What Does a Weight-Distributing Hitch Do? - Page 5 - Fiberglass RV


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Old 03-05-2007, 02:02 PM   #57
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...His feet are spanning the belly, which corresponds to the rear axle (one foot is the effect of the hitch head/receiver bolts on the TV frame and the other is where the chains mount to the trailer frame). As he pulls up to get the belly off the ground, his feet push down transferring the weight to the hands and toes.
But the WD system is mounted entirely behind the rear axle (south of the guy's belly), while if this midget's feet are spanning the belly he haa a foot on the guy's back... that's cheating. Anyway, same point, the midget (or the WD hardware) doesn't get to touch the ground, and has to apply forces only to the body (or tug and trailer).
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Old 03-05-2007, 02:08 PM   #58
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Maybe this explanation on a different group would help:

WDH
You have to get to the fourth post in that discussion before the explanations become correct, ignore that last part of the fifth one, and then the sixth is pretty sound... but if you can follow that sixth one, you probably didn't need an explanation!

Frankly, I'm not looking for an explanation for me, but anything which will help others who might not yet have a (wheelbarrow) handle on the situation.
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Old 03-06-2007, 08:02 PM   #59
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Quote:
But the WD system is mounted entirely behind the rear axle (south of the guy's belly), while if this midget's feet are spanning the belly he haa a foot on the guy's back... that's cheating. Anyway, same point, the midget (or the WD hardware) doesn't get to touch the ground, and has to apply forces only to the body (or tug and trailer).
You're right, the analogy is flawed -- I put his foot on the other side of the belly because that's where the effect of the WDH is felt because of the leverage on the receiver hitch to the frame...
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Old 03-10-2007, 02:27 PM   #60
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The lull in posts suggests that everything so far is now making sense, so I'll take a shot at applying the principles to an example, with drawings to illustrate. Since CD Smith generously offered a set of dimensions and weights for his Cadillac Escalade and Casita 17', as well as his weight-distribution intentions, I'll use his rig for the example. I hope that's okay, CD...

First, here are the tug (Escalade) and trailer (Casita 17') separately:
Separate Vehicles

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All of the downward force on the two vehicles is their weight, which is shown as a single force acting at each centre of mass - the point at which the whole mass of the vehicle would be concentrated if it could be squeezed down to a single point in space.

The upward force is the road holding up each axle, and a stand located at the coupler of the trailer. The distribution of load between the axles (and stand) is entirely determined by the distances to those supporting points from the centre of mass. Those centres are just calculated, and since we measure the axle loads directly on a scale there's really no need to know them... they're just there for interest.

In this and the drawings to follow, the forces are in pounds, the dimensions in inches, and while the calculations are based on CD's dimensions, the drawing is slightly off in some cases because the background grid didn't quite cooperate... the principles are unchanged.

The size (in pixels) of the drawing is larger than the FiberglassRV guideline (500 pixels wide), but a drawing like this is not clear enough to read in a small size, and the file size is small because it is mostly white space. I hope this is okay...
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Old 03-10-2007, 02:39 PM   #61
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Now we hook the trailer up to the tug. There's no stand under the coupler anymore - the tug takes the load:
Coupled Vehicles - no Weight Distribution System

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The structure of the tug acts as a lever, pivoted on the rear axle, so that the force of the trailer coupler down on the ball lifts load from the front axle of the tug. The rear axle load must increase to balance the total of the load on the ball and the load relieved from the front axle.

There is a caution message at the bottom of the drawing, and I'll repeat it here:
Relative proportions of front axle load reduction and tongue weight do not agree with wheelbase and axle-to-ball distance: significant measurement error must exist.
Weighing is always inexact, and calculations (such as the tongue weight) involving several measurements can accumulate more error. Hopefully, the principles are still clear.
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Old 03-10-2007, 02:55 PM   #62
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Finally, we add the spring bars and pull-up hardware of the weight-distribution system and crank 'em up in an attempt to hit CD's target of having every one of the three axles carry 150 lb more than they did as separate vehicles - an even 1/3 of the calculated 450 lb tongue weight for each.
Combined Vehicle with Weight-Distribution System Applied

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I approached the calculation in two steps:

First the trailer - blue in the diagram
The chains at the end of the spring bars and tightened so they are pulling hard enough to add 150 lb of load to the trailer axle. The trailer frame is a lever, the chain point the pivot, and the added axle load is balanced by more load on the coupler... since the coupler is much closer, most the load is on the coupler. The chain tension is the total of loads added to the coupler and axle.

Then the tug and WD system - green in the diagram
There are only three points of support for the combined rig. The three must remain in balance, so using the tug's rear axle as a pivot, the front axle load must increase with the trailer axle load, in a proportion determined by their distances from the rear axle. The total of those two increases must be the decrease on the rear axle.

The result is nearly CD's desired net 150 lb carried by each axle. A rig with different dimensions would distribute load changes in a different proportion.
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Old 03-10-2007, 08:40 PM   #63
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I believe that anyone reading your interpretation of my analysis will be mislead.

I never stated that I "desired net 150 lb carried by each axle".

My statement was "These weights 'assume' the hitch weight is neutralized by the WDH and equally distributed to the three axles ...".

It does not take a 739 lb. pull on the spring bars to 'neutralize' the 450 lb. measured hitch weight. If fact the necessary lift by the spring bars is closer to 360 lbs. and does not result in adding 150 lbs. to each of the three axles.

In fact the measure lift at each of the spring bars when my WDH is installed is 216 lbs. (108 lbs. on each bar) which only neutralizes 231 lbs. of the hitch weight.

I don't know anyone that trys to set their WDH bars to completely neutralize the measured (none WDH) hitch weight.
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Old 03-11-2007, 12:11 AM   #64
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I was told when I got the wdh installed that I would notice the same intense and dramatic difference in towing as I did when I got my brakes operating on my 13.

I can tell you, that was absolutely correct! I picked up the 17 on friday, the wdh being one of the service items I had attended to, and noticed the difference before I had even traveled 10 feet. The freeway travel was like the trailer wasn't even there.

Altho I never had any issues without one on the 13/Element combo, I sure wish I could have had one on it. The travels would have been much more pleasant and relaxed.
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Old 03-11-2007, 03:33 PM   #65
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See, Gina (I love this...) I TOLD you so!

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Old 03-11-2007, 04:24 PM   #66
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Gina, I'm glad your WDH is working for you. Did you get axle weights for the Liberty without the trailer, and for the Liberty and trailer with and without the WDH hooked up?


Even without any of the dimensions, this information would help explain the effect of the WD system.
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Old 03-11-2007, 06:12 PM   #67
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Charlie and Brian, this is a great team effort to get the weights and see the effects of using a WDH.

Charlie, if you ever get back on the scales again, I'd like to see the weight effects on the axles of disconnecting the leveling system (with the WDH disconnected).

Brian, altho the system really does hinge on the weights at the axles, I believe that the instructions use the fender measurements because the average owner (and even RV shop) doesn't have the scales at hand for adjustments -- The fender measurements aren't as accurate as the weights but have the advantage of being convenient, close enuf and easier to understand than ratioing weights (I, for one, don't even know what the weight objectives would be...).

That seems to be a lot of overhang on the Escalade, esp compared to my old short-bed pickup where I moved the receiver forward and shortened the ball-mount bar, so I would expect changes that far out would really make some differences.

PS Brian, what program are you using to make the sketch?
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Old 03-11-2007, 07:41 PM   #68
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Pete, I'm using Microsoft's Visio (2002 version). My package isn't really well suited for this purpose (it is not intended as a serious CAD program, although it does have dimension symbols) but it generally works well... and I have a copy.

I can e-mail the original file (.vsd) directly to anyone who has Visio and is interested; the file is much smaller and more clear than the JPGs, but of course is not usable by most people so it is not posted here. I can also export a version in the formats used by AutoCAD (.dwg or .dxf), or Adobe Illustrator (.ai). I think that the .dwg works decently, but I have no way to check the Illustrator file.
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