Ron Gratz
full time RVer
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MacMan wrote:Since the connection is "virtually" solid, couldn't it be considered that the "virtual pivot point" is non-existant, due to the excessive force that would be required to cause the Hensley to pivot at its hitch point when being reacted upon by the TT?
The 4-bar linkage connection is not "virtually" solid. It is not rigid in any sense. You would have to add a diagonal link to transform it into two triangles to make it rigid. The whole point of using a 4-bar linkage is to allow the orange portion to translate and rotate relative to the black portion or vice versa.
I think the most difficult thing to grasp in this discussion is the fact that, while the orange portion does translate and rotate, the effective pivot point is some 47" forward when the TT and TV are aligned straight ahead. The linkage does not prevent the orange portion from moving. The linkage does require that, in order for the orange portion to move to the side, it must simultaneously rotate.
A PullRite hitch produces the same result. In order for the hitch head to move to the side, the PR's draw bar must pivot about its pin. In fact, everything that is rigidly connected to the PR's draw bar must simultaneously translate and rotate if it is to move at all. The rotation (yaw angle) is the same at all points. The amount of translation is proportional to the distance from the pivot point.
It is not the HA's linkage which prevents the TT from swinging behind the TT. It is the lateral force on the TT's tires. Take the tire side force away and the TT can swing freely.
A main point to remember is that when the TT and TV are aligned straight ahead (or within several degrees) the pivot point is not at the hitch head.
Ron
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2lMan
Brewer, ME
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Joined: 09/18/2002
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Quote:The linkage does require that, in order for the orange portion to move to the side, it must simultaneously rotate.
This simultaneous rotation is sort of what I was trying to draw a parallel with about the Motorcycle swing arm. The swing arm rotates in an arc, in addition to going up. It would seem that the rotation of the hitch head could be what makes it very difficult for the TT to react on the TV.
Or, maybe not...
2003 GMC Yukon XL 2500 8.1L 4.10 Axle - 2002 Sprinter Quad Bunkhouse
Home is where you bring it!
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Ron Gratz
full time RVer
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Joined: 12/27/2003
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MacMan,
The TT can induce forces on the TV regardless of the rotation of the hitch head.
The HA is most effective at reducing the magnitudes of the force and "steering moment" when the TT and TV are aligned straight ahead since the VPP is at its maximum forward projection.
When going around a highway curve or changing lanes, the orange portion will rotate relative to the front part of the hitch. The hitch does not prevent the TT from swinging. The hitch does continue to reduce the force and moments - just not as much as when travelling straight ahead.
Ron
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sfdon
Novato CA 94947
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What an outstanding topic. I have been out of touch for two months. It is a bit much to absorb the entire thread in on sitting.
In a quick scan, I did not see a discussion that addresses a question of mine.
There have been comments in the forum regarding the weld failures of the GM receiver. I orginally assumed that the likelihood of failure would be based on tongue weight or reaction to severe bouncing of the TT. Now I wonder if a HA creates a larger moment on the weldment compared to a conventional ball hitch. The latter would have a moment arm of about 12".
Deviating from the thread, I offer:
This forum has successfully tackled the complex subjects of weight distribution hitches and now the HA system. What is next? Whatever it is, I look forward to it.
Last comment: I'm surprised that Mr. Hensley hasn't contributed to this threat.
Last question: I have not read the various Hensley patents, but is there any evidence that the newer patents have led to design changes?
Don and Gail
2005 Forest River Surveyor SV-230
2003 Yukon with Autoride, Honda EU1000i
Hensley Arrow
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Ron Gratz
full time RVer
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sfdon wrote:...There have been comments in the forum regarding the weld failures of the GM receiver. I orginally assumed that the likelihood of failure would be based on tongue weight or reaction to severe bouncing of the TT. Now I wonder if a HA creates a larger moment on the weldment compared to a conventional ball hitch. The latter would have a moment arm of about 12".
Don, welcome back to the ORF. Yes, the HA will create a significantly larger yaw moment on the receiver than will a conventional hitch. You can get an ideas of how large the moment is by going back to Tim's or my force calculations. If you multiply the amount of lateral force acting at the VPP times a distance of about 47"-12"= 35" the result will approximate the amount of moment which is transferred via the HA's linkage.
Quote:...Last comment: I'm surprised that Mr. Hensley hasn't contributed to this threat.
I hope you meant "thread".
Quote:Last question: I have not read the various Hensley patents, but is there any evidence that the newer patents have led to design changes?
The latest patent deals with a means of achieving weight distribution without having to transfer the load via the HA's 4-bar linkage.
Ron
P.S. We close on our property sale tomorrow and will join the ranks of the "homeless". This likely will be my last post for a while.
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sfdon
Novato CA 94947
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DonRon wrote:I hope you meant "thread".
Yes I meant thread.
Don
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jdwhittaker
York, SC
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Joined: 06/01/2005
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What a great thread!!
I’ve been lurking around this and other similar forums for several years trying to dig up any information that I can find regarding how the HA works. This is the first time I’ve felt compelled to jump in (Long time listener…First time caller). I think that this is the first HA thread that I’ve ever seen on any forum that has had so many people working together to understand this hitch. I was up until 3:30 AM last night following this thread.
To date I have spent numerous hours contemplating how this hitch works, and I have a theory that I’d like to bounce off all of you to see if there might be any validity to it.
Here goes… I think that the virtual pivot point, or projected pivot point is a mistake in the theory of how this hitch works, in my opinion there is no VPP or PPP. In my theory there are 4 pivot points, each of which move together in a way controlled by “linkage design” to allow turning initiated only by the TV not the TT.
Simple to say, a little more difficult to explain…
If you look at the 4 bar linkage diagrams posted several times above (sorry don’t know how to post a diagram), and ignore the theoretical pivot point part of them. Now look at the trapezoid linkage shape in the straight diagram, and compare that to the trapezoids shape in the turning diagram. Notice how the right side linkage collapses to the left when a right turn is initiated by TV (top of the trapezoid rotating CW), try to imagine how the same thing will occur with the left side linkage when the TV initiates a left turn (top of the trapezoid rotating CCW). The explaining gets a little more difficult from here. If you imagine trying to initiate a turn in either direction with the TT, notice that the force created by turning the TT(base of the trapezoid CW or CCW) causes a force that would be pushing directly on the linkages. The trapezoid CANNOT collapse like when turning is initiated by the TV. I make this motion easier for myself to visualize by imagining picking up the TV and turning it with a hinging type motion at the trapezoid, then by doing the same thing with the TT. With the TT the hinging can’t occur. Within this visualization turning motion created by the TT (base of the trapezoid) is eliminated causing the TT to follow behind the TV like a box truck. Not to say that cross winds cannot affect the TV/TT combo, but it will be handled more like a box van or straight truck.
The pictures of the “model” above(the ones with the man) came from the Hensley video, in that video he effortlessly turns the TV part of the model while no turning motion at all can be initiated by the TT part of the model(like a straight truck or box van). The motion that he can and cannot apply to the model is not influenced by the wheel positions or caster types or angles, but by the hitch linkage design.
By the way I just bought a HA off EBay a couple of weeks ago and am really looking forward to using it. First trip is July 4th to the Outer Banks, about an 8 hr drive from here, should be a good test.
Feel free to let me know what you think of my theory, or ask for more info if I wasn’t clear enough (sleep deprived). I’m sure that we can figure this out.
Thanks for your time,
Dave
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tluxon
Kirkland, WA
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Welcome to the discussion, Dave. As far as your "straight truck" theory, you might want to re-read some of the posts by Ron and I. Are you sure you caught this one (quoted below)?
tluxon wrote:I completely agree with Ron's assessments so far.
I'd like to address this impression of "rigidity" in the Hensley hitch. I'm guessing that most Hensley owners at one time or another have seen the marketing video that has the hitch model segment I posted captures from back on this page. The first time I saw that video four or five years ago, I also had the impression that lateral forces acting on the trailer wouldn't create a bend at the hinge, while turning inputs from the tow vehicle would readily bend the same hinge. That same impression I had seems to be held by just about anybody who's ever towed with a Hensley or seen the video. However, whether intentional or not, the test model shown is deceiving in that it misrepresents the actual geometry involved.
To follow what I'm talking about, please take another look at the test model.
![[image]](http://home.comcast.net/~tbluxon/HensleyTestModel_sm.jpg)
Pay particular attention to where the "wheels" are on both the pretend TT and the pretend TV. Do you notice how much closer the TV's rear axle is to the hitch than you'd see on a real TV. Considering the projected VPP of the Hensley being some 47 inches forward of it, the VPP on this model is considerably forward of the rear axle, which requires that the reactionary moment of the TV be exactly opposite of what the most common reality would be. Not only does this make it impossible to bend the hitch when applying lateral force to the TT, but based on the moment the VPP is putting on the TV, would cause the TV to try to bend the hitch the OPPOSITE direction!!!
Now, take a look at where the TT's axle is. This model places the theoretical TT's axle much, much closer to the hitch than would ever happen in reality. This serves to give an extremely short lever to the lateral force applied, which limits the resulting moment about the TT's axle to a scant fraction of what would happen under real conditions.
I hope that by describing these characteristics in the Hensley test model, you can see how it is possible that ALL the lateral forces applied are reacted to by the wheels and NOT by the linkage of the hitch itself.
So, if you follow my reasoning here, you'll agree that there really isn't anything magical about the linkage in the hitch other than how it projects the actual pivot point forward.
I'd like to point out that the 4-bar linkage can only act like a rigid "box" when both vehicles are perfectly aligned and there is no translated lateral force.
Tim -
wife Beverly & 2 boys who love camping
2002 K2500 Suburban 8.1L 4.10 Prodigy
2005 Sunnybrook 30FKS HP Dual Cam
Replaced 2000 Sunnybrook 26FK on 8/6/04
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tluxon
Kirkland, WA
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There seem to be a lot of people who have been so confused by Hensley's test model that they really believe that there is some kind of magic in the 4-bar linkage that allows a bend to be initiated from only one end (the TV end). It just isn't possible for a 4-bar linkage to freely pivot in one direction while being rigid in the other. It doesn't have anything to do with the angles between the points. It has everything to do with the ability for those angles to change.
I'd sure like to hear from the skeptics some plausible explanation for how they think the angle of the linkage can prevent a change in any of the angles at the 4 linked points?
As far as the pivot point, remember that a pull on one of the side links also involves a push on the other. Remove any contact with the ground from the equation and there will be a change in the linkage angle regardless of which end initiates the change.
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Stressor
Whitefish Bay, Wisconsin
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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. All of the talk about the VPP is simply nice to know, and it only applies if the tow vehicle is turning.
For practical purposes, despite all statements to the contrary, if the linkage only works one way, from the tow vehicle to the trailer, the connection that the travel trailer sees is rigid when it is under tow.
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.
If the 4 par linkage is angled such that an attempt to turn it from the tow vehicle end allows it to do so readily, the same angles will act against it turning from the travel trailer end, or as Tkluxon previously posted, greatly resist such turning, by dint of its having to move the tow vehicle to do it.
Only the two links move, and unless the tow vehicle is turning, both cross bars are held parallel.
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.
Milton Findley (and Kerene)
A small piece of my mind...
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