How The Weight Distributing Hitch Works

[Barb&Tim]

brulaz, thanks!

After you posted a reference to Ron Gratz, i went looking around and discovered a post over at rv.net concerning some changes in the setup instructions that have been made by various TV manufacturers and WDH makers in the last year.

My original instructions (2003) for my Reese WDH called for adjusting WDH tension so that the front and rear of my TV settled by the same amount front and rear compared to an unhitched state.

With the newest instructions out it appears that with my GM truck the WDH should be adjusted so that the TV front returns to the same distance (wheel well to ground) as the no hitched condition and the amount of settle on TV rear will be ignored. Appears they want the same load on the TV front axle while towing that I would have while not towing.

Here in the info i am referring to -->(interesting read):

http://www.rv.net/forum/index.cfm/fu...pd/pging/1.cfm

Tim

Thanks for posting that. And for my F150:

"Ford specs for 2011 and newer trucks say the w/d should be adjusted to eliminate about 50% of the front end RISE... IOW, if the front rises 1.5" initially, the RISE should be reduced to around 3/4"

Which is what I've read in the Ford Towing brochure as well.

Apparently these truck's suspensions are designed for carrying lots of weight in the rear. And can handle some lift in the front. When unloaded, they even appear slanted toward the front cause of the extra clearance in the rear (and some guys "level" their trucks by raising the front).


When comparing the rise measurements to axle weights you may run into some discrepancies. I first reduced the F150's rise to only about 25%. But when I went on the scales, I found that I was still putting some weight (20#) on the front axle.

How could I have a front rise but also extra weight on the front axle? I was stumped. But then I realized that when I weigh the truck on the CAT scale, both my wife and I were in the cab. And nobody was in the cab when I measured the rise. That's about 325# unaccounted for.


Anyway, I'm now going for even more rise, at least 50% as Ford suggests. This will also reduce the amount of weight on my trailer axle, which would be a good thing according to my latest CAT scale readings.

But what about handling? The current P rated tires are pretty squirmy no matter what I do. When I add more weight to the rear axle, they seem to get a bit worse. I'm looking forward to when I can justify getting some LT rated tires.

[Mr. Adventure]

Brulaz,
I believe I have the WDH spreadsheet working, and the new version is posted in the original post of this thread. It now gets great answers for both of us and hopefully everyone else, as well. If not, I'll get good at this eventually. Thanks for getting good weights and measurements for us.

Ron Gratz and I were obviously covering some of the same ground, but I didn't see a post of his much further along than the sketch you posted, where his weight distribution numbers look to be way out of whack on the high side (I'd like to run his weight numbers on the scale along with his measurements to see how the spreadsheet does with them, if you know of a post of his that has them).

One difference between your approach and Ron Gratz's is that you calculate the spring bar force from actual scale #'s for the weight added to the trailer axle by the WDH. Ron Gratz just assumes a spring bar force and calculates the additional trailer axle load added by the WDH. You both use the same formula for calculating the trailer axle load but go in different directions.

But when it comes to the front and rear TV axle load changes with the WDH, your formulas and results are quite different from Ron Gratz's. I plugged the calculation of my spring bar force (1215#) into the Ron Gratz formulas and obtained the following weight differences due to the WDH (your calculations and the CAT scale weights are also listed):

Source: CAT, Gratz, Adventure
Trailer: 160#, 160#, 160#
Front: 280#, 292#, 260#
Rear: -440#, -452#, -420#

I have NO idea which calculation method is correct.

But the Ron Gratz calculations are a bit closer to the observed #'s.

(Some details:
Like you and Ron Gratz, I calculated the "Additional Trailer Axle Load" as:
1215# x "WD spring bar length" / "Ball coupler to Trailer Axle" = 160#

As a simplification, once the trailer axle's additional WDH load is calulated, you just have to calculate the load on a single TV axle. The other TV axle can be calculated from: Front+Trailer+Rear=0

So, according to Ron Gratz, the additional Front axle load is:
"Additional Trailer Axle Load" x "Rear Axle to Trailer Axle" / "Wheelbase" = 292#

And the weight removed from the rear axle is then -(292+160) or -452#)

[Mr. Adventure]

Originally Posted by brulaz

One difference between your approach and Ron Gratz's is that you calculate the spring bar force from actual scale #'s for the weight added to the trailer axle by the WDH. Ron Gratz just assumes a spring bar force and calculates the additional trailer axle load added by the WDH. You both use the same formula for calculating the trailer axle load but go in different directions.

But when it comes to the front and rear TV axle load changes with the WDH, your formulas and results are quite different from Ron Gratz's. I plugged the calculation of my spring bar force (1215#) into the Ron Gratz formulas and obtained the following weight differences due to the WDH (your calculations and the CAT scale weights are also listed):

Source: CAT, Gratz, Adventure
Trailer: 160#, 160#, 160#
Front: 280#, 292#, 260#
Rear: -440#, -452#, -420#

I have NO idea which calculation method is correct.

But the Ron Gratz calculations are a bit closer to the observed #'s.

(Some details:
Like you and Ron Gratz, I calculated the "Additional Trailer Axle Load" as:
1215# x "WD spring bar length" / "Ball coupler to Trailer Axle" = 160#

As a simplification, once the trailer axle's additional WDH load is calulated, you just have to calculate the load on a single TV axle. The other TV axle can be calculated from: Front+Trailer+Rear=0

So, according to Ron Gratz, the additional Front axle load is:
"Additional Trailer Axle Load" x "Rear Axle to Trailer Axle" / "Wheelbase" = 292#

And the weight removed from the rear axle is then -(292+160) or -452#)

Bruce,
Thanks for digging up some more about Ron's formulas.

There are several ways to do these calculations (more wrong ones than right ones, it turns out). Several may provide useful ballpark approximations. Like Ron, I use the trailer axle contribution from the WDH on the scale to calculate the load on the hitch bars. My new approach sums the forces on the hitch ball, then figures the rear axle weight from them, and from that the front axle. I had to do this because the flaw in the first release was that I got my equations all tangled up when I moved spreadsheet cells around to make them read better before going to press in the first version, and then I didn't adequately check it. Besides not working any more, my first approach was trying to calculate the TV front axle, overly dependent on very fine measurements of the hitch bars, and hard to debug.

So I took it apart and put it back together again with a more direct approach to the problem. The way to tell which one is right is to see which one can produce the the right answers.

Ron multiplies the Rear axle to trailer axle distance divided by the wheelbase, treating the rear axle as the fulcrum. This might be closer to truth for those with stiffer rear suspensions and really jacked up WDH's, but it's pretty far off for me. For example, this approach would have my 86# boost to the trailer axle exert 183# on the front axle instead of the 100# we actually saw on the scale.

My WDH spreadsheet in current version (3-9-12) in the original post of this thread is showing what I believe to be your numbers. if anything's different, as these should be the same numbers for your 580# WDH tongue weight distribution that you reported from the scale: Front, rear, and trailer: 20, 400, 160 in the column labeled "Tongue WDH Distribution" (however, I confess to one teeny piece of editorial license: I used 60" for your overhang instead of 59.5"). I'm also projecting 1312# on the hitch bars for you (where did 1215 come from?), although that remains a number tremendously sensitive to the measurement of the point half way between the two hitch bar chains to the center of the hitch ball.

The total of the load changes to the two tow vehicle axles and the trailer being equal to the tongue weight isn't just a simplification, it's one of equations we need to be able to solve the math problem (the tongue weight always has to be the sum of the differences between the unhitched axle weights and the resulting axle weights however they might be hitched). And yes, I take that shortcut as soon as I have 2 out of 3 axles figured out.

I believe the 1215# comes from the 27" that I put in as Spring Bar length (not 25").

[Mr. Adventure]

Originally Posted by brulaz

I believe the 1215# comes from the 27" that I put in as Spring Bar length (not 25").

Thanks for bearing with me, and no problem on the hitch bars, just trying to stay on the same page with you.

The hitch bar length and their effective fore and aft components are just an interesting side trip and not really critical since we're basically using the same values on both sides of the hitch ball, so to speak (27 x 1215 works out to the same number of inch-pounds as 25 x 1312). A much more accurate measure of the force that loads the hitch is the trailer axle weight attributable to the WDH times the hitch ball to the trailer axle distance.

My numbers for your front axle come from your scale numbers (check out the Scale Calculator spreadsheet in the original post of this thread, and please let me know if I missed something). The front axle is 3260# unhitched (tow vehicle without the trailer) and 3280 hitched with the WDH, for a net 20#.

Source: CAT, Gratz, Adventure
Trailer: 160#, 160#, 160#
Front: 280#, 292#, 260#
Rear: -440#, -452#, -420#

That's looking at rear axle changes caused by the WDH, which is actually how my spreadsheet works. I'm more comfortable bringing these back to the tongue weight for display purposes, which is the total for each column below:
Source: CAT, Gratz, Adventure*
Trailer: 160#, 160#, 160#
Front: 20#, 32#, 20#
Rear: 400#, 388#, 400#

*The extra 1/2" I'm presuming upon you in the overhang measurement saves a 2 pound error. Two pounds is inconsequential on the scale (it's astonishing that the scales are accurate enough for us to be having this discussion), and our measurements can only be so accurate, anyhow (actual measurement errors are additive and could conceivably be elsewhere on the list of measurements -- think of this as the fine tune knob I'm using in looking at tape measure work to validate a spreadsheet).

Here's the same table for my vehicle:
Source: CAT, Gratz, Adventure
Trailer: 86#, 86#, 86#
Front: 100#, 183#, 100#
Rear: 320#, 237#, 320#

Thanks for your help in making this better!

Originally Posted by Mr. Adventure

I'm more comfortable bringing these back to the tongue weight for display purposes, which is the total for each column below:
Source: CAT, Gratz, Adventure*
Trailer: 160#, 160#, 160#
Front: 20#, 32#, 20#
Rear: 400#, 388#, 400#

Gratz never provides calculations for distributing tongue weight. He says:

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.

Did you get those Gratz Front and Rear #'s by:

(a) Front Axle weight (no Trailer) = 3260#
(b) Front Axle weight (trailer but no WDH) = 3000#
(c) Gratz Front axle WDH load = 292#

Front = b+c-a = 32#
Rear = 580-160-32 = 388#

If that's how you did it, and if you do the same calculation with your "Impact of WDH" front axle numbers:
(a) Same
(b) Same
(c') Cell J22 Front axle WDH load = 260#

Front = b+c'-a = 0#
Rear = 580-160-0 = 420#

Different from the 20#,400# you report above. So two different results, that depend upon how you do the calculations.

Like Gratz, I'm not sure that it makes sense to use the tongue load (measured without a WDH) when doing WDH calculations. Tongue load seems to be only a property of an unhitched trailer (or non-WDH hitched tv+trailer). Changes in axle weights/loads seem more relevant to me when talking WDH properties.

[Mr. Adventure]

Originally Posted by brulaz

Gratz never provides calculations for distributing tongue weight. He says:
Quote:
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.

Tongue weight is one of many loads on the tow vehicle. It would be goofy to say that it doesn't enter into the calculations, so we must have this out of context somehow.

The WDH is able to distribute weight though, not just tongue weight (Thought experiment: take the TrailManor off the TrailManor frame, and you have one very long tag axle). Distributing weight happens by boosting the hitched vehicle from the middle with spring bars, having the effect of moving weight off the rear axle as stated. But tongue weight and its effects are why we use WDH's so it's the most appropriate point of reference for us.

Did you get those Gratz Front and Rear #'s by:

(a) Front Axle weight (no Trailer) = 3260#
(b) Front Axle weight (trailer but no WDH) = 3000#
(c) Gratz Front axle WDH load = 292#

I got Gratz's numbers from your post. The only official Gratz calculation I've ever done is the one posted in that same message for my rig that shows that the Gratz approach is nowhere close to the right answer (183# added to the front axle instead of the 100# we see on the truck scale).

In the end we all have to get the same answers as the scale. It may be possible to figure things using the moving load numbers as a frame of reference, but you'd have to do it in a way that includes appropriate calculations at the WDH. The problem is that Gratz hasn't got it right yet, as far as we know from what's been posted here.

Front = b+c-a = 32#
Rear = 580-160-32 = 388#

If that's how you did it, and if you do the same calculation with your "Impact of WDH" front axle numbers:
(a) Same
(b) Same
(c') Cell J22 Front axle WDH load = 260#

Front = b+c'-a = 0#
Rear = 580-160-0 = 420#

Different from the 20#,400# you report above. So two different results, that depend upon how you do the calculations.

Whoa. The spreadsheet shows a net 400# increase on the rear axle with the WDH (versus an 820# increase on the rear axle without one). The column next to it shows the net -420# rear axle impact of the hitched trailer with the WDH versus not (820# impact on the rear axle without a WDH minus 420# improvement from using the WDH equals net 400# impact of the hitched trailer with a WDH on the rear axle). If you set the unhitched axle weights to zero in the left column, you’ll see that the numbers for each of the 3 axles in the first 4 columns exactly total to the tongue weight (the last column totals zero).

Loads are additive, and become simpler if analyzed separately. My approach is to focus on the tongue weight effects, holding the rest of the TV and the trailer exactly the same in theory, as well as in practice on the truck scale. The solution is based on realities (which ultimately are defined in simultaneous equations which are implemented in the spreadsheet):
Equation 1) The total weight of the combined vehicle is always equal to the sum of the axle weights. The effective weight on the hitch ball is always equal to the sum of it's axle effects on the tow vehicle.
Equation 2) Hitch ball load x (overhang + wheelbase)/wheelbase) = Rear axle impact of the net hitch ball load

The WDH is where the action is, and getting the loads correctly at the hitch ball gives you the right input for the axle calcs.

Sequence of calculation:
- The WDH effect on the trailer axle applied by the WDH is useful in two ways: 1) It's one of 3 axles 2) this value times the hitch-ball-to-trailer-axle distance defines the load the WDH applies to the TV at the hitch ball.
- Use the sum of the hitch loads to calculate the rear axle impact of the WDH, and add it to the unhitched rear axle weight (the tongue weight is one of these loads) (all this happens in cell H23 of the current spreadsheet version).
- Everything else is addition and subtraction.

Like Gratz, I'm not sure that it makes sense to use the tongue load (measured without a WDH) when doing WDH calculations. Tongue load seems to be only a property of an unhitched trailer (or non-WDH hitched tv+trailer). Changes in axle weights/loads seem more relevant to me when talking WDH properties.

Understanding the changes in axle weights/loads caused by trailers is the purpose of this exercise. The tongue weight is the property of a hitched trailer that creates the load on the hitch ball that makes all this necessary. It isn't possible to analyze axle loads and WDH's without analyzing what's happening to tongue loads in hitched vehicles, regardless of your preferred frame of reference as you do it. If it makes you feel any better, the spreadsheet takes the tongue weight along with the rest of all this and then calculates the rear axle load first.

Well, I think we've reached an impasse, and I've run out of things to say on the subject. Cheers.