to be extremely popular, accounting for 43 percent of vehicle sales in 2017. Station wagons have become practically extinct, and sales of sedans decline every year, threatening to make them an endangered species.

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