tluxon
Kirkland, WA
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tluxon wrote:BurbMan wrote:...Of course, the same wind is applying its force to an infinite number of points along the same side of the TV also, so does that produce a series of counter moments? Dealing with the wind effect on the TV is a much more complex issue due to the fact that the front axle has turning wheels attached, but the TV is not affected nearly as much by the wind as the TT. You want to break the problem down to the simplest terms possible, so I would think it sufficient to concentrate on the lateral force the TV needs to adequately react to a given lateral force from the TT.
I want to make sure it doesn't sound like I'm diminishing the importance of the tow vehicle here. It's my opinion that the TV is the most important part of the equation when using a conventional sway control hitch. Based on many years of towing trailers from hay wagons and combines behind a tractor to RVs behind a truck, I'm convinced that the worst sway events are not the fault of the initial swing of the TT, but of the way the TV reacts to it. With a conventional hitch where the TV's lever to the coupling point is anywhere from 3 to 6 or more feet long, there is almost a springback, or lateral bouncing, effect. Soft, spongey tires compound the problem. If that's not bad enough, the natural reaction of the driver is to correct for the change in course they sense in the TV. What would otherwise (without a trailer) be a normal course correction can then exacerbate the situation, much like pushing someone on a swing when they're already going as high as they feel comfortable. Experienced tow-ers learn to undercorrect to help dampen the swing of the TV's lever to the coupling point.
Now just as driving a car solo requires regular course correction just to stay between the lines on the road, there is a continual need also to keep a TV/TT combination in line and between the lines. For a less experienced driver or a less than ideal TV/TT setup, this task becomes increasingly uncomfortable and tiring and there is a greater chance of too strong a correction, perhaps leading up to a dangerous building sway event. As a driver's experience increases, the more natural these corrections will become to the point that they may barely notice any difference from driving a car solo (I feel this way with our HP Dual Cam). Nevertheless, those corrections are still there.
Here's where the Hensley comes in again. By shortening the TV's lever to the coupling point and reducing the lateral force the TT imparts to that coupling point due to the longer lever from the TT's axles, that "bounce" I characterized above never really happens with the Hensley, and the correcting of the TV to stay between the lines should feel no different than when driving the TV solo. I haven't experienced this myself yet, but I'd bet I'm pretty close.
* This post was
edited 05/17/05 03:52pm by tluxon *
Tim -
wife Beverly & 2 boys who love camping
2002 K2500 Suburban 8.1L 4.10 Prodigy
2005 Sunnybrook 30FKS HP Dual Cam
Replaced 2000 Sunnybrook 26FK on 8/6/04
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Ron Gratz
full time RVer
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BurbMan wrote:...If we assume that the crosswind is blowing at 20mph, changing the length of the lever is not going to make the wind blow slower. ...
That's correct, the wind force does not change. But, if you increase the TT axles to pivot point distance, the TT tires will react more of the force to make up for the decreased force at the pivot point
Quote:...Further, we can assume that the wind gust exerts pressure at all points along the side of the trailer, not just at the coupler, so theoretically, there are an infinite numbers of points where force is applied, each with corresponding moments from the TT axle.
Of course, the same wind is applying its force to an infinite number of points along the same side of the TV also, so does that produce a series of counter moments?
The wind acting on the TV will produce a yaw moment acting on the TV. Most likely, the resultant wind force will act forward of the TV's rear axle producing a clockwise moment which must be countered by a left steering correction. The lateral force imposed by the TT on the TV also will produce a moment on the TV which must be countered by a steering correction. Whether the TT-imposed force acts to the right or to the left will determine the direction of this second correction.
Ron
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Ron Gratz
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How the HA Reduces Lateral Loads Imposed by TT on TV
When external loading is imposed on the TT, via wind forces for example, some of the force is reacted by the TT's tires and some by the ball coupler. If the resultant external force on the TT acts forward of its tires, a clockwise moment will be imposed on the TT. If the resultant force acts behind the tires, a counterclockwise moment will be imposed. In the special case when the resultant force acts in line with the tire reactions, there will be no moment; all of the external force is reacted by the tires and there is no lateral load on the coupler.
The moment imposed on the TT by external forces can be countered in two ways:
1. By a lateral force acting at the coupler multiplied by the distance from coupler to TT axles and/or,
2. By a moment applied via the coupler (which the HA can provide, but a conventional hitch cannot).
With the HA, both moment and lateral force can be transmitted via the hitch. Since some of the moment acting on the TT can be countered by the moment transmitted via the hitch, the HA's lateral force does not have to be as large as it would be with a conventional hitch. The reduction in lateral force results from the effective increase in the TT's tongue length due to the forward projection of the virtual pivot point.
For example, consider a TT connected with a conventional hitch where the ball to TT axles distance is 20'. The actual moment arm is 20'. Now consider the same TT connected via a HA and assume the HA can project the VPP forward by 4'. The distance from the VPP to the TT axles has been increased to 24', so the virtual moment arm is 24'.
Now, for a given moment, the force decreases if the moment arm increases. Therefore, the force acting at the VPP is reduced to 20/24 = 83.3% of the force which would be imposed on the TV using the conventional hitch.
Now, it this makes it seem as though the hitch has caused the external force on the TT to be reduced, I must say that not even the Hensley Arrow can calm the wind. The amount of external force which is not being reacted at the coupler has been transferred to the TT's tires. For a given lateral load on a TT, as the distance from the TT axles to the pivot point (actual or virtual) increases, the TT's tires pick up more and more of the lateral load.
Ron
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BurbMan
Indianapolis, IN
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Now, Tim, you know the sad truth about why I never made it through Physics....thanks for clarifying!
Don
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Jack Marley
Ft Lauderdale, FL
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Claude: I think we're talking about 2 different hitches; the one I had had the ball at the diff, that was the pivot point. It had a fixed ball which went on the trailer: it didn't move, of course. No bars or any kind but the weight was all on the rear axle thus elimimating the car springs. As I said, it worked fine but had some draw-backs. Anybody else familar with that unit?
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Claude B
Montreal, Canada
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Jack, you're wright, i don't remember the hitch you had but do you remember the one i described (wishbone)? They where quite popular in the 60's
Your's was maybe an early Pullrite model.
Claude
2013 Gulf Stream VISA 19ERD
VW Touareg TDI 2013
VW Passat 2012 TDI
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Jack Marley
Ft Lauderdale, FL
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claude,etal: This was in th '50s, never knew the brand as I bought it used and had been re-painted. I'm not familar enough with Pull-rite to know if it could have been it. What was the one like you were talking about from the '60s? Jack
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Stressor
Whitefish Bay, Wisconsin
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Ron wrote: The reduction in lateral force results from the effective increase in the TT's tongue length due to the forward projection of the virtual pivot point.
Longer pendulums move slower, but they still move.
I am thinking that the linkage itself resists the angular change in the travel trailer due to outside influences. In operation, there are effectively two links between the TV and the TT, 7 inches and a little bit apart, with the TT being the entire load. As pointed out previously, one link is under tension and one is under compression as the trailer angle changes. The rear cross link is fixed in place by the struts, and the only force that can be applied to the TV must travel through the links.
This must decrease the input from the TT to the TV, and while the diagrams are informative, in actual operation, they only apply when we turn the TV. When towing straight, the VPP stays in a very short arc, left and right.
The connection between the TT and TV is rigid during forward straight line travel, with a heavier trailer and a larger frontal surface increasing that rigidity, and the forces are applied to all of the wheels, TT and TV, as a result.
Or so I think. So far.
Milton Findley (and Kerene)
A small piece of my mind...
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Ron Gratz
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Who said this thread was only for the engineers? First we have a marketing guy and now we have a psychologist and both of them have postulated rational theories about the physics of the HA. Milt's reference to a HA-coupled trailer acting, or not acting, like a pendulum opens up some of the "contentious stuff" I alluded to a few posts back.
If one could vertically hang a semi trailer or a 5er or a PullRite-coupled trailer from its pin, the trailer could be made to swing like a pendulum about the pin. A vertically suspended trailer coupled with a conventional hitch could be made to swing like a pendulum about the ball. And, a vertically suspended trailer coupled with a Hensley Arrow could be made to swing like a pendulum about its VPP. Other than a small amount of friction, there is nothing inherent in the pinned or ball connectors which would prevent the oscillations. The same is true for the HA's 4-bar linkage.
If this sounds outrageous, consider that there are clocks, especially mantle clocks, in which the pendulum is suspended from a 4-bar linkage which is governed by the same physics as is the HA. These clocks use the 4-bar linkage to effectively increase the length of the pendulum. The objective is to achieve a one second or a one-half second period with a much shorter physical pendulum length. The 4-bar linkage in a clock is not "rigid" and does not resist the swinging of the pendulum. The same is true for the HA's linkage.
The HA's linkage does not provide a "rigid" connection at any angle, zero or otherwise. A heavier trailer and larger frontal area could cause a greater longitudinal "pulling" force on the linkage. However, the HA's linkage does not generate a moment in response to a longitudinal force. It only generates a moment in response to a lateral force. When the TV and TT are moving straight ahead it is the lateral force on the TT tires which prevents the TT from moving side to side. The HA only controls how lateral loading from the TT can affect the TV.
Ron
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tluxon
Kirkland, WA
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I completely agree with Ron's assessments so far.
I'd like to address this impression of "rigidity" in the Hensley hitch. I'm guessing that most Hensley owners at one time or another have seen the marketing video that has the hitch model segment I posted captures from back on [url=http://www.rv.net/forums/index.cfm/fuseaction/thread/tid/15531727/srt/pa/pging/1/page/4.cfm]this page[/url]. The first time I saw that video four or five years ago, I also had the impression that lateral forces acting on the trailer wouldn't create a bend at the hinge, while turning inputs from the tow vehicle would readily bend the same hinge. That same impression I had seems to be held by just about anybody who's ever towed with a Hensley or seen the video. However, whether intentional or not, the test model shown is deceiving in that it misrepresents the actual geometry involved.
To follow what I'm talking about, please take another look at the test model.
[img]http://home.comcast.net/~tbluxon/HensleyTestModel_sm.jpg[/img]
Pay particular attention to where the "wheels" are on both the pretend TT and the pretend TV. Do you notice how much closer the [i]TV's rear axle[/i] is to the hitch than you'd see on a real TV. Considering the projected VPP of the Hensley being some 47 inches forward of it, the VPP on this model is considerably forward of the [i]rear axle[/i], which requires that the reactionary moment of the [i]TV[/i] be exactly opposite of what the most common reality would be. Not only does this make it impossible to bend the hitch when applying lateral force to the [i]TT[/i], but based on the moment the VPP is putting on the [i]TV[/i], would cause the TV to try to bend the hitch the OPPOSITE direction!!!
Now, take a look at where the [i]TT's axle[/i] is. This model places the theoretical TT's axle much, much closer to the hitch than would ever happen in reality. This serves to give an extremely short lever to the lateral force applied, which limits the resulting moment about the [i]TT's axle[/i] to a scant fraction of what would happen under real conditions.
I hope that by describing these characteristics in the Hensley test model, you can see how it is possible that ALL the lateral forces applied are reacted to by the wheels and NOT by the linkage of the hitch itself.
So, if you follow my reasoning here, you'll agree that there really isn't anything magical about the linkage in the hitch other than how it projects the actual pivot point forward.
[edit - reduced image to more appropriate size]
Note: Due to invalid formatting, all formatting has been ignored.
* This post was
edited 05/19/05 02:16pm by tluxon *
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