Hensley Arrow: How does it REALLY work?

Ron Gratz wrote:
Will, I'm still waiting for Milt to explain how a 4-bar linkage, whose links are connected via bearings, can be rigid ("locked up")when the hitch is aligned straight ahead, but be free to move otherwise.

Perhaps the linkage utilizes magic bearings which sense the articulation angle and "lock up" at zero angle but "free wheel" at non-zero angles. Or, perhaps, a magic diagonal brace materializes at zero angle to convert the 4-bar linkage into two rigid triangles. And, the brace then vanishes at non-zero angles. If it ain't magic, what is it?

Obviously, this was said tongue-in-cheek, since clearly if the hitch became locked at angle zero, how could even the tow vehicle initiate a turn? You may as well weld the TT frame to the TV frame and remove the TV's rear axle. Arguing the point about a possible one-way hinge makes me feel like I'm arguing with a child - I know no honorable way to do it.

Look - if it was possible to design a one way hinge or pivot point, Hensley wouldn't have needed to go to all the trouble to utilize a 4-bar link. Or WOULD they?

I say this because I think it's ironic that we're arguing over whether or not the hitch is rigid from the TT's perspective when it actually benefits the combo that it's not. Take a look at the diagrams I've attached below - I think they will help reveal something very interesting about how the linkage uses lateral input from the trailer against itself.



In this figure I'm noting a lateral force at the rear bar of the hitch that's a result of the counterclockwise 1000 foot-pound moment used in the previous moment diagram using dimensions from Will's trailer. It's intriguing to note that the advantage of the hitch is that it ALLOWS the lateral force from the trailer's moment to turn into an easy translation of the rear bar. From an aligned orientation (straight TV and TT), the front bar of a free 4-bar linkage is virtually helpless to oppose any lateral translation until AFTER the rear bar has translated noticeably off-center.

But this is actually a GOOD thing. Remember that with the 4-bar linkage you can't get translation without rotation. Please note that in order to get sufficient translation to require much lateral reaction by the front bar attached to the TV, the trailer would have to rotate significantly to even allow that kind of translation. Refer to figures 2 and 3.




Please note that Figure 3 is exaggerated to illustrate that the TT tires would have to move significantly off-center to accommodate the seemingly small rotation the translation has tried to induce. But more importantly note what direction the trailer would have to rotate. That's right, CLOCKWISE - which is exactly OPPOSED to the original CCW moment that caused the rear bar translation in the first place!

For this reason I would contend that there's no need for a rigid connection from the TT's perspective to the TV, as the free-swinging linkage performs much better anyway.

Tim