tluxon
Kirkland, WA
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jdwhittaker wrote:...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...
Hi Dave, I don't like that you call my linkage video, a "theory", but you've got a good grasp of the picture now. The lack of constraints on the model in the video was INTENTIONAL. I wanted as few constraints as possible to help people understand that just because the linkage is trapezoidal in geometry doesn't mean it can't bend from certain inputs. If I begin to add restrictions to the degrees of freedom of my model then I am introducing external components to the nature of the linkage. Then we're starting to talk about the behavior of the applied system. Of course the HA as a hitch system (which includes the attached vehicles) adds HUGE restrictions to the allowed degrees of freedom - that's the key to its effectiveness! On that I don't think anyone disagrees.
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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jdwhittaker
York, SC
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Joined: 06/01/2005
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Tim,
tluxon wrote:...I don't like that you call my linkage video, a "theory", but you've got a good grasp of the picture now...
Sorry, I didn't call your model a theory to belittle it, or to make it seem as if it made no sense, but rather because I thought that the model represented your theory on how the hitch actually works. It seems to me now that you were actually using it to dispute what you thought to be my theory on how all of this works.
tluxon wrote:...but you've got a good grasp of the picture now...
My understanding of the hitch hasn't changed since I first jumped in, I think that maybe I have conveyed my thoughs clearly enough now, that you understand what I've been trying to say all along.
My bad, admittedly I have struggled to clearly and accurately convey my thoughts on how the HA works. It was never my belief that the 4 bar linkage could not be moved in the way that your video depicts, however the actual hitch properly mounted between the TT and TV cannot move this way. I think that we agree on that now, and from the sound of things may have all along.
I do still have one question. How does the VPP from your earlier diagrams play into this mechanism?
Thanks
Dave
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2lMan
Brewer, ME
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OK, now that I am home, with a broadband connection, I have looked at Tim's video of the four bar linkage.
I see what you are showing here, but, for the TT to make the same kind of movement that you are simulating with your hand, it would essentially be pivoting at the ball. As Milt has said here, the hitch is prevented from pivoting at the ball by the two strut rods. These must be perfectly tight, or sway WILL most definitely occur.
Since the TT coupler is prevented from moving by the struts, the TT must essentially transmit all movement through the linkages, or it must sway more in a side to side motion. The struts would tend to prevent this swaying, or yawing movement of the TT tongue. So, perhaps the Hensley is only a two bar linkage.
I should try to build a model myself to show this. I am simply trying to visualize it.
Now, the movement or turning is perfectly able to be initiated from the front of the hitch, due to no linkages holding it in place.
So, based on my own use of the hitch (I have had it for about two months, and towed 2000 miles), and based on my observation here, I would maintain that the connection is, to use my own phrase, "virtually solid" when the TT is attempting to act on the TV. When the TV is initiating the movement, the hitch pivots freely. I believe that Tim's statement about the hitch is a theoretical application, but Milt's (and my own) oppinion about how the linkages work is a "real world" application.
Of course, this is only my humble oppinion, and is certainly not the voice of an expert. I certainly welcome any and all discussion about this, as it has been alot of fun to debate without everyone getting all excited.
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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bettered
UpCountry SC
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Joined: 07/26/2004
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I have followed this thread with keen interest as an HA owner of about 9 months with a rusty Mechanical Engineering degree that's 40 years old.
It's unfortunate that Mr. Gratz was recently forced to leave the discussion. It is apparent to me that he is an unusually capable engineer.
That said, the discussion went off track when the pictures of the 4 bar linkage were put up and the notion about "turning around the linkage" poked out of the weeds. This hypothesis is faulty and I'll explain why directly.
I recognized the confusion that erupted when Mr. Gratz was looking at the mechanism from the standpoint of the "black part" of the hitch, while others were focused on the "orange part." Much of this uncertainty and some confusion derives from one's point of view. Sitting on the back of the TV watching the Orange piece, one would see it move back and forth in turns. Sitting on the TT, one would see the black part move back and forth, while Mr. Orange was fixed, and stationary. Mr. Gratz has seen all of this but he has also looked at it from the standpoint of a bird flying over the hitch at the same speed of travel looking down. During turns, the bird has to fly a wavy path because of the Virtual Pivot Point phenomenon, which is not imaginary, it provides an explanation for the functionality of the linkage.
In the demonstration wmv, when the linkage is reversed, the pivot point used is the center of the horizontal bar in both cases. The bar pivots to and fro. The TT does this because it has effectively one fulcrum - it's 2, 4 or 6 wheels act as a single pivot point. The TV cannot approximate this motion unless its front wheels are scrubbed to and fro from side to side in a wide arc.
The idea of turning the linkage around doesn't work because the presumption then is that the TT is turning the vehicle, and we've already seen why that cannot happen. The friction between the 4 tires and the ground defines a fact that the tow vehicle cannot "pivot."
Much is made of the theoretical difference between a simple 4 bar linkage and the practical application. If there are no constraints on any of the links, we have merely a toy. A linkage becomes a mechanism only when one or more links are fixed to something.
* This post was
edited 06/03/05 09:01pm by bettered *
BetterEd
DW + 2 grandkids + Mini Schnauzer
2005 Chev 3500 Crew D/A 6.6L LLY, 6 x 6 DRW, 3.73
Tru-Flow + Banks, 2005 Flagstaff 831FKSS
Hensley + Prodigy
"Genius may have its limitations...." E. Hubbard 1856 - 1915
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tluxon
Kirkland, WA
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I'm glad BetterEd finally joined our discussion. I'm sure there are many others who have followed the thread but haven't chipped in yet.
Ed is right on. The thread really began to digress when we isolated the linkage and discarded the constraints built into the way the hitch attaches to a TV and TT. I think I lost a couple people who had real-life experience with the hitch at that point. I apologize for venturing in that direction, but I had good intentions at the time of attempting to support Ron's assertions about it not acting like a straight truck when hanging like a pendulum (no contact between TT and ground).
I think it would be good to compose some kind of summary of the ground we've covered in this thread. As far as I can tell, for the most part we agree on the basic premises of this discussion. The points where there have been disagreement seem to be largely a matter of semantics and subjective observation. None of these contentions appear to be critical in describing the effectiveness of the hitch even to the point of quantifying reactionary forces.
Maybe I can start something in the next day or two. If anyone else wants to take a stab at it, please go ahead.
Thanks,
Tim
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bettered
UpCountry SC
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Joined: 07/26/2004
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When the idea of suspending the "rig" vertically from the TV to see how the TT and hitch would behave, I was interested. My first thought was: What would happen if we were to suspend the TV from the TT? I was intrigued with that idea and delighted when it was posted. There has been some conjecture around this question which I followed closly. I don't think we have the answer to this question, but I believe it has to do with the arc through which the TT is turned. I suggest that for small angles (as when towing at speed), the rig hung vertically would act as a rigid body. I'm not clever enough to offer a suggestion to what point the TV would turn, but we have already discussed the result that the Hensley delays the onset of sway because of the reduction in forces and moments.
Although the advertising is "no sway", Hensley admits that the biggest problem with the HA is that people become overconfident and tend to forget they're towing. When this happens, it's possible to overdrive the capabilities of the TV to control the rig. We know from Mr Gratz efforts that this will occur when the tow angle allows one of the side links to "collapse" by going overcenter AND there is heavy breaking of the TV with insufficient breaking at the TT. This could occur at the bottom of a hill with a significant curve - but only if the TT breaks were faulty or maladjusted.
I'm pretty close to my CGW with a TV at 5700# and a TT with a GW of 7900. I'm driving a '00 GMC 1500 with a small V8 and a 4:10 rear end. It do slow down going up hills, and the gas mileage is about 15 around town (not towing) but when towing, I still get about 12 or 13 mpg because of the rear end ratio. As far as handling with this package, I will admit to some displacement when an 18 wheeler sails past at what seems to be twice my speed, but there really is no sway, and the resulting displacement is probably due to the effect of the wind on my TV, not on the trailer. In fact, I've experienced similar displacement under those conditions when not towing.
As a footnote, I had no idea I'd be towing an RV at CGW when I bought this truck, otherwise I'd probably have done something different. As it is, my only real regret is that I got the 4 door version but not the crew cab. It's a little tough on the grandkids (who are now six) and will get tougher as they get bigger. We're taking them to Dollywood in two weeks. But unless a money tree lands in my back yard, this is going to me my rig for the foreseeable future and frankly, I'm pretty well satisfied with it.
My wife couldn't understand why I'd pay $1500 for a HA (www.hitchtrader.com) + $200 shipping + $500 to extend the factory warrantee. My knuckles stay pink as a result (not white) in 'panic' situations as the rig behaves itself very nicely when I fail to adeguately anticipate an upcoming event - as when entering a curve at the bottom of a hill carrying too much speed.
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tluxon
Kirkland, WA
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I've updated the reactionary moment diagram I posted some 30 or 40 posts back with numbers updated to reflect Will's towing setup. I used the numbers I would get if I put my trunnion hitch head in Will's receiver for a "conventional" lashup.
![[image]](http://home.comcast.net/~tbluxon/Hensley_Moments_1a.jpg)
The resulting reactionary moment from Will's X to a 1000 ft-lb moment from his trailer with a conventional hitch would be 233 ft-lb compared to only 78.8 ft-lb with his HA.
This would result in such a slight flex in the sidewalls of the tires using an HA that a driver would be hard pressed to notice anything at all. I would submit to you that even the best friction-based sway inhibitors would have a negligible ability to reduce the 233 ft-lb moment felt with a conventional hitch. Since the sidewalls of the tires would flex more reacting to this moment, I'm certain that most drivers would feel something.
Additional good news with the HA is that even if there was a slight flex of the tire sidewalls, the recovery of the sidewalls to that flex would act through such a relatively short lever that it would result in virtually no lateral movement at the front of the trailer even if the linkage allowed it (which I believe it does).
With a conventional hitch, not only is there going to be a greater flex in the tire sidewalls, but the inevitable recovery (rebound) of the sidewalls will act through a longer lever and would result in a measureable lateral movement at the front of the trailer. An effective friction-based sway control greatly dampens this give-and-take to prevent a dangerous recyling sway event.
Tim
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Stressor
Whitefish Bay, Wisconsin
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![[image]](http://home.ripway.com/2005-1/238631/Folder%20Name/Hensleytext005.JPG)
![[image]](http://home.ripway.com/2005-1/238631/Folder%20Name/Hensleytext006.JPG)
These two pictures were taken to demonstrate to non-Hensley owners what the word rigid and one way means in actual practice. It seems clear that it is difficult to gain an understanding of the operation of the hitch through the use of graph paper and popsicle sticks, although it is an entertaining theoretical endeavor. A Hensley is an application of a rocker-rocker 4 bar linkage, not one of the innumerable possible designs of a 4 bar linkage. With that in mind, let us proceed...
The hitch is connected via the stinger to an F-250 pickup reciever rated at 1500/15,000 pounds. The travel trailer end of the hitch is free floating, attached only via the two links. The wires attached to the strut pin on the right hand side are attached to a black willow tree, 24 inches in diameter, through a MAASDAM POW-R-PULL rated at 4000 pounds and a recovery strap rated at 15,000 pounds.
The come-along is under tension sufficient to pull the 3/4 inch strut clevis pin into the shape of a banana, rendering it useless. When the vehicle recovery strap is plucked, it emits a tone which is reminescent of one of the keys on the far left of a piano keyboard.
This is intended to simulate a side load on the travel trailer end that is approximately what would be experienced if a large elephant was leaning on the rear four feet of the travel trailer.
In actual operation, the rear of the hitch is attached to the travel trailer by a hitch ball connector, and prevented from moving side to side by two struts. This is nice, but as you can see, the linkage works just fine without being attached to anything.
What this proves:
The linkage from the TT to the TV is as rigid as a 2x2 inch steel bar is rigid. During a towing operation, the travel trailer and the tow vehicle move as one rigidly connected unit from the travel trailer end of the rig. Any forces applied to the travel trailer are applied to the TV receiver through a steel bar, more or less any effects related to leverage, flexing sidewalls, aerodynamics, or the like.
Note that the front linkage has not moved, and that the rear linkage has not moved. No pivoting has occurred anywhere. All of the very considerable forces being applied in this demonstration are being felt at the TV receiver, and transmitted to the TV frame and other components.
Close observation will indicate that all of the play has been taken out of the receivers, both the one on the TT end and the one on the TV end, and that the hitch has attempted to line up the linkage with the direction of the pull. Again, no pivoting has occurred. In actual operation, this play is limited by the operation of the struts on the TT end. The linkage is locked to inputs from the travel trailer end.
The much loved virtual pivot point can only be seen from the drivers seat of the vehicle, (use binoculars, it is out there somewhere).
Where does it actually pivot? Like anything else, if you look for where it bends, that is the actual pivot point. When towing ahead, it bends at the hitch when the tow vehicle initiates a turn, and not at all if the travel trailer initiates a turn. No Sway. ![[emoticon]](http://www.coastresorts.com/sharedcontent/cfb/images/biggrin.gif "biggrin")
* This post was
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edited 06/06/05 03:57pm by Stressor *
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Milton Findley (and Kerene)
A small piece of my mind...
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Stressor
Whitefish Bay, Wisconsin
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Here is a link to a 4 bar demonstration 4 bar
Set the values at 200.0, except for BC, which can be set at 180.0 or so to simulate the Hensley linkage. You will get to see the links move as they do.
Note also that the Travel trailer end is set in concrete.
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willald
NC
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Wow! Excellent illustration, Milt!! LOL, did you really sacrifice one of your link pins on the Hensley, just to prove this point? Hope you have a spare to use, so you can get out camping soon. ![[emoticon]](http://www.coastresorts.com/sharedcontent/cfb/images/smile.gif "smile")
I'll be curious, to see what Tim has to say about this......
Will
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