Load distribution hitches-an analysis

[markdoane]

Originally Posted by dancepancha

I really enjoyed your load distribution analysis. I was wondering if the equations changes if you put an extended hitch on the Dodge truck? I assume it just changes "H" but I want to make sure. I have a '31 that I will be hitching to a Dodge Cummins extended cab with a cab over camper on it and I need to extend the hitch so I can turn without damaging the cabover. Thanks, Frances

Frances,

I was hoping Nick would answer, but it looks like he is offline this evening.

The answer to your question is "yes". I would ask, however, that you pay attention the caution Mr. Crowhurst stated earlier:

" . . . . The equations are simple practical tools to assist in the selection of a load distribution hitch. An analogy would be the decision as to which paddle a kayaker should buy. The vendor should give advice based on the height of the paddler, the beam of the kayak, the type of water being traveled. The kayaker should then be equipped with a paddle “fit for purpose”. However, it doesn’t mean she’ll be safe or efficient in the real world situation of moving water. That will require skill, experience and judgment. Just like towing heavy."

It's great that you are working through the math. If you need help, feed me the numbers, I have all the equations loaded onto a spreadsheet.

Be careful about where you let the numbers take you. I have a gut feeling that towing a 31 ft airstream behind a pickup with a cab-over camper might be quite a load. But if it's a nose heavy diesel with a dual axle it might work.

[Druupy]

Experience with Dodge diesel. 4dr 2wd short box.
When I got the truck, one week before the airstream arrived, and put the weight dist hitch setup together, I had "help" from the trailer/mech/sales/helper/guy. The truck/trailer package was the worst riding vehicle, other than a forklift, that I had ridden in. Sway, wiggle and bam on the freeway expansion joints.
I repositioned the ball mount 25 miles from the dealership. It got better.

The thing that really helped, weighing the truck when empty, 4200# front axle, 3000# rear axle. Tounge weight 900#. So using the calculations, short box truck, no WD bars needed? I could not believe it, but did a test drive anyway. Just hooking the 1000# WD bars, hand tight. 50# air in all truck tires to match the tire mfg load specs, 45# in the stream tires, no fresh water on board.
Smooth, nice, stable, and fast=== but never over the 65 mph? tire max?

If you want optimum performance, you have to know what it weighs. Weight it first and most of the guess work is gone. Thanks for the calcs.

[nickcrowhurst]

I'm grateful for the interest and encouragement from Ryanh, dancepancha, lewster and
markdoane. I'm across the Atlantic, so I've only just woken up in my time zone to see the
recent posts on this thread. I'd like to make a few comments:

1. To dancepancha: as markdoane said, the equations will work with the extended hitch. Just
input the larger value for H, the rear overhang. However, I support markdoane's warning.
Increasing the hitch distance, H, from the rear axle of the tow vehicle will decrease the
stability of the rig. The trailer just has a longer lever with which to rotate the tow
vehicle about its rear axle in a horizontal plane, making "trailer sway" more likely.
Hitches such as the Pullrite and the Hensley Arrow owe their effectiveness in dealing with
sway to their designs which, in effect, move the ball pivot point very close to the rear
axle. You will be going in the opposite direction, and you need to be aware of this. This is
not to say that you should not use an extended hitch. It just means that you need to
consider how close you already are to the maximum safe trailer load for your combination of
tow vehicle, trailer, and their loads. The weigh station, with the rig fully loaded with camper, passengers, fuel, tools etc, together with the loading data for your vehicles, is your starting point, IMHO.

2. To Druupy: I'd appreciate the opportunity to see your calculations. It may be that you
need a softer set of bars, set up with some deflection, rather than a hard set of bars with
virtually no deflection. This could assist a smoother ride. Could you please post the three
dimensions, B, H, and L, so that we can look at them? For those who are new to this thread,
B is the length of one load bar, H is the distance from the hitch ball to the tow vehicle
rear axle, and L is the distance from the hitch coupling to the center of the axle system of
the trailer. You give the tongue weight as 900 pounds. Is this from the Airtream manual, or have you
measured by going to a weigh station, or by using the "lever and bathroom scales method"?
Nick

[dancepancha]

Thank you very much for your reply to my question. I don't have a dual axel but another airtreamer I know carries a similar rig but his is the 3 axle Excella behind a Dodge diesel and it seems to work but I don't want to depend on just one person's experience. Thanks again, dancepancha

[ljmiii]

I'm having difficulty finding a neat set of equations to solve for an equal front and rear axle force, F.

I CAN just flip the rear axle equation around. To recap...where
....C = undistributed load on front axle
....R = undistributed load on rear axle
....A = load on trailer axle

You can solve for a force Z that balances the load between the front and rear axles of the tow vehicle

starting Front Axle + C/(A+C) * Z = starting Rear Axle - Z
-C + C/(A+C) * Z = R - Z
-C + (1 + C/(A+C)) * Z = R
(1 + C/(A+C)) * Z = R + C
Z = (R + C) / (1 + C/(A+C))

Since
F = R - Z

the equal front and rear axle equation is
F = R - (R + C) / (1 + C/(A+C))

Which is very useful since I tend to belive that many people adjust their WD hitch to balance the load on their front and rear tow vehicle axles. But this is also somewhat ugly. When you expand the equation to measured values, you get

F = T*(W+H)/W - (T*(W+H)/W + (T*H)/W) / (1 + (T*H)/W / ((T*H)/(H+L)

I can't help feeling that I'm failing to understand some physical manifestiation of the equal load situation which would yield a simpler equation.

Review of this work would be appreciated as well as any suggested mprovements.

[nickcrowhurst]

I now understand that the above post relates to post 25 on thread:
http://www.airforums.com/forum...r-hybrid-28016-

As explained at the beginning of my analysis, I spent a long time deciding where to make the simplifications necessary to make the analysis sufficiently accurate to be meaningful, while being simple enough to be useful in practice.
One problem in your attempt to evaluate forces to equalise the lowering of the tow vehicle at the front and rear axles is that the spring ratings will not be the same at the front as at the rear. The load necessary to lower the front of my truck by one inch is vastly more than the load necessary at the rear. It was partly to avoid the complexity of spring ratings that I chose the method of measuring the chain tension required to bring the front axle back down to its original position. This bypasses the question of spring ratings. (Neatly, I would like to think! )
I believe my analysis is still appropriate for your situation, as you have returned the front axle to its original height. I do not believe the height of the rear axle is important in this case. Put simply, rear ends of cars are designed to cope with going down under load. Front ends are not designed to be lifted up in the air by hitch loads, and we must bring the front end back down to its original position, or lower.
Nick.

[3DogNight]

Hi Nick,Thanks for the referal to your thread of the mathematical analysis of WD hitches. I had more fun studying it than sitting with the inlaws for Thanksgiving. It would be my guess that somewhere along the line you taught Mechanical Engineering. Your explanations were clear and I think understandable.
Please correct me if I am wrong but as I understand from your equations, D is in direct relationship to L. I didn't try to figure out if this was 1 to 1 and am quite sure it is not but as trailer hitch to axel length decreased the the necessary chain tension to level the truck deminished.
Sooo-- with not yet taking delivery on my 20' Safari or the Equal- I- zer hitch and it's still the Holiday I used the garbage in garbage out principle. I guestimated L at 140". T is 600# per specs. Having a F250 crew cab short bed diesel with a W similar to your TV setup I used your numbers as well as B of 33" and came up with D= 763#. A number which is not worth the time it took to type but it leads me to the conclusion that the 1000# spring bar from Equal-I-zer is a better choice than the 600# alternative. Do you see a flaw in my reasoning? Thanks, Jerry

[59toaster]

Originally Posted by 3DogNight

Hi Nick,Thanks for the referal to your thread of the mathematical analysis of WD hitches. I had more fun studying it than sitting with the inlaws for Thanksgiving. It would be my guess that somewhere along the line you taught Mechanical Engineering. Your explanations were clear and I think understandable.
Please correct me if I am wrong but as I understand from your equations, D is in direct relationship to L. I didn't try to figure out if this was 1 to 1 and am quite sure it is not but as trailer hitch to axel length decreased the the necessary chain tension to level the truck diminished.
Sooo-- with not yet taking delivery on my 20' Safari or the Equal- I- zer hitch and it's still the Holiday I used the garbage in garbage out principle. I guestimated L at 140". T is 600# per specs. Having a F250 crew cab short bed diesel with a W similar to your TV setup I used your numbers as well as B of 33" and came up with D= 763#. A number which is not worth the time it took to type but it leads me to the conclusion that the 1000# spring bar from Equal-I-zer is a better choice than the 600# alternative. Do you see a flaw in my reasoning? Thanks, Jerry

Since you are towing with a 3/4 ton truck You need the more flexible bars. The tow vehicle has stiff suspension so it doesn't need the rear to lift as much to get the weight transfer to the front.

The reason you want the more flexible bars is if the bars become unloaded you loose sway control. The more flexible bars will take a greater attitude change between tow vehicle and trailer before you loose tension on the bars.


Now you are going "More Flexible" ...what the heck is this guy talking about.!?!?!?!?!?!?!

I believe that the weight rating currently on the bars is misleading. That weight rating was designated when people were towing with cars with soft suspension. When you get into a truck especially a 3/4-1 ton the stiffer suspension plays heavily into the system.

There has been quite a few wrecks analyzed on this forum and if you get down to the core problem what happened is in an evasive maneuver if the sway control is lost you are a goner. Its often found that "over hitching" (coined by Andy if Inland RV who use to be an insurance adjuster for Airstream and had done extensive testing and relates to the load bars being too stiff for the application) to be at blame.

So the bottom line is you need the "most flexible" bars you can get that will get the tow vehicle back to level. If you can't get the bars to level the vehicle then you need to go up to the next level of stiffness.

The above formula is good guide. If towing with a half ton or a car you probably want to go on the stiff side but on a HD truck like yours you want to go on the flexible side.

[3DogNight]

Originally Posted by 59toaster

Since you are towing with a 3/4 ton truck You need the more flexible bars. The tow vehicle has stiff suspension so it doesn't need the rear to lift as much to get the weight transfer to the front.

The reason you want the more flexible bars is if the bars become unloaded you loose sway control. The more flexible bars will take a greater attitude change between tow vehicle and trailer before you loose tension on the bars.


Now you are going "More Flexible" ...what the heck is this guy talking about.!?!?!?!?!?!?!

I believe that the weight rating currently on the bars is misleading. That weight rating was designated when people were towing with cars with soft suspension. When you get into a truck especially a 3/4-1 ton the stiffer suspension plays heavily into the system.

There has been quite a few wrecks analyzed on this forum and if you get down to the core problem what happened is in an evasive maneuver if the sway control is lost you are a goner. Its often found that "over hitching" (coined by Andy if Inland RV who use to be an insurance adjuster for Airstream and had done extensive testing and relates to the load bars being too stiff for the application) to be at blame.

So the bottom line is you need the "most flexible" bars you can get that will get the tow vehicle back to level. If you can't get the bars to level the vehicle then you need to go up to the next level of stiffness.

The above formula is good guide. If towing with a half ton or a car you probably want to go on the stiff side but on a HD truck like yours you want to go on the flexible side.

59toaster,
Thanks for the quick repy. I see where you are coming from on the springyness of the tension bars and it does make sense. The flex in the bars is really what provides the tension over a range of movement. A stiff bar althought under tension in the static position would easily unload with any movement between trailer and TV. Thus especially for smaller trailers and stiff TV's the smallest size tension bar that keeps the TV level is the best.
Am I understanding what you have said? Also if possible please point me to Andy's thread on the subject. I'm new at these forums and still a bit lost in finding my way around them.
Somehow life was easier with my old Streamline Prince. I just hooked it up with the Reese hitch that came with it and towed it all over the place forgetting it was behind the truck. Ignorance was bliss. Jerry

[nickcrowhurst]

Originally Posted by 3DogNight

Please correct me if I am wrong but as I understand from your equations, D is in direct relationship to L. Jerry

Jerry, I'm glad you enjoyed the analysis. I'm the dreaded in-law at my kids' place for Thanksgiving, so I'm hiding out of the way on the PC. Unfortunately, D is not in a direct relationship to L. Equation 8 shows that it is much more complex than that:
D= (T*H*L)/(B(H+L))
You can see that B, H and T are also involved, and L is on the top as well as on the bottom of the equation.
59toaster gives excellent advice in his thread. As he says, the important thing is to get the front axle back to where it was before hitching up. If you put the accurate figures into the equations, you will establish the minimum chain tension required for this. You then require the minimum strength bars that will provide this tension, so that the connection remains flexible. As 59toaster said, this is particularly important with the Equalizer, where sway control depends on loaded springs. One point to bear in mind is that manufacturers may define the capacity of their hitches in different ways. If they are defined in terms solely of hitch weight, then this is oversimplifying, as can be seen from the analysis. In practical terms, though, it is probably an acceptable compromise. The only accurate way, as Markdoane says earlier in this thread, is to obtain the spring deflection/chain tension figures from the manufacturer. It would be interesting for you to input your accurate figures, when known, and to come back to us after that. The figures obtained can be combined with the practical experience of forum members like 59toaster, and a body of knowledge can be established. I hope this helps, even if it doesn't simplify this complex issue.
Nick.

[3DogNight]

Nick,
As for the direct relationship of D and L, I think my problem is semantics. What I was trying to say is D and L are not related inversely. D gets smaller as L gets shorter not larger as L gets shorter.
My REAL problem is that I live in San Diego and am taking delivery of my new 20'SafariSE in Salt Lake City. The Dealer, Ardel Brown, threw an Equalizer hitch into the deal. As anal as I am I wanted to know for myself why they give me a 600# or a 1000# spring arm and more importantly if they are giving me the proper one. I plan to take equation #8 along with me but I was hoping for a seat of the pants answer to the problem. I think 59toaster's sugestion of measuring the rise of the front end of the TV and then using the lightest spring arm that again gets it level is that answer. Do you agree? My other problem is that the 20'Safari is new to the line up this year and I have not found any "this works" posts for it. On top of that you have made it perfectly clear that what works for some one else's 20' Safari may not work for me if the TVs are not the same. Looks like I haven't graduated from Tow 101 yet.
Jerry

[2airishuman]

hi jerry

seems 2 me you've made this too complicated....

your trailer has a 600lb tongue mass?

select 600lb w/d bars....

or 800 IF the reported tongue doesn't include lpgas weight, or the spare tire.

no need for 1000lb bars for a 600lb tongue...

at 600 the tongue mass will reduce steering axle weight...a very small amt..

how much? drive on a scale to determine...

it will not be much...

measure ALL wheel well heights for the truck before hitching and after...

tighten w/d chains enough to approximately even out the drop but this isn't gonna be much...1 inch perhaps?

yes focus is restoring weight/height to the front end but with such a small trailer leveling is more likely..

this drop is affected by payload too...how much is going in the truck bed?

the 1000lb bars would be too much for this trailer for all but a small number of tow vehicles...

cheers
2air'

[TIMETOGO]

Sooooo....... When Do I Use The Flux-capacitor?

[SilverGate]

You’re right 2air’… it shouldn’t be this complicated.
Us newbies are just trying to make the right choice…
And do our homework before we pick up our first Airstream…
Based on the information at hand.
2007 Airstream brochure states Hitch Wt. without options or variable weight
for the 23’ Safari SE is 600 lb.
Equal-i-zer WD Hitch is availbe at a rating of 600# and 1000# (not 800).
My pending factory order 23’ Safari SE will come with 2 Solar panels, Full awning pkg. (3 sides), 2 AGM batteries, spare tire, 2 30-lb. LP tanks and other options.
Plus my tanks will approach full when boondocking.
My 2006 F-250 is a 2-wheel drive with a comfortable ride (not off-road with stiffer suspension)
Other 23’ Safari owners on these forums seem to be happy with their 1000# Equal-i-zers.
This all leads me to believe that the 1000# Equalizer should be my choice… and yet…
Nickcrowhurst, 59toaster, 2air’ and others with knowledge and experience…
Which Equalizer would you use if you had my tv and pending trailer?

[nickcrowhurst]

Originally Posted by 3DogNight

Nick, I think 59toaster's sugestion of measuring the rise of the front end of the TV and then using the lightest spring arm that again gets it level is that answer. Do you agree?

Absolutely. That is the basic principle. Ideally you need to try out both. I know nothing about the Equalizer, but I suspect ( a guess) from what I have read that 600 will be fine for a 20 ft trailer. I'm on the road for 3 days heading south, so will be offline.
Nick

[2airishuman]

Originally Posted by SilverGate

Which Equalizer would you use if you had my tv and pending trailer?

well i've never used the equal-i-zer. i see no 800lb bar...bummer.

i am a haha devotee....even for single axle or smaller a/s....
and btw they only had 1000 and 1400 w/d bars until now. a 750 is available...

the eqz' uses proprietary bars.... if customers bug 'em they can offer an 800.

given you have a 250 psd one would think the 600 would be plenty. as nick suggested trying both on a scale would be ideal.

does the degree of flex in eqz' spring bars affect the sway control or is it all about the washers...

this will read harshly but i'm not a fan of friction based antisway. yet i respect the many very happy eqz' users here.

i just wonder IF the airstream or the hitch or the tv are responsible for their smooth towing experience....

hey silvergate
maybe you can take both sets of bars, do the weigh in and adjustments.
i have no doubt you'd elevate the user info base by doing this...

cheers
2air'

[SilverGate]

2air' - Thanks for your input (always appreciated)!
I'd like to elevate user info by doing that test...
But I'll have a bit more on my mind when picking up my first Airstream...
Inspection, Walk-through, Orientation, paper work, and the drive home.
So I'd like the dealer to correctly install the right Equal-i-zer the first time around!

All other Equal-i-zer users: Let's elevate our knowledge base:
If you have been reading this thread and have an Equal-i-zer WD Hitch…
Please take a moment and go to "Community Polls" here:
http://www.airforums.com/forums/f232/what-size-equal-i-zer-hitch-do-you-use-28204.html
And add to our knowledge base under thread, "What size Equal-i-zer Hitch Do You Use?"
The size of your Equal-i-zer Hitch (600# or 1000#)
Your tow vehicle model and size
Your trailer size… and why you like your hitch.

Thank you!
SilverGate

[SilverGate]

Well, I went with the 1000 lb. bars and don’t regret it.
Equal-I-zer weight distribution hitch has worked fine for me.

But I do want to acknowledge Nick for starting this fine thread.
Thanks Nick – You have elevated our knowledge and understanding of load distribution hitches.

Reads like a college professor’s discourse and likewise should be required reading for newbies!

Thank you for elevating the quality of the forums here by this and your other 1000+ posts!

SilverGate

[slowmover]

Yes, that post is amazing (thanks!!!!), and the thread is the kind of quality one hopes to find in an Internet search where the details of safety are being examined.

[DFord79]

OMG.....if Wally could read this thread He would go back to tent camping.