# Weight Distribution Hitches



## Nathan (Jan 2, 2007)

Ok, we got WAY off topic







in another thread and were discussing this. Here is my take of a 1-D static analysis of how a standard WDH works. I simplified several items and assumed that everything was level, etc. The result however I think rather accurately reflects the physics in play. Anyone with detailed weights can check my numbers (you'll have to solve for the tension on the bars). Ok, so lets let the discussions begin.


----------



## matty1 (Mar 7, 2005)

Do you need to take into account all the other variables in play, such as the hitch, length of hitch on undercarriage, how the hitch is attached..if it connects to the bumper...ie...my OEM hitch attached to the bumper, no matter what i did to the WD setup I was limited to the amount of weight I could "throw" to the front of the truck...my new hitch (a Putnam) is not, and it attaches further down the undercarriage, without changing any setting on the bars or WD setup, I was able to significantly "throw" more weight to the front of the truck, as measured on scales.


----------



## Nathan (Jan 2, 2007)

matty1 said:


> Do you need to take into account all the other variables in play, such as the hitch, length of hitch on undercarriage, how the hitch is attached..if it connects to the bumper...ie...my OEM hitch attached to the bumper, no matter what i did to the WD setup I was limited to the amount of weight I could "throw" to the front of the truck...my new hitch (a Putnam) is not, and it attaches further down the undercarriage, without changing any setting on the bars or WD setup, I was able to significantly "throw" more weight to the front of the truck, as measured on scales.


This is a simplified analysis. It assumes that everything is infinetly rigid and in 1 line. I also assumed that the two axles of the trailer act as 1 through the equalizer between them. Again, it's not exact, but I think it is close. I know GM has the hitch issues, but if you want to start taking into account flexing of components or an unlevel setup, it will become a much more complex problem and will require some software to solve. I don't have that much free time right now. I did this between/during meetings.


----------



## BoaterDan (Jul 1, 2005)

See here to "catch up" in the previous thread if you want to jump in: Previous discussion



> WDH doesn't really shift weight... it basically is using lever arms to apply torques to keep everything straight


It shifts forces... but it can't apply torque through a chain! I'll study your drawings tonight, but it is physically impossible that the bars exert an upward force directly on the tongue through a chain. You can't have two upward forces, one on the TV and one on the TT, come out of thin air. The only thing physically possible is transferring one downward force to an equal and opposite upward force.

If you take the TT out of the picture for a minute, you basically have a simple fulcrum. You pull up at the back end, which is the end of the spring bars, and rotating around the fulcrum somewhere in the vacinity of the hitch head you're pushing down on the front axle of the TV. The upward force on the back of the spring bars can't come from nowwhere, it has to be met with an equal force DOWN on the tongue.

However, to the extent that the back of the TV is lifted, which is usually the case with WD, the TT tongue obviously lifts as well, and as I said I can see how THAT shifts weight to the TT axles.

I still content that it's misleading for a hitch manufacturer to claim that a WD hitch reduces the tongue weight. People will surely misinterpret that, and it won't always be the case. If you were hitching to the side of a building, where there wasn't any TV suspension sag in the first place, the shift on the trailer would be exactly zero.


----------



## PDX_Doug (Nov 16, 2004)

BoaterDan said:


> it is physically impossible that the bars exert an upward force directly on the tongue through a chain.


True enough. But it's not impossible to exert a downward force on the back of the trailer through the chain.









Happy Trails,
Doug


----------



## egregg57 (Feb 13, 2006)

This is going to be a good thread!


----------



## Nathan (Jan 2, 2007)

egregg57 said:


> This is going to be a good thread!


And people were just saying that we don't talk enough technical stuff... Where's Jim???

Dan, see if you agree with my approach in breaking it down into separate free body diagrams. I don't claim to be a physics professor.... so I could always be wrong (just ask my wife!!!)


----------



## bill_pfaff (Mar 11, 2005)

[/quote]
This is a simplified analysis. 
[/quote]

And I'm supposed to do what with this?


----------



## Nathan (Jan 2, 2007)

Ok, I was thinking about this on the way home (and they think cell phones are distracting...) and ok, I see Dan's point, I left off some of the force in the trailer diagram







. I'll have to work on it more. Anyone else want to join in solving this one?


----------



## California Jim (Dec 11, 2003)

Oh my!







Now we're getting into the nitty gritty for sure. Since I aint never had no learnin I'll just watch this one


----------



## biga (Apr 17, 2006)

I remember now why I dropped out of engineering school.


----------



## Nathan (Jan 2, 2007)

I might have just gotten it







. The family just got home though, so I'll have to post after bedtime.


----------



## Carey (Mar 6, 2012)

BoaterDan said:


> See here to "catch up" in the previous thread if you want to jump in: Previous discussion
> 
> 
> 
> ...


If the bars are attached with a chain the wd hitch cant keep everything straight. The chains allow some flex fore and aft. The initial idea of a wd hitch was to help distribute weight, not keep everything staright.

The wd hitches that use chains can use a sway eliminator like the reese cam system or something similar. But without this have no ability to keep everything straight.

The chains have no upward force. They only are the connector of downward pressure to create the fulcrum needed to apply upward force to the hitch head. If you ever cut a chain when the wd is installed the bars will go down, not up, right?

So what is taken at the rear is multiplied thru the spring bars, then that force is exerted up on the hitch head. The bigger the bar, the bigger the torque is multiplied to pull up on the hitch head.

Going over a bumpy road allows these weights from the chain thru the bars to change going to the hitch head. This is the part I have never like about wd hitches. They do not ride like I want.

For instance, the reese pro series use a friction pad at the bottom of the bracket at the rear of the bars. They are using the downward pressure to make friction to eliminate sway.

Carey


----------



## Carey (Mar 6, 2012)

BoaterDan said:


> See here to "catch up" in the previous thread if you want to jump in: Previous discussion
> 
> 
> 
> ...


If you hooked up a wd to a wall, the amount of pressure from the spring bars would take weight from the tv and aply it to the wall. You would remove unladen weight from the tow vehicle in your scenario.

Everyone thinks a wd hitch applies weights to the front axle and trailer axles. But in reality the wd hitch is just giving the weight back. If there is less tongue weight there will be more trailer axle weight or tv front axle weight.

For instance, my truck has a 2540 front axle weight when the trailer is hitched up. It has an empty front axle weight of 2820.

When I air my hitch up all but 15% of the tongue weight is now supported. My front axle weighs 2840, just 20 pounds heavier than if it was empty.... My truck only feels a 200 lb hitch weight. It now pulls the trailer thinking it only has a 200 lb tongue weight.

Now my hitch didnt apply weight back to the (edit: front axle of the) truck did it? No, it just gave it back.

The tongue was pulling weight off of the front axle. My hitch removed that pressure, so now the front axle can return to its normal weight.

A wd hitch does the same, but to a much lessor degree.

Carey


----------



## Rubrhammer (Nov 27, 2006)

"I still content that it's misleading for a hitch manufacturer to claim that a WD hitch reduces the tongue weight. People will surely misinterpret that, and it won't always be the case. If you were hitching to the side of a building, where there wasn't any TV suspension sag in the first place, the shift on the trailer would be exactly zero."
I have to dissagree with this. if you were hitched to a building and cinched up the bars you would still transfer weight ti the axles because you are pulling down at the point of connection with the chains. Depending on how much force the spring bars impart on the chains will determine how much the the weight will increase on the axles.
The hitches do reduce tongue weight. I weighed my rig with the bars unloded and drawn up. Truck with dead weight on hitch 5880lbs,truck with bars cinched up 5840 lbs. 40lbs were sent to the trlr axles. At the same time the truck front axle weight went up by 60lbs. Rear axle weight weight with dead weight on hitch 3220lbs, with the bars cinched up 3120lbs. so I removed 100 lbs from my rear axle. The one thing I regret is that I didn't weigh the trlr by itself. I took the increase in total truck weight(400lbs) as my tongue weight though for all practical purposes should be very close to 400.
As for the direction of the forces, I look at this as the ball connection being not a fulcrum but a point of rotation. Because there is a vertical distance from the ceter of the ball to the bottom attachment point of the wd bar, this causes the upward force of the chain on the bar totry to pull back on the bottom attachment point of the wd bar, this induces rotation around the hitch ball sending weight forward to the front axle of the truck. This is shown on reese hitches by where the bars need lube. (on the frunt of the upper lug and rear of the lower lug) The pivot point isn't fixed and will rise because the resistance of the front spring to compress is stronger than the down force on the ball(plus the upward force of the rear springs helps).
To help visualize the load directions picture a shelf on a wall with a bracket underneath it. (cantalevered load)The load is on the outer edge of the shelf and pushes down, the top screw in the bracket tries to pull out of the wall(horizontal force) the bottom edge of the bracket puts a dent into the wall(opposite horizontal force) this is the rotation of forces in the hitch only the load direction on the rear end of the bar is up not down.
Does this make sense to anyone else?
Just my 6 cents.
Bob


----------



## Carey (Mar 6, 2012)

quote name='Rubrhammer' date='Jan 11 2008, 08:56 PM' post='268065']
"I still content that it's misleading for a hitch manufacturer to claim that a WD hitch reduces the tongue weight. People will surely misinterpret that, and it won't always be the case. If you were hitching to the side of a building, where there wasn't any TV suspension sag in the first place, the shift on the trailer would be exactly zero."
I have to dissagree with this. if you were hitched to a building and cinched up the bars you would still transfer weight ti the axles because you are pulling down at the point of connection with the chains. Depending on how much force the spring bars impart on the chains will determine how much the the weight will increase on the axles.
[/quote]
-------------------------------------------------------------------------------------------------------
You say hooking a wd to a wall adds weight to the axle, I say it removes weight. lol

The chains will be pulled downward which causes the spring bars to flex up at the other end based on hitch head angle.. This would remove weight from the tow vehicle attached to the wall..

A wd hitch does not care what it is attached to, it will always work in the same physics. If there is no tongue weight, the wd hitch will still remove weight from the tow vehicle.

Tongue weight has nothing to do with it. Hitch head angle, spring bar forces, rear bar height, and the distance from the front spring bar attachment to the rear chains is what makes up a wd hitch, and give it its physics and capabilities.

The same wd hitch will work the same on any hitch. Tongue weight has no bearing on what weight the wd removes.

Put a 1200 lb wd bar system on a 750 lb hitch, and then put a 1200 lb wd bar system on a 1200 lb hitch. The weight it removes will be the same using the exact same settings for both.

This is why there is an adjustable hitch head, smaller and bigger spring bars, adjustable rear settings for length and height.

By you, the owner playing with these settings is how weight is removed. By moving the hitch head angle back, you are creating more flex/torque in the spring bars, By moving the chain attachment points front or back you are allowing more or less flex/torque respectivly. By lowering or highering the rear of the bars in respect to the hitch head allows the bars to flex/torque more or less.

Carey


----------



## Sayonara (Jul 23, 2007)

I think i learned this stuff at GMI 10 years ago. forgot it after the final exam. glad i have never needed to use it since computers make things much easier......well till Nathan blind sides us with these sketches.


----------



## MJRey (Jan 21, 2005)

Nathan said:


> Ok, I was thinking about this on the way home (and they think cell phones are distracting...) and ok, I see Dan's point, I left off some of the force in the trailer diagram
> 
> 
> 
> ...


Oh man, you're going to make me dig up my engineering books from 25 yrs ago! I recognize the approach your using but I think I may have been in management







too long to be any help.


----------



## Oregon_Camper (Sep 13, 2004)

Nathan said:


> This is going to be a good thread!


And people were just saying that we don't talk enough technical stuff... Where's Jim???

Dan, see if you agree with my approach in breaking it down into separate free body diagrams. I don't claim to be a physics professor.... so I could always be wrong (just ask my wife!!!)
[/quote]

Which Jim?


----------



## BoaterDan (Jul 1, 2005)

PDX_Doug said:


> it is physically impossible that the bars exert an upward force directly on the tongue through a chain.


True enough. But it's not impossible to exert a downward force on the back of the trailer through the chain.









Happy Trails,
Doug
[/quote]

True, but you can't pull down on the tongue and shift weight OFF the tongue and onto the axles. Besides, I don't think it's reasonable that a relatively small downward force 18 inches behind my ball is going to shift a significant amount of my 800+ pound tongue to my axles 20 feet away.


----------



## BoaterDan (Jul 1, 2005)

Colorado~DirtBikers said:


> "I still content that it's misleading for a hitch manufacturer to claim that a WD hitch reduces the tongue weight. People will surely misinterpret that, and it won't always be the case. If you were hitching to the side of a building, where there wasn't any TV suspension sag in the first place, the shift on the trailer would be exactly zero."
> I have to dissagree with this. if you were hitched to a building and cinched up the bars you would still transfer weight ti the axles because you are pulling down at the point of connection with the chains. Depending on how much force the spring bars impart on the chains will determine how much the the weight will increase on the axles.


-------------------------------------------------------------------------------------------------------
You say hooking a wd to a wall adds weight to the axle, I say it removes weight. lol[/quote]

The point of using the wall is to set up a situation where the hitch is not going to budge, so any TT weight shift caused by changing the angle of the frame is taken out of the equation. In this situation, pulling down on the tongue 18 inches behind the stationary ball (on the stationary hitch) can't possibly transfer weight OFF the tongue and onto the TT axles. It's impossible.

Which is why I content that if any transfer takes place under normal circumstances, it's just because the angle is changed, not something the WD does inherently.


----------



## BoaterDan (Jul 1, 2005)

Ok, folks, I think I have to admit I'm wrong here.

First, I answered my challenge myself. Found this on the draw-tite site:

"Rather than merely supporting the trailer tongue weight (TW), Weight Distributing Hitches apply leverage between the towing vehicle and trailer causing the TW to be carried by all axles of both the tow vehicle and the trailer."

and this

"The weight is then more evenly distributed between both axles of the towing vehicle and the trailer itself."

This post, which I think was dismissed by some of you because it comes from the dark side, explains it a little more straightforward than Nathan for those of us that would rather forget some of our college courses. How a WD hitch works

My error that this explanation points out is that the chains pulling down on the tongue are doing just that - they are pulling DOWN on the tongue, and because of that some of the downward force is transfered to the TT axles. Now, that force is transferred through the spring bars to cause a weight shift in the TV from rear axles to the front, which often causes the hitch (and front of the tongue) to raise. There isn't really a direct transfer of weight OFF the tongue and onto the TT. I think others may be misintrepeting the change in axle weights to mean this is the case. We're shifting from OFF the rear TV axles, not the tongue _per se_.

What's really important is that the concept of tongue weight with a WD hitch can't really be directly compared to that of a weight carrying hitch. One should not say "Oh, this TT has a tongue weight that's over my TV's rating, but that's ok because I'll add a WD and that will lower the tongue weight." (As I recall, that was the premise that started this whole thing). A TV hitch is rated for both weight carrying "mode" and WD "mode". If it is rated at 200 pounds in WC mode and 500 pounds WD mode, then that's the tongue weight it can handle with a WD hitch - period. If a TT has a tongue weight of 700 pounds it can't safely be towed with that hitch - period.

Everyone agree with that?


----------



## CamperAndy (Aug 26, 2004)

I stayed out of this until now but I guess I have to drop in and at least try to see if what I say will clarify what occurs. BTW what I say may have been said before but in a much more wordy way.

WDH do not defy gravity - The tongue weight is the same no matter what is done with the chains and bars.

WDH are at a soft pivot point in a 3 point suspension system (where the tires touch the ground) - The tension applied to the chains at the pivot point cause the perceived weight at the rear wheels of the TV to be less as the pivot point is stiffened and raised.

I wish I could find some of the photos from the 60's when WDH first came out as there were some advertising pictures that show a trailer/car combination that had the rear wheels removed from the car and the entire combination supported by the trailer and the on the cars front wheels.


----------



## PDX_Doug (Nov 16, 2004)

BoaterDan said:


> I wish I could find some of the photos from the 60's when WDH first came out as there were some advertising pictures that show a trailer/car combination that had the rear wheels removed from the car and the entire combination supported by the trailer and the on the cars front wheels.


I think I remember those ads! If memory serves, they used an Olds Toronado as the tow vehicle (one of the few front wheel drive cars of the day). Wow! Flashback!

Happy Trails,
Doug


----------



## Carey (Mar 6, 2012)

Good stuff guys!

Carey


----------



## Nathan (Jan 2, 2007)

BoaterDan said:


> ...
> 
> This post, which I think was dismissed by some of you because it comes from the dark side, explains it a little more straightforward than Nathan for those of us that would rather forget some of our college courses. How a WD hitch works
> 
> ...


Thanks Dan. It looks like I might have been overcomplicating things. That post does a decent job(Although I just read the first page....). Definetly all weights must be accounted for!









Sorry I was absent this weekend. Too many items on the Honey-do list...










BTW my reference to Jim was targeted at California Jim who was just commenting that the site was changing and not focused on the trailers. Now maybe I went too far the other way.







I'm just always the type that wants to understand how to solve the problem from a theoretical standpoint as well as a practical one


----------



## BoaterDan (Jul 1, 2005)

CamperAndy said:


> I stayed out of this until now but I guess I have to drop in and at least try to see if what I say will clarify what occurs. BTW what I say may have been said before but in a much more wordy way.
> 
> WDH do not defy gravity - The tongue weight is the same no matter what is done with the chains and bars.


Right. That's the point I kept getting stuck on.

I think the bottom line is everybody was right, just not expressing it very effectively.

- There _is_ a shift of weight from the TV rear axle to the TV front axle and TT axle.

- There is _not_ a reduction of tongue weight, per se, at least in the weight-carrying sense.

(BTW, the increase in TT axle force would be inversely proportional to the distance from the bar to the axle, right? The effect is less on my 31-footer than on a 21-footer. Right?)


----------



## BoaterDan (Jul 1, 2005)

PDX_Doug said:


> I don't think it's reasonable that a relatively small downward force 18 inches behind my ball is going to shift a significant amount of my 800+ pound tongue to my axles 20 feet away.


I don't know the exact numbers, but I think you might be surprised just how much force those spring bars actually do exert. I know on my Equal-i-zer there is visible bow in those 1-1/8"x1-1/8" steel bars. That's not easy to do! Also, the increase in weight at the trailer wheels is not all that much. I think in my setup it amounted to about 40-60 pounds. Just enough to throw the numbers off a bit.[/quote]

I meant relative to my 5000 pound TV and 8000 pound TT. But, you're right, it's still a lot of force and we're not talking about a tremendous difference at the axles.


----------



## biga (Apr 17, 2006)

PDX_Doug said:


> I wish I could find some of the photos from the 60's when WDH first came out as there were some advertising pictures that show a trailer/car combination that had the rear wheels removed from the car and the entire combination supported by the trailer and the on the cars front wheels.


I think I remember those ads! If memory serves, they used an Olds Toronado as the tow vehicle (one of the few front wheel drive cars of the day). Wow! Flashback!

Happy Trails,
Doug
[/quote]
It was actually a Equal-I-zer ad. They still had that picture in the showroom of the dealer when we bought our Roo. I'm trying to find it on the internet.


----------



## Nathan (Jan 2, 2007)

BoaterDan said:


> Right. That's the point I kept getting stuck on.
> 
> I think the bottom line is everybody was right, just not expressing it very effectively.
> 
> ...


That makes sense to me Dan. I guess that is the advantage of a 31'er!


----------



## Sayonara (Jul 23, 2007)

whos building the popsickle stick model? i think most everyone can understand the physics when seeing it in person. Not everyone can explain it clearly though. That is where things can get a little confusing. Good discussion though! now lets go do some winter camping !!


----------



## BoaterDan (Jul 1, 2005)

LOL. Funny you shouuld mention that - I came across this alleged Hensley demonstration with Legos in my travels for this topic.

Hensley Arrow with Legos


----------



## tdvffjohn (Mar 10, 2005)

Found the Olds, Airstream pic.....not great but interesting

http://www.airforums.com/forums/attachment...mp;d=1046733152


----------



## CamperAndy (Aug 26, 2004)

tdvffjohn said:


> Found the Olds, Airstream pic.....not great but interesting
> 
> http://www.airforums.com/forums/attachment...mp;d=1046733152


That's the picture. I want to see a clean one though, I wonder if there is one on the web.


----------



## tdvffjohn (Mar 10, 2005)

That one took me an hour to find


----------



## camping canuks (Jan 18, 2007)

Perhaps an interpretive dance could sort out the hitch debate?


----------

