Andersen Weight Distributing Hitches - Page 5 - Fiberglass RV


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Old 07-28-2013, 01:49 PM   #57
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Quote:
Originally Posted by Brian B-P View Post
The friction between the cone and the head applies torque about the vertical axis, or if you prefer, controls motion in the horizontal plane... which is what is needed for sway control. As far as sway control is concerned, the chains and elastomeric springs are just to force the triangular plate, and thus the cone, to rotate with the trailer; as long as they don't go slack, the tension in them is irrelevant.

If the chains were in the same horizontal plane as the ball, then it would be nothing but a sway control (and that variation might be an interesting related product for Andersen); however, the chains pull several inches below the ball, so the rearward force on the plate, and corresponding forward force on the ball, combine to apply torque about the lateral horizontal axis to transfer load between axles - that's weight distribution. There is no movement required, but if there is movement about the axis you could call it movement in the longitudinal vertical plane. This has already been well described multiple times above - this is just my version, with the horizontal/vertical issue addressed.
If you look closely at the way the system is designed you'll see that the forces applied to the ball are equal and opposite. The top of the ball has the same forces applied as the bottom via the plate, chains, etc. Therefore the ball has a lot of torque with the top forces in the forward direction and bottom forces in the rear direction. Since these are equal and opposite forces there cannot be any transfer of force to the tow vehicle.
If they are not equal and opposite please explain how they are not.
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Old 07-28-2013, 02:02 PM   #58
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Originally Posted by Byron Kinnaman View Post
I think you're seeing some magic. The torque on the ball is created by equal forces on the top and bottom therefore how can any force be transferred to the tow vehicle.
The forces producing "weight distribution" (that is, forcing a shift from the middle axle - tug's rear - to the end axles) are not both on the ball; they're forward against the ball and rearward against the plate at the bottom of the post/cone on which the ball sits. These are horizontal forces, vertically separated and thus forming a couple. That's a couple, no net force - the hitch isn't trying to push the rig anywhere.

This is just like the more conventional WD design, in which vertical forces at the bar ends and the ball, horizontally separated, form the same force couple (with lower forces but longer lever arms).

No magic, just basic physics applied to solve a problem... which is engineering.
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Old 07-28-2013, 02:20 PM   #59
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Sorry, I posted by response above before I realized that Byron had responded to my previous post. So... step by step:

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Originally Posted by Byron Kinnaman View Post
If you look closely at the way the system is designed you'll see that the forces applied to the ball are equal and opposite.
I agree. In fact I think I've posted that a few times.

Quote:
Originally Posted by Byron Kinnaman View Post
The top of the ball has the same forces applied as the bottom via the plate, chains, etc.
I agree. Again, I've posted that a few times.

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Originally Posted by Byron Kinnaman View Post
Therefore the ball has a lot of torque with the top forces in the forward direction and bottom forces in the rear direction.
Okay, you're calling the spherical bit plus the long cylindrical post it sits on - or maybe the whole system head - as the "ball". Given that, we are saying exactly the same thing: the forces by the coupler and chains on the ball/head together apply a (potentially huge) torque.

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Originally Posted by Byron Kinnaman View Post
Since these are equal and opposite forces there cannot be any transfer of force to the tow vehicle.
Correct. The hitch is not trying to push the tug forward, or pull it rearward; there is no net horizontal force. It is also not fundamentally trying to lift it up, or squash it down... although tug is lifted somewhat by the trailer due to the reactions to this torque. In this respect, the Andersen design is just like every other WD system.

The tug must react to this torque on the hitch head. The reaction comes from increased force downward against the ground at the front tires, and reduced force downward against the ground at the rear tires and at the hitch. Just as with the hitch components, this is parallel forces separated by distance - in this case vertical forces of one or two hundred pounds separated horizontally by several feet for a typical FiberglassRV hitch user - forming a force couple.

The trailer is on the other side of the WD mechanism, so it must react too, in the opposite direction. The forces are reduced net force downward on the hitch and increased downward force at the trailer tires. Typically for travel trailer and tug, these forces are still smaller than ones on the tug, because they are more widely separated.


Although everything about WD systems - including this one - is relatively basic physics, most people who don't take an active interest in mechanical design have difficulty understanding them.
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Old 07-28-2013, 02:44 PM   #60
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Originally Posted by David78073 View Post
A conventional system can practically transfer all of the weight
There's the problem - all what weight? The amount of load to transfer to properly use a WD does depend on the hitch load (if the intent is to return the front axle to non-trailering load), but it also depends on the tug's wheelbase and rear-axle-to-hitch distance.

It doesn't matter if a system can be modified and abused to lift the tug's rear axle - enough capacity for operation is what matters, and there is no common definition of that. Since the Andersen No-Sway is intended by the manufacturer for use with typically configured rigs having up to 1400 pounds of hitch weight, it presumably has enough capacity for any of our trailers if properly installed.

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... my 22' HiLo has a 750# system, my 29' HiLo has a 1400# system...

***the 1400# system will make the vans rear tires just barely touch the ground as the tounge weight of the 29' is only 700#...
That's a colourful description, but I'm sure if you grab one of those rear tires you will be unable to budge it. This is a common problem with discussion of WD systems, although usually regarding the front axle: people say that after the trailer is hooked up and without WD the front tires are "barely touching", when if fact they are carrying only a few percent less load than without the trailer. If the (overly stiff or not) WD system is applied to return the front axle to its non-trailer load, then the rear axle is still carrying even more than it would without the trailer - a ton or so depending on the size of the van.
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Old 07-28-2013, 02:56 PM   #61
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... I did learn how to discern between horizontal and vertical. I just don't see what horizontal movement has to do with weight distribution. Potential for sway, yes.
Jim, you don't see it because it does not exist. I agree, yaw oscillations (relative motion between the tug and trailer about the vertical axis, or in the horizontal plane) do not change the WD action of the Andersen No-Sway system.
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Old 07-28-2013, 03:24 PM   #62
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There are many sources that talk about the WD limitations of the Andersen. Post #684 here sums it up the best. Also review the many weigh scale slips posted by Andersen users and you will see it is very limited by it's design.

The Andersen WD Hitch User Thread - Page 49 - Airstream Forums
Thanks for the direct link to the page of interest... post #684!

That post is by the well-known Andy Thompson of Can-Am RV. It is interesting that this is called out as the best summary, when it is immediately followed by posts by others who do not find the same limitation. Also, Andy doesn't show any illustration of the system setup beyond the degree of spring compression: we don't know if the chains are angled properly, for instance. He also reports no axle loads or hitch weight; this is typical of Can-Am RV, who sell $90,000 trailers but can't come up with ten bucks for a truck scale check of axle loads, and sell $3000 hitches but don't have a $150 tongue weight scale (I know, I asked them).

Without facts, Andy's comments are as Andy's comments about any towing subject usually are: interesting, but not fully developed enough to be of value. Just my opinion, of course... Months later, Andy did rejoin this discussion, and even discussed axle loads of a completely different rig with a completely different WD system (despite this being specifically the Andersen WD users thread) - so we know he can use a scale - but he never addressed any questions about his Charger/Airstream/Andersen setup. Such a missed opportunity...

I'll admit to not looking for those scale slips by Andersen users who do believe in measurement; the one that I noticed in earlier research showed axle loads and WD action right in the typical range for many FiberglassRV members' rigs, without requiring excessive compression of the urethane spring. That one was properly installed; I don't know about others.

My comments about chain angle and proper installation refer to a likely installation problem: if the chains are angled up toward the tongue, instead of level, the vertical component of the chain tension and increased downward load on the ball will produce no useful action, but will require more spring force for the same useful horizontal component of tension. This will happen if the frame brackets are not set to extend downward far enough, limiting the effectiveness of the WD system in much the same way as placing the chain bracket of a conventional WD system too far forward or back (instead of directly above the bar ends). Post #704 in the same Air Forums thread shows a moderate instance of this error by another member... although after 400 miles pulling a 27' Airstream he had still no problems with the setup.
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Old 07-28-2013, 03:32 PM   #63
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Originally Posted by Brian B-P View Post
This is a common problem with discussion of WD systems, although usually regarding the front axle: people say that after the trailer is hooked up and without WD the front tires are "barely touching", when if fact they are carrying only a few percent less load than without the trailer.
Glad to read the above statement. Our front wheel drive vehicle has most of it's weight (60%) on the front axle and our 200 lb tongue weight is small compared to the existing load on the front axle.

I have enjoyed the discussion of the Anderson WD system.
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Old 07-28-2013, 03:55 PM   #64
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... our 200 lb tongue weight is small compared to the existing load on the front axle.
In addition, if the distance from rear axle to ball is - for instance - 40% of the wheelbase, the reduction in front axle load due to the trailer's weight on the hitch will be only 40% of that 200 pounds... or 80 pounds. Of course, most people considering WD with an egg are talking double that hitch weight or more, and thus maybe a couple hundred pounds front axle load change. In my case (and I don't use WD), it's 160 pounds, or about 4% of the front axle load.

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I have enjoyed the discussion of the Anderson WD system.
It's entertainment for mechanical techies, also hopefully useful to trailer towing people.
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Old 07-28-2013, 05:09 PM   #65
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Originally Posted by Brian B-P View Post
Andersen design is just like every other WD system.
I disagree with this statement. Most WHD systems use spring bars (often incorrectly call torsion bars) to apply forces in the other axis. If looked from the side at the Anderson system the forces are applied from front to back along the tongue and the chains.
Standard WHD systems apply forces as seen from the side are applied up and down, along the chains and at the ball.
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Old 07-28-2013, 05:41 PM   #66
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Wheel base versus ball to axle distance.

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Originally Posted by Brian B-P View Post

In addition, if the distance from rear axle to ball is - for instance - 40% of the wheelbase, the reduction in front axle load due to the trailer's weight on the hitch will be only 40% of that 200 pounds... or 80 pounds. Of course, most people considering WD with an egg are talking double that hitch weight or more, and thus maybe a couple hundred pounds front axle load change. In my case (and I don't use WD), it's 160 pounds, or about 4% of the front axle load.
The above statement is the kind that's helps new trailer towers showing the importance of wheelbase and the importance of distance from the ball to axle.

We recently moved our ball 2+" closer to the rear axle. It's obviously a small difference but in the right direction.
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Old 07-28-2013, 05:46 PM   #67
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I disagree with this statement. Most WHD systems use spring bars (often incorrectly call torsion bars) to apply forces in the other axis.
Trivia... Back in the 60's my dad obtained a dealership for the Equalizer brand of WDH's. At that time the company rep called them torsion bars. On the Eq web site they now call them spring bars. Reese calls their bars round bars and trunnion bars.
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Old 07-28-2013, 05:58 PM   #68
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The bottom line is that with the well proven torsion(or whatever your preferred terminology is) bar WDHs and the new Andersen ones, is that they both provide forward rotational torque on the ball hitch, effectively redistributing the hitch weight. They both use a different approach, but the resultant is the same. Maybe the older system can provide more torque, but for my trailer, the Andersen hitch has proved to provide more than adequate torque, and I had to even dial it back a bit.
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Old 07-28-2013, 08:04 PM   #69
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The bottom line is that with the well proven ... bar WDHs and the new Andersen ones, is that they both provide forward rotational torque on the ball hitch, effectively redistributing the hitch weight. They both use a different approach, but the resultant is the same.
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Old 07-28-2013, 08:08 PM   #70
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Trivia... Back in the 60's my dad obtained a dealership for the Equalizer brand of WDH's. At that time the company rep called them torsion bars.
Arggggh. One of many anecdotes which demonstrates why sales people should not be taken seriously on anything but pricing.

Byron is right - the spring bars are not used in torsion. They are cantilever bar springs (meaning that they are anchored at one end and pulled on the other end to bend).

Can we all promise never to use the "T-word" in connection to these spring bars again?
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