Weight Distribution Physics

[ohmman]

Yes, yet another weight distribution thread. But before you roll your eyes and think that this poster hasn't researched and read as much as possible, please let me assure you that's not the case. I've read many threads here and in other forums as well as technical documents about the physics. The problem is that, like with many things, the lore outnumbers the evidence by a large margin.

To the question.

I understand the mechanism by which WD makes the fulcrum more rigid and transfers weight to the front axle of the TV and to the TT axle. In my case, it's a Bambi, so we truly have three axles to consider. What I'm having a hard time determining is whether the upward force into the receiver is merely offsetting the downward tongue weight by some degree (for instance, for a 500# downward component of the tongue weight, it transfers only 300# downward and 200# settles back to the trailer axle). I understand that the tongue weight is the same no matter what, but if the upward force of the torsion bars is offsetting that downward onto the trailer axle and upward into the receiver, doesn't that imply that a smaller downward force is placed on the receiver of the TV? If so, why is the common response to this question that it's the same downward force?

I want to be clear that I'm not trying to get around tongue weight limitations of my TV. I've got a tongue scale and everything is within my limits. I'm just trying to understand what the forces (upward and downward) look like at the receiver on the TV.

I have drawn a force vector diagram and it seems to me that the downward force at the receiver is offset by the upward force of the torsion bars. This upward force is what allows the front axle to be reloaded. I'd like to hear why this is mistaken or simplified..

[Al and Missy]

Here's my take. It is at least partially a semantics problem. In my view, the "tongue weight" is the force exerted by the trailer coupler on the hitch head ball. This is not the same as the weight placed on the receiver. When the weight distribution is not engaged, the weight on the receiver is the trailer tongue weight plus the weight of the WD hitch components attached, at least the hitch head and maybe the bars. With WD engaged, in addition to the downward force there is an upward torque. TV manufacturers don't deal in this distinction except some provide different receiver specifications for regular hitches and WD hitches.

If you work the classical sum of forces and moments on the system there appears to be an equivalence between the case with WD engaged and the case with WD disengaged and less tongue weight. While the effect as measured at the tires is the same, I haven't studied it in enough detail to convince myself the forces on the receiver are identical, although I suspect they are. My suspicion is that, since there is not a single point connection between the shank and the receiver, the force exerted on the receiver by the hitch shank is actually a torque generated by the downward force at the rear of the receiver and an upward force on the front of the receiver. I think the WD tends to counter this torque with one in the opposite direction.

Al

[rostam]

Ron is a forum member and is very knowledgeable in this area. Here is his post on RV.net

https://www.rv.net/forum/index.cfm/f...d/14265335.cfm

Hope this helps.

[HowieE]

You are both grossly overthinking this.

First off the tongue weight limit on the TV is only a consideration when the trailer is ball mounted only. Tongue weight has no consideration while using a WD hitch.

The shank and the receiver is a rotational connection with the rear of the receiver forced upwards and the front downwards. That rotational torque is what divides the load and shifts it to the front TV and trailer axles while reducing the load on the TV rear axle. The amounts transferred is a function of the load put into the chain hangers and the length of the 2 vehicles.

If you were to measure the load on the ball itself it would be greatly increased when hitched.

[ohmman]

Originally Posted by HowieE

You are both grossly overthinking this.

First off the tongue weight limit on the TV is only a consideration when the trailer is ball mounted only. Tongue weight has no consideration while using a WD hitch.

The shank and the receiver is a rotational connection with the rear of the receiver forced upwards and the front downwards. That rotational torque is what divides the load and shifts it to the front TV and trailer axles while reducing the load on the TV rear axle. The amounts transferred is a function of the load put into the chain hangers and the length of the 2 vehicles.

If you were to measure the load on the ball itself it would be greatly increased when hitched.

Thanks for all the responses. I had seen Ron's diagram and description on RV.net, but thanks for linking it - it's great information.

Regarding the above quote, here's where that's not as cut and dry for me. I understand there's an upward force (though I see it being upward at the butt of the shank where it bottoms into the receiver), but it cannot be a net upward force with no downward force, otherwise zero (or less than zero) tongue weight would make its way to the rear axle of the TV. There is still a downward force on the receiver. It nets out to less than the tongue weight of the TT - this is what I am positing in the thread, but which is absolutely not the general consensus here on AF or elsewhere.

What that would suggest is that the often-shamed advice that a WD hitch will allow you to exceed a TV's tongue weight rating actually has some merit. Assuming that the TV's tongue weight limit is calculated by rear axle loading and/or receiver attachment and build quality, netting out upward and downward forces may indeed allow one some leeway on the limits of the TV when it comes to tongue weight. I think the issue is that it's very difficult to calculate all of the variables at play to determine the net of the force on the receiver, so staying inside of the rated numbers is safe.

[DryFly]

This may help, it clarified things for me

https://www.youtube.com/watch?v=XBZu39pQ8Gg

[Vitaver]

Ignorance is bliss, God is kind to the ignorant, etc. for whatever is worth and the reason/s behind them, I towed over 2,000 miles including mountain roads with high grade, with a TV designed for 50% of the tongue weight placed on it without incident. Did use WDH BlueOx though, so the empirical take of this journey appears to indicate that WDH do reduce actual tongue weight. Or maybe it was luck or the TV (LR4) being built with tolerance of > +50%

[jcl]

Originally Posted by ohmman

What that would suggest is that the often-shamed advice that a WD hitch will allow you to exceed a TV's tongue weight rating actually has some merit. Assuming that the TV's tongue weight limit is calculated by rear axle loading and/or receiver attachment and build quality, netting out upward and downward forces may indeed allow one some leeway on the limits of the TV when it comes to tongue weight. I think the issue is that it's very difficult to calculate all of the variables at play to determine the net of the force on the receiver, so staying inside of the rated numbers is safe.

If the essence of the enquiry is to understand the forces on the receiver, then another way to think of it is that there are two types of forces applied to the receiver by WD equipment. First, there is the tongue weight and resultant downward force. Think of it as the same as the tongue weight if there were no WD equipment installed. This means that the receiver will not see less downwards force than it would otherwise, and no, you cannot put more vertical load on a receiver than you would with no WD equipment, in terms of the receiver strength. The second force is a rotational moment applied by the WD equipment, and this is a strong bending moment, upwards. It is what levers the rear axle of the TV upwards, and transfers weight to the front axle. This torque does not represent a reduced force on the receiver. It is relatively higher in terms of stresses, and is what results in some unreinforced receivers pointing upwards (ie WD equipment not able to transfer weight to the front TV axle successfully due to flex). This rotational moment is the reason that some receivers get reinforced. We continue with the assumption that the downward force on the receiver is the same, but what this rotational force means is that net weight on the TV rear axle is reduced.

So when some say that the weight on the receiver is the same with WD equipment, that is true for considerations of receiver strength. When others say that the weight is reduced, that isn't true for the receiver, but it is true in a resultant way for the rear axle of the TV, ie the net effect of the tongue weight.

I don't have any problem in reinforcing a receiver so that it doesn't flex. It will be safer afterwards, because it will allow better weight transfer, ie reduced flex. It isn't a sign of a weak tow vehicle, it is a sign of a receiver not designed for WD equipment.

There was a poster here a few months back with an SUV, who made a number of calculations, and decided not to reinforce his receiver, as it looked strong enough. He was primarily concerned about it bending downwards (from the tongue weight). He applied WD equipment, as was surprised to see it bent upwards. He had been thinking WD equipment was effectively a reduction in tongue weight, and thus a reduction in the downward bending moment on the receiver, when it wasn't. The upward bending forces will be higher than the downward forces of the tongue weight. They have to be to transfer weight effectively to the front axle.

In your freebody diagram, represent the loads on the receiver with a vertical load, and a rotational moment, instead of just with a combination of vertical forces.

[Countryboy59]

In order for 24" long bars to transfer weight to the front of a 120" vehicle something has to twist. It's not just weight. I used to have to hitch up, then raise the rear of my Grand Cherokee almost a foot with the trailer jack to hook up the WD bars. There is no way that was not applying a twisting moment to the frame.

Weight is weight, it doesn't go away.

[ohmman]

jcl - of course you're right. My FBD was netting forces and omitting torque. It's been 24 years since I took my course in statics in college. I assumed my memory was perfect, and now I realize how many cobwebs were there. I just went back and reviewed and I am revisiting the diagram this morning.

Countryboy59 - a downward force at a distance (as with a weight carrying hitch) creates its own moment. The WDH counteracts that moment. I'll hopefully have the diagram today and can submit it for everyone's teardown.

[PKI]

If you have the removable hitch shank currently associated with the European type hitch, you might look at the forces applied to the hitch shank socket. The engagement length must transfer all the tongue weight and WD torque to the chassis. That can be considerable force and the multiple forces will work and wear that connection over time. Pat

[ohmman]

Check the attached for my diagram. I isolated the FBD from the static weight of the TT and TV, since those do not contribute (statically) to the moment at the hitch. This is tongue-weight isolated. I also assumed a near-perfect distribution of weight across the three axles, something I realize is not necessarily realistic. But I don't have measured weights at this point.

Solving for the torque in the bars by assuming the sum of moments is zero at points A, B, and C, I came to approximately 11900 in-lbs of torque (just about 1000 ft-lbs) in order to achieve the desired weight distribution.

With that being applied at t(WD) on the diagram, solving for the moment at location R (receiver attachment point into the body of the vehicle), I get -4100 in-lbs, or approximately 342 ft-lbs of torque in a CCW direction.

The specification on my vehicle's hitch is 500 lbs on a weight carrying hitch at 8" from the pin (13.5" from the attachment point R on my diagram). That means it's rated for a torque of 562.5 ft-lbs of CCW torque.

So (and please do comment on any omissions or errors or general foolishness that you see), it appears that my WDH statically relieves the torque on the receiver as well as the rear axle.

[markdoane]

You may want to review this spreadsheet which was developed by Nick Crowhurst.

[HowieE]

Originally Posted by ohmman

What that would suggest is that the often-shamed advice that a WD hitch will allow you to exceed a TV's tongue weight rating actually has some merit. Assuming that the TV's tongue weight limit is calculated by rear axle loading and/or receiver attachment and build quality, netting out upward and downward forces may indeed allow one some leeway on the limits of the TV when it comes to tongue weight. I think the issue is that it's very difficult to calculate all of the variables at play to determine the net of the force on the receiver, so staying inside of the rated numbers is safe.

The max tongue weigh a receiver is rated for may not be governed by what you would think. I suggest is it the mounting of the receiver to the TV frame that is under consideration there. Most receivers mount with 4 bolts. 2 forward an 2 rearward. In a straight ball mounted towing it is the limits of the 2 rear bolts and the frame that have to be considered. With a WD hitch the load on those bolts is all but eliminated due to the torque produced by the WD hitch. So once you are using a WD hitch the you should see a second rating on the receiver that supersedes the max. tongue weight.

As for the above diagram you would have to look a lot further when analyze a WD hitch. This is not a simple beam loading calculation.

[ohmman]

markdoane - thanks for the spreadsheet link.

Originally Posted by HowieE

As for the above diagram you would have to look a lot further when analyze a WD hitch. This is not a simple beam loading calculation.

I recognize that I've left out some vertical offsets, but do you care to elaborate or provide a sketch that helps with the statics problem? I recognize that the dynamics are vastly different with the WD hitch and a WB hitch since the WB hitch allows relatively unimpeded pivot on the ball. However, I was aiming to look at the statics primarily.