bettered
UpCountry SC
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Joined: 07/26/2004
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I'll sign up!
Ed
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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Ron Gratz
full time RVer
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Joined: 12/27/2003
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willald wrote:Milt (Stressor) argues that the 4 bar linkage, in actual, practical use on the Hensley, basically prevents any pivoting AT ALL from the trailer's perspective, and from the trailer's standpoint, everything is locked in line, like a straight truck. Based on how us Hensley owners have observed our rigs behave, its very easy to believe this.
Will, I'm still waiting for Milt to explain how a 4-bar linkage, whose links are connected via bearings, can be rigid ("locked up")when the hitch is aligned straight ahead, but be free to move otherwise.
Perhaps the linkage utilizes magic bearings which sense the articulation angle and "lock up" at zero angle but "free wheel" at non-zero angles. Or, perhaps, a magic diagonal brace materializes at zero angle to convert the 4-bar linkage into two rigid triangles. And, the brace then vanishes at non-zero angles. If it ain't magic, what is it?
The HA does not, and does not need to, "lock up" at any angle. It works because it is able to transfer both force and moment between the TT and TV via the two side links of the 4-bar linkage. A conventional ball coupler can only transfer force. It is the HA's moment transfer capability that is the key to how it works. It is that capability which effectively projects the pivot point forward. In doing so, the magnitude of the lateral force is reduced, and its point of application is moved closer to the TV's rear axle. The end result is that the "steering moment" imposed by the TT on the TV is significantly reduced.
Lateral instability is a threshold exceedence phenomenon - hence the name "instability". The HA enhances the TV/TT stability so that "sway" does not occur for a wide range of TV/TT combinations operating under a wide range of conditions.
Ron
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willald
NC
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Ron Gratz wrote:Will, I'm still waiting for Milt to explain how a 4-bar linkage, whose links are connected via bearings, can be rigid ("locked up")when the hitch is aligned straight ahead, but be free to move otherwise.
Perhaps the linkage utilizes magic bearings which sense the articulation angle and "lock up" at zero angle but "free wheel" at non-zero angles. Or, perhaps, a magic diagonal brace materializes at zero angle to convert the 4-bar linkage into two rigid triangles. And, the brace then vanishes at non-zero angles. If it ain't magic, what is it?
hehehehe, alright, Ron, lets not get sarcastic here with the talk about 'magic'. Thats what makes these things get ugly.
I agree, that it does not 'lock up'. However, I think we can agree, that from the trailer standpoint it is somewhat 'virtually locked', in that the direction the trailer is forced to pivot is such that the trailer almost cannot pivot that way.
Can you agree, Ron, with the summary I provided in the last post?
Will
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Ron Gratz
full time RVer
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willald wrote:I would submit, that Tim is right in that pivoting does take place from both ends. However, due to the way the pivot point is projected, it is VERY difficult (almost impossible) for the trailer to initiate much pivoting at all through that pivot point/elliptical path.
So in essence, from the trailer's perspective, it cannot pivot. Or, the pivoting it DOES do, since it pivots through the virtual pivot point/elliptical path we have outlined, is pivoting that does not affect the tow vehicle at all, so it basically eliminates all swaying as we know it.
How does this sound? Can we all agree on this? Or, am I over-simplifying it too much? ![[emoticon]](http://www.coastresorts.com/sharedcontent/cfb/images/smile.gif "smile")
Will
Will, I think we're talking about two different kinds of "pivoting". On the one hand, we're talking about the TT swinging behind the TV, which it is free to do if there is no lateral force acting on the TT's tires. OTOH, we're talking about the TT initiating pivoting of the TV.
When we're talking about the TT "initiating pivoting", we should be referring to the ability of TT-induced lateral force to cause the TV to tend to pivot about its rear axle. In this second case, the HA significantly reduces the TT's ability to initiate pivoting.
In the first case, the HA does not prevent the TT from swinging behind the TV. It is the TT's tires which control the amount of swing. The HA does not allow the TT to pivot at the ball coupler. But, the HA's linkage does allow the TT to move as though the TT were pivoting about the VPP (or about the effective pivot point as the Hensley Patent Document refers to it).
So, from the TT's perspective, it CAN pivot. However, the lateral loading on the TV, which is a separate issue, is reduced by the HA. And, the HA causes the point of application of the lateral load to be moved closer to the TV's rear axle. The net effect is to significantly reduce the "steering moment" on the TV. The HA does not have to eliminate the swinging of the TT in order to reduce the loading on the TV. The HA controls the magnitude of the "steering moment" and the TT's tires control the magnitude of the swinging.
Ron
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Ron Gratz
full time RVer
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willald wrote:I agree, that it does not 'lock up'. However, I think we can agree, that from the trailer standpoint it is somewhat 'virtually locked', in that the direction the trailer is forced to pivot is such that the trailer almost cannot pivot that way.
Can you agree, Ron, with the summary I provided in the last post?
Will
Will, the HA linkage does not lock -- either actually or virtually. The rear unit is free to simultaneously translate and rotate. The TT is rigidly attached to the rear unit via the struts and ball coupler, so the TT also is free to translate and rotate. Of course, the TT's tires will control the amplitude of the swing.
Consider a PullRite hitch. One end of the PR's drawbar is attached to a physical pivot. The hitch head at the other end is free to translate and rotate. In fact, the rear end of the drawbar is supported on a circular track to accommodate the swinging. So, the PR's hitch is free to translate and rotate (pivot about the real pivot point). The TT is rigidly attached to the hitch head and the TT must swing as the drawbar swings. Of course, the TT's tires will control the amplitude of the swing.
The HA and the PR operate on the same principle -- reduce the lateral force and move it closer to the TV's rear axle. The difference is that the HA's virtual (effective) pivot point moves while the PR's remains fixed. There is nothing about either hitch which will prevent the TT from swinging about its respective pivot point. The hitch controls the "steering moment" on the TV and the TT's tires control the swinging of the TT.
Ron
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tluxon
Kirkland, WA
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So far I haven't seen anything I disagree with in Ron's past few posts.
However, I thought of something else that should be addressed.
As one who has never owned a Hensley, I'd like to ask if it's possible that the hitch has a mechanism in it that actually and literally "locks" the linkage before running the stinger bar into it to keep from damaging the hitch when backing it in? That could make it so it actually DOESN'T move at all until both ends are securely engaged. If this is so, that would explain why it's difficult for Hensley owners to comprehend what we non-Hensley owners insist - that there is freedom for BOTH ends of the linkage to trans-rotate through their respective restricted elliptical paths. Can anybody say for certain one way or the other if this is the case?
I bring this up because it's one of the few reasons I could come up with to explain the difficulty we've had trying to resolve this impasse. I believe most designers and engineers would come to the same conclusion that Ron and I have been insistent on - that the Hensley COULD (not that it WOULD) be turned from either end. In fact, I can think of no obvious way to make any kind of hinge that can pivot from one side but not the other. It's no more logical to me than freezing food with heat. If anyone can cite an example I really want to see it.
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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bettered
UpCountry SC
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tluxon wrote:So far I haven't seen anything I disagree with in Ron's past few posts.
However, I thought of something else that should be addressed.
As one who has never owned a Hensley, I'd like to ask if it's possible that the hitch has a mechanism in it that actually and literally "locks" the linkage before running the stinger bar into it to keep from damaging the hitch when backing it in?
Yes, and NO, but mostly no. There is nothing in the linkage or bearings that does any such thing. The front bar of the linkage (black part) moves quite freely in either direction to the extent of the side links interfering with each other. So that the hitch will be possible to back into without careening freely about, Hensley includes a gravity locking bar in the link fixed to the TT that drops down into a slot in the black part to hold it in one of three positions as long as and only if a hitch bar (stinger) is NOT inserted. In order to observe the free swinging motion, this drop down bar must be held in the up position (I use a 2 1/2" x 2 1/2" square wood stick for this purpose - mostly for initially aligning the receiver on the hitch with my hitch bar just prior to backing. Once the hitch bar is inserted the drop down bar is totally inoperative.
tluxon wrote:That could make it so it actually DOESN'T move at all until both ends are securely engaged. If this is so, that would explain why it's difficult for Hensley owners to comprehend what we non-Hensley owners insist - that there is freedom for BOTH ends of the linkage to trans-rotate through their respective restricted elliptical paths. Can anybody say for certain one way or the other if this is the case?
I think it's mostly semantics. The back link (orange part) is firmly affixed to the TT frame and cannot (ever) move anywhere relative to the TT. Of course the TT (and the hitch) does move back and forth behind the TV in turns and the back link goes along with it, thus moving back and forth. So it depends on your point of view. It never moves, and it always moves.
tluxon wrote:I bring this up because it's one of the few reasons I could come up with to explain the difficulty we've had trying to resolve this impasse. I believe most designers and engineers would come to the same conclusion that Ron and I have been insistent on - that the Hensley COULD (not that it WOULD) be turned from either end. In fact, I can think of no obvious way to make any kind of hinge that can pivot from one side but not the other. It's no more logical to me than freezing food with heat. If anyone can cite an example I really want to see it.
Tim
The only way the HA could be moved from the back link is if the TT tires are translated sideways (the hockey puck scenario). But as I explained in my earlier post, the forces required to do this are not trivial (say 400# minimum) and are unlikely to be realized in any but an extremely unlikely situation.
Ed B
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Ron Gratz
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bettered wrote:The only way the HA could be moved from the back link is if the TT tires are translated sideways (the hockey puck scenario). But as I explained in my earlier post, the forces required to do this are not trivial (say 400# minimum) and are unlikely to be realized in any but an extremely unlikely situation.
Ed B
Ed, There is a difference between tire static friction (when the tire is not rolling) and the lateral force generated by a tire when it is rolling. A rolling tire does not generate lateral force unless is has some degree of "slip angle" relative to the direction of travel. This is why a moving TT swings when subjected to a lateral force. The TT must develop enough yaw angle to generate sufficient side force to counteract the imposed lateral load. The side force is proportional to the tire's cornering stiffness, the load on the tire, and the amount of yaw.
Ron
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tluxon
Kirkland, WA
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Ron Gratz wrote:Will, I'm still waiting for Milt to explain how a 4-bar linkage, whose links are connected via bearings, can be rigid ("locked up")when the hitch is aligned straight ahead, but be free to move otherwise.
Perhaps the linkage utilizes magic bearings which sense the articulation angle and "lock up" at zero angle but "free wheel" at non-zero angles. Or, perhaps, a magic diagonal brace materializes at zero angle to convert the 4-bar linkage into two rigid triangles. And, the brace then vanishes at non-zero angles. If it ain't magic, what is it? Obviously, this was said tongue-in-cheek, since clearly if the hitch became locked at angle zero, how could even the tow vehicle initiate a turn? You may as well weld the TT frame to the TV frame and remove the TV's rear axle. Arguing the point about a possible one-way hinge makes me feel like I'm arguing with a child - I know no honorable way to do it.
Look - if it was possible to design a one way hinge or pivot point, Hensley wouldn't have needed to go to all the trouble to utilize a 4-bar link. Or WOULD they?
I say this because I think it's ironic that we're arguing over whether or not the hitch is rigid from the TT's perspective when it actually benefits the combo that it's not. Take a look at the diagrams I've attached below - I think they will help reveal something very interesting about how the linkage uses lateral input from the trailer against itself.
![[image]](http://home.comcast.net/~tbluxon/Hensley_Fig1.jpg)
In this figure I'm noting a lateral force at the rear bar of the hitch that's a result of the counterclockwise 1000 foot-pound moment used in the previous moment diagram using dimensions from Will's trailer. It's intriguing to note that the advantage of the hitch is that it ALLOWS the lateral force from the trailer's moment to turn into an easy translation of the rear bar. From an aligned orientation (straight TV and TT), the front bar of a free 4-bar linkage is virtually helpless to oppose any lateral translation until AFTER the rear bar has translated noticeably off-center.
But this is actually a GOOD thing. Remember that with the 4-bar linkage you can't get translation without rotation. Please note that in order to get sufficient translation to require much lateral reaction by the front bar attached to the TV, the trailer would have to rotate significantly to even allow that kind of translation. Refer to figures 2 and 3.
![[image]](http://home.comcast.net/~tbluxon/Hensley_Fig2.jpg)
![[image]](http://home.comcast.net/~tbluxon/Hensley_Fig3.jpg)
Please note that Figure 3 is exaggerated to illustrate that the TT tires would have to move significantly off-center to accommodate the seemingly small rotation the translation has tried to induce. But more importantly note what direction the trailer would have to rotate. That's right, CLOCKWISE - which is exactly OPPOSED to the original CCW moment that caused the rear bar translation in the first place!
For this reason I would contend that there's no need for a rigid connection from the TT's perspective to the TV, as the free-swinging linkage performs much better anyway.
Tim
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bettered
UpCountry SC
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Ron Gratz wrote:
Ed, There is a difference between tire static friction (when the tire is not rolling) and the lateral force generated by a tire when it is rolling. A rolling tire does not generate lateral force unless is has some degree of "slip angle" relative to the direction of travel. This is why a moving TT swings when subjected to a lateral force. The TT must develop enough yaw angle to generate sufficient side force to counteract the imposed lateral load. The side force is proportional to the tire's cornering stiffness, the load on the tire, and the amount of yaw.
Ron
I agree with you Ron. But before we can get into sliding friction, we have to get past static friction. Now I'm not claiming to be an expert on tire dynamics, but even though the contact patch is moving (forward), I'm thinking it has a static friction coefficient in the lateral direction (sideways).
I assumed that some force was induced on the side of the TT forcing the TT to move sideways (much of which could be momentum) and that if the static friction coefficient (sideways) was merely 0.05 (I'm thinking conservatively), causing the TT to "break loose" the lateral forces on the TT would have to be 400# minimum. Until the point of "break loose" occurs, we're not into the sliding friction coefficient.
It is only after the TT has broken loose that it can apply any translation or turning movement to the rear bar of our 4-bar system. Forces yes, but movement of the rear link without tires going sideways - can't happen.
I'm very interested in your take on this postulation.
Ed B
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