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Knee deep in america, where the plague of SUV's has grown to endemic proportions, there is a particularly interesting phenomenon. As you travel the intestates and other multi-lane highways on days when there is a modest amount of snow, you will observe that the majority of the vehicles that have come gently to rest in the ditches are SUVs!

The morals:

It had started snowing about nine, and kept at it all day. We all crawled into my friend Chris's Chrysler Concorde for the long drive home after work. I drove, while Chris slept. It was slippery, but not all that bad.

As we turned south on I-71 I noticed brake lights on the ramp. Two people standing to the left, holding cellphones. A new Jeep Liberty perched atop a fractured guardrail. Ahead on the right, a Jeep with a mashed nose stood parked behind a semi-tractor, with personal information being exchanged. Across the road, a Chevy 4WD was sideways in a ditch. Three four-wheel-drive vehicles. Three accidents. What happened?

Many SUVs and four-wheel-drive pick-ups are sold using marketing that shows pictures of some pickup a top a mesa or some other place that would challenge a mountain goat. The sad truth is that the vehicle probably got there slung under a helicopter.

These vehicles will take you places no car will go. With a spotter, a winch and excellent technique. Four wheel drive gives you traction under conditions where two wheel drive won't. They have a higher ground clearance. Higher ground clearance offers enormous advantages in crossing obstacles and snow, because the higher the bottom of the vehicle the less likely it is to hang up, or be held by by drag from snow or mud. Approach and departure angles allow for better obstacle crossing. But those abilities come at a price.

The price for high ground clearance is a high center of gravity.

Understand that all cars and SUV's go, stop and turn only because their tires grip something. The contact patch between rubber and road makes the truck go. Because of inertia, whenever there is change in velocity, via acceleration, braking, or turning, the vehicle will resist the change in direction around its center of gravity Cg. The higher the Cg, the more leverage that resisting mass will have. Weight wants to transfer more quickly and abruptly. This is why SUV's and many other four wheel drive vehicles are likely to overturn under all conditions. Abrupt shifts at the limit encourage breaking traction. And once traction is broken, the amount of grip drops like a stone.

On the race track race car drivers use the properties of controlled weight transfer to maximize their car's speed around the track. Increasing weight on a wheel, increases the size of the contact patch, and thus the amount of traction produced by the wheel. Often when a race car spins, it is because the wheel that weight was being transferred from breaks loose.

The high center of gravity means that these shifts in traction will be abrupt relative to a car, and thus harder to control. In addition, the large wheels and tires of these vehicles gives SUVs a high percentage of unsprung weight, or weight that is not controlled by the suspension. The vehicles are relatively heavy, meaning the tires and suspension have a lot of weight to conrol. Under normal driving conditions at reasonable speed control is rarely a problem. But low-traction situations are often unpredictable, with the actual available grip becoming hard to determine. Breakaways can be uncontrollable. The high center of gravity characteristic of SUVs, Jeeps and 4WD pickups makes them more susceptible to abrupt spins, or loss of control. In other words, they're a lot more likely to find themselves atop a guardrail.

This disadvantage of these vehicles is made worse by the reasons for which they are purchased. Many purchases like their Hummer because they want a macho vehicle, which explains all the rocks in SUV ads though very few SUV owners venture off-road. Many also believe that 4WD will make them invincible in winter.

Frankly, that isn't true. Every car built after World War II has four wheel brakes. Everyone who has driven in winter has experienced sliding while trying to stop. That Dodge Ram will get you across that snowdraft and to the grocery when my Ford SVT Focus will not. But it's not invincible. 4WD vehicles still can get stuck. They still spin, they still crash. If you want invincibilty, drive a tank.

Many also purchase SUV's because they want improved crash safety. Research has shown that SUV owners often have less confidence in their driving than car or minivan owners. Unfortunately for them, research also shows that SUV and heavy truck drivers suffer a greater percentage of fatalities and severe injuries than their automotive counterparts.

Partly that is because SUV's are inherently more likely to flip than a car (see below). But another reason comes from the type of construction used for most SUVs. Most, and until recently all, SUVs were made with body-on-frame construction. This is a very old way of making cars. With the exception of the rare cornelian almost all cars made before 1960 used that a conventional frame. Body on frame construction is very flexible, as different bodies can be dropped on the same frame. Frames are great for tow vehicles. Frames carry weight well, and the pieced-on bodies are easier to repair after a low-speed collision. However, modern unibody vehicles are much better at absorbing heavy impacts, as crumple zones are easily designed into the structure. The crumple zones absorb crash energy, so it is not transmitted to the passengers.

Finally, despite some really clever engineering, cars go, stop and handle better than SUVs, particularly when price is figured in. The easiest way to survive an accident is to evade it.

Really, it should come as no surprise that SUVs are often involved in more accidents than their automotive bretheren. They do certain things very well, and they will keep you going when things get really awful.. But they offer no panacea, for their off-road abilities come at a real price.

Rollovers under normal conditions occur because when weight transfers to one side of a vehicle, it transfers {off} the other. Now if a vehicle is going fast enough, the force of said transfer is a simple mass times velocity calculation, with the center of gravity adding a leverage. The higher the Cg the higher the leverage, in effect increasing the force of the weight transfer. The vehicle also has a roll center which is a property of axle height and Cg. While the vehicle has an actual roll center, under acceleration that point shifts relative to the wheels. Rollovers occur when the force-- seen as weight on one set of wheels-- is great enough to overcome the weight on the other pair. Once 'weight' moves outside the track of the loaded wheels, over she will go. The high Cg characteristic of SUV's and trucks means they will always want to roll over more than an automobile. This same principle also explains why weight shifts in trailers or trucks can cause accidents, even at apparently reasonable speeds

Noders are advised to check out the following nodes for further information; center of gravity, weight transfer and traction circle.

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