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?

[Ron_Gratz]

Tim,

Thanks for the very informative plot and the excellent references.

What parameter did you vary, and in what increments, to get the different instantaneous centers?

Ron

[tluxon]

I've updated my model to show the dimensions Ron pointed out from the patent. I've also added points that represent many of the instantaneous center projections of the two side links as they go through their hinge angle sweep.


There are many ways that engineers use 4-bar linkages (http://designcenter.ucsd.edu/resources/subject.asp?subid=13 - go about 1/2 way down the page), but most of them won't remind you a whole lot of a Hensley. I did find one use that was quite interesting to me - the mechanism used in prosthetic knees. In an abstract titled, "The Influence of Four-Bar Linkage Knees on Prosthetic Swing-Phase Floor Clearance" on the American Academy of Orthotists and Prosthetists website, there is a figure that shows a path of projected instantaneous centers much like we've discussed here.

[Ron_Gratz]

Modeling the Hensley's 4-Bar Linkage

Some of you have suggested that having a physical model of the HA's linkage would aid the understanding of how the hitch works. I agree. The following is a way to construct such a model.

Start with a 3' piece of lath or wood of similar dimension. Cut four pieces with lengths: 1 @ 9 1/8", 2 @ 7", and 1 @ 9 7/8". Drill a hole on the centerline of each piece 1" in from each end; i.e. holes spaced at 7 1/8, 5, 5, and 7 7/8" center-to-center.

Insert a flathead bolt, pointing down, through one hole of a 7" piece and also put the bolt through a hole of the 9 1/8" piece with a washer between the pieces. Do the same for the other 7" piece placing that bolt through the other hole in the 9 1/8" piece. Put a nut on each bolt.

Insert a flathead bolt, pointing up, through the second hole of one of the 7" pieces and place one hole of the 9 7/8" piece over the bolt with a washer between the pieces. Do the same for the other 7" piece. Put a nut on each bolt.

You now should have lath pieces in three levels with each end of a piece pinned to another piece at a level above or below. The lower level (9 1/8" piece) represents the front link of the HA which actually is the hitch head. The middle level (two 7" pieces) represent the side links. The upper level (9 7/8" piece) represents the rear link to which the TT tongue is connected and restrained from yawing relative to the link.

Glue a short piece of lath to the top of the 9 7/8" piece at its midpoint and perpendicular to it. This will represent the TT's tongue.

If you got the bolts inserted in the right directions. You should be able to hold the front link and cause the other links to rotate about their bolts.

If you project a straight line through the longitudinal center of each side link, the point of intersection of the lines defines the instantaneous center of rotation of the linkage (the Virtual Pivot Point). When the rear link is parallel to the front link, the projected lines should intersect about 52" ahead of the rear link. As you allow the linkage to turn, the VPP will move to the side and rearward.

Ron

[Jack_Marley]

I thot maybe some of you younger RV's might like to hear about an early type of hitch which did the same thing as the Hensly(in a way) at less money but with some drawbacks. When we moved to FL from IA in 57(yes l957) I bought a 35' trailer and a used hitch. It had a ball and A frame which went on the trailer tongue. It had long tube which went under the car to a saddle which clamped on the differencial. Thus all the weight was on the rear car wheels but was lessened by the length of the tongue. The car thus rode normally as no extra weight was on the body. It did, of course, have some drawbacks such as extra weight and poor clearance. I don't remember any trouble with turning radius, but must have limited it. I pulled it with a '57 Chev station wagon with the l65 hp v8 and it did fine, straight xmission and didn't use over-drive.

[Ron_Gratz]

Virtual Pivot Point versus Apparent Pivot Point

It appears that the virtual pivot point and the apparent pivot point converge to the same location when the angle between TV and TT is zero.

Define:
Virtual Pivot Point (VPP) is the location at which straight lines projected through the centers of the side links will converge.
VPPmax is the location of the VPP when the angle between TV and TT is zero.
Apparent Pivot Point (APP) is the location at which the longitudinal centerline of the TT intersects the longitudinal centerline of the TV.
APPmax is the location of the APP when the angle between TV and TT is zero.

My AutoCAD models indicate that VPP and APP converge to the same point, VPPmax = APPmax, when the angle between TV and TT is zero. Also, for non-zero angles, APP never lies forward of VPPmax. Please disregard my earlier incorrect statement about the APP being at infinity in front of the TV when the angle is zero.

I hope that Tim can use his modeling software to confirm these findings when he returns. If they are correct, our task of answering how the HA really works should be simplified.

Ron

[PSDExcursion]

Claude B wrote:
PSDE,

And i guess that with a 41' TT, you don't need a water pressure regulator ! The hose is soooooo long that the pressure drop just take care of it !!! 😉 LOL

Yep, first time out I had to go to Walmart and get an extra water hose. I use the brass water pressure regulator that in much stronger than the plastic ones.

[tluxon]

Ron Gratz wrote:
Can you tell from the video if any or all of the wheels are caster wheels? Is the front wheel of the "TV" always at 90 degrees?

Throughout the duration of the video, I never saw any of the wheels swivel. The front wheel of the "TV" was always at right angles to the other wheels. Based on how well it resisted his substantial lateral push at the hitch, I'd have to assume that at least the wheels on the "TT" and the rear wheels of the "TV" were not caster wheels.

We're heading out for the weekend. I'll have to rejoin the discussion on Monday.

Tim

[Claude_B]

PSDE,

And i guess that with a 41' TT, you don't need a water pressure regulator ! The hose is soooooo long that the pressure drop just take care of it !!! 😉 LOL

[PSDExcursion]

The bottom line is I never had any sway even when passing 18 wheelers or in crosswinds while towing my 41 ft TT with the Hensley Arrow.

[Ron_Gratz]

Tim,

Thanks for posting the "test model" images. They pretty much confirm what others have explained to me verbally.

I suggest that anyone who is trying to interpret the significance of this "demonstration" should keep in mind that the probable pivot point for the "model" is out in front of the "TV" and not 20" or so behind the rear axle as it would be with an actual TV.

Can you tell from the video if any or all of the wheels are caster wheels? Is the front wheel of the "TV" always at 90 degrees?

Ron

[tluxon]

Here are captures from the Hensley video where the demonstration model is manipulated. In the first two images, the "hinge" did not bend or move at all.





This should help.

Tim

[tluxon]

Ron Gratz wrote:

tluxon wrote:
...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.
Tim

Tim et al.,

I don't think there is any significance that can be attached to the point of intersection between the centerlines of the TT and the TV.

There's only no significance if you disregard what it looks like to the layman from a birdseye view. I would submit that it is sometimes useful and helpful to provide descriptions to things even though they may not be considered "significant" for calculations or for understanding the primary interactions.

Ron Gratz wrote:

tluxon wrote:
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.

Tim,

In a previous thread, I made several postings (running across two pages) offering my opinions on Hensley's "model" TV and TT coupled by a full size HA.

I would appreciate your comments on my remarks since you have seen the video and I have not.

Ron

You have a remarkable understanding of how the Hensley works considering that you haven't even seen the video. In the demonstration on the video, the guy only pushes laterally on the front of the TT tongue portion of the model. The only other thing he does is go to the front of the TV portion of the model and rotates it to simulate the TV turning the TT. I have snapshots that I just took off of a video capture that I'll post on here next.

[Ron_Gratz]

Claude B,

Please refer to my previous post.

Ron

[Ron_Gratz]

tluxon wrote:
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.

Tim,

In a previous thread, I made several postings (running across two pages) offering my opinions on Hensley's "model" TV and TT coupled by a full size HA.

I would appreciate your comments on my remarks since you have seen the video and I have not.

Ron

[Claude_B]

Ron,

Did you had the chance of to see the HA promotionnal video ?

I'm talking about the scene where there is the hitch mounted on a demo stand on wheels and the guy showing how it works. This is where I start to figure out why the TT is lock behind the TV in a straight position on the road. Of course, if you try the HA on a very small utility trailer and you are able to lift the trailer by the rear bumper (wheels of the ground), no it's not lock but if you push laterally on the tongue (sway forces) with the wheels on the ground, it is lock.