tluxon
Kirkland, WA
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Stressor wrote:I totally agree with you Mr. Whittaker. The linkage only works one way, and Mr. Gratz continues to want it to work 2 ways, and insists that it does, despite two great big steel bars. Absolutely not true. You're sounding like one of the early day scientists who insisted that the world was flat or that the Sun orbited about the Earth. It would be helpful for yourself and others to put together your own model built with popsicle sticks and screws (make sure holes in "linkage" are clearance holes so pivoting resistance is negligible) or something similar before making such "authoritative" statements.
Stressor wrote:All of the talk about the VPP is simply nice to know, and it only applies if the tow vehicle is turning. This is absolutely not true. All forces - turning, lateral, and otherwise - must translate through the side links one way or another, making the projections of these side links (VPP) pertinent at ALL times.
Stressor wrote:I spent a couple of hours playing with mine over the long weekend, and after getting all greasy, and suffering back strain, it looks like a simple explanation will suffice. Your back strain was for naught if you didn't remove the effect of all the tires on the ground. You'd have to do that to isolate the behavior of the hitch itself. Again, it would be so much simpler to just build yourself a model of the linkage.
Stressor wrote:It is not possible to describe how it works unless it is described dynamically, in short, do not remove any contact with the ground, but take the various loads and friction into account.
It takes a little mental calculus to do it, but heck, even a psychologist can do that. Engineering school must've got the best of you when they got to free body diagrams. You must be able to isolate elements of a system in order to truly understand what they are doing. That's fundamental. Just go make a stick model of the linkage. Then we'll talk some more.
Tim
Tim -
wife Beverly & 2 boys who love camping
2002 K2500 Suburban 8.1L 4.10 Prodigy
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Replaced 2000 Sunnybrook 26FK on 8/6/04
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Stressor
Whitefish Bay, Wisconsin
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Tkluxon,
How it works is a function of all of its parts working together. That is what a free body diagram seeks to explain. Adding in vectors that are canceling each other out, or vectors that exist only under certain conditions, does not increase my understanding of what is actually going on there, and that is what we wanted to investigate in this thread.
I will also submit that playing with the real thing is a superior method of investigation than playing with popsicle sticks.
A good explanation has to make sense to the person that it is presented to, has to explain that which is easily and directly observable, and it has to apply to what is being explained under real world conditions.
I do not think that we have arrived at that point yet, although your previous explanation and diagram certainly made great progress in that direction.
I made a living as an electrical engineer in the area of computer controlled processing equipment and actuators. An understanding of mechanical principles was certainly part of that activity, and my training. I have also written over a hundred technical manuals on items ranging from large walking draglines to EKG machines, and put together several training programs on those and other devices and machines.
While the math is admittedly beyond me, baffling me with BS is impossible. ![[emoticon]](http://www.coastresorts.com/sharedcontent/cfb/images/biggrin.gif "biggrin")
Milton Findley (and Kerene)
A small piece of my mind...
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tluxon
Kirkland, WA
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Stressor wrote:Tkluxon,
How it works is a function of all of its parts working together. That is what a free body diagram seeks to explain. Actually, free body diagrams are typically used to isolate components of a system, to make it easier to understand what's happening in smaller, simpler, "bite-size" pieces.
Stressor wrote:Adding in vectors that are canceling each other out, or vectors that exist only under certain conditions, does not increase my understanding of what is actually going on there, and that is what we wanted to investigate in this thread. ???? I'm not sure what you're trying to say here. I haven't spoken about vectors at all - just the usefulness of isolating the geometric behavior of the Hensley.
Stressor wrote:I will also submit that playing with the real thing is a superior method of investigation than playing with popsicle sticks.
Interesting. How exactly did you observe the contribution of the hitch without isolating it? If you were able to isolate it, please explain how you did it. Otherwise, apart from free body analysis how can you possibly know where the contribution of the hitch ends and that of the rest of the system (primarily forces through the tires) begins? Using your whole system method, how would you propose to understand the difference between a Hensley and a conventional hitch other than through subjective observation?
I'm not sure, but I really do think the reason for our lack of agreement is mostly because you're having difficulty separating the 4-bar linkage concept from your experience of how effective the Hensley solution is as a whole. Trust me - if you make a stick model you WILL gain a better understanding of the contribution of the hitch. Without it, I'm not confident that a discussion between us will make the headway we should get. If you want to PM me with a mailing address, I'll be happy to send a stick model your way. Otherwise, I'll make a short video clip and post a link to it where all can benefit. Either way, I think you'll wonder what the big deal is because it's so unbelievably simple.
FWIW, there's no "k" anywhere in my actual name or my username.
Tim
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Stressor
Whitefish Bay, Wisconsin
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Sorry about the K Tim, I am getting nearsighted in my old age. I am glad you think you have figured it out.
A good explanation has to make sense to the person that it is presented to, has to explain that which is easily and directly observable, and it has to apply to what is being explained under real world conditions.
I believe that I have a perfect understanding of the way that the linkage works, derived through manipulation of it from both the travel trailer end and the tow vehicle end, and through observation of the hitch in use.
Isolating the hitch to observe its operation is easily accomplished by either suspending it from the travel trailer as it is normally, or by suspending it from the hitch bar for manipulation of the linkage from the travel trailer end.
I have the same reservations about your understanding as you do about mine.
Here is what I do know:
The travel trailer end of the hitch does not move relative to the trailer tongue.
The Tow vehicle end of the hitch does not move relative to the centerline of the tow vehicle.
(If you think that either of these things occurs, drop the popsicle sticks, and go play with the hitch.)
All angular change between the two vehicles is through the two links, with the forward two pivots being slightly closer together than the rear two pivots.
This geometry allows the tow vehicle to change direction, collapsing one link inward to do so, and therefore the trailer turns with it.
The same geometry does not allow the travel trailer to turn the tow vehicle. The link will not collapse from the travel trailer end of the hitch. Any forces transferred through the linkage are therefore applied as a sideways force to the tow vehicle at the location of the VPP.
The tow vehicle resists this force due to a number of factors including traction, moment, and plain old dead weight to name but a few. So does the travel trailer, and the good news is, since the trailer continues to track directly behind the tow vehicle, there is no sway.
This is a perfectly good mechanical explanation, and it has the benefit of being easily observable, easily demonstrated, and it explains the behavior of the travel trailer in combination with the tow vehicle.
What did I miss?
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2lMan
Brewer, ME
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A virtual pivot point is just that... virtual. It does not move at this virtual pivot point, because there is no place for it to pivot there. It is just a place that is thought of as where the pivot point would be... if it were mechanically possible for it to pivot.
So, even though (theoretically) the trailer is or is not (depending on your view here... I offered my explanation, and am not going to get involved with this particular point of the debate) able to act upon the tow vehicle, do you think that it would make sense to call the connection "virtually" solid. After all, we refer to virtual pivot points, which are not actual pivot points. Is the connection "virtually" solid, even though it is capable of movement?
OK, I'll let you engineering students go at it, and I will just try to understand it with my "high school graduate" mind.
2003 GMC Yukon XL 2500 8.1L 4.10 Axle - 2002 Sprinter Quad Bunkhouse
Home is where you bring it!
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Stressor
Whitefish Bay, Wisconsin
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I am with you Mac.
Virtually solid is what it acts like from the travel trailer side, and if it was a Pull-rite, it would be truly solid, and work exactly the same way.
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willald
NC
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Wow, this topic has taken a really interesting turn. Stressor and Tluxon both present very convincing arguments, I'm not sure which one to side with, LOL.
Think I'll just sit back and let the discussion continue. Really is great to see this discussion has gone for so long, and so much great information has been exchanged. And, for once, we have a good, informative, interesting thread talking about the Hensley, without anybody getting mad or getting the thread locked. ![[emoticon]](http://www.coastresorts.com/sharedcontent/cfb/images/smile.gif "smile")
I think once we determine which analysis (Stressor or Tluxon's) is correct, this thread should be ready to get 'stickied' to the top, for all to see that truly want to learn and understand how the Hensley works. Perhaps, the truth is somewhere between the two....
Keep up the great discussion.
Will and Cheryl
2021 Newmar Baystar 3014 on F53 (7.3 V8) Chassis ("Brook")
2018 Jeep Wrangler JK ("Wilbur")
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vandave
jamestown, ohio
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I think I'm back in the 50's listening to "Mr. Wizard". Almost too much for my little brain but I am following it.
Vanda and Dave Ludwig (Vandave) Spoiled King Charles Spaniel, Ultra spoiled Cock-a-Poo
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tluxon
Kirkland, WA
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I'm not so sure we're as far off as it might seem. Remember I'm looking at this in the most simple terms to begin with as a building block to comprehending the entire system. I think it's important to begin by seeing how the angles at the 4 pivot points change with respect to each other.
See if you can get this short video clip isolalating the 4-bar linkage. It's a little under 900KB in size.
http://home.comcast.net/~tbluxon/4-bar_linkage_model.WMV
This model is using the dimensions Ron provided in his stick model directions several pages back in the thread.
If you'd like to see any other interactions shown in addition to this I can try to show that as well. For example, I can also show how the linkage behaves when swung as a pendulum from either end.
Tim
* This post was
edited 06/02/05 10:54pm by tluxon *
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tluxon
Kirkland, WA
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Stressor wrote:...This geometry allows the tow vehicle to change direction, collapsing one link inward to do so, and therefore the trailer turns with it.
The same geometry does not allow the travel trailer to turn the tow vehicle. The link will not collapse from the travel trailer end of the hitch. Any forces transferred through the linkage are therefore applied as a sideways force to the tow vehicle at the location of the VPP... Let's see if I can explain the error here in a little clearer manner. Pretend for a moment that you could reverse the hitch by swapping places between the tow vehicle and the trailer. If the hitch could be affixed to the TV and TT in a reverse manner, I assure you that the tow vehicle would still be able to pivot the linkage and turn the trailer. It would NOT act like a straight truck. Unfortunately, I'm not sure you can test this out with the actual hitch and that's one of the reasons I put up the video of the linkage being pivoted from both ends.
BTW, I appreciate how clearly and organized you made your post. I wish I could explain myself as clearly.
Tim
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