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Old 10-12-2014, 09:14 PM   #1
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Weight Distributing Hitch Adjustment

How do I adjust my WDH for correct load transfer?

That is a very frequently asked question on this and other Towing Forums. Unfortunately, there is no agreement as to what is “correct”.
For some, “correct” means that equal load should be added to the TV’s front and rear axles.
For others, “correct” means the WDH should be adjusted to yield approximately equal GAWs for both TV axles.
For others, “correct” means that the front axle load should be restored to its unhitched value.
And, since about 2010, a group of major TV and WDH manufacturers have been advocating restoring between 50% and 100% of the load which was removed from the front axle.
50% restoration corresponds to a net decrease of about 12.5% of tongue weight, and 100% restoration corresponds to a net axle load change of 0%.
The following will examine the origins, practicalities, and ramifications of the different definitions of “correct”.

Equal Added Load on TV’s Axles (a.k.a. “Equal Squat”)
First use of the term, “equalizing” in connection with a trailer hitch most likely can be attributed to Martin H. Mathisen, founder of EAZ-Lift Spring Corporation. His
Safety load equalizing and draft stabilizing coupling device for automobile towed trailers
US 2597657 A
patent contains several mentions of the term.
Terrell J. Reese was awarded a later patent, Load transferring trailer hitch US 2952475 A, which contained no reference to “equalizing”. However, the term had gained familiarity with the RV towing community and was destined to remain in use – aided by Progress Mfg. Inc. trademarking and using the brand name, Equal-i-zer.

Variations of “Equal Added Load”
Although a numerical definition of “load equalizing” has not been found in any patents, a special definition, of unknown origin, stated that 2/3 of the tongue weight should be carried by the TV and equally distributed over both axles. The remaining 1/3 was carried by the TT.
The so-called 1/3,1/3,1/3 (33%,33%,34% in Airstream manuals) distribution has been advocated with apparent lack of understanding that such a distribution only can be obtained for unique combinations of TV/TT dimensions – combinations which rarely exist for real combinations.
Some people have devoted time and effort trying to achieve such a distribution – which was impossible for their combinations.
Being able to transfer a load equal to 1/3 of the tongue weight to the TT’s axles is beyond the capability of many WDH/receiver combinations.
And, a test program conducted for US DOT-NHTSA – using an Airstream trailer – concluded that transferring large amount of added load to the TV’s front axle was undesirable from the standpoint of stability. This will be discussed further in a subsequent post.

A more general definition of “load equalizing” only requires that the TV’s front and rear added loads should be equal. It is not required that the TV should carry an added load equal to 2/3 of the tongue weight. This means that a wider range of combinations of TV/TT dimensions could satisfy the “equal squat” objective. However, this also tends to be undesirable because you either end up with more than 1/3 of TW added to the TT’s axles or more than 1/3 of TW added to the TV’s front axle. Again, this definition of “equal squat” might not be achievable or desirable.

Equal TV GAWs
Sometimes used interchangeably with “load equalizing” is the belief that, when hitched with WD applied, the TV’s axles should be at equal gross load. In the special case of having equal unhitched GAWs for front and rear axle, you could end up with equal GAWs when hitched with WD applied if equal amounts are added to both axles. However, in general the unhitched GAWs are not equal, so adding equal amounts would not yield equal GAWs when hitched.
This WDH adjustment specification might stem from the often-disputed belief that sports cars and racing vehicles should have a “50/50” weight distribution.
However, adjusting a TV’s front/rear load distribution via a WDH is far different from adjusting a race car’s weight distribution by moving the car’s center of gravity.
Unless it has been shown that a race car pulling a trailer performs best with 50/50 axle loads, I would not blindly accept that what’s good for a race car is good for a tow vehicle.

Which Equal Loading Definition is Correct?
There are some who, when asked what is the correct tongue weight distribution, might state that it should be 1/3,1/3,1/3 or they might say it should be equal load added to both TV axles or they might say the WDH should be adjusted to make the GAW of both axles equal or nearly equal. In general, these are three different answers. And, depending on the particular case, they might all be incorrect.

WDH Adjustment Based on Front Axle Load Restoration
A study conducted for the US DOT-NHTSA concluded that:

“---hookups resulting in a small hitch-low attitude (corresponding to 0 percent hitch load transfer to the front axle in conventionally sprung tow cars) is more desirable than a level attitude which provides approximately 25 percent hitch load transfer.”

IOW, adjusting the WDH to restore the front axle to its unhitched load (a.k.a. 100% Front Axle Load Restoration) is more desirable than adjusting the WDH so that a net load equal to 25% of TW is added to the front axle.
This finding, and perhaps similar findings from yet undiscovered test programs, could explain why Ford, GMC/Chevrolet, Toyota, Progress Mfg. Inc. (Equal-i-zer), and Reese now are specifying that a WDH be adjusted to give 100% (or even 50%) FALR rather than using the old recommendation based on “equal squat”.
More about the US DOT-NHTSA study will be presented in a subsequent post.
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Old 10-12-2014, 09:17 PM   #2
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Caravanner Insurance Company “Loss of Control” Investigation

It has been claimed that – for Airstream trailers – a study conducted by the Caravanner Insurance Company division of Airstream Inc. has proven that “1/3,1/3,1/3” is the correct definition of load transfer.
Unfortunately there is virtually no publically-available documentation of this study, so it is not possible to ascertain why this conclusion is substantially different from results of the test program conducted by US DOT-NHTSA which will be described in a subsequent post.

The only Caravanner Insurance investigation information provided to date was presented in Sections 2 & 3 of this TOWING MYTHS article.
The investigation appears to have been based on information provided in response to questionnaires sent to persons involved in loss of control accidents.
The information requested was:
1. Year, brand & type of tow vehicle?........................
2. Brand of hitch?............................
3. Model of hitch (rating)?.......................
4. Was hitch equipped with full sway control?...........................
5. Who installed the hitch?...........................
6. Was hitch bolted or welded?.......................
7. Were rear springs on tow vehicle standard or heavy duty?................
8. Type of rear shock absorbers, (standard, heavy duty, air shock, air lifts,
load leveler or other.
9. If air shocks or air lifts, what pressure was used?..................
10. Was anything mounted on the rear of the trailer?......................
If yes, what was attached?....................... Weight of item.............
11. How full was black water tank?...............Gray tank?...................
12. How full was water tank?......................

No explanation was provided as to how answers to these questions could be used to determine either the loaded tongue weight or how much of the tongue weight was distributed among the TV’s and TT’s axles.
It is stated that the questionnaire responses were supplemented with interviews of some of the persons involved in the accidents. However, it seems unlikely that any significant number of those persons would have known the distribution of tongue weight at the time of the accident.
Even if the TW and TW distribution at the time of the accident could have been determined, there is no explanation of how that information could have been used to prove that a distribution of 1/3,1/3,1/3 would have prevented, or minimized the likelihood, of the accidents.
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Old 10-12-2014, 09:20 PM   #3
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US DOT-NHTSA Test Program

The Final Report No. DOT HS 804 248, EFFECTS OF WEIGHT DISTRIBUTING HITCH TORQUE ON CAR-TRAILER DIRECTIONAL CONTROL AND BRAKING, is available free of charge from National Technical Reports Library. Found under "NTIS DATABASE". You do need to register as a User to access the free PDF download. The report comprises 127 pages.

QUOTE
This report presents the results of a full scale test program, with supporting computer simulation analysis, aimed at determining the effects of weight-distributing hitch forces (or torque) on combination vehicles (C-V) handling and braking. Two C-V configurations were used; a full sized station wagon plus 31 ft travel trailer, and a compact sized sedan plus 18 ft utility trailer. Test procedures included a step steer (for tow vehicle understeer changes and transient response), a pulse steer (for trailer swing mode damping), straight line braking (for stopping distance), and braking in a turn (for transient understeer changes). Over 800 test runs were performed in which load leveling torque, hitch load, and tire pressure were the primary independent variables. Results showed that increasing hitch load and/or load leveling torque degrade the tow vehicle understeer and reduce the speed for incipient jackknifing. On the other hand, trailer damping is somewhat improved with load leveling. Front/rear tire pressure differentials (front lower than rear) have a significant beneficial influence when the hitch load is high.

Several devices and procedures were investigated in order to provide the user with a technique for determining the load leveling torque applied by a weight-distributing hitch. All methods provided usable results, the simplest being measurement of differential bumper height. In this respect it was found that hook-ups resulting in a small hitch-low attitude (corresponding to 0 percent hitch load transfer to the front axle in conventionally sprung tow cars) is more desirable than a level attitude which provides approximately 25 percent hitch load transfer. This level was arrived at by first minimizing the hitch load consistent with maintaining acceptable trailer swing mode damping. Load leveling is then applied, as necessary, to bring rear tire loads back within rated limits. Further leveling should be accomplished with air shocks.
UNQUOTE

The 1973 Caprice station wagon used in the control tests was considered a large tow vehicle in its time. Its wheelbase, ball overhang, and empty weight were nearly identical to today’s Suburban.
And for those who might think that a TV connected to an Airstream will respond differently than if connected to SOB, the Caprice was towing a 31’ Land Yacht TT.
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Old 10-12-2014, 09:23 PM   #4
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Conclusions – Caravanner Insurance vs. DOT-NHTSA

The Caravanner Insurance investigation was deemed by the investigator to have proven that a TW distribution of 1/3,1/3,1/3 is the “correct” distribution.
Results of the DOT-NHTSA test program were deemed to have shown that 0% net TW distribution to the TV’s front axle was more desirable than a “level attitude” which corresponds to approximately 25% of TW load transfer to the front axle.

Comparing apples to apples – for the Caprice/Airstream combination, 0% net TW distribution corresponds to a TW distribution of approximately 0%,75%,25% for front,rear,trailer versus 1/3,1/3,1/3 which was deemed by the Caravanner Insurance investigator to be the “correct” distribution.

One could ask why the DOT-NHTSA conclusions (or other similar conclusions) were not incorporated into WDH-adjustment specifications prior to 2010.
It’s a fair question, and the answer would have to come from the TV, TT, and WDH manufacturers who should have had access to the public information.

However – better late than never – Ford, GM/Chevrolet, Toyota, Equal-i-zer, and Reese now are specifying that the front axle load when hitched with WD applied should not be greater than when unhitched.
That means, in their assessments, the load transferred to the TV’s front axle should not be more than zero.
Ford and CMC/Chevrolet now specify that only 50% of the front-end rise should be eliminated. That corresponds to a TW distribution of approximately -25%,+112.5%,+12.5%.

People will continue to ask how they should adjust their WDH.
This information is provided in the hope that it will help them in deciding which approach is best for them.
Of course, there is no one-answer-fits-all.
Different TV and TT loading conditions and different TV and TT dimensions require different approaches.

Ron
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Old 10-13-2014, 12:52 PM   #5
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Ron,
Really excellent posts clarifying an intriguing issue. Jim


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Old 10-14-2014, 11:11 AM   #6
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Ron great information. I went back and looked at my spreadsheet (based on your method of weighing). Front axle no trailer hookup is 4800#. Hooked up, no WD engaged, the front changes to 4260#. 50% of the delta is 270#. So my 50% target would be 4530#. My actual is 4620#.

From the spreadsheet it calculates my 'tongue' weight at 1160#. 25% of that is 290# So that also pretty well aligns.

Interesting how this all comes together and your review gives a very easy way to understand what is going on when you have the information from the scales.

Thanks for this. Most outstanding
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Old 10-14-2014, 05:52 PM   #7
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1977

I've been following this with interest. I looked up the report. It was done in 1977. The Caravanner stuff was 1969 or so wasn't it? So I have to ask, if it's been around that long, why did they (hitch makers) wait 37 years to make a big deal of it? I bought my Equal-I-Zer in 2007, so that report had been around for 30 years even then.

Not trying to be a smarty pants. I just know a little about vehicle dynamics, and I'm wondering why all of a sudden a report from that long ago trumps what seems to have worked well for a long time?

If I'm reading this correctly, the issue is increased tendency to over steer; as in the front end doesn't plow ahead straight now as you keep dialing in more steering "at the limit" (under steer, which most car makers put in their standard alignment settings to protect Johnny Public from hurting himself in a car that swaps ends) but rather keeps on turning in. And it will continue to turn in longer now than it would have before because of the weight distribution putting more weight on the front axle.

Well, it makes sense that if you push down on the front axle harder, it will have more traction, and so it will have more "bite" when giving steering inputs. I'm not really sure that that's a bad thing though. As long as you don't over control it, I'd think it would be a good thing. I don't think I'd want to lighten the front end too much.

On the other hand, by putting zero additional load on the front axle, it makes you kind of like a semi truck, where the vast majority of the tongue weight is taken by the rear axles. And they drive a lot more miles than we do.

But, it's generally accepted that fifth wheels handle better than travel trailers, and the "magic" spot seems to be to put the hitch about 3-6" ahead of the rear axle. So, you would get a little extra load on the front, but not much.

So if we were to summarize all of this stuff, are we basically saying:
Only use weight distribution enough to not take weight off the front axle?

I am guilty of never weighing my individual axles. I set my EQ up (and there is a long and painful thread on here where I detailed how I did it) according to their old directions, just trying to get the front and rear squat to be close. I cranked in enough to prevent the front from rising and the back only went down a little, but it sits tail high empty anyway (Dodge 2500HD, Avion 34X with tongue weight guestimated around 900-1000lbs). I've towed it at least 20,000 mile like this and it's done very well. I've even had a few panic swerves and it did OK.

Anyway, I'm just curious about this. It's good data, but it's not exactly new. I'm just wondering why it's coming to light now and not before I bought my hitch?

I'd like to do this test myself, with modern instrumentation. But I don't want to risk my truck or trailer. Any donors ?
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Old 10-14-2014, 09:14 PM   #8
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Quote:
Originally Posted by JimGolden View Post
I've been following this with interest. I looked up the report. It was done in 1977. The Caravanner stuff was 1969 or so wasn't it? So I have to ask, if it's been around that long, why did they (hitch makers) wait 37 years to make a big deal of it? I bought my Equal-I-Zer in 2007, so that report had been around for 30 years even then.
Jim, I anticipated this question in the 4th post:
"One could ask why the DOT-NHTSA conclusions (or other similar conclusions) were not incorporated into WDH-adjustment specifications prior to 2010.
It’s a fair question, and the answer would have to come from the TV, TT, and WDH manufacturers who should have had access to the public information."

I don't know the answer to your question.
I do know that it takes some businesses many years to make a paradigm shift in how a product should be used.
Some of the WDH manufacturers had advocated the "equal squat" for 25 years before the test program results were reported.

As far as I can tell, Ford didn't have a spec for WDH adjustment until around 2000, and Chevrolet didn't have a spec until around 2004.
In both cases, when they did decide to put a spec into the Owners Manual, they opted to specify returning the front end to the unhitched height (100% FALR) instead of adopting the "equal squat" approach which still was being used by most, or all, WDH manufacturers.
Ford switched to 50% FALR around 2010 for some models, and GMC/Chevrolet switched to 50% for some models around 2013.
Equal-i-zer adopted a 50-100% FALR specification around 2010, and Reese began using the 100% spec in 2013.

It might be only a coincidence that the SAE introduced the FALR concept into the SAE J2807 drafting process beginning around 2010.
The principal investigator of the 1977 DOT-NHTSA test program also has been involved in the development of SAE J2807.

Quote:
Well, it makes sense that if you push down on the front axle harder, it will have more traction, and so it will have more "bite" when giving steering inputs. I'm not really sure that that's a bad thing though. As long as you don't over control it, I'd think it would be a good thing. I don't think I'd want to lighten the front end too much.
I think "over control" is a fundamental concern. Most, if not all, vehicle stability experts advise against trying to steer out of a potential sway event. They recommend holding the steering wheel straight and applying the trailer brakes. If you try to use steering to control the "sway", and if the overloading of the front axle makes the steering more "responsive" than it should be, the situation goes from bad to worse.

Quote:
So if we were to summarize all of this stuff, are we basically saying:
Only use weight distribution enough to not take weight off the front axle?
Ford, GMC/Chevrolet, Toyota, Equal-i-zer, and Reese now are saying it is okay to have the front-axle load less than when unhitched.
For example, if the tongue weight causes the front-axle load to be reduced by 400#, the new approach says it's okay to adjust the WDH to restore only 200-400#.

Ron
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Old 10-14-2014, 10:14 PM   #9
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Very interesting results. It is obvious that you put quite a bit of time into these posts. Sharing them with us is greatly appreciated!
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Old 10-15-2014, 05:21 AM   #10
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Ron - you have certainly earned a PhD in hitch adjustment. I look forward to giving this narrative more time than 2-pieces of toast and a glass of orange juice allows.

dennis
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Old 10-15-2014, 06:24 AM   #11
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Nice!
couple of questions:
What is "trailer damping?"

What does "conventionally sprung" mean?

What was the "compact" car?


I also wonder if the results would be different if done with a modern vehicle with independent suspension, fwd, lower-profile tires, etc.
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Old 10-15-2014, 06:48 AM   #12
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Ron,
Thanks for posting this information!
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Old 10-15-2014, 07:06 AM   #13
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All interesting information, Ron. You've done a lot of research on the matter of adjusting WD hitches.

Years ago when I first joined this forum, and after having been setting up WD hitches and towing travel trailers for even more years, I got into, uh lets call it a "discussion" with the person on here most noted for advocating the 1/3, 1/3, 1/3 weight distribution "rule". I told him the rule might have worked well in the 60's when it was common place to tow with a sedan, or a station wagon that was designed to carry it's load toward the center of the vehicle, but not with a pickup truck that is designed to carry it's load over the rear axle. The "discussion" digressed from that point to almost the name calling level.

It's great to see some work done on the matter by the US DOT-NHTSA. However, I still believe not all setups are the same. A 1/2 ton pickup is different than a 3/4 ton, and they are both obviously different than a front wheel drive minivan. Each user must actually experiment with their own setup trying different adjustments to end up with the best driving, riding, and handling possible for their own rig.

I have frequently thought it would be nice to be able to load the tow vehicle with the anticipated tongue weight of the trailer, placed where you want the tow vehicle to carry the weight, i.e. like 6" in front of the rear axle, and then set the ball height for a level trailer. Then it would be a simple situation to adjust the weight distribution to get the trailer level, and it would be done. However, loading 1000 pounds or so into and then removing it from a vehicle is not a simple or easy task.
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Old 10-15-2014, 07:10 AM   #14
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This is interesting stuff. I'm intrigued now and will have to go measure all my individual axles. I'd be willing to bet that I'm actually a lot closer to the newer criteria than the old with my current setup.

See you guys on the road!
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