Hensley Arrow: How does it REALLY work?

jdwhittaker wrote:
...Thanks for posting the link to your model video; I think it gives me a much clearer idea of how your theory works. Please allow me to explain where I think that the flaw in your theory lies. In your video model you allow too many degrees of freedom. There are several types of 4 bar linkages, most usually have anchor points to restrict unwanted movement. In your example you allow full unrestricted movement of the linkage by anchoring it only by pivot points at the midpoint of the top and base of the trapezoid. In the HA this is not the case. To demonstrate you’d have to glue or fasten perpendicular members to the model at the midpoint of the base and top of the trapezoid, then move the model again the same way you did before, and watch how wildly the perpendicular members move back and forth. Now for the TT or the TV to be able to move in this fashion they would need to have swiveling casters for wheels. Since the wheels on a TT don’t swivel, they act as an anchor point that is attached to a 20’ (based on TT size) lever arm. Based on that premise if you go to your model again and pin down the TT end of the hitch so that it can not pivot (based on our new anchor point) and apply inward force to either the upper left or upper right corner of the trapezoid in a direction that approximately bisects the corner that you have chosen you see motion that is the same as that of an HA Remember not to pin down the TT end of the model so tight that there cannot be motion as needed through all 4 pivot points in the linkage. The resulting motion is a little bizarre, but the way that the HA moves is a little bizarre...

Hi Dave, I don't like that you call my linkage video, a "theory", but you've got a good grasp of the picture now. The lack of constraints on the model in the video was INTENTIONAL. I wanted as few constraints as possible to help people understand that just because the linkage is trapezoidal in geometry doesn't mean it can't bend from certain inputs. If I begin to add restrictions to the degrees of freedom of my model then I am introducing external components to the nature of the linkage. Then we're starting to talk about the behavior of the applied system. Of course the HA as a hitch system (which includes the attached vehicles) adds HUGE restrictions to the allowed degrees of freedom - that's the key to its effectiveness! On that I don't think anyone disagrees.

Tim