Talk About Tongue Weight

[mstephens]

That could be one way to define empirical. The more typical definition as used in engineering is: Verified by direct observation, testing and experience. The phrase "without due regard to theory" isn't appropriate here, because all the "theories" of towing are being obeyed. e.g. Power, torque, weight distribution, hitch design, vehicle dynamics, and so on. What is being disregarded is just the manufacturer's specs, of which we know nothing about in theory or practice.

One of the primary reasons these cars don't get tested by the maker for a tow rating is that there is no convenient "bolt on and walk away" hitch solution. Each of these vehicles has to have a custom designed hitch, which takes some thought, some fabrication, some expertise to install. The current hitch market is a bolt-on only market. There is almost no welding gear at the hitch installers. They want a $8/hr kid to be able to follow the directions and bolt on your $229 Curt receiver. Now, you can't have Mom and Pop buying a car rated at 5000# towing showing up at U-Haul to get a hitch, and the U-Haul man says, "I ain't gots no hitch fer that!" Mom and Pop would haul their shiny new Chrysler back to the dealer and want a refund! This whole drama is primarily about the hitch. Funny, huh?

[rostam]

Originally Posted by Phoenix

Definition: Empirical -- Relying on experience or observation alone, often without due regard for system and theory.

Just my opinion, but this sounds like a recipe for a potential towing disaster; or, at the very least, mechanical and/or towing problems.

Let us know how this works for you...

(Now, I'll be quiet...)

Agreed. This is serious stuff. Your safety and the safety of people driving around you are at hand. Not a good idea to cut corners, or rely on unverified theories.

[dkottum]

There's always plenty of help telling you why you can't do something and so very few telling you how you might. Achievements are made by the innovators. Press on.

[mstephens]

Originally Posted by dkottum

There's always plenty of help telling you why you can't do something and so very few telling you how you might. Achievements are made by the innovators. Press on.

Thanks Doug! I am an old, thick skinned engineer. So, I am not insulted or discouraged at all by skeptical comments. All views are welcome and appreciated. I know that one has to press ahead and think it though, and try it. I am an experimenter from way back, although not so much with cars. The principles are all the same though. There's no magic involved.

[Ganaraska]

If you are in communication with Can Am Andy have you asked him these questions? He is the authority on towing an Airstream with a Chrysler 300.

[Jim Flower]

Originally Posted by mstephens

Thanks Doug! I am an old, thick skinned engineer. So, I am not insulted or discouraged at all by skeptical comments. All views are welcome and appreciated. I know that one has to press ahead and think it though, and try it. I am an experimenter from way back, although not so much with cars. The principles are all the same though. There's no magic involved.

I hope there are some young engineers reading this thread. They could learn a lot from your excellent approach to problem solving. Jim

[mstephens]

Originally Posted by Ganaraska

If you are in communication with Can Am Andy have you asked him these questions? He is the authority on towing an Airstream with a Chrysler 300.

I am. He has been an absolute gem in describing for me the right mods to make it all work. That part is taken care of.

This thread was intended to separately explore the "tongue weight" concepts, and try for a better understanding of exactly what is happening. I don't want to bug Andy on all these millions of side question like this. I know in advance that I can use the "published tongue weights." My goal here is to know why they are what they are, and if there are opportunities to fine tune them to my advantage. In other words, thinking out loud.

One of the immediate curiosities regarding tongue weight is that if it is all about reducing sway, which is the common claim, then what happens if one uses a 3P hitch, which mechanically locks out sway? Does that mean the tongue weight can be reduced by some amount? I would think that is the most obvious question to probe. If a 3P hitch could operate with say 25% reduced tongue weight, that would be a huge boon to smaller vehicles. Even the conservative move of stepping down to a Traverse/Acadia from a Tahoe would be handsomely rewarded by taking 200 pounds off the payload. It also might be a huge inducement to me to invest in that kind of hitch!

The next most obvious question is trailer design. Given an 25 foot SOB with a weight of 6,000 pounds and an 25 foot Airstream of 6,000 pounds, do both designs require the same tongue weight? That almost seems ludicrous on the surface. Suppose you arrange the SOB TW so it "just stops swaying." Isn't it likely that the AS would require even less TW? If not, what's so superior about the design?

I think the number one cause of swaying is speed, not tongue weight.

I suppose one simple experiment is to tow first with full propane tanks, and then remove the propane tanks and the batteries and tow again to compare sway. If there be no difference, that is at least 100 pounds of differential right there.

[SteveH]

I think part of what you are ignoring about tongue weight is rear axle tow vehicle traction, and the fact that the tongue weight is not a constant force, but one that varies as the road conditions change.

With lower tongue weight, the tow vehicle will loose rear axle traction sooner on roads that make the rig bounce up and down, and during one of the up cycles of the bounce, sway could more easily be induced. In other words, there are dynamic loads on the hitch, not just static weight.

With regard to using a Hensley or ProPride hitch, keep in mind they weigh at least an additional 175 pounds above most of the 'standard" WD hitches. Diminishing returns?

[andreasduess]

Originally Posted by SteveH

With regard to using a Hensley or ProPride hitch, keep in mind they weigh at least an additional 175 pounds above most of the 'standard" WD hitches. Diminishing returns?

That would assume that a standard WD hitch has no weight.

The Hensley weighs in at 190lbs, all in, if I remember correctly, about 60-80lbs heavier than a standard hitch.

Very quickly about brake performance, through my own stupidity I found myself without trailer brakes this summer, doing about 45-50 downhill. My vehicle stopped the trailer without any drama whatsoever.

[SteveH]

Originally Posted by andreasduess

That would assume that a standard hitch has no weight.

The Hensley weighs in at 190lbs if I remember correctly, about 60-80lbs heavier than a standard hitch.

The ProPride that I had weighed 220lbs., but they have been proven stronger than the Hensley.

[dkottum]

Originally Posted by SteveH

With regard to using a Hensley or ProPride hitch, keep in mind they weigh at least an additional 175 pounds above most of the 'standard" WD hitches. Diminishing returns?

A couple of thoughts on the Hensley/ProPride hitch.

The distance from tow vehicle receiver to trailer axle is increased with these hitches, compared to "standard" WD hitches, thereby lightening the weight the tow vehicle receiver actually carries. It would not amount to an additional 175 pounds, but something less. For example, Mojo weighed his Hensley equipped Airstream at the tongue jack, and then at the forward side of the Hensley stinger, and reported a 200 pound difference.

Another factor comes to mind. These hitches project the pivot point forward nearly 4 feet compared to a standard hitch. My experience indicates a longer distance from hitch pivot point to trailer axle reduces tendency of the trailer to sway. So does this projected pivot point result in reduced sway tendency in a similar manner?

It appears the Hensley/ProPride design is an advantage in many ways when examined beyond the usual considerations.

[m.hony]

What he said. I left camp and drove around a week before someone pointed out to me that my brake lights and turn signals weren't working. The umbilical was not hooked up for a week and I couldn't tell the difference.

[mstephens]

Originally Posted by SteveH

I think part of what you are ignoring about tongue weight is rear axle tow vehicle traction, and the fact that the tongue weight is not a constant force, but one that varies as the road conditions change.

With lower tongue weight, the tow vehicle will loose rear axle traction sooner on roads that make the rig bounce up and down, and during one of the up cycles of the bounce, sway could more easily be induced. In other words, there are dynamic loads on the hitch, not just static weight.

With regard to using a Hensley or ProPride hitch, keep in mind they weigh at least an additional 175 pounds above most of the 'standard" WD hitches. Diminishing returns?

Trying not to ignore anything, but yes, I agree that at some point in the vertical travel of the ball, traction will be lost. Certainly there are two obvious contributors to that, road gullies and speed. More of each will cause large swings in the inertia of the ball junction and at some point traction would be lost.

Here's a very interesting demonstration. In this demo, the key effect to control sway is concentrating the trailer weight on the yaw point. That's entirely counter to a heavy TW!
DEMO: Bailey of Bristol - Caravan Stability Studies

As we are uncovering, there is a lot more to this than simply, "put 10% to 15% of your weight on the tongue."

[mstephens]

I suppose a more basic question to ask is whether "they" even considered WD or sway control hitches when concocting the 10% to 15% rule of thumb? Would be a fun question to pose to AS engineers.

So far, my intuition tells me that 10-15% is a gross number that takes nothing much into consideration. A CYA sort of number that doesn't have much engineering behind it. Wouldn't it be great if 7% was the right number for properly hitched AS trailers?

[SteveH]

Yes, and if you watch and analyze that video, when he spreads the weight farther apart, he also reduces the weight on the tongue. In addition, he does not present any undulations in the road surface.

What he does do to effect the sway more is increase the speed. I am a firm believer than any tow vehicle/trailer combination will sway, if the conditions are right and the speed is high enough.

I have experienced it with a 3/4 ton truck pulling a 31' Airstream using a ProPride hitch when going down a hill too fast, and entering a curve when I hit a hump in the road. Fortunately, I was fast enough on the trailer brakes.

And the reverse of this is no trailer/tow will sway if driven slow enough, just like it's hard to have a crash into something if you are stopped.

[SteveH]

Originally Posted by mstephens

I suppose a more basic question to ask is whether "they" even considered WD or sway control hitches when concocting the 10% to 15% rule of thumb? Would be a fun question to pose to AS engineers.

So far, my intuition tells me that 10-15% is a gross number that takes nothing much into consideration. A CYA sort of number that doesn't have much engineering behind it. Wouldn't it be great if 7% was the right number for properly hitched AS trailers?

Boat trailers typically have much less tongue weight than travel trailers, and rarely sway. BUT, they have a much farther to the rear spaced axle, and a much lower center of gravity than a travel trailer, and less side surface for wind to effect.

[Ganaraska]

I get what you are driving at but it is not an either/or question, more of a gradient. 100% stable @ 10% tongue weight to 0 stability @ 0 tongue weight to negative (completely unstable) @ negative tongue weight.

You have to experience it. I suggest you get a SMALL trailer, like a luggage trailer, and experiment with cement blocks or other movable load. If you go to a 0 or negative tongue weight you may be surprised how that little trailer can whip your tow vehicle back and forth. DO NOT DO THIS ON A PUBLIC ROAD.

Once you have done this you will appreciate having enough tongue weight. Since it is easy to get and costs nothing, might as well have plenty which is 10% of the trailer weight.

British and European trailers (caravans) seem to be stable with quite low tongue weights by our standards. I'm not sure how they manage it except their caravans are light and are limited to quite low speeds.

[Chuck]

I think some of this equation is the importance of keeping the center of mass of the trailer forward of its center of gravity. (think: badminton birdie---heavy end goes forward). Its really hard to know where either of those things is, exactly...but you can easily know the weight of the vehicle, and the tongue weight; 10-15% insures a forward center of mass.
(may be totally wrong, but it seems like that might be the source of the rule-of-thumb, here).

My old vw bus really wanted to be going the other way. (back-end first!). And it would do just that, very easily, on a snow covered road.

[andreasduess]

Originally Posted by Ganaraska

are limited to quite low speeds.

Same as most anywhere here, 100km/h or 60mph. Most people seem to tow at around 110-120km/h.

[mojo]

Originally Posted by SteveH

The ProPride that I had weighed 220lbs., but they have been proven stronger than the Hensley.

Please share that documentation.