tluxon
Kirkland, WA
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I scratched together a quick sketch to help illustrate the difference between a Hensley and a conventional hitch and how they work with a trailer's moment, or twisting force. The source of this moment is not really important. It could be caused by a gust of wind or a quick lane change or a number of other things.
Assumptions:
All the lateral forces on the trailer combine together into a moment of 1000 Ft-Lb about the center of its axles.
The distance from the trailer coupler to the midpoint of the two axles = 20 feet. For reference, the axles of our Sunnybrook 30FKS are centered about 18-1/2 feet from the coupler.
To simplify the illustration, I've assumed that the trailer coupler is the same distance (67 inches chosen since Will had mentioned that he thought the VPP was about 20 inches behind the rear axle of his Excursion) from the rear axle of the tow vehicle with either hitch. I don't believe this is all that accurate, since a number of Hensley owners have said that the hitch adds about a foot or so to the overall length when compared to a conventional hitch.
There are other deficiencies in this sketch that don't perfectly reflect reality, but it should suffice for a general comparison.
![[image]](http://home.comcast.net/~tbluxon/Hensley_Moments_1.jpg)
Note that the trailer's moment converts to a slightly smaller lateral force (42 lb.) at the tow vehicle with the Hensley's VPP than with the conventional pivot point (50 lb.). However, the reacting moment of the TV is 4 TIMES as much with the long lever of a conventional hitch than it is with the much shorter lever created by the Hensley's VPP.
I think this could be taken a lot further, but this is good enough for me for now. Thoughts?
Tim
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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Ron Gratz
full time RVer
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Tim,
I completely agree with your methodology. For the Excursion, I am guessing that the TV axle to ball distance would be closer to 60" for the conventional hitch and 72" for the HA. Using these numbers with your other dimensions, the conventional hitch would have a reactionary moment of 250 ft-lb and the HA's would be 87.1 ft-lb.
While my guesstimates do not give such a large reduction percentage as do yours, I believe these figures easily explain why the HA is able to control the lateral stability as well as it does.
Lateral instability is a threshold exceedence phenomenon. The forces and moments on the TV must exceed a certain level in order for the system to go from being stable to being unstable. By its ability to reduce the magnitude of the TT-imposed forces and moments, the HA raises the level at which the instability will occur.
Contrary to what the Hensley advertising materials would have one believe, the HA does its job without creating a rigid connection between TT and TV and without causing the TT/TV combination to, in the words of the HA marketers, '"think" and "act" like a straight unit, a straight truck if you will.'
Ron
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tluxon
Kirkland, WA
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Ron, I believe your ball locations are closer to actual than my guesses, so your reactionary moment figures should be more accurate.
Will, do you happen to know the actual measurements on your Excursion? If you still have your conventional hitch perhaps you could install it in the receiver and get a pretty close measurement from the rear axle to the conventional ball. Then next time you hook up to your Hensley you could measure the distance from the rear axle to approximately where the trailer coupler is. To complete the package, you could measure the distance from the TT's axles to the trailer coupler. I would love to see good numbers for an actual setup.
Tim
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Stressor
Whitefish Bay, Wisconsin
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Ron Gratz wrote:Stressor wrote:
My point exactly. The leverage of the linkage prevents a change in geometry.
The linkage does not prevent a change in geometry. The linkage does not prevent the TT from swinging behind the TV. It is only the lateral forces on the TT's tires (and inertial effects) which prevent the TT from swinging. If a gust of wind hits the side of the TT, it will swing to a new yaw angle. When the wind goes away, the TT will swing back. The linkage prevents neither swinging to nor swinging fro. However, the effective lengthening of the TT tongue will have some effect on the period and amplitude of the swing.
Ron
Sure it does, from the trailer to the TV. From the TV to the TT, the angles allow it to change.
If a gust of wind hits the travel trailer, it will not yaw at all, unless you loosen the strut bolts a turn or two, other than that, the trailer does not swing either way.
This is easily observed in the rear view mirrors, and a simple experiment demonstrates what happens when the hitch head is allowed to move.
Let us not trivialize or ignore the statements made. The orange head of the Hensley does not move, relative to the trailer tongue, period. Do not misunderstand what the patent document states, and do not assume that a patent document is intended to be a straight forward explanation of how something works. If this were true, engineers would do the interpreting, not patent lawyers.
Only the lower linkage moves, all moment is transferred through the linkage, and if the linkage greatly resists yaw, it effectively prevents the trailer from yawing.
As illustrated.
* This post was
edited 05/23/05 09:32am by Stressor *
Milton Findley (and Kerene)
A small piece of my mind...
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willald
NC
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tluxon wrote:Will, do you happen to know the actual measurements on your Excursion? If you still have your conventional hitch perhaps you could install it in the receiver and get a pretty close measurement from the rear axle to the conventional ball. Then next time you hook up to your Hensley you could measure the distance from the rear axle to approximately where the trailer coupler is. To complete the package, you could measure the distance from the TT's axles to the trailer coupler. I would love to see good numbers for an actual setup.
Tim, I cannot remember the specific measurements for the Excursion, it was some time ago I made the measurements/estimates, and came up with 20" behind the axle. That number could be way off, and I too, would like to see some good numbers from an actual setup (mine).
Sooo, in the interest of keeping this (excellent!) discussion going and getting some good numbers from an actual setup, I will go out tonight, and measure such. I will try and post back here sometime this evening, the actual numbers. Not sure I can hitch up the trailer tonight, but I can definitely put the drawbar in the receiver of the Ex, and measure from the rear axle to the point where the drawbar goes into the Hensley head, then from the Hensley head opening to the trailer ball hitch. That, along with measuring from the trailer wheels to the ball, should give us some real good numbers to work with. Not sure I have a measuring tape long enough to measure all the way from the trailer hitch to the trailer wheels, but I'll find a way.
BTW, excellent illustration, Tim. I think it shows very well, why the Hensley works so well. ![[emoticon]](http://www.coastresorts.com/sharedcontent/cfb/images/smile.gif "smile")
Will and Cheryl
2021 Newmar Baystar 3014 on F53 (7.3 V8) Chassis ("Brook")
2018 Jeep Wrangler JK ("Wilbur")
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tluxon
Kirkland, WA
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Milt, I don't think Ron disagrees with you. He was just talking about the linkage by itself without any tires on the ground providing frictional resistance. That's why he had to suspend it vertically like a pendulum. The reason you don't see any movement when you're towing is because the quite small lateral force that results from a gust of wind is easily handled by friction between the tires and the ground and the tire sidewall stiffness.
Will, do you still have your conventional hitch drawbar and ball? That measurement from the rear axle would also be useful, but if you'd like to just estimate the distance from the rear axle to the receiver's drawbar pin hole, that'd be good to. I could just add the distance of my conventional drawbar to that for a good approximation.
Tim
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Ron Gratz
full time RVer
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Stressor wrote:Ron Gratz wrote:Stressor wrote:
My point exactly. The leverage of the linkage prevents a change in geometry.
The linkage does not prevent a change in geometry. The linkage does not prevent the TT from swinging behind the TV. It is only the lateral forces on the TT's tires (and inertial effects) which prevent the TT from swinging. If a gust of wind hits the side of the TT, it will swing to a new yaw angle. When the wind goes away, the TT will swing back. The linkage prevents neither swinging to nor swinging fro. However, the effective lengthening of the TT tongue will have some effect on the period and amplitude of the swing.
Ron
Sure it does, from the trailer to the TV. From the TV to the TT, the angles allow it to change.
Milt, are you saying that, if a force is applied to the TT end of the hitch (the orange portion), the linkage will not permit the orange portion to move relative to the front part? Instead of our continuing to debate this ad nauseum, why don't you conduct an experiment. Hook up your truck to the trailer and then remove the WD bars and disconnect the trailer from the ball and from the struts so that you just have the HA hitch head hanging on the stinger. Then, without the TT attached, push laterally on the orange portion and see if the linkage prevents the orange portion from moving.
Ron
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Ron Gratz
full time RVer
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Stressor wrote:...If a gust of wind hits the travel trailer, it will not yaw at all, unless you loosen the strut bolts a turn or two, other than that, the trailer does not swing either way.
Milt, perhaps we each need to define what we mean by "yaw". I define "yaw" as a pivoting of the TT about some vertical axis. When subjected to a strong cross wind, a semi trailer or a 5er or a PullRite-coupled TT will pivot (yaw) about the pin. A TT coupled with conventional hitch will yaw about the ball coupler. And, a HA-coupled TT will yaw about the VPP. The trailer will swing until its tires have enough slip angle to develop sufficient lateral force to counter the force of the cross wind. How do you define yaw?
Ron
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Ron Gratz
full time RVer
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Stressor wrote:...Let us not trivialize or ignore the statements made. The orange head of the Hensley does not move, relative to the trailer tongue, period. ...
Milt, I'm afraid other members are going to think you do not read my posts before responding to them. I have never said that the orange portion moves relative to the trailer tongue. In fact, I have said that the tongue is rigidly connected to the orange head via the struts and the ball coupler.
Ron
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Ron Gratz
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Stressor wrote:...Do not misunderstand what the patent document states, and do not assume that a patent document is intended to be a straight forward explanation of how something works. If this were true, engineers would do the interpreting, not patent lawyers.
Milt, actually, they usually are both attorneys and engineers. During the three years I served on the Patent & Licensing Committee of our R&D Department, I had the pleasure of working with several very capable patent attorneys all of whom also had engineering degrees.
When a patent application dealt with a primarily mechanical invention, it was assigned to an attorney with a mechanical engineering degree. And, when the patent application was sent to the US Patent Office, it most likely was assigned to a reviewer who was a mechanical engineer.
It is the job of the patent attorney to ensure that the patent application presents a highly detailed and accurate description of the invention. A patent application contains a myriad of claims designed to define the uniqueness of the invention. The attorney, working closely with the inventor, will try to include every single claim that they believe will be accepted by the reviewer. If the person(s) who prepared the Hensley Hitch application believed they could substantiate the claim that the HA makes the combo "think" and "act" like a straight truck, they would have included it. If there is any important aspect of the operation of the device that is not included in the patent application, then the patent attorneys and the inventors are not doing their jobs.
Ron
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