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]

tluxon wrote:
...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,

The numbers stated in the Hensley patent document are:
Front pins 7 1/8" c-c
Rear pins 7 7/8" c-c
Side links 4 1/2" c-c

Using these numbers, I got a distance of 47.1". However, with these dimensions, I was not able to reproduce Hensley's stated maximum turning double angle of 165 degrees.

When I changed the side links to 5" c-c, I got a pivot point distance of 52.4" and was able to reproduce the 165 degree angle.

Ron

[Ron_Gratz]

Claude B wrote:
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)..

Claude,

I must respectfully disagree with this part of your post.

Yes, the HA does "lock" the TT tongue to the upper (rear) part of the hitch via the struts. In similar manner, the PullRite locks the TT tongue to the hitch via the WD bars. However, both hitches effectively extend the TT's tongue forward to a new pivot point.

With the HA, although the TT is "locked" to the rear part of the hitch, the rear part of the hitch is not "locked" to the front part of the hitch. The linkage allows the rear part to simultaneously rotate and move sideways relative to the front part. Therefore, the TT is not "locked" to the TV.

The key point to remember is that when the TT and TV are aligned nearly straight ahead, the HA and the PullRite act in the same manner except that the pivot points generally are at different distances from the TV's rear axle.

Ron

[Ron_Gratz]

Stressor wrote:
The virtual pivot will describe an arc in space on a horizontal plane as the side bars change direction.

Milt,

I fully agree. My AutoCAD models indicate that the loci will approximate an ellipse. Depending on which HA pin spacings you choose to assume, the major axis of the ellipse might be in the range of 47" to 54", and the minor axis will be around 38" meaning the pivot point can be as much as 19" to either side of the TV's centerline.

Ron

[Kenneth]

The HA looks something like this: (use your imagination...it looks fine on the compositon screen)
__|__
/_____\
|

The tow vehicle is at top and the trailer is at bottom. All four corners pivot. The upper inverted T (in this diagram) and lower T are rigid. The upper T is solidly on the drawbar, and the lower T is on the ball for lateral and transverse tilting, but held in position by the struts.

(The pivots are tapered roller bearings much like wheel bearings. I had my '95 HA apart for new grease and new seals. The old grease was hard and dry, but no bearing damage.)


Ken

[Ron_Gratz]

willald wrote:
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.

Will

Will,

Now you've done it. You used the s... word.

I think it is quite possible that, by having the pivot point at 20" behind the TV's rear axle rather than at the approximately 70" for a conventional hitch, you have reduced the lever arm enough to eliminate any perception of s... for the conditions you have encountered.

Ron

[Ron_Gratz]

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.

Please disregard the RED portion of the following paragraph. I now believe that it is incorrect. See my 5/13/05 4:59pm post for an explanation.

When towing, the usual condition is for the angle between the centerlines to be zero. In this limiting case, the "apparent" pivot point would be at infinity ahead of the TV. This means that, as the TT pivots through zero angle with a finite angular speed, all points on the TT would have to be moving at infinite tangential speed.

Ron

[Ron_Gratz]

Kenneth wrote:
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.

Ken

Ken,

I admire your ability to think outside the box. However, UniCacher's statement was, "The pivot point is actually BEHIND the ball, just forward of the trailer axles."

If UniCacher is following this thread, I would like to hear more about his model.

Ron

[Ron_Gratz]

tluxon wrote:
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.

Tim et al.,

Here is a link to information about Instantaneous Centers of Rotation.

The right side of Figure 2.15 shows how the centers (pivot points) are defined.

Ron

[Ron_Gratz]

Wow, this has turned into a great exchange of ideas and information!

Thanks to Will for getting it going, and thanks to Barney and Les for helping to keep the discussion on track.

My input is going to be somewhat limited (perhaps for the better) because Jean and I currently are in the process of disposing of 40 years of accumulated "stuff" with the goal of becoming fulltime Rvers on May 31st. Jean says I must keep off the internet during the daytime so people can respond to our ads. 😞

I do think we will find, as this topic continues, that there is only one "pivot point" of interest. I'm guessing it will be the "instantaneous center of rotation" which Tim referred to. And, it will make the analysis much simpler if we can show that the force vectors carried by the side links converge at this location making it a point at which only force and no moment is transferred to the TV.

Just some food for thought.

Back to the disposal process.

Ron

[Claude_B]

Doe's this make sense ?

My humble theory

BTW, i'm sorry for all my english mistakes, i'm french canadian but i can tell you that i sure work hard 🙂

[tluxon]

yepitsaCummins wrote:
...Is it correct that two of the bars are effectively rigidly attached to the tow vehicle and the other to the trailer?

I don't have a Hensley, but I made the assumption that the centerline of the TT and the back bar form a fixed "T" as do the centerline of the TV and the front bar. Can anyone confirm this to be true?

Thanks for the links to Hensley's patents, Kenneth. Other than being designed to handle higher tongue weights, do you know of anything else that's different with the new Hensley?

Thanks,

[Kenneth]

Hensley's new 2005 patent hitch drawings

The new Hensley hitch Patent #6,851,696
--quote--
Briefly stated, the invention is a hitch assembly comprising a hitch bar assembly coupled with a hitch receiver of a tow vehicle for transferring pulling and stopping forces to and from the tow vehicle. A hitch box assembly couples with the hitch bar assembly for transferring pulling and stopping forces to and from the hitch bar assembly, the hitch box assembly having a first pivot point. An overcenter latch assembly secures the hitch box assembly to the hitch bar assembly. A front support member pivotally connects to the hitch box assembly at the first pivot point for transferring pulling and stopping forces to and from the hitch box assembly and for pivoting during turns. A strut assembly pivotally connects to the front support member for transferring pulling and stopping forces to and from the front support member wherein the strut assembly can pivot vertically for accommodating uneven roads during driving. Also, the strut assembly includes a second pivot point. A ball mount assembly pivotally connects to the strut assembly at the second pivot point for transferring pulling and stopping forces to and from the strut assembly. The ball mount assembly laterally pivots about the second pivot point within the strut assembly during turns. The ball mount assembly includes a tail tube extending rearwardly. A ball plate assembly attaches to the ball mount assembly for transferring pulling and stopping forces to and from the ball mount assembly. The ball plate assembly includes a hitch ball for removable attachment of the trailer for transferring pulling and stopping forces to and from the trailer. A tail support assembly attaches to a trailer frame and couples with the tail tube whereby the tail support assembly restricts lateral movement of the tail tube and the ball mount assembly so the trailer remains relative to the ball mount assembly at all times. A slide assembly resides within the ball mount assembly such that forces inherent in towing the trailer are not transferred through the slide assembly. The slide assembly slides forwards and backwards to accommodate the change in radial movement of the converging links during turns. Converging links pivotally connect between the hitch box assembly at the first pivot point and the slide assembly whereby the angular position between the first pivot point and slide assembly can be varied. The converging links effectively move the pivot point between the tow vehicle and trailer forward of the hitch assembly. In addition, forces inherent in towing the trailer are not transferred through the converging links. A hanging support assembly attaches to the strut assembly including vertical links pivotally attached to the ball mount assembly for transferring tongue weight from the ball mount assembly through the strut assembly and front support member to the hitch box assembly and hitch bar assembly so tongue weight is not exerted on the converging links or the slide assembly. A jack assembly attaches between the trailer frame and the front support member for distributing tongue weight among tow vehicle wheels and trailer wheels.
--end quote--

The original hitch Patent #4,722,542
--quote--
Fishtail sway is caused by the large distance between the rear axle of the tow vehicle and the hitch assembly and is aggravated by lateral forces against the vehicle caused by winds or passing vehicles. Previous attempts to solve fishtail sway involve stiffening the connection between tow vehicle and trailer by using various methods of friction. While these methods help some, none completely correct the problem.

Fortunately, in U.S. Pat. No. 4,722,542, hereafter referred to as the "Hensley hitch", the sway problem is effectively corrected by forcing the hitch to turn through converging links that effectively move the pivot point between the tow vehicle and trailer to a point near the rear axle of the tow vehicle. Therefore, this design provides better steering and control of the trailer by eliminating trailer sway. While the converging links do this very well, the gross trailer weight is limited by the size and design of the converging links in '542 because so much of the tongue weight is supported by the converging links. As a result, increasingly heavier tongue weights require larger links, larger bearings, larger spindles, and larger related support systems. Increasing the size of these parts also increases both the hitch weight and the cost of manufacturing. In addition, the Hensley hitch needs workable brakes on the trailer controlled from the tow vehicle. Without trailer brakes or even with surge brakes the converging links tend to move to one side or the other due to the trailer pushing on the hitch assembly when the tow vehicle brakes are applied.
--end quote--

[Kenneth]

From the patent:
--quote--
Fishtail sway is caused by the large distance between the rear axle of the tow vehicle and the hitch assembly and is aggravated by lateral forces against the vehicle caused by winds or passing vehicles. Previous attempts to solve fishtail sway involve stiffening the connection between tow vehicle and trailer by using various methods of friction. While these methods help some, none completely correct the problem.

Fortunately, in U.S. Pat. No. 4,722,542, hereafter referred to as the "Hensley hitch", the sway problem is effectively corrected by forcing the hitch to turn through converging links that effectively move the pivot point between the tow vehicle and trailer to a point near the rear axle of the tow vehicle. Therefore, this design provides better steering and control of the trailer by eliminating trailer sway. While the converging links do this very well, the gross trailer weight is limited by the size and design of the converging links in '542 because so much of the tongue weight is supported by the converging links. As a result, increasingly heavier tongue weights require larger links, larger bearings, larger spindles, and larger related support systems. Increasing the size of these parts also increases both the hitch weight and the cost of manufacturing. In addition, the Hensley hitch needs workable brakes on the trailer controlled from the tow vehicle. Without trailer brakes or even with surge brakes the converging links tend to move to one side or the other due to the trailer pushing on the hitch assembly when the tow vehicle brakes are applied.
--end quote--

The original patent, #4,722,542
http://patft.uspto.gov/netacgi/nph-Parser?Sect1=PTO1&Sect2=HITOFF&d=PALL&p=1&u=/netahtml/srchnum.htm...

The new hitch and its 2005 patent, #6,851,696
http://patft.uspto.gov/netacgi/nph-Parser?Sect1=PTO2&Sect2=HITOFF&p=1&u=/netahtml/search-bool.html&r...

[yepitsaCummins]

Tim, I believe you are right, in that free body diagrams is the only accurate way to look at it. But, I think in the end, the VPP will be the intersection of the side bars of the 4 bar linkage as you diagram above. That pivot point will move as this linkage flexes. Is it correct that two of the bars are effectively rigidly attached to the tow vehicle and the other to the trailer?

[tluxon]

willald wrote:
...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.

If we really went crazy, we could break it down to free body diagrams and increase the scope from simple theoretical points to analyzing specific forces, points of application, centers of mass, moments of inertia, etc. Personally, I'd rather just look at the 4-link hitch as a sophisticated hinge that allows twisting forces to create an angle and prevents lateral forces at the "hinge" from doing the same.