Coast Resorts Open Roads Forum: Weight Distribution (WD) Hitch --- How it Works
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 > Weight Distribution (WD) Hitch --- How it Works

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Ron Gratz

full time RVer

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Posted: 08/26/04 11:21pm Link  |  Print  |  Notify Moderator

Quote:

--- The gist of the quotes you agree with imply that a 900# weight gets distributed roughly in thirds. You never stated that in your original explanation and that is what confused me.

Thomas,

You're twisting my words. I said, "I have no major disagreement with the gist of either post." 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

thomas malenich

sound beach, new york 11789

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Posted: 08/27/04 06:57am Link  |  Print  |  Notify Moderator

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.
_____________________________________________________________________


Ron, thank you for your continuing examples and explainations. I understand and agree with most everything you are saying but I still have one problem. You are saying that the 900# load is distributed, but not the 900# tongue weight. Let me please entertain one more example:

You put a scale on the ground directly under the TV's ball. You put a wood post between the scale and the ball just the right length to make contact. Now the scale just reads the weight of the post(lets say 10#s). Next you put the tongue of the TT onto the hitch ball. You look at the scale and now it reads 910#s. I assume you agree with me so far. Next you hook up the WD hitch and tension the bars. This distributes the load more equally on all the axles which I ageed with all along. Now are you going to tell me when I look at the scale it is still going to read 910#s?? I can see how the actual pounds down on the ball has increased by 1700#s, but that is TENSION force from the WD system and is countered by the 2000# upward tension force.

All that said, I have a feeling that the scale would read much less than 900#s now, as the rear axle moves back upwards.

If this is true then you have to admit that the gravity weight between the tongue and the ground has lessened, which is why some say tongue weight has been redistributed or is less. Do you see that point of view??

* This post was edited 08/27/04 08:48am by thomas malenich *


Thomas and Laura Malenich
1988 Suburban 1500, 4WD
Scotty 16 1/2' , smaller and loving it
2 kids and 3 dogs

Ron Gratz

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Posted: 08/27/04 06:05pm Link  |  Print  |  Notify Moderator

Quote:

--- You put a scale on the ground directly under the TV's ball. You put a wood post between the scale and the ball just the right length to make contact. Now the scale just reads the weight of the post(lets say 10#s). Next you put the tongue of the TT onto the hitch ball. You look at the scale and now it reads 910#s. I assume you agree with me so far. --

This sounds like a trick question from a Statics 101 exam. The trick is that in order for a scale to measure a force, the scale must completely support that force. Therefore, if there is sufficient "slack" between the hitch head and the receiver so that the receiver does not "receive" any of the load, the scales would read 910#.

However, there is no point in continuing the analysis of your example because you have changed the support conditions. In addition to the TV's front and rear axles and the TT's axles, you have introduced the scales and post as an additional support. This changes the reactions which would occur when the WD bars are tensioned. I could attempt to explain what the new support loads would be under this unrealistic scenario, but I am afraid that would only further confuse the issue.

Quote:

--- If this is true then you have to admit that the gravity weight between the tongue and the ground has lessened, which is why some say tongue weight has been redistributed or is less. Do you see that point of view??

The only "gravity weight" which enters into this analysis is the gravitational force resulting from the mass of the TT which is assumed to act at the TT's center of gravity. Since the center of gravity lies some distance ahead of the TT's fulcrum point, some of the gravitational force (weight) is supported by the axles and some by the ball coupler. The mass of the TT does not change or move. The reactive force at the ball coupler which supports some of the TT's weight does not change or move. The hitch load (vertical force between hitch head and receiver) will change because the WD hitch produces a net UP force on the hitch head.

If you want to define "tongue weight" as being equal to the hitch load, then you can say that "tongue weight has been redistributed or is less". I do not agree with that definition for tongue weight.

I don't think there are any more ways in which I can attempt to make my point.

Ron

thomas malenich

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Posted: 08/27/04 07:45pm Link  |  Print  |  Notify Moderator

However, there is no point in continuing the analysis of your example because you have changed the support conditions. In addition to the TV's front and rear axles and the TT's axles, you have introduced the scales and post as an additional support. This changes the reactions which would occur when the WD bars are tensioned. I could attempt to explain what the new support loads would be under this unrealistic scenario, but I am afraid that would only further confuse the issue.
_____________________________________________________________________

So now your going to pull the ole no point in continuing because......and do not want further confuse the issue routine.


Ron, I respect and value what you are doing here - it seems I may be the only one who does since no one else is responding. I think that it is important to understand this concept and I am learning much from you as we correspond. Unfortunately you are not answering a valid question/example. What's wrong with introducing a scale and means to transmit weight to the scale directly underneath the very thing we are talking about -THE TONGUE. We only need the post to transmit to the earth. If the post bothers you then build up the ground so that the scale will be directly under tongue/ball.

Your drawing and figures are superb and I agree with you on the applied forces, however I continue to disagree regarding tongue weight. I still say it DOES get distributed when a WD hitch is engaged. And it don't really matter if it is distributed in exact thirds or some other percentages. Your position is that tongue weight is still right there where it always was, yet you cannot comment on what the scale in my previous example would read after a WD hitch was engaged.

* This post was edited 08/27/04 07:53pm by thomas malenich *

thomas malenich

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Posted: 08/27/04 07:51pm Link  |  Print  |  Notify Moderator

If you want to define "tongue weight" as being equal to the hitch load, then you can say that "tongue weight has been redistributed or is less". I do not agree with that definition for tongue weight.

Yes!! that is what I am saying. I am saying that "tongue weight" - weight measured right at the tongue gets distributed after a WD system is engaged. So we agree, we just disagree as to what to call it???????

Ron Gratz

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Posted: 08/27/04 09:15pm Link  |  Print  |  Notify Moderator

Quote:

--- Your position is that tongue weight is still right there where it always was, yet you cannot comment on what the scale in my previous example would read after a WD hitch was engaged.

Thomas, your post/scale becomes a fulcrum which supports the hitch head. The hitch head supports the rear end of the TV and the ball coupler of the TT.

The WD hitch, with bars 30" long and tensioned to a total of 2000#, produces a torque of 60,000 in-lb at the hitch head. This torque cannot be transmitted through the ball. The torque therefore is all transmitted to the TV.

The TV must balance the torque with an UP force on the front axle and an UP force on the rear axle. To calculate the forces we must know the relative values for the spring constants on the TV's axles. To make things simple, lets assume the spring constants are equal although we know they are not.

Given torque = 60000 in-lb, distance from hitch head to front axle = 195", distance from hitch head to rear axle = 65", and equal spring constants, we calculate a front axle load INCREASE of 277# and a rear axle load INCREASE of 92#. And, as always, the WD bar loads will cause a load INCREASE of 300# on the TT's axles.

So how have things changed? Well, several posts back, we started with a "tongue weight" of 900# sitting on the tongue jack. We now have replaced the tongue jack's UP force of 900# with
1) an UP force of 277# on the TV's front axle,
2) an UP force of 92# on the TV's rear axle,
3) an UP force of 300# on the TT's axles, AND
4) an UP force of 231# on the post supporting the hitch head,
giving a total of 900#

We have done nothing more than replace ONE support reaction (that of the tongue jack) with FOUR support reactions. None of the TT's weight has been moved and the portion of the TT's weight carried on the ball coupler has not changed one iota. We have distributed 900# of support reaction. We have not redistributed any mass.

Perhaps the reason this is so hard to grasp is due to the fact that normally in the analysis of structures the structural characteristics are fixed and the loads are moved around (redistributed). Changing the loads changes the support reactions. In the case of a WD hitch, the structural characteristics are changed. A large rotational spring is inserted to "bridge" across the ball coupler and a large torque is introduced into the system.

The WD hitch does not redistribute any weights. It does change the structural characteristics of the TV/TT connection so that, even without any changes of weight, the support (axles) reactions do change. That's the whole point. You can change the axle loads without performing any kind of "weight distribution". Too bad they didn't call it an Axle Load Distribution Hitch and maybe we could have avoided this whole discussion.

Ron

thomas malenich

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Posted: 08/28/04 06:38am Link  |  Print  |  Notify Moderator

Ron,
Your numbers make sense. However like all things there should be a way to prove your claim. There should be a way to scale the weight at the tongue BEFORE(which there is) and AFTER(which you claim you cannot) the WD system is engaged. You can claim anything on paper. If my experiment with placing a scale directly under the tongue ruins your claim, then can you think of another way to prove yourself with scales or similar? Scales under the axles do not prove your point, as they only prove that weight has been distributed and everyone agrees with that.

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Posted: 08/28/04 07:33am Link  |  Print  |  Notify Moderator

Thomas I understand your point but Ron is exactly right.
The original tongue weight (the weight sitting on the ball before the WD bars are tensioned) cannot possibly lessen when the bars are tensioned. The bars pull up on the rear of the TV and down on the front of the TT. The weight on the pivot point at the ball has no choice but to increase as down force from the tongue has increase and up force from the TV has increased. The original weight is still there plus the added weight from these forces.


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Ron Gratz

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Posted: 08/28/04 03:15pm Link  |  Print  |  Notify Moderator

Quote:

--- If my experiment with placing a scale directly under the tongue ruins your claim, then can you think of another way to prove yourself with scales or similar?

Thomas,

I have made several claims in this thread. Can you please

1) be specific as to which claim you refer, and
2) be specific as to how your experiment disproves my claim?

And, if you use the phrase "tongue weight" in your reply, please provide your definition of same.

Ron

thomas malenich

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Posted: 08/28/04 09:13pm Link  |  Print  |  Notify Moderator

Quote:

Quote:

--- If my experiment with placing a scale directly under the tongue ruins your claim, then can you think of another way to prove yourself with scales or similar?

Thomas,

I have made several claims in this thread. Can you please

1) be specific as to which claim you refer, and
2) be specific as to how your experiment disproves my claim?

And, if you use the phrase "tongue weight" in your reply, please provide your definition of same.

Ron


Hi Ron,

1) Here is the quote I refer to: " However, there is no point in continuing the analysis of your example because you have changed the support conditions. In addition to the TV's front and rear axles and the TT's axles, you have introduced the scales and post as an additional support."

I am trying to come up with a real life way to measure weight AT THE TONGUE before and AFTER a WD system is engaged. You said it can't be done with my method because I changed the rules. So, now I am asking you to come up with an experiment of your own that proves that the weight measured at the tongue is the same after the WD system is engaged, which is your claim. I am not asking for an experiment on paper, rather an experiment that can be done with scales.

2)My experiment does not disprove your claim because you are not allowing it as a legitimate experiment.

Here are two more examples of quotes by respected members that stick with me regarding tongue weight and WD systems. I could send you about 100 of them. I know you are going to say that you do not like to comment on posts taken out of context, but here goes anyway......

Caymann, Hiya.

Yes, the WD system distributes tongue weight over both the front and rear axle (that is the purpose of it). That is why, when you set up your wd system, you measure a front and rear fender well, down to the ground, and then adjust the wd system so the settling, after being hooked up, of both front and rear of the vehicle is as close to the same as possible. As far as the percentage of the tongue weight that is transferred, I don't know, but checking axle weights on a scale before and after hooking up will tell you."


"WD is just that. Weight Distribution of the TT's tongue weight. From dead weight on the TV's tail (rear axle) and distributed to the TV's front axle. The WD "bars" do this. It's like putting a 6 foot steel bar into the receiver and lifting the end while standing on the TT's tongue. Lifts the TV's tail and moves weight to the TV's front. TT's tongue gets whatever your feet transfers, then redistributes it on both TT & TV hitch system."

The following is a common definition of a Weight Distribution Hitch:
WEIGHT DISTRIBUTION HITCH
This hitch is used for heavy trailering. With its equalizing bars and snap-up brackets, this hitch applies leverage between the tow vehicle and the trailer to help distribute trailer's Tongue Weight evenly to vehicle and trailer instead of carrying the load mostly on the rear of the vehicle. The brackets and spring bars raise the hitch point parallel to the ground, equalizing the load onto all axles. This results in a more level ride, reduced weight on the rear suspension, and provides improved steering and braking control.

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