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Here is an article on the physics of SUV rollovers, written by a physicist: The Physics of Sports Utility Vehicle Rollover Accidents
A must-read for all SUV owners, even if possibly biased against SUVs. Here is a quote:
| In our example, at just 20 mph, our vehicle would roll over when trying to turn in our 25-foot radius circle! |
Actual SUVs are much heavier than we have used. However, the heavier weight increases the centripetal force proportionally, which increases the force that acts to roll the vehicle, EXACTLY in the same proportion that the necessary lifting force is increased for the heavier vehicle. The actual overall weight of the vehicle does not alter these results! It is not actually necessary to know the weight of the vehicle, and heavier vehicles are no more stable than identical lighter ones. The track-width, the center-of-gravity height, the turning radius, and the vehicle speed are the ONLY variables!
Now, a rollover accident at 20 mph might not be that dangerous. The problem arises when a higher speed tall vehicle makes a sudden steering adjustment, such as to avoid a deer or some object in the roadway. At 60 mph, the sideways (centripetal) force is NINE TIMES as great, because it depends, as explained above, on the SQUARE of the speed and we have tripled the speed. This means that, even if the radius of curvature of a sudden turn is NINE TIMES AS GREAT (225 foot radius, half a block), the vehicle WILL suddenly roll over! At 80 mph, it is 16 times as dangerous, and a 400-foot-radius (nearly one full city block) turn will result in the vehicle rolling over. In other words, an SUV at 80 mph, simply changing lanes fairly quickly on an Interstate highway, can result in a sudden and unexpected rollover! And, at such a speed, whether the SUV hits any other vehicles or not, very bad results are likely for the occupants.
Long ago, Ralph Nader wrote a book "Unsafe at any Speed" about the poorly designed Corvair cars. It is hard to see how that is not even more true today about SUVs! Since some States now have Interstate highway speed limits of 75 mph, SUVs can commonly be traveling at 80 mph. The families inside them have NO idea of what a dangerous situation they are in! The worst part of all this is that all of the SUV manufacturers have always known this simple Physics! In order to sell the extremely popular SUV vehicles, they have chosen to ignore the amazing danger involved in driving them!
The really sad part is that countless thousands of people are going to die while the SUV manufacturers continue to deny all culpability. At least, with the Corvair, Mr. Nader got public outrage started after only a few hundred people died from their failings. This SUV situation will cause the deaths of far more people, and already has! But they are so popular, and the buyers and drivers of them BELIEVE they are safer inside them, that SUVs will continue to be extremely popular, until gasoline prices get so high that people start buying smaller vehicles.
(I realize that the above paragraphs are essentially an Editorial Opinion, and not usually heard from a Physicist, but it seems outrageous that so many people are dying just because giant corporations insist on selling popular products, even though they are extremely dangerous. If that is not a deja vu regarding cigarettes, I don't know what is!)
And here is one more quote regarding the effect of wider track (defined as the distance between "the points where the tyres contact the road":
The PBS FrontLine program included some "manufacturer's experts" claiming that widening the track by two inches would do miracles. That was clearly a lie! It is easy to do this math again to show that it would only make an incremental difference. In our example, widening the track by 2" makes our horizontal 30" into 31", while leaving the center of gravity height of 30" the same. It is easy to check that this geometrical change makes our 45 degree angle now become 44.06 degrees. The tangent of that angle is 0.9677. When put in our problem, we would now have 3400 * (0.9677) or 3290 pounds, STILL making the vehicle roll over! The dangerous lift is only minimally reduced, from 3400 to 3290 pounds! A two inch wider track would have a VERY slight benefit as to safety from rollovers. A benefit, true, but so small as to not provide significant extra safety for passengers. Actually, where our original vehicle could have made our circle at up to 19.3 mph while being on the verge of rolling over, the one with the two inch wider track would be able to do it at 19.6 mph instead, less than half a mph improvement!
So as per this quote, wider tyres would only marginally reduce the tendency of SUVs to roll over.
Also read the section on "Low Tire Pressures and Soft Suspensions". This will remove some prevalent misconceptions.
Last edited by rks : 27th June 2007 at 17:25.