Weight Distribution Hitches - some good info

[billmoore]

Found a link to this really good discussion of how WD hitches work while on the F150 Forum:

https://www.rv.net/forum/index.cfm/fuseaction/thread/tid/14265335/srt/pa/pging/1/page/1

The topic comes up frequently here, so I thought this might be useful. Also, on the F150 Forum, someone took the information from that thread and applied it to a real world example that really brings it home, so I'm cutting and pasting that info below. This is not mine, it is from here:

http://www.f150forum.com/f82/excellent-discussion-how-wd-hitch-works-338841/

That example would be much better if it started with a theoretical tongue weight and how that changes loading on the axles to begin with and then went into how the WDH works to return weight to the front axle.

So, for example:
Tongue weight: 1000#
TV wheel base: 145"
TV rear axle to coupler: 65"
Ball to TT axle; 228" (19ft)

Just by dropping the tongue on the ball:
Weight removed from front axle (using rear axle as fulcrum)
1000# * 65" / 145" = 448.27#

Weight added to rear axle
1000# + 448.27# = 1448.27#

So, to add 448.27# back to the front we have two variables (I could probably reduce this but it was quicker to use his formulas)
X = spring bar force
Y = weight transferred to rear axle

448.27# = Y * (228+64)/145 (using rear axle as fulcrum)
448.27# = 2.02Y
Y=221.84#

X*(30/228)=Y=221.84#
X=221.84# / (30/228)
X=1685.98#

So the WDH
ADDS 448.27# to the FRONT axle
REMOVES 670.11# from the REAR axle
ADDS 221.84# to the TT axles.

In this example, the net weight added to the rear axle is 778.16# (1448.27-670.11) once everything is hooked up. Now, I did ignore the weight of the WDH itself. In practice, that would remove some weight from the front axle as well and would need to be accounted for in the calculations.

This is where the head tilt comes in to play as that is where fine adjustments to the force the spring bars exert are made. Those 1000# bars are not always exerting 1000#.

[billmoore]

I found it particularly interesting that dropping 1000 lbs of tongue weight on the hitch actually added 1450 lbs to the rear axle, and unloaded the front axle by 450 lbs.

And the effect of the WD hitch is pretty dramatic - transferring that 450 lbs back to the front axle, transferring 220 lbs back to the trailer axle, and removing 670 lbs from the rear axle. So in the end, only 780 lbs is added to the rear axle, not 1450.

[mitch]

I have not done the math but from a purely theoretical standpoint this means you could go over the trailer allowed axle weight.  On the new 16 footers that switched to single axle the use of a WDH would throw some weight on that axle. Wonder how close the trailer is to the axle limit as built.

[charliem]

I have not done the math but from a purely theoretical standpoint this means you could go over the trailer allowed axle weight.  On the new 16 footers that switched to single axle the use of a WDH would throw some weight on that axle. Wonder how close the trailer is to the axle limit as built.

[font size="3"]Good point, Mitch. I encountered this situation on my single axle RPOD. I used a WDH to even up the Pilot loads and weighed the rig. I was surprised to find the RPOD axle loaded near its 3500# limit.

The 16TBS now has a 4400# axle with 2128# (3370-442) as built. That's with one empty propane bottle, no battery, and no water in WH or tanks. Now add 1 or 2 batteries and 2 full propane bottles to the tongue and redistribute it. Then add 6 gallons in the WH, some traveling water in the tanks, and a bunch of "stuff" and you're adding it up.[/font]
[font size="3"]
A good argument for retaining some design margin.[/font]

[daplumbr]

This is very interesting info. I had never thought about weight transfer from front to rear on the TV due to the increased loading on the hitch. As a practical matter, many vehicles used now as TVs are operating close to their hitch weight limits even with Camplites. That makes front-rear weight transfer even more relevant and is a strong argument for a WDH. 

[pinstriper]

The real eye opener for me was the WDH adding weight to the TRAILER's axles.

So anybody that tells you a WDH allows you to run close to, or Mo forbid, overload the trailer, because you have a WDH is uninformed. No we know better.

[david]

I was shocked by the 448 lbs of weight transfer in the example above. While I think that I agree with the geometry and the math presented in the example, the parameters selected are rather extreme. The rear axle to coupler dimension in the example is 65". For my Pathfinder it is 45". The longer dimension exaggerates the weight shift from front to rear when weight is placed on the coupler. To paraphrase Archimedes, with a long enough lever, I can move the world. Or said another way, with a 100' lever, I can raise the front wheels with my foot.

Also the 1,000 lb tongue weight is 2-3 times what we see on LLs.

So, not having done the math, I am guessing that putting 350 lbs (my 16TBS) on the hitch of my Pathfinder adds no more than a hundred pounds or so to the axle load in addition to the tongue weight, all of which was taken into account when Nissan specified 500 lbs max tongue weight.

David

[billmoore]

David, the example came from an F150 forum, and the numbers are very close to reality both on lengths and typical tongue weights for most of the trailers people are towing on that forum.

And the bigger LivinLite toy haulers can approach that 1000 lb tongue weight when loaded. Even our 7x20 would be over 700 lbs if we were loaded all the way to GVWR, probably closer to 800 lbs.