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Old 12-17-2006, 09:26 AM   #1
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Weight Distribution Questions

Hi All,

I've been doing the Search function here and reading some interesting posts, especially those by Nick Crowhurst, regarding weight distribution.

I'm an engineer myself and I'm trying to understand something:

I always hear people say you "need" to have 10-15% of the weight of the trailer on the tongue? Why?

What I see is you have this 10,000lb trailer and then you drop 1500lbs onto the ball. But, you can't have that much weight on the ball, and so you end up lowering the back of the truck 10". You then have to monkey around with weight distribution bars to get the weight shifted back to the trailer's axles and lighten the "apparent weight" on the tongue.

What is the point to this? Why not design the trailer to put less weight on the tongue in the first place?

I've heard guys say the 10-15% rule is for stability, but I don't see how that really makes a difference. I would think stability would come from the side to side plane, not the vertical plane. I could see you wanting some amount of weight on the hitch so that if you hit a bump the right way you don't actually try to lift the rear of the truck off the ground. But why must you have so much (such a high percentage)? Especially if you're only going to try and "respring" it back to the trailer's axles with the bars?

I've got an idea to make a built-in hitch similar to a Hensley that is an integral part of the trailer frame. It would be pretty easy to do. The weight distribution part complicates it some. And, this is what makes me wonder why you need it at all? (Other than the obvious reason of if your trailer's tongue weight is so much that it makes the back of your truck go down a foot).

I pull a 10,000lb utility trailer behind my Ram 2500, just on the ball. It tracks as stable and true as anything. Never had a bit of problem with it, even at 80mph. It's 22' long.

I really think Hensley's design with the 4-bar type linkage (similar to a car's non paralel double wishbone type suspension but turned 90 degrees) is really the best way to go. That takes care of sway control by design, and there is no need for tension or friction. The geometry of the hitch itself causes stability.

So if the requirement for tension is removed, why do you need weight distribution at all? Why not design a built in hitch where the tow vehicle sees about 300lbs of hitch weight and that's it? And the geometry of the hitch takes care of all the sway control?

Why do you need to have 10-15% of the trailer's weight on the hitch, only to redistribute it back to the axles?

I'm not trying to be difficult, just trying to understand this. I've heard the "10-15%" rule mentioned as long as I can remember, but never really heard a good explanation of why. Can you all help me?

Thanks,
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Old 12-17-2006, 09:38 AM   #2
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did you put the bars on before you put the weight on the TV ?
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Old 12-17-2006, 09:41 AM   #3
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what is your TV ?
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Old 12-17-2006, 10:08 AM   #4
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Jim,

I know we have discussed this before. I think you are right that under some conditions (specifically location of hitch point relative to TV rear axle), you can have a safe configuraton with low hitch weight.

Although he doesn't treat the Hensley design directly, I still think the Bundorf paper (SAE 670099) is the mathematical model you need. The geometry of your new hitch linkage should be able to give you the effective hitch location, you could plug in the other numbers and calculate a lateral acceleration gain number that is better than the gain with a "normal" hitch and tongue weight.

I would be glad to fax you a copy of the paper if you need it.
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Old 12-17-2006, 10:18 AM   #5
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What tires do you have on your utility trailer that are rated at 80 MPH?
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Old 12-17-2006, 10:30 AM   #6
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Jim,

From a practical perspective, regardless of the mathematics, I think the limitting factor for low tongue weight will be traction of the drive wheels of the tow vehicle. Low weight+large tires = more tire slideslip under low traction conditions.

Imagine trying to tow your 10,000 pound trailer with a chain instead of a hitch. On wet grass with no weight in the truck bed, you might not be able to move the load before the tires start to spin.

The same applies when you are at speed and the trailer starts to push sideways. The tires will be less likely to sideslip if there is more weight on them.

You could get around this if you keep a good load in the truck bed.
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Old 12-17-2006, 01:01 PM   #7
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Quote:
Originally Posted by markdoane
Jim, I know we have discussed this before.
you have here...

http://www.airforums.com/forum...zer-22480.html

and posts 26-34 are some of the most interesting i've ever read here...

first rate exchange and good learning 4me...

great example of the pearls buried here.

cheers
2air'
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Old 12-17-2006, 01:56 PM   #8
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Hello Jim ,

using my travelall and trdwnd for towing as an example : the trailer ,especially a single axle needs to be able to track steady behind the TV.So its design is to have the rear axle placed behind the center of the frame as to reduce the teatertotter that would be prevalent if the axle were in the center ,thus built in tongue weight (axle farther back to the rear)
more weight will be in front and reduces the tetertotter effect.If you pulled it
with the tongue weight very low ,axle in the center ,the tendency for the trailer to want to wander or sway around will be great ,at first it doesn't seem like it would and with a very heavy duty TV with a small trailer like a utility trailer or a vintage bambi it may not be prevalent .The physics of it are that the mass will be more controlled if there is weight added to the front .
In my case and with the older vintage cars and trucks such as my travelall
they cannot hold up the trailer toungue without some squat which of course
pulls up on the front of the Tv and causes instability there ,takes weight off the front wheels.The WD effectively will lift up the rear of the TV and Trailer
by way of the tension of the spring bars and "tie" the frames together and
provides both to act together as one long frame ,making both intregal to
provide stable towing ,in my case if my trdwnd had little tongue weight and with its mass and length it would be whipping me around kinda like the tail waggin the dog is a great example,a tandem trailer I think would be less prone to this effect as the double sets of tires will lessen the trailers tendency to want to track around on its own behind the TV.The idea of
tieing the two frames together in a loaded or tensioned state (loaded)if you
will with the bars effectively provides the control needed ,sway can and does
of course still happen ,so the sway control comes into play ie:Hensley or
the reese dual cam ,Im still using the friction sway control (works fine )
but no doubt the other units are much better by far in controllong sway as
they tend to (lock in) the control by design.The larger mass and tallness of
a travel trailer (airstream to us )is much more prone to want to go ahead
and (do what it wants ) but the WD and correct hitching will allow the TV
and trailer to manuever and be much more controlled in an emergency
situation such as Barrys spartan car hauler ,once the round bar fell out
and his tongue weight was low as he had the cars inside to the rear of the
axles (as he has posted ) the thing had a mind of its own .He has a very
trick setup there and all .but the physics of it are the same ,still need to have the weight mass forward of the axles to provide stability and controll .
and the 10 to 15% rule have become the standard ideal amount to effectively
provide the weight transfer to the tongue as needed .

Scott
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Old 12-17-2006, 02:09 PM   #9
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All I know is that:

1) I don't tow at 80mph
2) Without the weight bars there is terrible bounce
3) My TV barley moves more than an inch or 2 when I put nearly 850 or more lbs on it. However, the pitch of both the TV and RV is not toally level. Putting the bars on, even though my TV can deal with the weight without the bars makes it more level and less bouncy.
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Old 12-17-2006, 02:42 PM   #10
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Definately agree ,because when we first bought the trdwnd some years ago in nebraska on a vacation (by chance) we hitched and hit the freeway ,man
I knew we wouldn't make it too far without the WD as it was lighter on the front end and no sway controll .I stopped at anRV place that had everthing
new and old, had the right bars ,we were saved and it went well .The WD
as it raises up the rear of the TV shifts weight to the front of your TV as well
as the trailers ,so all axles share the given load across the frames .

Scott
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Old 12-17-2006, 03:16 PM   #11
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Jim, I don't know of any RV's that have a 1500# tongue weight , 5th wheels will have that and more for a pin weight , but that is a whole other system . My understanding is that most of the duty of a WD hitch is to transfer load from the rear axle of the TV to the front to maintain steering control . I don't think it takes that much weight off the trailer axle(s). Good question . I know from experience what not having enough tongue weight will do to control , but cannot supply the engineering answer.
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Old 12-17-2006, 03:37 PM   #12
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I know the slide outs might. I say might because after testing the hitch weight, I found that typically the hitch weight of the RV is higher than what it has on paper. The slides are about 1100lbs, which to me would put a slide out coach near the 1500lb hitch weight....or at least possible FWIW.
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Old 12-17-2006, 06:25 PM   #13
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It isn't the tongue weight itself that's significant, it's what the tongue weight represents, and that's the location of the the trailer's center of mass between the tongue and the center of the axles.

The further forward of the axles the trailer's center of mass lies, the less influence it will have to sway the trailer and the less lateral force it can put on the ball.

The primary purpose of a weight distributing hitch is to put back onto the front axle, the weight leveraged off it and transfered to the rear axle, by the tongue weight. This prevents change in the tow vehicle front end alignment set up for more weight, and it reduces the possibility of understeer caused by the loss of weight on the front end. It can also put some of the tongue weight on the tow vehicle front axle if it has reserve capability the rear is out of.

In the process of doing this, yes, some amount of tongue weight is leveraged back onto the trailer axles. However, this does not change the trailer's center of mass even though it affects the downward force of the tongue and the trailer axles.

Without a weight distributing hitch, mass in a 10,000 pound trailer would have to be distributed less safely aft to keep the tongue weight below a typical 500 pound maximum non-weight-distributing rating of most hitches. With weight-distribution, you can get it up to 1,000 pounds, 1,250 pounds, or even 1,500 pounds with the right hitch setup.

The spring bars also help keep the trailer's porpoising from affecting the tow vehicle as much.
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Old 12-17-2006, 07:15 PM   #14
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I think I figured it out

Guys,

Thanks for the responses. First let me say that I am NOT the SUPREME TOWMEISTER yet and am just trying to understand all this. I've towed a lot with my utility trailer, but am still green with travel trailers.

SCF31, my current tow vehicle is a 2004 Dodge Ram quad cab long bed 4x4 with the Cummins. It's long as a freight car, very heavy, and tows very well. I'm very happy with it. I had a 2001 Ford Excursion before the Ram. It wasn't quite as long, but it towed very nicely in its own right.

Cats, I don't routinely run that fast. There's a stretch of interstate 81 where I live where it's six lanes and everybody runs that fast. You either run 75-80 or get run over. I've run that speed for about 10-15 miles at a clip. It was on new ST tires. Yes, I know now (I didn't at the time) they're rated for only 65mph, but I was lightly loaded. (Tire speed ratings are supposed to be for 12 hours...at least for car tires) My only point with that was that the utility trailer (a Patriot 16' flat bed equipment hauler...it's 22' long from ball to bumper) tracked straight as an arrow. On that topic, though, I do plan on following the lead of one gentleman I saw posting on here and going to larger wheels where I can run E rated truck tires, like Goodrich Commercial TA's (of which I've had good luck with myself in the past). Most of those are S or T speed rated, which are both over 100mph. Not that I'd ever go that fast, but I like knowing the tire is capable of more than I'd ever throw at it.

Scott, very good explanation of things. Thanks!

Twinkie, I think I'm in a similar boat as you with the tow vehicle to trailer ratio. And if you're using bars because you've seen a difference with vs. without, then I better use them too. Unfortunately, my trailer's not really roadworthy right now. I'm working on it, just want to make sure I get everything the best I can.

2Air, I give you credit for being the one man almanac I have trouble remembering what I had for breakfest...I knew I'd touched on this topic before but goodness knows I'd never be able to find it again. Sure enough, you had the link right there. It was after that post that I'd gotten with Don to read that Bundorf paper.

Don,
Thanks for the offer on the Bundorf paper. Actually, you'd kindly faxed it to me a few months ago and I'd just not had a chance to go through it in great detail. There is a LOT of math in there and it takes a while to go through it properly. Well, before RoadkingMoe posted, I took a detailed several hour look through this paper and started putting together a spreadsheet of my own based upon it. That paper pretty well explains it all, and RoadkingMoe is right on the money.

From what I get out of that paper, the weight distribution part really isn't the key component if you have enough tow vehicle. But if you don't (as in most of us with less than a Freightliner), then it leverages weight back onto the front axle, which gives you resistive force to offset the moment from the trailer cranking on the ball, then about the rear axle, trying to scrub the front axle sideways.

I want to get the SAE paper by Kom, which is referenced in the Bundorf paper, and go through it too.

But what I saw in Bundorf was that an aft CG is inherrantly unstable (just like an airplane). As well, too far forward a CG is bad too because it makes the gain too high on the tow vehicle; i.e. when the trailer tries to move laterally, it muscles on the tow vehicle too much if the CG is too far forward. So what you want is some happy medium on the trailer CG that is just ahead of the axles. And, good old trial and error and experience has shown that somewhere around 12-15% is the magic number. They used 14% in their example.

So if your tow vehicle is OK with that and it doesn't offload the front axle, then you wouldn't really need the bars. But if the geometry is such that 12-15% of the trailer's weight does offload the front axle a bit, then you better have them.

On my Ram, the wheelbase is 161", the distance from the center of the rear axle to the ball is 56", and it's 228" from the ball to the center between the tandems on the trailer. I'm going to have to run the numbers and work it all out, but I think that if I were to run a conventional hitch setup (like the Dual Cam that I have now), I'd definitely run the bars.

But like you said, I can also design a 4-bar setup similar to the HAHA and work it all out to get a gain that's as good as the tow vehicle alone. I'm going to run the numbers and see what I come up with. In the end, I should probably just by a HAHA and be done with it (although it costs as much the new frame and axles). But since I'm making a new frame anyway, I'd at least like to explore this possibility before I write it off. I still think the idea of a Hensley-Like unit built into the trailer itself would be really slick. It may still need the weight distribution bars, but at least I understand why you often need them now.

Looks to me like you want to keep tension on the system to bring the tow vehicle back to its original balance. And to my way of thinking, you would be better to do this with bars that deflect a lot, rather than a little, so that when you do hit the humpdebumps, you don't lose the effect due to offloading the bars by moving the trailer 2" or something. So light bars deflected a lot work better than stiff bars deflected a little.

Proves Inland Andy right yet again.

Bottom line, sounds to me like the weight distribution bars better be there for any big travel trailer attached to a rear mounted hitch.

The paper did go into detail about how stability is supremely improved by moving the hitch point forward. If you get it ahead of the rear axle, the math says it's basically "like magic" how much better the handling gets. Hence the reason people say fifth wheels tow SO much better (my dad included). And, that's why the Hensley Arrow with its four-bar non parallel linkage setup which projects the turn center up ahead of the pumpkin handles so well.

Yeah, I sound like an ad-man for Hensley. I'm not; can't even afford one right now. But I am very impressed by their engineering. Just wish they cost less...

Anyway, thanks guys for pointing me in the right direction. You really can find out just about anything by asking on here
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