It started with the cop in the swamp.
She was a good driver, track-trained in what cops like to call “pursuit of the suspect vehicle.”
But this time, when the motorcycle passed her, going in the opposite direction on a rural stretch of Vermont road, she was driving an SUV. Now SUVs are practical members of many police and fire department fleets. But you can’t drive them like cars, as SUV owners should know.
She went into a three-point turn, trying to reverse direction and catch the speeding motorcycle. The next thing she knew, she was in the swamp.
Baffled by what had happened, her chief called Bud Stanley, a partner at Stevens Advanced Driver Training in Merrimack, N.H. The Stevens staff has taught lots of cops to drive.
Stanley, watching SUV wrecks down the years, had developed an anecdotal theory about a key danger inherent in SUVs. Using the cop’s car, he put it to the test.
He drove 45 m.p.h. on a runway, slammed the brakes and turned to change lanes. No problem.
Did the same thing again, turning first then hitting the brakes.
“The next thing I knew, I was off the road and 75 feet out into the grass,” Stanely recalls.
SUVs, he says, can be dangerous to drive because they have high centers of gravity, high-roll axis, stiff frames and unforgiving suspension systems. People who buy them–looking for car-like feel, perceived added safety and the ability to carry lots of passengers–often ignore the fact that they are basically trucks, and they drive like trucks.
Put yourself behind the wheel of a big SUV. You are doing 50 miles per hour on an expressway. You are distracted for a moment and when you look back out the windshield, you are racing upon a slow truck. With your hands on the wheel and your foot on the gas, your first instinct, and quickest option, is to steer away from the problem.
“The more danger, the greater tendency to crank the wheel,” says Stanley.
Only after turning the wheel do you try to slow down.
And now you are in big trouble.
Car wrecks are all about the transfer of weight and energy, and because you’re driving a heavy SUV, it’s Transfer City.
Say the sharp turn you have just made is to the right.
Your vehicle’s weight will shift in two ways. First, it goes cross-axle, from the inside of the turn to the outside. This is accompanied by a diagonal shift, from the inside rear tire to the outside front tire. Physics at work.
“Now you’ve got an enormous weight transfer,” says Stanley.
When you hit the brakes after you initiate your turn, you accelerate and amplify this transfer to that one tire. Now it is the primary tire for braking and steering the auto.
At this point, says Stanley, one of two things happens.
The driver freezes and the car carves an arc off the road. Or, the driver tries to steer back the other way. All the pent-up energy that has settled down on the outside front tire is released as it unweights. It moves to the other side of the car, is amplified by the car’s motion and shifts to the outside of the new turn. There is a good chance you will roll.
So, besides the braking-steering mistake, what went wrong?
First, consider the anti-lock brakes. After your first mistake, with most of the weight on the front outside tire, you have to rely on the other three tires to save you. If you could lock them up, you’d probably skid, but you’d get a gradual pullback of some of that weight, and you wouldn’t carve that wide arc off the road.
But because the three tires whose help you need are unweighted, ABS won’t let them lock up. They are of little use to you as they spin-clamp, spin-clamp merrily along.
Second, when you try to correct, you are transferring weight back through a truck-like ladder frame. Rigid and non-absorbing. Stand with an aluminum ladder at your feet, running away from you. Grab the bottom of one leg and lift it. Do you sense an equal redistribution of weight to the other three legs? Or do you sense a diagonal shift to one far leg, along with a shift to the opposite side of the ladder from the one whose leg you are holding?
Back to the SUV. Weight is shifting, pent-up energy is being released by coil springs and in some cases leaf springs.
Keep in mind that this is happening in a top-heavy vehicle. The motor, transfer cases, transmission are all up high. Not to mention the tall passenger cab.
This gives you a lot of weight rotating on a high roll axis, the imaginary line through a vehicle that is the center of a rollover.
Roll axis? If a car rolls over, there is a point in its circle that, as you look at it straight ahead or from behind, does not move–an imaginary axle. If you put a skewer dead center through an apple, the apple would be stable, right? What if you put the skewer through the apple one third of the way from the core? Hold the skewer with the upper two-thirds of the apple above this axis and see how stable it is.
Keep in mind that cars will have the same initial reaction to the incorrect steer-brake sequence. But cars are lighter, so there is less energy being thrown around; they do not sit as high; their roll axes are far lower.
But many of us drive SUVs and we like them.
So what do we do?
First, don’t drive them like a car. They may feel like a car–all those fancy interiors, the quiet, smooth ride down the freeway–but they remain trucks.
Don’t drive them fast. It takes longer to slow them, and in a crash situation speed increases energy and weight shift.
Don’t tailgate. Ditto.
And if you do get into trouble and have to avoid a crash:
Slam on the brakes fast and hard and keep them on. Don’t pump them because this confuses your ABS. This puts weight on both front tires as suspensions contract. Then steer away.
“There’s not guaranteed success, but you’ll have the best opportunity to miss whatever’s ahead of you and keep your car under control,” Stanley says.
