Kenneth Russell, Professor Emeritus, MIT --
Design News,
June 4, 2007
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The 1987 Olds 88 rammed into the back of a tractor trailer stopped at a traffic light. The driver reported seeing a long line of traffic behind him, then being struck from behind with such force he was thrown into the steering wheel, causing whiplash. He, of course, sued. I was retained by the defense to determine whether or not the force of the collision was likely to have caused the claimed injury.
The Scene of the Crime
I visited the accident scene and examined both the trailer and the car (now repaired) involved in the accident. I also saw a copy of a picture of the car taken after the accident. The front end was pretty well banged up, but the damage did not extend back to the radiator and fan.
The Investigation
The driver of the Olds claimed to have been stopped in “drive” with a foot on the brake when the car lurched forward, with the engine revving wildly, striking the trailer. I believe the driver hit the accelerator rather than the brake, causing the collision.
The Olds had struck the trailer in the “sissy bar,” which is slang for the ICC bar, a device meant to keep vehicles from running under the trailer. The claim was the collision bent the sissy bar and the car's bumper had driven the right rear tire mud flap into the rear tire. The collision was alleged to have caused the whiplash. The sissy bar was indeed bent and the mud flap damaged.
The whiplash allegation is nonsense on several counts. The figure shows a side view of the trailer rear and car front, roughly to scale. The figure, above right, clearly shows that if the car bumper had driven the mud flap into the tire, the sissy bar would have shoved the radiator back into the engine fan. Neither the radiator nor the fan was damaged.
The sissy bar misses the car bumper by more than 8 inches, so the kinetic energy of the car was transmitted only by the flimsy sheet metal and plastic of the grill and environs. The car bumper contacted neither the sissy bar or the tire, so there was thus no frame-to frame contact nor any serious shock transmitted. The sissy bar was most likely bent by drivers backing the trailer into the loading dock. I took note of similar sissy bars, every one of which was bent the same way as the subject bar.
In addition, the plaintiff's claim violates Newton's Laws of Motion. A heavy blow to the rear of the trailer would have tended to press the driver back in his seat, not throw him into the steering wheel, as claimed.
I explained in court how the car bumper could not have reached the trailer tire to give a serious shock and the only contact was with flimsy body materials. I also invoked Sir Isaac Newton and left the court feeling pretty good, unlike some earlier cases. (See The Case of the Ruptured Rim.) But, we lost the case.
The Smoking Gun
My client's paralegal told me of the plaintiff's attorney describing the towing of huge airplanes just by hooks attached to the fuselage. If a huge airplane could be towed by flimsy metal, the accident could have resulted in a serious shock. The jury bought it and, having concluded I was wrong on this point, disregarded my entire testimony. In fact, the towing force is transmitted to structural members inside the fuselage. The defense attorney for some reason did not make this point.
The Menace behind Mismatched Bumpers
The ICC bar, or “sissy bar” is designed to keep other vehicles from running under the backs of tractor-trailers, thereby preventing possibly dire results. However, the sissy bars are so high that they miss car bumpers by a wide margin. A car-trailer collision then causes serious front-end damage to the car and may well result in bodily injury.
The mismatch is not limited to tractor-trailers. The bumpers on some SUVs and pickups totally pass over automobile bumpers. A friend of mine got the front end of her sedan smashed by the rear of her sister’s big SUV in this way.
I figured that the idea behind bumpers was to prevent or minimize damage to person and property during collisions. Or, as Robert Frost put it, “Good fences make good neighbors.” The federal government issued standards some years ago requiring that car bumpers match in height and are able withstand impacts of a couple of miles an hour with limited damage. I figured the same would be so for SUVs and pickups.
Well, I was really wrong in that idea. A short search of the Web showed that there are NO regulations for bumpers on such vehicles. (See www.consumeraffairs.com/news04/bumper_tests.html.) The manufacturer is free to put the bumper at any height, to make it out of any material whatever, or to have no bumper at all.
As a result these behemoths may be harder on one another than on passenger cars. The sporty-looking protruding spare tires on the backs of some SUVs are a particular menace. A tail ender on one of these transmits the force to the tailgate of the struck SUV and the grille of the striking SUV with expensive consequences.
I just did a quick survey of SUV and pickup bumper heights in a local parking garage and found neither rhyme nor reason. Some bumpers match up well with cars, and others, particularly on big new pickups sit way up in the air.
I am generally opposed to the "nanny state," but here make an exception. It seems a proper function of government to make sure that bumpers match.
Author Information
Ken Russell (kenruss@mit.edu) is Professor Emeritus of Metallurgy and Nuclear Engineering at MIT. He specializes in physical metallurgy, forensic metallurgy and failure analysis. Cases presented here are drawn from his actual forensic files.
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