Trailers gone wild (fishtailing)

[48Rob]

I don't know exactly how you are planning to distribute the weight in your trailer in its "ready to hit the road state" but the criteria you reference above (trailer 3000#, 360# tongue weight, 500# behind the axle, and 2500# in front of the axle) seems, intuitively, like it would difficult/impossible achieve. I'd like to see details on your "weight distribution plan" and perhaps some calculations. PPPPPPP. PM me if you don't know what PPPPPPP means.

Cheers,

Gus

Gus,

I posted that without thinking it out so I could clearly explain...my bad.

To be exact...
Using Andrew's weight calculator;

Total weight is 3150 pounds when going down the road.

For now, in the design stage, I'm using 2700 pounds as overall weight.
The trailer is 168" (box)
Trailer with tongue is 208"
Axle/wheel center is 63" from the rear.

If I plug in 3150 pounds as the trailer weight, I get a tongue weight of 514 pounds (16.3%)
That is too much...

After calculating how much weight will be forward, and rear of the axle, I'm using 2700 pounds as the overall weight of the trailer.
By reconfiguring the interior to take a number of heavy items behind the axle, I then added that weight, which is 450 pounds, to reduce the tongue weight to a reasonable 357 pounds (11.3%)

Seemed a good way to me to help "balance" the load, without moving the axle so far forward that it looked odd, or interfered with my doorway.

Since Andrew cautioned against the "Dumb-bell" or see-saw effect, I wanted to try to understand at what point does this "effect" come into play.
As Andrew is much better at math than I, I was hoping he might explain the theory behind the statement, so I could apply it and be a happy camper.

Rob

[angib]

If a trailer weighs say, 3000 pounds, and you have 12% (360 pounds) tongue weight, but you have 500 pounds behind the axle and 2500 pounds forward of the axle, does this constitute "dumb-bell" weight distribution?

I think one reason people use x% tongue weight as a check is that it's simple and easily measurable. Measuring inertia is much, much more difficult and simple checks aren't possible.

Almost all trailers will hit sway problems if they are towed fast enough - all we need to do is push the sway limit above the normal towing speed and we're OK.

Anyone looking at trailer stability should visit this website and play the 'game' - which is an engineering model as well as a game - Caravan Stability Studies

I think the rules of thumb are that a constant weight distribution along the length of a trailer is OK. Any individual weights (fixed or mobile) then need to be added while thinking about their effect - so ideally all the weights will be just in front of the axle. Adding weights at the ends is a definite problem, though loading up just the tongue usually seems to be OK.

What is not OK, and fits the dumb-bell description, is to add weights to front and back ends in the hope that they 'cancel out' and give the 'right' tongue weight. One guy on the FiberglassRV forum reported that he added a second propane tank to his tongue to compensate for the multi-bicycle rack he'd fitted behind the trailer but that he still had the sway problem when the bicycle rack was loaded - which shows how near the stability limit some trailers are.

Rob, I doubt that helps much but, like I said, it's a tricky subject with no simple tests!

[bobhenry]

My Spin......

Level or nose down NEVER nose up!

U-haul hitch hook up rules !

[MushCreek]

The axle is 5" behind the CL of the original 10' rectangular deck, but there's 4' of tongue, and half of that is Vee-shaped body, so the axle is in fact further back in relation to the grand scheme of things. By comparison, the axle on my 2000 lb. pop-up is dead-center under the body, and the tongue is very light- about 75 lbs. It towed dead solid at 70 mph, all day long. Even on rough roads, or when a truck passed, it stayed right in line. The only two major differences is that the pop-up is much lower, and it rides level, not tongue-up.

[48Rob]

Rob, I doubt that helps much but, like I said, it's a tricky subject with no simple tests!

Andrew,

Every scrap of info I can collect ads up to the knowedge I need, or can then seek to get the job done correctly.

Thank you!

The game is neat, and helpful.
The short video showing the car and trailer on the treadmill, with the adjustable weights really drives the point home in a hurry.

I was surprised by the basis used on the game though... the tongue weight of the trailer, as built, before accessory weights are added, is well below the accepted "safe" threshold of 10%

(1000 KG trailer with only 62 KG tongue weight)
Adding weight to bring it to 10% makes it less stable.
I may be missing something though...

Rob

[mikeschn]

I'm surprised there hasn't been any discussion on axle placement.

Having owned a Fun Finder where the axle was too close to the center of the trailer, I know first hand how wild and white knuckled fishtailing can be.

And when the axle is too close to the center of the trailer, weight distribution can make the difference between between being able to tow, or fishtailing all the way to your destination.

Mike...

[48Rob]

Mike,

Your question is why I'm searching for clarification.

Using Andrew's calculator, and the standard 10-15 % rule, I must place my axle at or very near the center of the trailer in order to get the proper tongue weight.
I've moved my axle back to an appropriate location, but must reduce tongue weight by rearranging weight (redesigning floor plan and weight distribution) so that I have a reasonable amount just behind the axle to balance the two evils.
A few inches one way or the other on axle placement makes a huge difference!

Jay's trailer isn't level, which can make a lot of difference.
The axle placement sounds like it isn't correct, but until the rig is leveled out, I think it is just a guessing game as the weights can change so much when the trailer is tilted one way or the other.

Rob

[mikeschn]

Rob,

I believe axle location is important. Problem is, there isn't any way to calculate the sway factor.

For teardrops there was a generic formula for axle placement, and for the many hundreds (thousands) of teardrop that were built, I don't know of any that complained about fishtailing.

But with my fun finder I quickly found out that the fishtailing was due to the axle being too close to the center of the trailer.

So I sold that, and built another teardrop, using the old tried and true teardrop rules. Perfect! No fish tailing!!!

Here's the rule: This formula is from a fellow Teardropper, William Adkins, who uses a measurement of 34 1/2" for a 8' body and add 6" for each additional foot in body length. Here's the website page where Larry was so kind to share that: http://www.outbackteardrop.com/chassis.html

Mike...

[Corwin C]

I believe axle location is important. Problem is, there isn't any way to calculate the sway factor.

There are equations, but it takes much more than college algebra to put them to use. In fact, 18 years out of college I would have to do some serious reviewing to do it myself. For example:



This is the equation for a simple damped oscillation ... there are additional equations to find damping ratios, oscillation frequency, rotational inertia in each axis and so on. Not something that is intuitive or usable for the typical builder. AND if you change something, you have to start all over again. In practical use, moving as much mass as you can to just ahead of the axle (reduces yaw inertia), making sure your balance is correct, and using an appropriate drop/lift hitch to make the trailer level would be the easiest things to try. Next would be load equalizing hitch and an anti sway system.

This is one reason why trailers seem to pick up that tandem axle at just over 3500#. It's not about capacity as much as stability. The tandem configuration damps out the yaw movements very quickly and effectively.

[madjack]

Here's the rule: This formula is from a fellow Teardropper, William Adkins, who uses a measurement of 34 1/2" for a 8' body and add 6" for each additional foot in body length. Here's the website page where Larry was so kind to share that: http://www.outbackteardrop.com/chassis.html

Mike...

...that is basically how I have figured all of my trailers...I do however, use 36"s as a start point and 4-6" for each additional foot...as short as these trailers are to begin with, 2-4"s can make a BIG diff in their balance......
madjack

[Martiangod]

the hitch on the van is higher than the trailer, so it slopes back a good 4" or more.

Any and all trailers I set up are with a slight nose down attitude. I don't like level and absolutly will not send someone out nose up.
At any speed nose up will cause air exiting from under van to lift on the front of the trailer, in effect negating tongue some or all tongueweigth, like a wing. Level can cause somewhat the same effect.
A slight nose down attitude, minimizes air uder the trailer but also provides downforce from the roof, spoiler effect.
If the van rides high off the ground, a large side to side rubber mudflap may lessen ait exiting from under van getting to the trailer

[dh]

If I plug in 3150 pounds as the trailer weight, I get a tongue weight of 514 pounds (16.3%)
That is too much...

Too much for your tow vehicle to handle? IF YOUR TV CAN HANDLE THE TONGUE WEIGHT there is no harm in being tongue heavy.

[angib]

I was surprised by the basis used on the game though... the tongue weight of the trailer, as built, before accessory weights are added, is well below the accepted "safe" threshold of 10%

Rob, that's the 'safe' threshold in the US, but the caravan study is from Europe where much lower tongue weight %ages are normal - 4-7% is the usual range quoted here.

"But that's completely unsafe" I hear you say, "the roads of Europe must be lined with wrecked trailers" - well, curiously they aren't. But it does explain why European towing speeds are lower than the US (50-60mpg legal maximum) and I think European tow-ers expect to pay more attention while towing.

It does illustrate too why those who want to use European tow ratings, generally double the US rating for the same tow vehicle, need to also consider European towing practices.

[48Rob]

Thanks Andrew.

Perhaps in the US. it is simply "safer" to be sure everyone has a wide margin of error.

"Limits" can be reduced I suppose, if the owner/operator is better educated about cause and effect, and thus has the knowledge to apply if the threshold is approached.

Rob

[48Rob]

Too much for your tow vehicle to handle? IF YOUR TV CAN HANDLE THE TONGUE WEIGHT there is no harm in being tongue heavy.

DH.

Probably not too much for safe towing, but too much for my personal taste in towing.

300-400 pounds is the top of my limit for enjoyable towing with our vehicles.
We can do more, but why have a front heavy trailer that requires weight distribution if we can redesign the trailer, or adjust the weights so we don't need it.
That is where I'm at now, in the design stage. I'm trying to gain a good balance so I can have sufficient tongue weight without creating a domino effect in all the other design considerations.

Rob