What Does a Weight-Distributing Hitch Do?

[Brian B-P]

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




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.

[Brian B-P]

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




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.

[CD Smith]

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.

[Gina D.]

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.

[Roger H]

See, Gina (I love this...) I TOLD you so!

Roger

[Brian B-P]

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.

[Pete Dumbleton]

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?

[Brian B-P]

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.