jdwhittaker
York, SC
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I have read through most of the thread again tonight and am of the opinion that we may all be on the same page as far as how the HA works except on one point…the VPP. Maybe it’s my misunderstanding, but I don’t think that the VPP is relevant to the way that the HA prevents sway. To me the VPP, as Tim, and Ron have explained it is just the intersection point of the sides of the trapezoid as the linkage moves through its range of motion. Please don’t take this as a knock, but rather as my misunderstanding of this part of the theory. Feel free to try to explain the VPP, and it’s affect on sway control to me, in terms that I might understand, and I’ll try to explain my thoughts to you in a way that you might understand. While I’m not an engineer, I have been a CAD operator for nearly 15 years doing design and CNC programming, and I was a machinist for more than 10 years before that, so I’m no stranger to mechanics and mechanical principles. Linkages are used to control, restrict, or enhance motion. I think I have a pretty good understanding of how the HA works, but I can’t get the VPP part of it. I too have mounted up my HA and played with it to “see what makes it tick”, and it does move exactly as Tim illustrates in his drawings. I just believe that the design of the linkage that limits the TT’s ability to cause sway, not the VPP.
Please don’t be offended by anything that I’ve said, I’m just trying to help provide a clear understanding of how the HA works for everybody, myself included.
Dave
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jdwhittaker
York, SC
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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...
As Tim said, a very well organized, clear explanation.
tluxon wrote: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.
Tim,
It would seem to me that if the HA was to be reversed as you described the only motion that would be permitted would be that same motion that causes sway(TT initiated). I think any TV initiated motion would break the hitch, receiver, trasiler tongue, or other weak link.
BTW I must have missed the video you posted, can you provide me a link?
Thanks
Dave
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tluxon
Kirkland, WA
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jdwhittaker wrote:As Tim said, a very well organized, clear explanation. Don't read more into it than what I said. I said it was clearly presented. I also said it wasn't completely accurate.
jdwhittaker wrote:Tim,
It would seem to me that if the HA was to be reversed as you described the only motion that would be permitted would be that same motion that causes sway(TT initiated). I think any TV initiated motion would break the hitch, receiver, trasiler tongue, or other weak link. No, nothing would break as a result of the linkage being reversed. A 4-bar linkage by itself has no built-in resistance to rotation as a result of an input imposed on any link (either end). It's very much like flexing a bottomless trapezoidal box.
jdwhittaker wrote:BTW I must have missed the video you posted, can you provide me a link?
Here's another link to the video.
http://home.comcast.net/~tbluxon/4-bar_linkage_model.WMV
Tim
Tim -
wife Beverly & 2 boys who love camping
2002 K2500 Suburban 8.1L 4.10 Prodigy
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Stressor
Whitefish Bay, Wisconsin
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I like that Tim. The Hensley may not be a 4-bar linkage, it may only be a 2-bar linkage, in that only 2 bars can move. Your demonstration allows the upper and lower bars to pivot freely, but on the hitch, this is not allowed by virtue of a lot of steel, and the geometry of the links.
A more appropriate demonstration would occur if you grasped the top bar, (TV), in your left hand, and the bottom bar, (TT), in your right hand, and then, pulling outward with both hands, tried to turn the ends of either one in towards the other. If the geometry is correct, only the bar with the shortest distance between the pivot points will allow a "Turn" to occur.
On the hitch, suspended from the TV, this is simulated by grasping the weight distribution bars or the strut connecting pins, leaning back, and pulling less on one side than the other. On a true 4-bar, that you have demonstrated, this would result in a turn, but on the Hitch, it does not.
And in the interest of helping clarify the VPP concept, it is useful in explaining how the TT tracks behind the TV, and why the trailer backs up the way it does. It is also useful in explaining where the side loads are felt when the trailer attempts to shift its angle relative to the Tow vehicle because it represents one end of a lever.
Reversing the hitch is easier than one would think, just put the TV in reverse and watch what happens to the trailer as the steering wheels are turned and the forces are reversed.
* This post was
edited 06/03/05 07:51am by Stressor *
Milton Findley (and Kerene)
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tluxon
Kirkland, WA
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Stressor wrote:I like that Tim. The Hensley may not be a 4-bar linkage, it may only be a 2-bar linkage, in that only 2 bars can move. Your demonstration allows the upper and lower bars to pivot freely, but on the hitch, this is not allowed by virtue of a lot of steel, and the geometry of the links. No, all 4 bars are allowed to move freely on this hitch. The only thing that makes you think the top and bottom bars aren't moving is the fact that they're fastened securely to a relatively mammoth TT and TV and both of those vehicles are very well connected to the ground via gravity and the friction in their tires.
Stressor wrote:A more appropriate demonstration would occur if you grasped the top bar, (TV), in your left hand, and the bottom bar, (TT), in your right hand, and then, pulling outward with both hands, tried to turn the ends of either one in towards the other. If the geometry is correct, only the bar with the shortest distance between the pivot points will allow a "Turn" to occur. No, this would not be appropriate for isolating the behavior of the hitch from the contribution of the ancillary vehicles and their intimate connection with the ground. If anything, your method would come closer to demonstrating the behavior of the entire system, which I was hoping to avoid in this elementary step.
Stressor wrote:Reversing the hitch is easier than one would think, just put the TV in reverse and watch what happens to the trailer as the steering wheels are turned and the forces are reversed. Sorry, but that would not be a good way to see what happens when reversing the hitch alone unless you have a very unique trailer that has a long wheelbase and wheels on the far side from the hitch that turn. The fact that most trailers don't have an engine and driveline to pull with is an added difficulty.
The bottom line is that I think you're going to have trouble understanding the contribution of the hitch ALONE unless you are able to totally blank out your experience of observing the behavior of the whole system with at least one vehicle attached. That changes EVERYTHING because of the all the translations that the 4-bar linkage requires.
I can't continue to invest this much time trying to explain a relatively simple concept, so if you're going to insist on disagreeing with me I'll just have to let you have the last word and trust that impartial observers will be able to discern for themselves what they want to understand. I was hoping to eventually drop in a free body diagram focused at the hitch, but without a basic agreement on the nature of the linkage (such as what is free to move and rotate and what is not - WITHOUT anything attached), it would serve little purpose.
Let me ask you this. Would you believe a Hensley engineer if he confirmed that there was nothing built into the linkage that prevents any of the 4 bars from translating? I can try to work from that angle if you think it would make a difference.
Thanks,
Tim
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2lMan
Brewer, ME
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Tim,
I, for one, would LOVE to see a Hensley engineer's comments about this thread. It would let us all know just how close we have come.
Craig
2003 GMC Yukon XL 2500 8.1L 4.10 Axle - 2002 Sprinter Quad Bunkhouse
Home is where you bring it!
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jdwhittaker
York, SC
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Tim,
I hope that you’ll stick around; I can see that you’ve invested a lot of time and energy into this thread, and I appreciate your efforts, as I’m sure that everyone following does. Try not to get too stressed out by those of us who are not engineers challenging your ideas, but also try to keep an open mind. It seems to me that Stressor and I are of very similar opinion as to how the HA works, while you and Ron are of a slightly different opinion. Maybe the truth about how the HA works lies somewhere in between all of our ideas.
Thanks for posting the link to your model video; I think it gives me a much clearer idea of how your theory works. Please allow me to explain where I think that the flaw in your theory lies. In your video model you allow too many degrees of freedom. There are several types of 4 bar linkages, most usually have anchor points to restrict unwanted movement. In your example you allow full unrestricted movement of the linkage by anchoring it only by pivot points at the midpoint of the top and base of the trapezoid. In the HA this is not the case. To demonstrate you’d have to glue or fasten perpendicular members to the model at the midpoint of the base and top of the trapezoid, then move the model again the same way you did before, and watch how wildly the perpendicular members move back and forth. Now for the TT or the TV to be able to move in this fashion they would need to have swiveling casters for wheels. Since the wheels on a TT don’t swivel, they act as an anchor point that is attached to a 20’ (based on TT size) lever arm. Based on that premise if you go to your model again and pin down the TT end of the hitch so that it can not pivot (based on our new anchor point) and apply inward force to either the upper left or upper right corner of the trapezoid in a direction that approximately bisects the corner that you have chosen you see motion that is the same as that of an HA Remember not to pin down the TT end of the model so tight that there cannot be motion as needed through all 4 pivot points in the linkage. The resulting motion is a little bizarre, but the way that the HA moves is a little bizarre.
I realize that I’m a Johnny come lately on this thread, but that is only because I didn’t know it existed, or I definitely would have been here for the duration.
MacMan I too would be interested to see what the Hensley engineers would have to say about our thread.
Dave
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BurbMan
Indianapolis, IN
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Tim and Milt, I think you are both correct. I don't see disagreement here as much as I see a difference in perspective.
Tim is looking at the hitch design by itself, as a simple 4-bar design not attached to anything. He is correct in his analysis of the design and why the design is effective in controlling sway as a trailer hitch.
Milt is looking at the hitch itself, which is not simply a 4-bar design, but a 4-bar design applied to this application by the way it mounts to the trailer with the struts.
A four bar design in itself does not restrict movement, but this particular ones does when it's attached to the trailer via the rigid struts.
Tim is talking 4-bar theory, Milt is talking 4-bar application.
Let's not get bogged down on this point....I suspect that both Tim and Ron have more info to share with us as this discussion unfolds. I, for one, am staying tuned...
Don
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tluxon
Kirkland, WA
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Don, you are mostly correct. I'm sure we're in agreement on all but one of the attributes of the Hensley hitch.
I believe the only point of disagreement between the Hensley owners and the non-Hensley owners (Ron and I) is whether or not the Hensley's 4-bar linkage is a one-way hinge. By that I mean the concept that the TV can initiate an angle between the TV and TT at the hitch but the TT cannot or the TV could not if the hitch was spun around 180°.
I'm certain that the Hensley owners are convinced that an attribute of the trapezoid is that it allows the 4-bar linkage to only be translated from one end (that of the TV). I put up my video clip to illustrate that there's nothing magical about the geometry that prevents the trapezoid from being bent from the long-bar end OR the short-bar end. Unfortunately, the Hensley owners seem to be so loyal to their real-life experience with the hitch that they are having difficulty seeing this linkage in its true simplicity. If they would keep an open mind they would see that the reason it acts like a one way hinge is because of components outside the linkage, not the geometry within it.
Tim
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BurbMan
Indianapolis, IN
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Tim, exactly! It's not the 4-bar linkage itself, but the application of the linkage in conjunction with the other elemnets of the hitch and the way it's mounted to the trailer.
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