There have been several discussions on this hitch that have become somewhat 'controversial'. I am NOT trying to stir this controversy/bashing back up. Instead, I'd like to see a good, intelligent discussion on the specific geometry/physics behind it, and how/where it projects the pivot point when towing a travel trailer. Perhaps even some discussion, of why the pivot point being projected to where it is, seems to eliminate sway completely (or at least give the perception of such).
For starters: HERE is a basic illustration, showing the trapezoid/4 bar linkage, that the Hensley uses to eliminate sway. Some of the details of this description have been accused of being inaccurate, but this still provides a good starting point. I have actually measured our Hensley head, and found that the two side bars are about 4.5" long, the rear (longer) bar on the trailer side is about 8", and the front bar is about 7.25" long.
Many different theories have been suggested, on specifically WHERE this trapezoid will project the pivot point. Hensley's advertising suggest the Hensley projects the pivot point forward, close to the rear axle. The question they don't answer is, specifically HOW FAR forward? This is what I'd like to discuss.
Some have said its projected to a point infinitely forward of the rear axle (when trailer is in a straight line). Another analysis (provided by Ron Gratz) implies its projected about 47" forward from the trailer ball hitch. Yet another recent model I heard of from 'UniCacher' on here, suggests the pivot point is actually BEHIND the trailer ball hitch. Not sure I understand that, but would like to hear the reasoning behind that.
Soooo, those that own a Hensley, or have some understanding of how it works: Just where DOES a Hensley project the trailer's pivoting point? And, if this point is actually behind the rear axle, why does it give the perception of zero sway? Is there some other piece to the geometric 'puzzle' here, that we are missing, and is not explained with the '4 bar' problem/illustration?
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Yet another recent model I heard of from 'UniCacher' on here, suggests the pivot point is actually BEHIND the trailer ball hitch. Not sure I understand that, but would like to hear the reasoning behind that.
Depends on where you stand when you look at it. If you're standing behind the hitch looking forward, I guess one could say that the pivot was behind the hitch...actually meaning closer to the tow vehicle's front.
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primarily only address the Apparent Pivot Point. Part of the reason it's called the "apparent" pivot point is because it's not a true pivot point in every sense.
True, it's handy for illustrating how the TT will track behind the TV in a turn, but the forces involved in pivoting behave much differently than with a conventional ball.
The way the linkage is configured inside the Hensley's hitch head causes forces induced from the TT to be reacted on differently than when forces are induced from the TV. That's because the forces involved in pivoting are actually transmitted through the links connected by the 4 pivot points within the hitch head. For the forces involved (as opposed to the Apparent Pivot Point), you actually have to start talking about something called the instantaneous center of rotation.
The ramifications of these two concepts are far from simple, and perhaps that's why it seems that discussions seem a bit controversial.
Am I on the right track?
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I can't help but comment here. Only about three persons have addressed the original question posed by Willald. The rest have been "off topic" and if I had moderating privileges on this forum, they would be deleted! Why can't you just discuss the question of "How" the hitch works without debating the pro's and con's of the hitch? This has been debated so many times and every time it becomes meaningless discussion. The question posed is a very legitimate one and has NOT been discussed very thoroughly before. I would like to suggest that everyone can learn something here if you will stick to the original question in the first post. I know I would be very interested in the answers.
We had a discussion like this in the Towing forum which became one of the most intelligent and informative threads I have seen on the forums. How a Weight Distributing Hitch Works
I think this one can develop into a similar thread if you will just let it, instead of putting in your pro or con posts.
Think about it before you post and maybe we can all learn something here!
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This diagram has been cited several times in Hensley discussions.
The diagram is incorrect. The virtual pivot point (a.k.a. "center of rotation") for such a four-bar linkage is at the point of convergence of projections of the two side links and not as Bill Basham has depicted.
I have discussed this with Bill Basham, but he has not changed the diagram. Unfortunately, this leads to considerable confusion about the location of the HA's virtual pivot point.
But since few can even agree on what sway is, it may be hard to move on to the other topics.
I fully agree. So lets avoid talking about "sway" and focus on things that we might be able to agree on.
For example, I think we all can agree that a TT has the ability to exert a lateral force on a TV. Perhaps we even can agree that the magnitude of the lateral force and the point at which it is applied will affect the stability of the TV/TT combination.
In this topic, Will is hoping to address the location of the point of application of lateral force when using a Hensley Arrow. With a conventional hitch, the point of application is at the ball coupler. This is called the "pivot point". As Tim has pointed out, the HA does not have a physical pivot point. Instead, we refer to an "apparent" or a "virtual" pivot point.
So, lets avoid the topic of "sway" and focus on the point at which the TT applies a lateral load to the TV.
I don't think we could do this discussion justice without you, Ron. I'm glad your joining in, and I know you'll bring a lot to the table that will help us think through the very intriguing design of the Hensley hitch.
I can understand the concern about the misleading nature of Bill Basham's diagram, but I also think it serves a purpose at an elementary level. Those who have towed with a Hensley will attest that the TT doesn't track behind their TV the same as it does with a conventional hitch. If you ignore the terminology Bill uses, his diagram is a good representation of the angle created between the TV and the TT with the Hensley and how it differs from the angle created with a conventional hitch.
I think it is useful to give a name to this angular behavior, and I don't have any problem with using the term "apparent" pivot pointin the same way Bill did. Also, it makes sense to me to use the word "apparent" since it implies that it's simply what it looks like - as opposed to what it really is. Clearly, as the angle between the TV and the TT change, the point of intersection between their centerlines changes, and this way we have a name to reference that moving point.
Now, there is another very important dynamic point to consider that needs to be defined, and that is that "point of convergence of the projections of the two side links" that Ron mentioned. He called it the "virtual" pivot point, and I'm okay with that. One reservation I have, however, is that some might confuse one with the other since "apparent" and "virtual" have similar meanings. Perhaps a more distinctive term would be virtual load point or something to that effect. Any thoughts on that?
As we have time to look at loads on the hitch in more detail, I think it would be quite helpful if diagrams were used to help us all follow along, especially since the ever-changing geometry is an all-important part of why this hitch is effective.
Great points/suggestions, Tim and Ron. Glad to see this topic generate some good, intelligent discussion. That was the intention.
I think Tim makes an excellent point, in the differentiation between the 'apparent' pivot point, and the 'virtual' pivot point. That might explain some of what I (and probably many others) don't understand.
I have seen two different very detailed analsysises (spelling?) done, to try to determine the 'apparent' pivot point for a Hensley. Both came up with very consistent, similar results, that showed a pivot point ~ 47" forward of the trailer ball. After doing some measuring on our rig, I found in my case that would mean the (apparent) pivot point would be about 20" behind the rear axle of our Excursion.
Given the way the Hensely completely eliminates sway for us, I find it hard to believe that having the pivot point 20" behind the rear axle could do this. Always thought there was more to it than that, and perhaps there is.
Even though the apparent pivot point is 20" behind the rear axle, maybe the 'virtual' pivot (or 'instantaneous point of rotation') point is different, and may be much further forward, at least when in a straight line? This would explain why the Hensley seems to eliminate sway completely, regardless of tow vehicle size or trailer length.
I am following along, and "virtual pivot point" is probably the most accurate and easily understood description, and we need to be able to visualize this point as moving both forward and back, and from side to side as Ron has pointed out for us in his remarks about the convergence point of the projections of the side bars.
The virtual pivot will describe an arc in space on a horizontal plane as the side bars change direction.