Hensley Arrow: How does it REALLY work?

TnAnFLA wrote:
Well Ron, we agree half way on this one. There is clearly no net lateral force applied to the coupler by the struts. However I don't think a rearward force is applied either. You are correct when you state that the struts push aft on the a-frame, but at the same time they are applying an equal forward push on the hitch head. Since the coupler is located on the upper element of the hitch head, mounted between the strut attachment points, the two opposing forces cancel out so there is no net force on the coupler in the longitudinal axis. This allows the coupler to provide two functions only. It acts as a "ball joint" in the vertical plane so the A-frame can pivot up and down on uneven road surfaces (e.g., when coming to the bottom of a sloped driveway) and it provides a point to transfer force from the TV along the longitudinal axis during acceleration and deceleration.

Andy,

Consider the situation where the HA is attached to the TT, but is not attached to the TV. Also assume the WD bars are not attached to the hitch head. If the struts are pushing forward on the hitch head, there also must be an equal force pushing rearward on the hitch head to achieve equilibrium. This rearward force can come only from the A-frame via the ball coupler and ball.

Now assume the TT tires are not chocked and do not offer any rearward rolling resistance. If the struts are pushing rearward on the A-frame, then there must be an equal force pushing forward on the A-frame to achieve equilibrium. This forward force can come only from the hitch head via the ball and ball coupler.

In order to have the struts in compression, in the absence of any external forces on the hitch head or A-frame, you must have the coupler pushing rearward on the ball and the ball pushing forward on the coupler. This will cause the A-frame channels to be in tension.

During towing, the wind and tire drag forces will cause tension in the A-frame channels rearward of the strut attachments. Some of this drag force will be reacted by a decrease of compression in the struts and some of it will be reacted by an increase of tension in the A-frame channels between the strut attachment points and the ball coupler. The amount of drag reacted by struts versus A-frame channels will depend on their relative axial stiffness. The increased tension in the A-frame will increase the rearward force on the ball.

The reason I tend to harp on the struts so much is that Hensley states in the installation manual (and I'm paraphrasing) that if the struts are set up incorrectly, you have reduced or no sway control. I think they are an integral part of how this whole system works.

I believe it is critical to have the struts adjusted so that the longitudinal axis of the TT is in line with the longitudinal axis of the TV when towing straight ahead. This means the HA's rear unit must be perpendicular to the TT's axis.

If the rear unit is not perpendicular to the TT's axis, the TT will still track nearly straight ahead; but the rear unit will be rotated and shifted to one side. If the rear unit is rotated by, say 3 degrees, the "straight ahead" virtual pivot point will only be moved forward about 24" instead of around 47". This means the "steering moment" will not be reduced as much as is should be. Perhaps even more critical is the fact that the VPP would be shifted to the side of the TV's centerline. This means that any forward thrust from the TT also would be able to exert a steering effect on the TV.

And I also think the preload they apply to the hitch head/four bar linkage is an important part of that.

Well, I can agree that compression in the struts will preload the ball and coupler. But, I do not see any way for the strut compression to preload the linkage. Again, consider the HA hanging on the end of the A-frame. If there is nothing to push rearward on the linkage, then there cannot be any forward force either. Perhaps we do need more discussion.

Ron