Dual Axle Trailer Tongue Weight

[BCooke]

Monte,
Just to verify. Loren stated to reduce tongue weight you lower the tongue and that puts more weight on the front axle.
I believe it adds more weight to the axle but would think it also adds to the tongue.
Bill
Figuring the weight is evenly distributed.
(More thought)
Never mind it finally sank in.

[Full.Monte]

Yes, when you lower the tongue, you lift the rear wheels. This removes weight from the rear wheels and it has to be supported somewhere. It transfers to the front wheels and removes tongue weight. Think of a teeter-totter, with the tongue on one side, the front wheels on the fulcrum and the rear wheels on the other side of the fulcrum. Because you are lowering the tongue, you are increasing weight on the other side of the fulcrum, which decreases the weight on the tongue.

[SmokeyWren]

Originally Posted by Full.Monte

Yes, when you lower the tongue, you lift the rear wheels. This removes weight from the rear wheels and it has to be supported somewhere. It transfers to the front wheels and removes tongue weight. Think of a teeter-totter, with the tongue on one side, the front wheels on the fulcrum and the rear wheels on the other side of the fulcrum. Because you are lowering the tongue, you are increasing weight on the other side of the fulcrum, which decreases the weight on the tongue.

No. Not with a decent design of the leaf spring tandam axle suspension. There is a quarter-moon shaped equalizing pivot between the axles. You can move the hitch up or down a few degrees without changing the weight on the trailer axles - because of the way that equalizing pivot works.

Take a close look at the leaf spring suspension between the axles to see that equalizing pivot gizmo.

Granted, if you have a tandam axle suspension designed by a simpleton who literally makes it a teeter-totter design, then your comments are valid. But I haven't seen a tandam-axle trailer with that design since the late 1940s.

[Francesca]

Originally Posted by SmokeyWren

No. Not with a decent design of the leaf spring tandam axle suspension. There is a quarter-moon shaped equalizing pivot between the axles. You can move the hitch up or down a few degrees without changing the weight on the trailer axles - because of the way that equalizing pivot works.

Take a close look at the leaf spring suspension between the axles to see that equalizing pivot gizmo.

Hmmm....
Dawn may be breaking in my head....

Does this drawing illustrate that point?



Thanks!

Francesca

[SmokeyWren]

Originally Posted by Francesca

Does this drawing illustrate that point?

Yep. I surffed around looking for somthing similar to include in my post, but after an hour or so I gave up. So you're a better googler than I am.

[Loren B.]

Monte,


Thanks for supporting me. I've generally given up on trying to explain several facts that are counter-intuitive. Things like, "The pressure your tires exert upon the pavement is essentially equal to the air pressure inside your tires." I heard that in sophomore or junior year engineering, and it took the professor only about ten minutes to prove it to everyone in the classroom. In order to prove that, though, he counted on the fact that all of his students understood how to use a free body diagram.


I own a flatbed tandem-axle trailer. When I retract the tongue jack (lowering the nose), I can lift the tongue of that 4000 pound trailer with one hand. When I jack the nose up as high as it will go, I can't lift it at all.
　　
That flatbed trailer is not sprung like the one in the diagram above, it has Dexter Torflex axles that basically result in an independent suspension for each wheel. The design above would tend to lessen the effect I have stated, but would not eliminate it.


------


Francesca,


You can believe me or not, but it's true.


-- Loren, BSCE

[Francesca]

Hi, Loren

I believe you!
I'm just having a hard time understanding.
Although it's probably moot as far as my friend's situation goes.
He's pretty definite about wanting to tow the trailer level- the way we travel, we often aren't in one place long enough to bother with unhitching.

And no! We're not getaway drivers for the mob!

The pivot-point illustration does answer the question about tandem axle weight distribution that I asked when I started the thread, though...
Doesn't it?

Thanks to everyone for your patience!

Francesca

[VanDiemen23]

The shackles on a spring trailer equalize it a bit over a small range. Torsion axles do not equalize. it is important to have the trailer level when loaded or hitched, because the axle loads on a dual axle torsion suspension can change dramatically with angle and overload the tires.

Monte is the "most right". If I had a fifth scale I'd set up my race car scales under the trailer tires and the tongue jack demonstrate how the weight moves around on both types of trailers. it's not linear.

When setting up a new trailer, I put the tongue jack on the scale and level the frame. Then load the trailer, estimating weight and looking for a decent tongue weight percentage. When I have it loaded and have the tongue weight, I have someone measure hitch height to the ground and then stand on the ball and measure the change. Divide that distance by 220 lbs and I have the effective spring rate at the hitch.

Multiply that by the tongue weight and you now have how far the hitch will drop (more or less) when loaded. Now it's pretty easy to calculate the hitch ball top height to produce a level trailer at that tongue weight, and buy the proper parts to achieve that.

[Loren B.]

Francesca,

You already know that you are doing the right thing by towing with the trailer level. My reason for posting was to point out that if you tow a dual-axle trailer that is not level you will affect the tongue weight.

-- Loren

[wingnut60]

Beginning to realize that I am several years short of an CE degree...

Loren, thanks. Taking your 4000 lb trailer, it should have 400-480lbs tongue weigth, correct? If you retract the tongue jack so that it is off the ground, the 400-480lbs is now on the axles? And, as you said, you can lift the tongue by hand--because the trailer is now in balance, but should be tongue-down somewhat because the axles should be located as to give that weight bias toward the front? At what point does the tongue weight begin to affect your ability to lift the tongue--I would think immediately and more so as you lift it higher?

While I did not 'splain it correctly, I do know that a trailer with weight bias toward the rear axle will not be as stable as one with the bias toward the front axle, when towing. Why is that, in layman's terms?

Joe

[Engineer Guy]

I'm a 'numbers-oriented Engineering type. So, while taking into account whatever spec'd Tongue Weight values you're encountering, I would keep an open Mind re: choosing various Trailers. The Manufacturers you're looking at likely have many years in the Business, and have no vested interest in churning out Trailers that whipsaw around, or behave/brake erratically. So, question your assumptions, and then question them again.

Case-in-point... On my lil 15' 1989 Trailer, the 'generous' 30 gallon fresh Water Tank is under the front end Couch/Bed. At ~8.3 lbs./gallon, I can 'instantly' alter Tongue weight of this Single Axle unit by putting ~250 lbs. into that Tank. I don't, typically. I get Water near our Boondocking destination. I pump up the TV in-Spring Air Bags I installed years ago, and level out the SUV rear end. I haven't weighed it yet; I'm content to go off of Road handling characteristics. The ease/difficulty of turning the Trailer Jack Crank also tells me all I need to know re: relative Tongue weight.

Where you pack Camping Supplies, and where I put my 2 'gotta have' Tool Boxes, or ~40 lb. Jerry Cans of Tap Water, allow me to vary Tongue weight to my liking. It's a big deal to 'get right', but it's not a big deal to vary optional item weights to your pal's liking/safety. Hopefully, they can choose the Trailer they like, and tweak Tongue settings and Equalizer Bar settings from there. Indeed, I put my Tool Boxes behind the Axle, near the rear Door, and put my 2 Jerry Cans' in the under-Dinette Seat cubby just in back of the Axle so that they don't move around in transit. I usually tow with ~1/4 Tank of fresh Water, and empty Black/Grey Water Tanks [in the Trailer rear].

Dual Axle Trailers are a little more 'forgiving' in that weight is distributed between those 2 Axles [hopefully ~equilaterally]. Yes, Tongue Weight is still very important, but you don't have the single 'hinge point' of the load as you do with a Single Axle.

I understand all the illustrations/points above. Your friend thinks as I do and as I've practiced over decades of Cargo and Trailer towing: get the load 'pretty' level. I have and use several drop Hitches - as on my low-slung Utility Trailer - and did the same leveling while towing a very heavy, high value Cargo Trailer full of Sound/Band Equipment. My point, vs. all the theory: your friend's TV and Trailer choice necessitate individual 'tweaks' to get everything 'right'.

[CD]

The Manufacturers you're looking at likely have many years in the Business, and have no vested interest in churning out Trailers that whipsaw around, or behave/brake erratically.

I have to question the above statement. RV manufactures are trying to sell RVs and a lot of people won't give up their 1/2T PUs etc so the manufactures are trying to accommodate them by moving the axles closer to the balance point. If the Rver doesn't load very carefully he will have an unstable TT. I have a 28' faltbed trailer that has the axles way back, more like a semi trailer. The only way I can load it wrong is to overload the tongue. I won't tow it with anything less than a 1T dually. I doesn't make any difference what or how I put something on it, it is stable.
JMHO

[Francesca]

Hi, All

I want to thank everyone that's contributing information here.
My pal's coming over for a BBQ today, and we're going to hash all this info over...

And I hope you'll still be with us if (when!) we come up with more questions...


Francesca

[Loren B.]

Originally Posted by wingnut60

Beginning to realize that I am several years short of an CE degree...

Loren, thanks. Taking your 4000 lb trailer, it should have 400-480lbs tongue weigth, correct? If you retract the tongue jack so that it is off the ground, the 400-480lbs is now on the axles? And, as you said, you can lift the tongue by hand--because the trailer is now in balance, but should be tongue-down somewhat because the axles should be located as to give that weight bias toward the front? At what point does the tongue weight begin to affect your ability to lift the tongue--I would think immediately and more so as you lift it higher?

While I did not 'splain it correctly, I do know that a trailer with weight bias toward the rear axle will not be as stable as one with the bias toward the front axle, when towing. Why is that, in layman's terms?

Joe

　
Joe,
　
The first thing to know is that tongue weight, by itself, does not affect the stability of a trailer. Tongue weight, relative to the trailer’s total weight, is a measure of how far the trailer’s CG (center of gravity) is forward of the axle(s). It’s a convenient way for us drivers to measure the stability of a trailer before we hitch to it. The farther forward the CG is, relative to the axle(s), the more stable the trailer will be and the less it will sway.
　
Explaining why that is the case is beyond my ability right now. I have never studied stability of road vehicles. I have had an introduction to stability as it relates to airplanes, and the concepts are the same, but I haven’t worked it out for trailers and cars/trucks. I just accept that it works this way. (As an aside, another issue I haven’t figured out yet is why a car/truck must steer from the front to be stable. Vehicles that steer with their rear wheels want to swap ends. Try it with a shopping cart sometime.)
　
It seems to me that you understand the basics of what I was trying to say. Yes, I can retract the tongue jack on my flatbed trailer enough to lift the jack completely off the ground. At this point the tongue weight is obviously zero. As soon as I start to lift the front of the trailer with the jack, the force (tongue weight) increases, and it doesn’t take too much height before the weight is too large for me to lift.
　
My flatbed trailer has the front axle pretty close to the balance point of the trailer. The rear axle, obviously, is well behind that point. This results in a situation where, if I lower the nose enough, all the weight of the trailer is transferred to the front axle and the tongue weight approaches zero. This doesn’t happen until the front of the trailer is well below a level attitude.
　
When I tow that trailer empty, I hitch so that it’s level. This results in the rear axle carrying a larger part of the weight of the trailer and a tongue weight that is high enough to ensure the trailer is stable.
　
This is the point I was trying to make for Francesca, that if the nose is too low, the tongue weight becomes too light and the trailer will sway.
　
With my flatbed trailer, as with all trailers, I must be careful to load it so that a significant portion of the total weight is forward of the axles. Otherwise it will sway. I wish I could explain why in terms of physics, but I haven’t figured that out for myself yet. Sorry.

Maybe Monte can help us out here.
　
– Loren