Hensley Arrow: How does it REALLY work?

[willald]

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?

[Dogarms]

This is a very interesting discussion and I am beginning to understand more cleraly why the Hensley is often referred to as a "sway prevention" hitch instead of a "sway control" hitch. If I understand correctly, when driving in a straight line, the hitch essentially locks the trailer in column behind the TV as if they were a single, non-articulated unit. (I think the Dual Cam sway control cams provide some of this locking action, though from an entirely different approach.) When the TV and TT move out of column, for whatever reason, the hitch geometry projects the point of articulation forward toward the rear axle, in the manner of a fifth wheel, reducing or eliminating the tendancy of the two units to get out of phase with each other. Very clever.

My hitch, which is a straight forward "sway control" hitch simply dampens, but does not prevent, oscillations before they can become a problem. I works very well, but admire the genius of the Hensley hitch. I wish they were cheaper.

TC

[tluxon]

willald wrote:

bryanl wrote:
Perhaps the best way to understand what is going on would be to put together a scale model and do a little bit of 'experimenting' with it. The model only needs to be 2 dimensional. You could do this easily enough with a few small sticks and some glue (and a hitch and rig to measure).

I was actually thinking the same thing, except going to a bit more detail. Perhaps if you could find/build a scale model of a good size TT, and a truck/tow vehicle, then hitch them up with something similar to the 4 bar linkage. You could make the linkage with just 4 sticks like you mentioned here. Then, you could do things like 'push' on the side of the trailer to simulate crosswinds, etc, and see how any pivoting it does, affects the truck. My bet is you'd find that the whole combination would be pushed to the side as one unit, and that the trailer could NOT induce any pivoting. That would answer the question of where the pivot point is, from the trailer's perspective.

I have the Hensley video where they use an actual hitch for demonstration purposes. They show that when the TV tries to create an angle with the TT there is no resistance and the angle is easily created. Then they demonstrate that applying strong lateral forces to the TT side of the hitch results in no angle being created - Yes, the combo moves as a single unit. I don't know that that answers where the pivot point is so well, but it sure addresses the concern with TT induced sway.

By the way, I'm not a Hensley owner - just a mechanical engineer who is impressed with how cleverly the Hensley addresses trailer-swinging induced sway. For the price, you'd think they'd be able to do it with a little less added weight by using more titanium parts.

Tim

[tluxon]

willald wrote:
...Hmmm....Is it possible/fair to say, that the 'virtual pivot point' from the trailer's perspective might actually be an infinite distance forward from the ball, since the trailer cannot induce any swaying/lateral movement on the tow vehicle?...

No, the VPP could not be infinitely forward from the ball or you'd run into a situation where lateral forces could get leverage on the front of the TV. The only place where you have NO twisting leverage on the TV due to lateral forces is exactly halfway between the rear axle and the front axle. However, anywhere between or close to the axles (probably within 12" on a typical wheelbase TV) will probably make leverage due to lateral forces almost nil.

Tim

[LAdams]

BarneyS,

BarneyS wrote:
The rest have been "off topic" and if I had moderating privileges on this forum, they would be deleted!

DONE!!!

Moderator

[willald]

bryanl wrote:
Perhaps the best way to understand what is going on would be to put together a scale model and do a little bit of 'experimenting' with it. The model only needs to be 2 dimensional. You could do this easily enough with a few small sticks and some glue (and a hitch and rig to measure).

I was actually thinking the same thing, except going to a bit more detail. Perhaps if you could find/build a scale model of a good size TT, and a truck/tow vehicle, then hitch them up with something similar to the 4 bar linkage. You could make the linkage with just 4 sticks like you mentioned here. Then, you could do things like 'push' on the side of the trailer to simulate crosswinds, etc, and see how any pivoting it does, affects the truck. My bet is you'd find that the whole combination would be pushed to the side as one unit, and that the trailer could NOT induce any pivoting. That would answer the question of where the pivot point is, from the trailer's perspective.

Of course, who has the time to do all that? lol! :B

Ron has the best analysis of this I have seen so far. But the other talk about 'eliminating sway' and other absolutes should raise the skeptic meter. The hyperbole should always be a warning that something is missing.

I agree, Ron has a great analysis. And, I also agree that something is missing - from Ron's analysis. His analysis does a great job of showing where the pivot point is from the tow vehicle's perspective, but I can't help but wonder if from the trailer's perspective, that 'virtual pivot point' may be at a different place.

Will

[tluxon]

willald wrote:
...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...

I sketched the geometry up in SolidWorks and plugged in the numbers you provided for the link lengths. The furthest forward the "virtual" pivot point (if we're using VPP to describe the projected intersection of the two side links) is of course when there is no angle between the TV and the TT. Using your numbers, that point is 43.35 inches in front of the "front bar" (the bar that makes a T with the TV).

Of course, this is using Will's numbers which I believe are only guestimates. We'd have to know the actuals to determine how far up the VPP really is.

Tim

[bryanl]

Perhaps the best way to understand what is going on would be to put together a scale model and do a little bit of 'experimenting' with it. The model only needs to be 2 dimensional. You could do this easily enough with a few small sticks and some glue (and a hitch and rig to measure).

Ron has the best analysis of this I have seen so far. But the other talk about 'eliminating sway' and other absolutes should raise the skeptic meter. The hyperbole should always be a warning that something is missing.

[willald]

Great points, Claude, and thanks for including the hand-drawn illustrations. 🙂

Hmmm....Is it possible/fair to say, that the 'virtual pivot point' from the trailer's perspective might actually be an infinite distance forward from the ball, since the trailer cannot induce any swaying/lateral movement on the tow vehicle? Then, the 'apparent pivot point' would only be from the tow vehicle's perspective, and would be about 47" forward of the trailer ball (when in a straight line)? This would seem very consistent with the reality of how this hitch works.

On another note, I wanted to clarify one thing, about what Claude discussed:

Claude B wrote:
This is also why with the HA, the TV has always the control of the TT. With this hitch, a jacknife situation is totally impossible in panic stop whatever the speed.

Thats true - with a Hensley, the TV always has control of the TT, as long as the tow vehicle is 'pulling' the TT, and the opposite does not occur (TT 'pushing' the tow vehicle real hard).

However, if during a panic stop, the trailer 'pushes' the tow vehicle too hard, it actually IS possible, for a jackknife to occur with a Hensley. During a panic stop, if the brake controller is not set aggressive enough, and the trailer 'pushes' the tow vehicle hard enough, the Hensley will let the trailer shift off to one side, resulting in a push slightly off to one side. This 'push' affect of the Hensley has been discussed, and is well known among Hensley owners. It just means that with a Hensley (like any other combination/hitch), you must have your brake controller adjusted right, to prevent too much trailer 'push' from occuring during a panic stop.

I will tell you, I once did not have our brake controller set right, and felt this 'push'. I was braking REALLY hard when it happened, and the push was not anything to worry about. Only way I can see it would cause a problem, is if it was a case of a VERY light/small tow vehicle towing a huge trailer, that it should not be towing in the first place, Hensley or not (like some of the combinations CanAm RV puts together..LOL).

Kind of goes back to what you said, Claude - even though the Hensley is such a perfect design, it should not be used as a 'cure-all' for a bad setup. You still need to set everything else up correctly, especially your brake controller. And, you still need to match the tow vehicle to the trailer appropriately.

Will

[Claude_B]

Ok, i am not a H-A owner but i surely agree on it's great technical quality. IMHO, it's a perfect hitch by desing. Now i see 2 discussions here. First, how doe's the H-A eliminate sway and second, the "apparent" or "virtual" point of pivot between the TV and TT. I made 2 hand drawing to explain my comprehension of the system.

I think we all agree that sway is the result of the TT pivoting aroud a point locate in the center of the axles forcing the tongue to the left and right (laterally) pushing the back of the TV sideways up to a point where control of the rig can be lost. Lots of factors are involved here like wheelbase, overhang of the hitch ball from the rear axle, tongue weight etc.

We also agree that sway only occur at speed over 45 to 50 mph (+/-)
So, at this speed, TT and TV are in straight line 99% of the time. This is where the HA desing is unique, it locks completely (from the TT point of vue) the TT and TV just like riding on a railroad. It has the same effect as welding a 6" I beam from the back of the TT to the front of the TV. Rotation is impossible if induce by the TT (sway). The HA trapezoïd 4 bars linkage combined with the 2 struts bars are locking the TV and TT. On highways or country roads, the TV and TT are always almost straight so with the HA, there is no apparent pivot point of rotation close, after or forward the TV axles. Lateral forces will be on the entire TV frame (from the HA to the receiver to the TV frame).

The apparent pivot point between the TT and TV appeers when and only when the TV induce a rotation and will occur only at low or very low speed in close manoevring (city driving, parking, backing-up) except on a panic and rapid lane change on highways.

Hand drawing of the HA without the strut bars : HA drawing 1

Hand drawing with the strut bars and what they do : HA drawing 2

This is also why with the HA, the TV has always the control of the TT. With this hitch, a jacknife situation is totally impossible in panic stop whatever the speed.

Now, here is my comment of a non HA owners 😉

Technically, it is a perfect hitch but it's to bad that in their advertising video, the let the impression that it's also a perfect "idiot proof" hitch. Don't take me wrong, HA owners are NOT idiots ! But when they claimed that you can load your TT with heavy stuff in the back not having any taughts about the tongue weight etc. After all, the HA will take care of any unstable rig due to an uninformed driver who know's nothing of the physics of towing (wide majority on the road).

P.S. The cherry over the cake was the ... lawyer (in the video)!

Best regards to all.

[Stressor]

Analyses is the word. :B

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.

[willald]

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.

Will

[tluxon]

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.

Cheers,

Tim

[Ron_Gratz]

rolnrolnroln wrote:
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.

Ron

[Ron_Gratz]

Will and Tim,

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.

Ron

[BarneyS]

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! 🙂
Barney