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Weight Distributing Hitches and weights explained...ish

16K views 16 replies 11 participants last post by  Monica B Muller 
#1 · (Edited)
I came across some light reading the other day that helps explain how the loading changes on a tow vehicle (TV) and trailer (TT) when using a weight distributing hitch. I was thinking of creating a force diagram for the exact same thing to illustrate how the forces are distributed, then I found that somebody else had already done it :)

From RV.net:

START THEORETICAL NUMBERS



A summary of the 150+ posts in this topic has been developed by several of the contributors 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.

Several recent posts have discussed WD hitches. One 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. 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.

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.

END THEORETICAL NUMBERS

Later in the thread, the author helps to define and differentiate a few terms such as tongue weight and receiver weight/load which often get incorrectly lumped together. From Woodalls:

Tongue Weight: That portion of the TT weight which is carried on the ball coupler as opposed to that carried on the axles.

Hitch Load: The vertical force exerted by the hitch head on the TV's receiver.

Now here a post from page two of the original post on RV.Net using real numbers.

START USING NUMBERS

Lets try using some numbers. Take all the dimensions from my diagram and add your 900# tongue weight.

Back the ball of the Burb under the coupler and raise the jack. What happens to the Burb's axle loads?
Rear axle load INCREASES by 1350#.
Front axle load DECREASES by 450#.

What happened to the "tongue weight"?
Nothing. The coupler still is pressing on the ball with a force of 900#.
Well, the loads on the TV's axles INCREASED by 1350-450 = 900#. Where did that come from? Some trailer weight must have been transferred from somewhere.
Nope, the 900# on the axles simply results from a relocation of the 900# reaction which previously was provided by the tongue jack. The loads on the TV's axles have changed, but the "tongue weight" has not changed.

Now, we are not too happy about the INCREASED load on the rear axle and the DECREASED load on the front axle. So, what can we do about it?
Well, we can install a WD hitch with 30" bars and tension them to 1000# each for a total of 2000#. This INCREASES the front axle load by 612# and DECREASES the rear axle load by 912# giving
TV front axle load = -450 + 612 = 162# INCREASE
TV rear axle load = +1350 - 912 = 438# INCREASE

This means the total load on the TV now has increased by 600#. Before we tensioned the WD bars, the total added load on the TV was 900#. What happened to the other 300#?
It was transferred by action of the WD hitch to the TT's axles. So, of the initial 900# added to the TV, we now have
162# = 18% on the front axle
438# = 49% on the rear axle
300# = 33% on the TT's axles

If we wanted to shoot for the 1/3-1/3-1/3 "distribution", we could increase the WD bar tension from 2000# to 2300#. This would INCREASE the front axle load by 703# and DECREASE the rear axle load by 1048# giving
253# = 28% on the front axle
302# = 34% on the rear axle
345# = 38% on the TT's axles
and this is about as close as you can get to equal thirds with the assumed dimensions.

So, have we distributed the 900# "tongue
weight"?
Nope, it's still where it was to begin with. But we have re-distributed the 900# load which was transferred from the tongue jack to the TV's axles.

Hope this helps.

Ron

END USING NUMBERS

What I gather from this, first of all, is that assuming tongue weight translates directly to payload on the TV is WRONG. The WD hitch distributes a portion of the load back on to the TT's axles, thereby reducing the load shouldered by the TV. A 900# tongue weight (as used in the example) translates to a 600# load on the TV (split 162#/438# front/back on the TV's axles).

Second, which coincides with the first, the simplification of terms we use are often inappropriate and insufficiently describe vehicle loading. It is easy to simplify a TT/TV loading scenario (that's why it's called 'simplify', right?) in to very few components: TV weight, TT Tongue weight, and payload. Add the three together and they can't exceed GVWR. According to observation 1, this is flat out incorrect because TT tongue weight does not equal the load seen by the TV using a WD hitch.

Third, Gross Axle Weight ratings will rarely be approached in a WD hitch setup.

Fourth, payload capacities of these trucks isn't NEARLY as limiting as they are made out to be.

And fifth, these numbers and load percentages DON'T apply to 5th wheel trailers (as much as we wish they would.

Let's use my truck, family, and gear with the numbers above to work out a scenario.

According to the stickers on my door jamb, Toyota says my truck weighs 5,714 lbs, including the payload deduction of the little yellow 'accessory' sticker (though scaled trucks have been known to weigh less). Specs are as follows:

Vehicle weight as delivered: 5714 lbs
GVWR: 7100 lbs
Front GAWR: 4000 lbs
Rear GAWR: 4150 lbs
Total payload capacity: 1374 lbs
Family + dog weight: 427 lbs
Coca Cola stashed in door cup holders and dewzer cups: 8.8 lbs
Upgrades (roll bar, tonneau, stereo, sound deadener): 27+30+13+45 = 115
Travel Trailer Tongue weight: 900 lbs
Travel Trailer weight: (assuming 11% tongue weight) 8182 lbs
My WD hitch: 130 lbs

Weight distribution from tongue weight using (2) 1000# bars:
162# = 18% on the front axle
438# = 49% on the rear axle
300# = 33% on the TT's axles

So, my 8182 lb trailer exerts a total force of 600 lbs on my truck. I have 550.8 lbs of people and gear in the truck (427 + 115 + 8.8) plus a 130 lb hitch. That means that my payload is 600+550.8 + 130 = 1280.8 lbs. Since my payload capacity is 1374, I'm still 100 lbs UNDER my payload. I believe that even keeps things under payload for most CrewMax trucks, as well.

And what's more, if I were to MAX out my towing capacity of 9800 lbs and use (2) 1150# bars instead of 1000# bars, according to the relative percentages presented above, the trailer would exert 668 pounds of force (9800*.11*.62) on my truck. Technically, that would exceed my GVWR by just 24 lbs. Of course, I could just quit drinking so much darn Coca Cola and lose 15 lbs. I'd lose the 8.8 lbs of Coke in the cab, too. Then I would be spot on! Also note that as long as I don't have more than 500 lbs or junk in my truck (which, unfortunately, I do in this example) I would be under the 16,000 lb GCVWR as well. The load that is transferred to the vehicle by the trailer (hitch load or tongue weight, depending on the context) does NOT factor in to GCVW; only the weight of the TT, TV, and passengers with gear are added together to find GCVW.

And just in case you think I started at the end result and worked backwards to 'doctor' the numbers, I didn't. I kept pushing my trailer weight up until I reached max payload. Just so happened it worked out to be max trailer rating. How's that for Toyota engineering! And as for the weight of the Coke, I assumed 4 liters from the beginning, which is where the 8.8 lbs comes from.

What I'm getting at is towing safety is about numbers and driver awareness. We can assume all is OK and just go for it; we can assume it's all too much for our trucks, park them in the mall parking lot while getting groceries, and dicker over rule's of thumb; or we can do the math and figure out what is safe. Either way, assumptions aren't helping anybody. Of course, you would need to start at the beginning by measuring actual distances on your trailer and truck instead of using the ones in the original excerpt (though they are pretty close).

That being said, if you think you are approaching any max capacities, weigh your setup and crunch the data. That will tell you if you are within the capacities of your truck.

Let the keyboard :boxing:, context :machinegunner: , noobie :nunu:, and general:whacky: begin...
 
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#4 ·
Thanks. I'm just trying to put reasonable, accurate, simple-to-understand information out there. Apparently, this is a hotly contested topic. It can be a real PIA to try and learn new things, sometimes :banghead: Especially when the info is just misunderstood - like trying to figure out how much dirt is in a 3'x3'x3' hole.. >:D
 
#5 ·
Wow!:eek: I just took my setup to the CAT scales.;) Everything within limits! :)
 
#6 ·
There are some TTs I want that would be near limits but this certainly helped. I will continue to do the math, will go get MY actual truck weighed, and will come back armed.

I would like to ask for input from someone with actual experience, not just following what the manual and internet says. Who has used a WDH and weighed their truck to see how much weight is actually moved back to the TT? Is 33% accurate? Once I start doing the math I will have to include some estimates prior to actually having the TT in my possession and being able to weight it.
 
#7 ·
I have scaled mine many times, mine is closer to 25% transfer to rear axle (200 lbs on a 825lb hitch weight) but I have my wdh set to return the lost weight only from the hitch back to the front. With my dc configuration, people and stuff in the cab I find tends to load the front axle more, So I dont want to add too much bar and overload the front or risk unloading the rear too much
 
#8 ·
Good insight DH. I assume 1/3rd is probably optimal with the perfect setup. 25% may be much closer to realistic.
 
#10 ·
A cubic yard, eh? Meehhhh... WRONG. :D

Thanks for weighing in in the conversation, Ditchhooker. It's nice to see some real numbers; I've been thinking of starting a thread for scale tickets, but I won't have my trailer back to weigh for a while.
 
#11 ·
I have a 2013 DC and 32 foot TT. This week I weighed my truck alone, and then the truck connected to trailer on the local truck stop CAT scale. I use a Equalizer hitch set within the fender HEIGHT measurements specified by Equalizer. PS - Even Equalizer technical service admits it is extremely hard to transfer much load to the front axle of a long tow vehicle like mine.

Solo F=3480 and R=2900

connected F=3440, R=3720, two trailer axles combined = 6500.

I have a tongue scale and the same day measured my tongue weight at 1,000 lbs. So when connected, my front axles lost 40 lbs and the rear gained 820 lbs. I did not have opportunity to weigh the trailer by itself, but calculating the statics problem indicates 146 lbs transferred to the trailer axles.

The sketch and logic at the beginning of this post comes from an 18 page exchange at another site that is 10 years old. That post is long since closed, but I continue to question the conclusion or assumption that tongue weight does not enter into the calculation.

The WD hitch transfers forces from the rear axle and to the front axle and trailer axle. This occurs because the WD hitch bars are trying to bend, and thus exert strong forces on the hitch head and the trailer tongue at the L brackets. However, the amount of the bending force on these bars is determined by how much the set up deflects (settles) when the tongue jack is retracted. And that amount of deflection depends on tongue weight. The more the tongue weight, the more the bending force, and the more weight transferred.

Anyone still reading this chain?
 
#15 ·
Ding! Ding! Ding! We have a winner! You would find NO dirt in a HOLE! I know, a dirty trick...
 
#16 ·
Blenton,

Pretty good write up. This stuff was pretty much all covered by the WDH companies in the 60’s.

In the 60’s I had a speech on this (science class) as a freshman in High School. Back then it was pretty cool to demonstrate these fundamental concepts with an early 60’s Olds Toronado. Kids thought it was cool to actually drive a Toronado with “no” back wheels on the car pulling a 35’ Airstream. Of course it was dangerous because you could not steer, but it did prove the point of how WDH works. Also the load forces would actually bend the frame to the Toronado.

To me it is pretty shocking how little peeps know today about Towing in general, such as:
1. WDH
2. Trailer Brakes – all the various kinds and how they actually work
3. Backing up a trailer. I would venture to guess that over 70% that “tow” are completely clueless.

Some of the funniest stuff I see today; it is the “truck experts” (auto journalist) that demonstrate how to back up a trailer and they have “no clue”. It is like a lost art (or science)?

Thanks for the write up, it can be a good source for those that want to learn about towing.
 
#17 ·
Very interesting but I still have a burning question. In my case, I added a spring because my 2015 Tundra's springs were starting to be too soft. I use the truck primarily for towing a 2 horse trailer - empty tongue weight 650 lbs, loaded tongue weight 900 lbs. Questions is this: The additional spring lifts the rear of the truck 1.5 inches - so I had to take the ball mount down a hole on the hitch shank - fine. BUT, given that the rear springs are now stronger than the front, how to I calculate how much weight distribution is now happening, if any?
 
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