Hensley Arrow: How does it REALLY work?

BurbMan wrote:
... NOW.....if we turn these examples upside down (literally), and hold the semi trailer in the air, the tractor would still swing like a pendulum around the hitch point. BUT, if we hold my trailer in the air, would the HA let my Suburban swing like a pendulum?

Logic tells me NO, since the VPP is within the mass of the truck (not above it) it can't swing like a pendulum, it would have to rotate around the VPP, which it can't do because gravity is pulling the truck down, and the connection to the trailer is stopping the rotation. ...

Don, the instantaneous center of rotation (virtual pivot point) for each bar of a 4-bar linkage lies at the point of convergence of the two bars which are attached to it. This means that for the HA, the VPP for the rear bar (the rear, upper portion) and the VPP for the front bar (the front, lower portion) are at the same point.

If you hold your TT in the air, the HA will let the truck swing about the VPP. In this case, the HA's rear unit which is fixed to the TT does not move. However, the HA's front unit which is fixed to the TV is free to simultaneously rotate and move side to side as the TV swings about the VPP. The difference now is that, since since the VPP now is between the HA's front unit and the front of the TV, the front of the TV will move left when the HA's front unit moves right.

If any one bar of a 4-bar linkage is fixed in space, all of the other three bars still are able to simultaneously translate and rotate. Therefore, with the HA, it makes no difference whether you restrain the front unit or the rear unit. The other three bars are free to move. These kinematics are much easier to understand if one constructs a model of the HA's 4-bar linkage and observes how the bars move relative to one another.

Ron