Load distribution hitches-an analysis

[Cracker]

2air':

Maybe it would help if I once again provided my loaded and unloaded scale weights - noting first that the truck's rear axle rating is 8,550 lbs and the front axle rating is 4,800 lbs. Light, including permanently mounted built-ins under the tonneau cover, the front axle weighs 4,400 lbs and the rear axle weighs 3,960 lbs. Loaded for the road, the front axle weighs 4,200 lbs, the rear axle weighs 4,900 lbs, the trailer axles weigh 7,400 lbs, and the tongue weight is 900 lbs. These weights are with the equalizer bars tensioned and my wife and I in the truck cab. Note that the front axle has unloaded 200 lbs and the rear axle has taken on 940 lbs. The truck rear has dropped about 1.25" - which makes it almost dead level. I'll repeat that I've made modifications to the rear springs which you (2air') commented on some time back herein - but that has nothing to do with this discussion. The unloading of the front axle has little or no effect on the truck handling and it would quickly return to it's unloaded weight of 4,400 lbs with the addition of a couple of passengers. As for the 940 lbs on the rear axle, that does nothing more than make the truck ride smooth right out. With 7,400 lbs on the trailer axles, cranking the equalizer bars up would simply bring their load closer to the trailer 8,000 lb axle rating along with adding to the TV's front axle load. Note that the front axle rating for these dualies, with the Duramax, A/C, 4wd, and crew cab doesn't have much of a reserve - only 400 lbs when light. That said, for me personally, and for many others towing with a heavy duty truck, the equalizer bars do, IMHO, serve more as a cushion and as a device for taking up mechanical slack. Since the chassis extensions on these vehicles should be more than capable of handling a 1,000 lb dead load (---and probably 1,500 lbs with the right receiver---) I cannot understand the reduction in trailer weight stated on the receiver placard (7,500 lbs without equalizers and 12,000 lbs with) when the only critical factor addressed by the inclusion of equalizers would appear to be tongue load.

This is bordering on a hijack of Nick's original topic and excellent presentation - which I never intended it to be, so, 'nuff said!

[slowmover]

Originally Posted by 2airishuman

to restore the pre existing weight to the STEERING wheels and axle.
most of these tow vehicles understeer in normal driving...

reducing the weight on the front axle makes under-steering worse....

along with loss of grip and changes in alignment.

this is the primary reason to use w/d bars, even with a rear end that can 'handle it'...

i think.

cheers
2air'

Well, yes, of course (as I understand it also.) I meant for it to read rhetorically, "why bother with W/D?"

If the distribution is 2/3 forward and 1/3 backward to all axles then the TW just sort of disappears as I see it. The hitch and the receiver now have a spring to counteract the nastier vertical movements.

It'de be nice to mine Rogozinski's brain to see how those early 70's accident predictive factors were arrived at in Caravanner's offices.

[Inland RV Center, In]

Originally Posted by REDNAX

Well, yes, of course (as I understand it also.) I meant for it to read rhetorically, "why bother with W/D?"

If the distribution is 2/3 forward and 1/3 backward to all axles then the TW just sort of disappears as I see it. The hitch and the receiver now have a spring to counteract the nastier vertical movements.

It'de be nice to mine Rogozinski's brain to see how those early 70's accident predictive factors were arrived at in Caravanner's offices.

The predictive factors were arrived at by very closely examining over 1000 loss of control accidents while towing an Airstream trailer.

Each loss was examined as to how the tow vehicle was rigged, what kind and brand of hitch was used, what rating or ruggedness the suspension system had, what was added to the suspension system of the tow vehicle, type of sway control, if any, etc.

Gathering that information, very quickly raised "red" flags as to what setup was an accident looking for a place to happen, and what on a small scale was still a mystery.

We documented, "and proved," the cause of 90 percent of those accidents.

I will look in my very old records to see if I can find the original 12 questions that were used in each and every one of those losses.

However, posting them today, in this forums, would probably cause a rash of arguments, that I do not wish to start.

Perhaps I could post them as an article in our web site, for everyones perusal.

Andy

[nickcrowhurst]

Cracker, I believe your posts fit in well with this thread as they give a practical application of the hitch calculator. For example, can you put your TV figures into the calculator, but with a 12000 pound trailer weight and a hitch weight of 1460 pounds ( (900/7400)*12000), and then repeat the calculation with a hitch weight of the normally recommended maximum of 15%, namely 1800 pounds? I would roughly guess that the front axle of your TV would be lifted by about 325 pounds in the first case, and 400 pounds in the second, without WD.
It seems to me that the GMC recommendation may coincide with your view. That is, your dually can safely tow up to 7500 pounds without WD. Your trailer weighs 7400, so no problem. If, however, you wish to tow up to 12000 pounds, your front steering axle could be lifted up by about 325 to 400 pounds . The engineers stipulate that this is not safe, (I assume because it is outside the safe design parameters of the steering and suspension) and that you should use WD to reduce or eliminate the front axle uplift. Perhaps it's as simple as that, and perhaps the strength of the hitch is not an issue in the GMC 3500 towing weight limits?
Nick.

[slowmover]

Originally Posted by Inland RV Center, In

The predictive factors were arrived at by very closely examining over 1000 loss of control accidents while towing an Airstream trailer.

Each loss was examined as to how the tow vehicle was rigged, what kind and brand of hitch was used, what rating or ruggedness the suspension system had, what was added to the suspension system of the tow vehicle, type of sway control, if any, etc.

Gathering that information, very quickly raised "red" flags as to what setup was an accident looking for a place to happen, and what on a small scale was still a mystery.

We documented, "and proved," the cause of 90 percent of those accidents.

I will look in my very old records to see if I can find the original 12 questions that were used in each and every one of those losses.

However, posting them today, in this forums, would probably cause a rash of arguments, that I do not wish to start.

Perhaps I could post them as an article in our web site, for everyones perusal.

Andy

I believe that you'd have done the RV community a signal service by posting such at Inland RV Center's website.

[SRW]

Nick:

Thanks for you analysis. It is quite impressive.

I have a new 28 ft. and use a Hensley Arrow Hitch, and have been wondering how this mechanical contraption (a parallelogram with pinned connections at the corners) functions as a antisway device. It uses tension bars for load leveling.

How do you feel about applying your spreadsheet calculator to an Arrow Hitch?

[slowmover]

Originally Posted by SRW

Nick:

Thanks for you analysis. It is quite impressive.

I have a new 28 ft. and use a Hensley Arrow Hitch, and have been wondering how this mechanical contraption (a parallelogram with pinned connections at the corners) functions as a antisway device. It uses tension bars for load leveling.

How do you feel about applying your spreadsheet calculator to an Arrow Hitch?

Some of what you may be looking for is here:

http://www.airforums.com/forums/f238...s-17984-2.html

I'm working my way through my non-A/S all-aluminum trailer with the help of others here (great help!)

By the way, SRW, I was wondering if there were a good lunch spot (table service) in your hometown where I can park this 61' rig. I don't often run I-35 through Georgetown, but occasionally do between Dallas and Corpus Christi.

Thanks

[nickcrowhurst]

Originally Posted by SRW

Nick:How do you feel about applying your spreadsheet calculator to an Arrow Hitch?

SRW, I have very little detailed knowledge of this hitch, but I know of no reason why the calculator should not work if the WD part of the hitch is of normal design.
Nick.

[SRW]

Originally Posted by REDNAX

Some of what you may be looking for is here:

http://www.airforums.com/forums/f238...s-17984-2.html

I'm working my way through my non-A/S all-aluminum trailer with the help of others here (great help!)

By the way, SRW, I was wondering if there were a good lunch spot (table service) in your hometown where I can park this 61' rig. I don't often run I-35 through Georgetown, but occasionally do between Dallas and Corpus Christi.

Thanks

Thanks for the info.

Are you talking about an 18 wheeler?

Let me know and I will check.

[SRW]

Originally Posted by nickcrowhurst

SRW, I have very little detailed knowledge of this hitch, but I know of no reason why the calculator should not work if the WD part of the hitch is of normal design.
Nick.

OK.

....and again thanks for the analysis.

[mustang]

Boy Nick I think your giving 2airish a run for his money. I bet he is thinking he has competition, HMMMMM

[JFScheck]

Nick - Superior Work!!!!

Wish I could have found this a month ago to help me - but after speaking with TONS of folks (to include Pro Pride & Hensley folks) and figuring out a formula - I get the same results...

Thanks to one of our fellow Airstream members who is also a Tourareg members...

Well - actually your formula is superior but I'm close (faulting a bit on the cautious side)....

See - all this time latter and your hard work is still helping out others!!!

[nickcrowhurst]

John, many thanks for taking the time to make your post. It makes the work truly worthwhile. Nick.