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Old 06-28-2013, 11:20 AM   #15
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Originally Posted by BFlinn181 View Post
Attachment 41420
In the diagram, the vertical movement of the back of the RV will change the angle A, but the net effect on the distance between the RV and toad will be very small. I'm not going to go to the effort to show all the math. I wonder it the 'tugging' you are feeling is actually the rattling of the tow bar in the receiver. Perhaps a receiver immobilizer might help.

Neat drawing. Thanks for that...

No, the tugging that I am feeling is not the slop in the RV to Towed joints. In fact I am not using the manufacturers punched holes in the hitch receiver or the tow bar tongue. I line-drilled my own DUAL .625 (5/8") holes to eliminate slop.

In fact, in my original post, I proposed introducing a controlled slop into the tow system. If one were to tow with a bungee cord, for example, one would never feel any slop or jerking from the tow linkage.

I agree with you that the RV to Towed straight line distance doesn't change much, but as I told another poster, "if you have a 20,000 pound object trying to move a 2,000 pound object even 1/4 of an inch in a few milliseconds. You WILL feel the amount of energy/work it takes to do that".

I guess that since no one seems to have experimented with a energy absorbing tow bar, I will just go ahead and built one. I already made a design.

Oh yea... I'll also slow down as suggested by another poster.



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Old 06-29-2013, 11:10 AM   #16
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Think of a right triangle... If you lengthen one of the legs, the other leg will become shorter.

I believe your analogy reasoning is flawed. Think of the towbar attachment point at the MH as a center of a circle. The attachment point on the towed as the radius of that circle. No matter where you go on the circle, the radius remains the same.
Not saying you don't have issues with the jerking and tugging, but I cannot believe if the towbar is working correctly that it is getting longer or shorter. It is locking open I presume?

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Old 06-29-2013, 10:53 PM   #17
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Hi Doc,

Yes, the tow bar is operating correctly, IS locking open, and is NOT getting longer or shorter. The fact that it is not getting longer or shorter IS precisely the reason for the jerking and tugging.

Let’s look at it this way. Lets say that you have a 25,000 pound Class A RV with leveling jacks and a 2,500 pound towed car. The tow bar between the RV and the car is exactly level.

Now with the RV and the towed car of absolutely flat ground, drop a plumb bob from the hitch receiver locking pin to very near the ground. Also drop a plumb bob from one of the baseplate-to-towbar attaching pins to very near the ground.

OK… Measure the distance between the two plumb bobs. Let’s just say for this example that it measures exactly 72 inches between the two plumb bobs.

IF we use your example and presume that the hitch receiver pin is a point, and the tow bar is a straight line and is the radius of a circle, any way you move the tow bar, up-down, right-left, I agree that the length of the bar will be the exact same 72 inches.

Now, to explain what I am saying, let’s use the leveling jacks on the RV to raise the whole RV UP 12 inches. Remember that the towed vehicle does NOT go up 12 inches and is still flat on the ground.

Because the tow bar is inflexible, if you now measure the distance between the two plumb bobs (and the 2 vehicles), it will measure 70.992 inches.

What has happened is that while the tow bar is still a 72 inch straight line, it is now also the hyopotenuse of a right triangle with an elevation of 12 inches.

The Pythagorean Theorem (A squared + B squared = C squared) dictates that because we raised one end of the straight line between the RV and the car, the BASE of the resulting right triangle MUST shorten. The distance between the RV and the car is now 1.008 inches shorter that it was when the RV was on the ground.

In normal driving, the rear of the RV bounces up and down. Part of the reason that we rarely feel any “tugging” (= the towed car trying to pull the RV backwards) is because the bounces are fairly small and the “slop” in all of the joints of the tow bar absorb the small front-back movement. But if the RV bounces a foot in a few milliseconds, as with going through a dip or over a RR track, the towed car (which is 1/10th of the weight (mass) of the RV), will be suddenly pulled forward, then suddenly pushed backward. The towed car will also try to pull the RV backwards, then push it forwards.

I probably feel the tugging more because I have engineered nearly all of the slop out of my towing link between the RV and the car.

In my original post, I theorized building a “spring or shock absorber” into the tow bar to absorb these larger than normal tuggs.

Have I cleared up what I was trying to say?


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