OT: Physics and Towing ?? - Off-Road Forums & Discussion Groups
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post #1 of (permalink) Old 10-17-2001, 08:36 AM
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OT: Physics and Towing ??

Here's a question to ponder.
If you take a vehicle...for ease, say it's a jeep, and you hook up a trailer to the rear bumper of the jeep...and the trailer has an insanely high hitch weight, the rear of the jeep will be pushed down, and the front will be lifted, right? It pivots on the rear axle.

Likewise, if you run a gooseneck hitch, and place the gooseneck right in the middle of the jeep between the driver and passenger, the jeep gets compressed evenly on all 4 corners (roughly).

So here's the question. Imagine we take a piece of the strongest metal in the world, unobtanium, and make a hitch out of it. THe hitch starts with a plate bolted to the front bumper of the jeep, extends forward from the jeep a foot, and then bends down in a U, goes backwards under the jeep, and extends out a foot behind the jeep in the rear. The only place it contacts the jeep is the front bumper attachment. This unobtanium is incredibly strong and will not bend or deflect. If we put a lot of tongue weight on the hitch now, what happens? I have two conflicting theories:
1. The jeep reacts like it would if the hitch were attached directly to the rear of the jeep and the nose raises.
2. The jeep reacts like it's got a heavy weight on the front end, and the rear raises.

Now logically, it seems like #2 can't happen...for the rear to raise (the hitch is attached to the frame in the front, remember), the hitch would raise as well. But if #1 happens, how? How does the frame/jeep "know" the weight is on the rear, if it is being applied to the front? And if #1 happens, it will raise the front of the jeep, which is where the downward force from the hitch is being applied...which doesn't seem right either.

If you can't imagine the hitch as I described it, imagine a J laid over sideways...the long end goes under the jeep, out the rear. The hooked end bends around 180 degrees and bolts to the front bumper.

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Something like that.

Any idears?



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post #2 of (permalink) Old 10-17-2001, 09:42 AM
 
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Re: OT: Physics and Towing ??

Intersting. I have taken classes on mechanics of materials and such, so this is what I think would happen:
While your thinking for #1 is logical and is actually correct. but not the way you describe it.
what is acutally happening is that the torque applied to the front bumper from having basicly a huge breaker bar attached to it, would either twist off the mounts, or if held ridgid, would turn the frame down, there fore pushing the rear of the frame towards the axle. its pivioting on the front bumper.
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post #3 of (permalink) Old 10-17-2001, 09:58 AM
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Re: OT: Physics and Towing ??

I'm no wiz kid or anything.. I think that the front would still come off the ground. What does it matter if the ball is on bumper or just under it with your under carriage hitch mounted to the the front bumper. All of the weight is still behind the rear axle..... Heck if I know though

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post #4 of (permalink) Old 10-17-2001, 10:05 AM
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Re: OT: Physics and Towing ??

It probably is something like the leverage argument made. The thing that gets me is that the downward force is being applied at the front bumper...which makes it counterintuitive that the front bumper would rise. But you're probably right. Any other theories?


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post #5 of (permalink) Old 10-17-2001, 10:39 AM
 
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Re: OT: Physics and Towing ??

What would really happen is the back is lowered more then the front. the front will still have some weight pushing down on it, but most will be transmitted to the rear through the frame.
Ofcourse, if this metal is so strong, how could we bend it into a J shape and put bolts through it? :-)
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post #6 of (permalink) Old 10-17-2001, 11:00 AM
 
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Re: OT: Physics and Towing ??

Doesn't balance come into play? The ball is still pivot/hinge point. So even though the weight would be expected to transfer to the front, the hinge is still at the rear. Unless the counterbalance of the Jeep's weight is more than the weight on the trailer tongue, the front of the Jeep would still lift.

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post #7 of (permalink) Old 10-17-2001, 11:01 AM
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Re: OT: Physics and Towing ??

Yeah yeah yeah...we just have H8 grab the end of it and bend it over his knee.

I liked the comment about the front of the jeep falling off first...


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post #8 of (permalink) Old 10-17-2001, 11:05 AM
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Re: OT: Physics and Towing ??

I agree with #1. When the load is applied to the rear of the hitch it would create a moment (pivot) at the front bumper. That means that all the weight pushing down on the other end of the hitch would push the front end of the hitch up. With the hitch now pushing up on the front of the Jeep, a moment is created at the rear wheels. Therefore the front of the Jeep would rise and the back end lower. If you are having trouble picturing this, use a paper clip to experiment with.

post #9 of (permalink) Old 10-17-2001, 11:59 AM
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Re: OT: Physics and Towing ??

Come on guys!! Do a free-body-diagram (FBD) of the Jeep. [img]/wwwthreads_images/icons/crazy.gif[/img]

We haven't moved the load, so the resposne of the Jeep doesn't change. The super-stiff-unobtainium hitch part becomes part of the Jeep, making really no difference at all for the FBD.

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post #10 of (permalink) Old 10-17-2001, 02:27 PM
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Re: OT: Physics and Towing ??

Surveyboy is the closest to having the right answer. Unobtainium is soooo strong it is unholey. Since the devil is in the details, you engineers can't build it.

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