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?

[tluxon]

The diagrams on the page you link to

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?

[Kenneth]

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.


Ken