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Old 04-03-2012, 09:42 AM   #1
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How a weight distribution hitch works

This summary tries to explain WHY a weight distribution system might be necessary and WHAT a WD system does to improve a rig's handling:

Without a WD system, the tow vehicle's rear axle load could significantly increase due to leveraging of the tongue weight. Conversely the front axle load will be decreased. These axle load changes will make most tow vehicles "unlevel". The decreased load on the front axle can cause a loss of steering control and braking difficulties. The increased rear axle load might exceed that axle's rating, and the load on the receiver might exceed its rating.

A weight distribution system enables a tow vehicle to more effectively handle the tongue weight of a trailer by removing some of the load from the tow vehicle's rear axle and distributing it to the tow vehicle's front axle and the trailer's axle(s). Note - When the WD system is engaged the actual tongue weight does not change. Recommended tongue weight is from 10% to 15%.

Consult your owner’s manual to determine if your vehicle is suited for a WD system. Some are not due to frame construction and/or suspension.

Several recent posts have discussed WD hitches. In one older post a member stopped using his because he felt it was contributing to sway by decreasing the "tongue weight". I hope the following will give a better idea of what the WD hitch does and does not do. Questions and comments are welcome.

Example assumptions:
TV wheelbase = 130”
TV rear axle to ball coupler = 65”
Ball coupler to TT axles = 200”
WD spring bar length = 30”
WD spring bar rear end load = 1000 lbs/bar = 2000 lbs total

How the WD hitch works:

Spring bar tensioner pulls UP on rear end of bar and DOWN on TT tongue. DOWN force of 2000 lbs on TT tongue adds a load of 300 lbs at TT axles.
This is calculated using ball coupler as the fulcrum: 2000x30/200 = 300.

Now, having added a load of 300 lbs at the TT axles, we must balance the TV/TT teeter totter. Using the TV’s rear axle as the fulcrum, to balance the 300 lbs at the TT’s axles we must add some load at the TV’s front axle.
The lever arm from the rear axle to front axle is 130”. The lever arm from the rear axle to the TT axles is 65+200 = 265”.
The required balancing load at the front axle is 300x265/130 = 611.54 lbs.

Or, we can calculate the reaction at the TV’s rear axle by treating the TV/TT as a lever with the fulcrum at the TV’s front axle.
The lever arm for the 300 lbs at the TT’s axles is 130+65+200 = 395”.
The lever arm for the rear axle is the wheelbase = 130”.
Since the TT axles are “lifting up” with a force of 300 lbs, this translates to an “uplift” at the rear axle equal to 300*395/130 = 911.54 lbs.

Summary of axle load changes:
TV front axle 611.54 lbs ADDED
TV rear axle 911.54 lbs REMOVED
TT axles 300.00 lbs ADDED

Now it is interesting to consider what happens at the hitch.

DOWN force of 2000 lbs on TT tongue adds a load of 1700 lbs at ball coupler.
This is calculated using TT axles as the fulcrum: 2000x170/200 = 1700.

The UP force of 2000 lbs on the rear ends of the spring bars produces an UP force of 2000 lbs at the hitch end of the spring bars.


The UP force of 2000 lbs minus the DOWN force of 1700 lbs on the ball gives a net UP force of 300 lbs at the hitch.

The vertical load on the receiver has been reduced by 300 lbs.
The vertical load transmitted through the ball has been increased by 1700 lbs.

SUMMARY

It is interesting to note that TT weight and “tongue weight” do not enter into these calculations. The WD hitch does not distribute “tongue weight”. It simply removes load from the TV’s rear axle and distributes it to the TV’s front axle and the TT’s axles.

Now there; wasn't that easy!
PS: This is NOT my original work. Not even sure where it came from.
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Old 04-03-2012, 10:35 AM   #2
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Quote:
Originally Posted by herk7769 View Post
Now there; wasn't that easy!
PS: This is NOT my original work. Not even sure where it came from.
Good post to have on Forest River Forums.

This is probably the most accurate description of the forces involved in a WDH. The original material came from a thread by Ron Gratz over at RV.net entitled Weight Distribution (WD) Hitch --- How it Works. I trust that its ok to link to it.
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Old 04-03-2012, 11:04 AM   #3
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Thanks Scott.

I do not want to take credit for another's work. I found this while organizing my hard drive. Apparently I had "cut and pasted" it into a doc file for my own use.

There have been a number of recent posts about WD hitch care and feeding; so I thought it would be appropriate.

The post is welcome to correctly attribute the source material.
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Old 04-03-2012, 11:15 AM   #4
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Lou

Nice summary on WDH to have on this forum. It would be good to apply this info to the real world results pg_rider posted in his reading from CAT scale of his 2306 TT.
Always enjoy your posts
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Old 04-03-2012, 11:20 AM   #5
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Originally Posted by rschoono289 View Post
Lou

Nice summary on WDH to have on this forum. It would be good to apply this info to the real world results pg_rider posted in his reading from CAT scale of his 2306 TT.
Always enjoy your posts
I thought pg_rider did an outstanding job all by himself
His 3 weighs demonstrated why a single weighing is "almost" as bad as no weighing when using a WD hitch on a travel trailer (hitched WD on; hitched WD off; and truck only).

2 weighs is the minimum on a 5th wheel (hitched and truck only).
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Old 04-03-2012, 12:25 PM   #6
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Excellent information! Thanks for posting and sharing!
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