Hensley Operation
Robert Carr
Introduction:
About a week ago, within the infamous "Trippleing, Hemlsey, wrecks, Yup"
thread, I proposed the idea that a trailer could swing relative to a tow
vehicle equipped with a Hensley hitch. In the quoted thread, I used an
example which had me lifting the back of the trailer until the tires cleared
the ground, walk a few steps to the right and set the trailer back down. All
of this accomplished without disturbing the tow vehicle.
This idea was developed using a model of the Hensley hitch linkage system,
that I constructed. The concept that a trailer could swing behind the tow
vehicle was met with very limited support
and much resistance. As a result, I concluded that, possibly, my model could
be suspect, particularly in the dimensions between pivot points and the
lengths of the links. I basically acknowledged that it was possible to
extract improper conclusions, if the characteristics of the linkage system
changed significantly, as a function of dimensions. The remainder of this
post describes the actions taken to resolve this issue and my conclusions.
Discussion:
First and most important is to describe what I mean, when I say swing the
trailer. To swing the trailer means to rotate the trailer about an axis,
located at the effective pivot point of the Hensley hitch system. This point
is not at not located at the hitch assembly. The point is typically located
very close to the rear axle centerline of the tow
vehicle. The point is created by the Hensley linkage system. I use the word
swing because it seems very descriptive of the process. It is much like, if
not identical, to the pendulum of a clock. This swinging action is not sway,
defined
as fishtailing, tail wagging the dog or other similar descriptions. These
descriptions of sway define a process in which the trailer is exerting
steering inputs to the tow vehicle, creating the all too common situation of
the trailer going one direction and the tow vehicle the other.
The next accomplishment was to obtain accurate dimensions of the linkage
system so that I could correct any errors in the model I constructed. I did
obtain dimensions, first from the patent disclosure and second from the
measurements on an actual Hensley hitch system. I would like to thank
Kevin("respk" on the newsgroup) for taking the time and expending the effort
to perform those measurements. The two sets of measurements agreed
reasonably closely, except for the length of the two links. The dimensions
differed by 1/2 in for the links and 1/8 in on the pivot distances. I
considered that the relatively large difference of 1/2 in on the links may
be reflecting a design change between the early development models and final
production models. That is pure speculation on my part. Either way, these
dimensions when applied to my model did not change the conclusion that the
trailer could swing behind the tow vehicle. The only real change was how far
forward the effective pivot point was from the actual hitch assembly.
Finally realizing that others could still question my model, I decided to
call Hensley. I briefly explained why I was calling and then posed the
question about swinging the trailer. The gentleman that I was talking to,
asked me to hold a few minutes. When he returned he said it works just like
you stated. He explained that they have a model in their office,
representing a tow vehicle, Hensley hitch and trailer. What they did is run
a test on what I was claiming. He had another individual hold the tow
vehicle up in the air(front bumper up) with the trailer hanging down from
the tow vehicle. He instructed the other individual to hold the tow vehicle
firmly(not allowing it to move or rotate). He then proceeded to swing the
trailer. He also characterized the motion at the front of the trailer as
describing an arc, rather than a single pivot point. I replied that my model
showed the same. It was concluded that the point at which the trailer was
swinging about, was the effective pivot point created by the linkage system,
located higher(in this test case) than the nose of the trailer. His final
comment was:(paraphrasing) We ran the test and it does what you said. I
found this gentleman at Hensley to be very cordial and helpful and I wish to
thank him for his assistance.
Conclusion:
What does all of this mean? The easiest way to describe the trailer swinging
in the real world is to describe a plausible situation. Let us assume that
we are towing a trailer, utilizing a Hensley hitch, down a very straight
road. Everything is proceeding very well. We come to a slight downhill grade
on a slight left-hand curve. As we proceed through the curve, the road
surface becomes somewhat banked(sloped downward to the outside) and very
slippery(ice and
water). Part way through the curve, the balding trailer tires lose traction
with the slippery surface. The trailer could very easily begin to slide out
to the right, swinging with respect to the tow vehicle. The hitch, just like
a Pullrite hitch, a fifth wheel hitch or even an 18-wheeler configuration
can do nothing to stop this action. The trailer is on its own. Furthermore,
while this is going on(early stages), the tow vehicle is still continuing on
it's normal
course. So far there have been no steering inputs to the tow vehicle.
Eventually, there is likely to be a wreck(jack-knife). Again, this is not
unique to the
Hensley or any other hitch. It is a fact associated with all articulated
vehicles, unless on rails.
My only reason for pursuing this subject to some logical conclusion was to
identify what I would call a myth. In other words, some factual operating
characteristics of the hitch, as related to sway(fishtailing), can not be
successfully extrapolated to cover any and all trailer events. The myth or
improper extrapolation of fact is derived, in my opinion, from the following
statements: "The Arrow allows movement on the ball in every direction except
side-to-side. Side to side movement is forced to go through the linkage
system which is one directional. From the trailer side, the linkage is
solid. Pivoting by the linkage must be initiated through the tow vehicle".
The key here is that they are talking about side-to-side movement, as if
pivoting on a conventional ball hitch. The hitch does exactly what they say.
However, you can not extrapolate those words of "solid from the trailer
side"
and "pivoting is initiated by the tow vehicle" to mean that the trailer is
somehow solidly locked to the tow vehicle for all events. It is, without
doubt, a novel implementation for significantly improving TT towing
performance. Just don't assign it some magical quality that it does not
possess. Remember, it is still an articulated vehicle combination.
I hope this is a sufficiently clear explanation and that I haven't burst
anyone's bubble.
Bob
wi...@epix.net
You done pretty good, Bob! I trust that all but the lunatic fringe will
agree with your conclusions. IMO you are a little too kind in view of
the actual Hensley statements, brochures, and the famous video, but I
can't see any factual errors in your post.
Will KD3XR
the Kindly Curmudgeon of Sill Hill
DJOsborn
>You done pretty good, Bob! I trust that all but the lunatic fringe will
>agree with your conclusions. IMO you are a little too kind in view of
>the actual Hensley statements, brochures, and the famous video, but I
>can't see any factual errors in your post.
Mr. Sill,
Mr. Carr, in describing the "swinging action" that could happen with a
Hensley hitch under the circumstances he described, wrote "This swinging action
is not sway, defined as fishtailing, tail wagging the dog or other similar
descriptions. These descriptions of sway define a process in which the trailer
is exerting steering inputs to the tow vehicle, creating the all too common
situation of the trailer going one direction and the tow vehicle the other."
Mr. Carr certainly seems to be saying (again) that the Hensley hitch, while it
allows the "swinging action" he described, does **not** allow sway (as defined
by you.
Congratulations on agreeing with us that the Hensley hitch doesn't allow
sway!
David, N8DO; FMCA 147762
djosborn at aol dot com
Charles H. Van Dusen
Thanks for your excellent description and for your tenacity. I posted earlier a
very similar situation on ice with a conventional TT/hitch setup and surmised
then that the PullRight, Hensley, and 5th wheel configurations would behave as
you describe. I see value in upgrading from the conventional hitch I now have
to a PullRight, Hensley or equivalent, but I am not expecting any "magic." Your
use of the term "myth" is particularly appropriate to the root of the
controversy around these improved hitches. While having many desireable
attributes and substantially improving the RV towing experience, they do not
remove the obligation of the DRIVER to be attentive, knowledgable and
rational....
Chuck
Hugh Darling
as I envisioned the operation to be.
Hugh
Robert Carr wrote in message
<6nPE3.5643$tJ1....@news.rdc2.occa.home.com>...
Robert Carr
I have to admit, I kind of borrowed your example. Thought it was pretty good
and about as non-hypothetical as possible.
Thanks
Bob
Donald J. Dickson
<charles....@tektronix.com> wrote:
Does a Hensley hitch maintain it's "straight line" characteristic when
braking as well as when pulling. A fifth wheel and a PullRight don't
change their characteristics because the pivot point is "actually" moved
close to the rear axle. The Hensley emulates this with the levers at the
pivot point which is still behind your rear bumper. It seems to me if you
are in a panic stop where the tow vehicle gets at an angle to the trailer
a Hensley would be no different than any regular equalizing hitch with
respect to steering the tow vehicle.
--
Don Dickson
Remove first "x" from xcx666 to reply by email.
Robert Carr
I may not be understanding your question properly. I read it, go think about
it and then read it some more. Let me try to answer.
The capability of the linkage system to lock out sway is still there,
whether towing or braking. This is true in a range of approximately 12 deg
either side of being lined up with the tow vehicle. For larger angles, this
sway lockout begins to degrade. This can also, I believe, be looked at as
the effective pivot point moving rearward toward the actual hitch assembly,
eventually being coincident with the hitch assembly for very sharp turning.
Is this what you were getting at?
Bob
Donald J. Dickson <xcx...@freenet.carleton.ca> wrote in message
news:xcx666-1809...@249.224/27.18.226.209.in-addr.arpa...
Donald J. Dickson
terms of a 25-30 degree angle and you answered that just fine.
Don Dickson
In article <Az7F3.5806$tJ1....@news.rdc2.occa.home.com>, "Robert Carr"