Thought I'd share some more info I got from the president of Superbumper.com concerning the energy forces when the hitch is hit in a rear-end collision.
Sue:
Sorry to hear about the accident and thanks for the tip on the web site…it is getting a little too cluttered.
Anyway, getting back to your question…Receiver hitches are bolted directly to the frame…unless it is a unibody construction vehicle. I believe your vehicle is built on a frame.
When you get hit from the rear, a lot of the crash energy is transferred directly to the frame via the receiver hitch (if the vehicle has one) and ultimately to the occupants. I say “a lot” because some of this crash energy may be absorbed by the bumper, bumper cover, end gate, etc. – unless it’s a direct hit on the hitch. Then all the force is transferred by the receiver hitch.
We have a video of the Chevrolet Trailblazer from 2002 that was crashed into a pole at 5 mph by the Insurance Institute For Highway Safety. The result was NO damage because the receiver hitch hit the pole directly. All of the force from this collision would have been transferred directly to the occupants because none of the vehicle’s other parts absorbed any of the crash energy. This is a typical example. If your receiver hitch protrudes out beyond the bumper (like most pickup trucks for instance) chances are all the impact energy will end up going through the frame to the occupants because there’s nothing being absorbed…by your vehicle. There is some energy being absorbed in a two vehicle collision but most of it is being done by the bullet vehicle as the front end crushes. A receiver hitch completely changes the crash dynamics. That’s why the IIHS removes all receiver hitches now before they do any rear bumper crash testing.
Most auto manufacturers do not want any “global buckling of the frame” at speeds less than 35 mph so this leaves out the so called “crumple zones” unless you get above that speed. And of course, a lot depends on the mass of the vehicle that hits you. It might only take 15 mph if you get hit by a Hummer.
Force goes strictly but the Kinetic Energy formula: ½ MV squared (where the velocity is squared). If you double the mass you’ll double the force. But if you double the speed you quadruple the force. In other words a 4 mph impact will generate 4 times more force than a 2 mph impact on the same vehicle. So if you hit that pole at 2 mph and then decide to try 4 mph you’re going to experience 4 times the amount of force.
As an occupant dealing with a rear end collision you’re going to experience an instant acceleration or “change in Delta V” of 2.5 times that of the bullet vehicle. So if you’re hit at just 5 mph from the rear, you will accelerate to 12 – 13 mph – certainly enough to hurt people. That’s why it is really hard for the occupants of the target vehicle to tell how hard they were hit and most “crash experts” rarely take into consideration the receiver hitch because they hide damage.
I would say if the hitch is not bent downward then you were probably hit at a speed less than 10 mph. If the hitch is bent downward then it’s definitely above 10 mph. If the frame has been tweeked so that the doors have a hard time opening then you’re looking at 20 mph or more. Just a general observation. But if you’ve been hit above 10 mph the least of your worries should be the vehicle.
Insurance companies will argue that unless your vehicle is damaged there’s no way you could have been hurt. Wrong. The video mentioned shows how a vehicle can absorb a huge amount of force and not show it.
Anyway, there are a lot of variables when dealing with rear end collisions but the force has to go somewhere and it goes directly to the occupants if there’s a direct hit on the hitch.
Jeff
P.S. 48% of the people that hit you in the rear are not slowing down and will hit you directly. Of the other 52%, about ½ will brake - and then hit you. Or they’ll brake and swerve to try to avoid the collision…but they still hit you. You just happened to get one that “woke up” at the last second and clipped your fender. But about 75% of the time they hit you directly. You just got one of those 25%.
Feel free to put this on your site…video too.
Super VXer
Originally Posted by Grif
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