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?

[Trail-Mate]

Go out and Price some good 5Th weel hicthes, They are not that cheap either.

[bettered]

Stressor wrote:
Bettered, you are another one who needs to buy a clue. :B

There are opinions, and there are facts.

The design of the Hensley Arrow uses double taper roller bearings to secure the links to the upper and lower elements (the front and back links of this 4 bar linkage.) The method of operation and application of roller bearings has been around longer than you yourself Milt. It's a proven design for securing members that rotate. There is certainly no wild theorem as to how they might work, the technology exists and has been a part of accepted design practice for well over 50 years.

You are free to argue your opinions and theories in the confines of your classroom, where you are the arbitrar of the grades your students receive. But in the real world, your opinions and theories can be readily tested by thoughtful people with far more experience than I.

You may pronounce a grade for me on this exam, but it really is of supreme indifference to the world or how it really works.

Cling to your opinions as you will, but facts are always stronger than opinions.

Was it the students who coined your forum name? Just curious.

[Ron_Gratz]

Stressor wrote:
The trailer cannot exert a yaw force in a Hensley equipped rig.

Milt, I just love it when you demonstrate your knowledge of physics.

Perhaps you would like to explain why the trailer cannot exert a yaw force?

[Stressor]

bettered wrote:

Stressor wrote:
The lateral force is applied to the whole rig under those conditions, because the Hensley linkage is locked up tight, and with the exception of whatever play there is in the components, the rig is as stiff as a two inch steel bar can be stiff.

If you actually own a Hensley Arrow, you can demonstrate this to yourself on your next outing. Drive along and watch the rear end of your trailer for a while. Stop for a break, and while you are there, loosen the struts on the Hensley by a turn. Drive on, and watch the rear end of your trailer. You will see movement that was not there before.

In a locked linkage, there is no virtual pivot point. Steering input from the tow vehicle unlocks it, and as soon as things straighten out, it is locked again. The trailer cannot exert a yaw force in a Hensley equipped rig.

Check out the Hensley web site, they explain how the hitch "actually works" very clearly.

:B

The links between the upper and lower components of the hitch are fixed in place about their axis of rotation by double tapered roller bearings (both top and bottom) in the hitch, entirely similar to the way wheels are mounted on axles.

If this system operates as you've suggested, why don't we have cars driving down the road suddenly "locking up" and skidding out of control to a panic stop. With millions of cars on the road, surely one of them would have experienced lockup by now.

Bettered, you are another one who needs to buy a clue. :B

[robsouth]

I know not. Hee Hee.

[bettered]

robsouth wrote:
Seems to me that if I thought I needed a HA, I would be inclined to switch to a 5th wheel and be done with it. I cannot imagine paying the price of an HA. I have never needed one and don't expect to. JMHO.

Sorry Rob. I can only hope to encourage you to trade in your TT for a 5er and then tell us how much you saved over the cost of an HA. But with a 22' rig, you're probably right, you may never wish you had an HA. At least I hope not.

[drfife]

robsouth wrote:
Seems to me that if I thought I needed a HA, I would be inclined to switch to a 5th wheel and be done with it. I cannot imagine paying the price of an HA. I have never needed one and don't expect to. JMHO.

Your logic does not make financial sense.

The cost of a 5th wheel trailer and 5th wheel hitch is much more than the cost of a comparable travel trailer and Hensley hitch.

[robsouth]

Seems to me that if I thought I needed a HA, I would be inclined to switch to a 5th wheel and be done with it. I cannot imagine paying the price of an HA. I have never needed one and don't expect to. JMHO.

[Ron_Gratz]

In another thread, Bryanl posted:

Bryanl wrote:
AFAIK there is no evidence of any sort that a Hensley is actually safer than any other properly installed and used trailer hitch.

Yes, I know. This one riles people something fierce. That's an interesting phenomena, too.

But the HA as inherently safer is sales hyperbole. Yes a bit better handling can enhance safety a bit. But an HA isn't going to help you make your rig stop on a dime or avoid rollover on evasive maneauver or prevent unexpected circumstance from happening.

It the safe and aware driver who properly handles his rig and maintains awareness of conditions that makes the difference.

Bryan,

The Hensley Arrow and the PullRite reduce the TT's ability to "steer" the TV by virtue of moving the point of application of lateral force closer to the TV's rear axle. Because there is less "steering moment" imposed on the TV, the driver does not have to make as large or as precise a steering correction to maintain yaw stability.

This means it is less likely that the driver will overcompensate with steering actions which will increase the yawing of the TV and TT. The overcompensation can lead to an unsafe condition. Therefore, if the HA and PR can reduce the potential for overcompensation, then the HA and PR can provide potential for increased safety.

Ron

[markc4]

Willald WHAT have you done???

I think the forum is gonna blow!!

[bettered]

Stressor wrote:
The lateral force is applied to the whole rig under those conditions, because the Hensley linkage is locked up tight, and with the exception of whatever play there is in the components, the rig is as stiff as a two inch steel bar can be stiff.

If you actually own a Hensley Arrow, you can demonstrate this to yourself on your next outing. Drive along and watch the rear end of your trailer for a while. Stop for a break, and while you are there, loosen the struts on the Hensley by a turn. Drive on, and watch the rear end of your trailer. You will see movement that was not there before.

In a locked linkage, there is no virtual pivot point. Steering input from the tow vehicle unlocks it, and as soon as things straighten out, it is locked again. The trailer cannot exert a yaw force in a Hensley equipped rig.

Check out the Hensley web site, they explain how the hitch "actually works" very clearly.

:B

The links between the upper and lower components of the hitch are fixed in place about their axis of rotation by double tapered roller bearings (both top and bottom) in the hitch, entirely similar to the way wheels are mounted on axles.

If this system operates as you've suggested, why don't we have cars driving down the road suddenly "locking up" and skidding out of control to a panic stop. With millions of cars on the road, surely one of them would have experienced lockup by now.

[bettered]

bettered wrote:

Interesting observation. With my hitch, I don't get that "initial" wave at all. Zip. The only push I feel is when the bow wave hits the front of my TV as the big guy passes me. Even then it's less of a blip when I'm towing than when I'm not. The whole rig seems more stable towing with the Hensley than not towing at all. Go figure.

RonGratz wrote:
Ed,

When a bow wave exerts a right-directed force on the rear of your TT, the TT responds by exerting a left-directed force on the TV. With a conventional hitch, the force is applied at the ball. The force will push the rear of the TV to the left and the relatively large resulting steering moment will cause the TV to yaw CW. It is primarily the yaw which is sensed by the driver.

With the HA, the lateral force from the TT is applied to the TV at a point closer to the TV's rear axle. The steering moment is considerably reduced and the yaw of the TV also is considerably reduced. The yaw might be reduced enough that the driver does not notice it.

As for the TV seeming to be more stable when towing, this could be due to the TT acting as a "yaw damper" for the TV. In order for the rear of the TV to be pushed to the right, the TT must undergo a CW yaw. The TT's yaw moment of inertia will resist the lateral push from the TV. This resistance will counter some of the bow wave force on the TV. Therefore, the response of the TV should be less when towing. At least, that's how I figure it.

Ron

I completely agree.

[sfdon]

We just finished our first road test of our new HA with trip from SF to Scottsdale AZ over the Grapevine and LA's 12 lane freeways. Early on the trip I had to brake hard and I felt a slight side thrust cause by a reaction of the TT. I tightened one of the struts a quarter turn and upped the setting on the brake controller slightly (6.2 to 7.2) and took the rig out for a test on a wide stretch of road. I accelerated up to 45 MPH and hit the brakes hard....no side thrust.

Trucks cause NO side sway, but I can feel the slightest reaction as large flat faced RV's pass by.

Based on this experience, I'm going ahead and list my dual cam setup on Craigslist.

Don

[Stressor]

The lateral force is applied to the whole rig under those conditions, because the Hensley linkage is locked up tight, and with the exception of whatever play there is in the components, the rig is as stiff as a two inch steel bar can be stiff.

If you actually own a Hensley Arrow, you can demonstrate this to yourself on your next outing. Drive along and watch the rear end of your trailer for a while. Stop for a break, and while you are there, loosen the struts on the Hensley by a turn. Drive on, and watch the rear end of your trailer. You will see movement that was not there before.

In a locked linkage, there is no virtual pivot point. Steering input from the tow vehicle unlocks it, and as soon as things straighten out, it is locked again. The trailer cannot exert a yaw force in a Hensley equipped rig.

Check out the Hensley web site, they explain how the hitch "actually works" very clearly.

:B