Dynamics of tongue weight questions

[TD Beej]

They even have a little simulator to test your loading patern http://www.towingstabilitystudies.co.uk/towing/caravansnake.html

[Trackstriper]

Here in Europe, as we generally have smaller tow vehicles, it's usual to tow with lower hitch weights (and much higher trailer weights) than in the US.

So trailer sway is a serious issue here and you might like to visit this website where they present some stability studies and have a simulator where you can try out your ideas:
http://www.towingstabilitystudies.co.uk/


Andrew

Many years ago I made the mistake of towing a small, 4'x5', open utility trailer with a negative hitch weight. I'd backed up to my garage and loaded some construction debris for a trip to the dump. I did not unhook the trailer from the car, why bother? So I pulled out and headed down the road. At only about 40 mph the whole thing became quite unstable. The trailer was wagging the little Fiat 128 around like a rag. Applying the brakes worsened the situation. I did get it slowed without damage and rearranged the cargo. Problem fixed.

I knew better in my head, but I wasn't thinking about that as a potential problem when I loaded up. I should have checked the tongue weight. There is a video on the website that Andrew linked to that illustrates what I did wrong.

http://www.towingstabilitystudies.co.uk/stability_simulator.htm

Unfortunately, unless I missed something, the good Doctor is telling folks that his demonstrated instability is caused by having a large polar moment. And this could be a potential problem. But, if you watch the demo carefully you will see that he tail loads the model trailer. He moves the rear weight farther than the forward weight, and the rear weight is the heavier of the two. It appears that he created a negative tongue weight on the models, and got the expected result. I don't think this was a polar moment issue, but it is a good visual of what can happen with a negative hitch weight.

Andrew, as always, thank you for that link.

Bruce

[TD Beej]

Bruce, also notice the little demo trailer does not have any suspension to store energies and amplify instability. Yes, agree he probably fudged but I am also sure that he left out big chunk of the behavior w/o the springs. It would be nice to reproduce with more realistic model.


A multilink suspension would probably help with its passive steering by decreasing the lean and by slowing the whipping because the wheels will tend to point forward more then torsion or rigid axles.

[eamarquardt]

I dunno, but it things would have to be pretty drastic to have the weight distribution such that the front end of the tow vehicle would be raised to the point that steering is affected. Air shocks or air bags will return a dragging tail end to an acceptable height and the cost is probably less than $125 if you shop around (cheap IMHO).

The real bugaboo is trailer sway! Sway can make things real exciting (and dangerous) very quickly. Having the ability to move the axle easily (see pics in my album) makes it easy to relocate the axle (even easier if you mount the fenders to the angle iron the springs are mounted to) to obtain an acceptable weight balance (better term rather than just tounge weight as the correct balance is what you're really looking for). After thinking about it (and the fact that I'll have a real reefer and oven in the galley and 20 gallons of wateron board) I think that adding shock absorbers to the suspension would be a prudent addtion. They will dampen any tendency to ocsilate and, hopefully, make things safer for a very modest investment. I see very few trailers with shocks but think this represents "false economy".

There are lots of factors involved: height of the CG, weight, stiffness of the suspension, weight distribution, stiffness of the TV suspension, and more. Proper prior planning should prevent most problems.

My 2 cents.

Cheers,

Gus

[caseydog]

A multilink suspension would probably help with its passive steering by decreasing the lean and by slowing the whipping because the wheels will tend to point forward more then torsion or rigid axles.

Yeah, but talk about overkill. It is so much cheaper, and easier, to just put the axle in the right place. A multilink suspension on a teardrop camper is a rather complex solution to a simple problem, and if the axle location is wrong, causing negative tongue weight, I have my doubts that a multilink suspension would keep the trailer from swaying.

[madjack]

A multilink suspension would probably help with its passive steering by decreasing the lean and by slowing the whipping because the wheels will tend to point forward more then torsion or rigid axles.

Yeah, but talk about overkill. It is so much cheaper, and easier, to just put the axle in the right place. A multilink suspension on a teardrop camper is a rather complex solution to a simple problem, and if the axle location is wrong, causing negative tongue weight, I have my doubts that a multilink suspension would keep the trailer from swaying.

...yep, way too much sugar forra dime...having said that, we each build what we want...if someone wishes to make an engineering study of all of this on their build...mo'power to 'em but me, I got better things to spend my money and time on(MORE TOYS!!!!)
madjack

[TD Beej]

A multilink suspension would probably help with its passive steering by decreasing the lean and by slowing the whipping because the wheels will tend to point forward more then torsion or rigid axles.

Yeah, but talk about overkill. It is so much cheaper, and easier, to just put the axle in the right place. A multilink suspension on a teardrop camper is a rather complex solution to a simple problem, and if the axle location is wrong, causing negative tongue weight, I have my doubts that a multilink suspension would keep the trailer from swaying.

Putting the axle in the right place is only step one. Yeah a multilink might be overkill but inward tilt steer would help stabilize the trailer and it can be done in other ways. Negative tongue weight isn't the problem, it is about increasing the range of stable operation, simply the faster you go the more energy that can be put into throwing the trailer and the harder it will be to bring things back under control. If by using the other design elements such as aerodynamics, suspension design, reducing polar inertia, lowering center of mass, lowering hitching points,... you can increase your safe operation range you will also be able to bring things back under control faster and more easily when something does goes bad.

[clarkbre]

Putting the axle in the right place is only step one.

And, step two is load the trailer properly so there is a 10-15% tongue weight.

Then there is no need for a 12 step program!

[caseydog]

A multilink suspension would probably help with its passive steering by decreasing the lean and by slowing the whipping because the wheels will tend to point forward more then torsion or rigid axles.

Yeah, but talk about overkill. It is so much cheaper, and easier, to just put the axle in the right place. A multilink suspension on a teardrop camper is a rather complex solution to a simple problem, and if the axle location is wrong, causing negative tongue weight, I have my doubts that a multilink suspension would keep the trailer from swaying.

Putting the axle in the right place is only step one. Yeah a multilink might be overkill but inward tilt steer would help stabilize the trailer and it can be done in other ways. Negative tongue weight isn't the problem, it is about increasing the range of stable operation, simply the faster you go the more energy that can be put into throwing the trailer and the harder it will be to bring things back under control. If by using the other design elements such as aerodynamics, suspension design, reducing polar inertia, lowering center of mass, lowering hitching points,... you can increase your safe operation range you will also be able to bring things back under control faster and more easily when something does goes bad.

Dude, you are right in theory. I am with you on that.

BUT (there is always a but), I have owned four boats, two cargo trailers, and two teardrops, and I can tell you this with no hesitation -- putting a lot of time and effort -- not to mention money -- into a sophisticated suspension design is not going to yield the kind of results that justify the time, effort and expense.

I am a car nut, and work for car magazines, and I drive a high-end German car with a five link suspension. And, on a car, all that engineering makes a big difference.

Unless you want to set a lap speed record on the Nürburgring towing a teardrop, don't let this suspension thing become an obsession.

I can't feel my TD behind my Audi. I could probably tow it it behind a Honda Civic and feel perfectly safe. And, I have a leaf spring suspended solid axle. That's as old school as it gets.

If you get the basics right, you don't need to go with uber-engineering. Like MadJack said, if you want to design and build a state-of-the-art teardrop with multi-link suspension and coil-overs, just for the fun of it, go for it.

But, it is not hard at all to build a safe trailer with very basic technology.

To be totally honest, my teardrop could handle the Nürburgring behind my Audi just as fast as my Audi could handle it alone. Unless your tow vehicle is a Murciélago, I wouldn't put too much worry into the suspension. Just put the axle in the right place.

CD

[TD Beej]

I am trying to understand the dynamics of trailer handling so I can make the best choices.

The point is to make informed choices and to do that one needs to be informed.

Now a multilink suspension is attractive for a number of reasons but it also has drawbacks and when the choice is made it will be done knowing why and not because anyone may think it is "overkill".

[madjack]

hmmmmmmmm, I don't think it to be overkill, I know it is overkill, based on experience...the worlds fastest TD, timed at Bonneville(don't know the speed) had a plain jane torsion type axle(I think)...placed in the proper position...and I do realize, it was a straight line speed...but, along with moderate attention to center of gravity, it is all you will ever need...all we are trying to do is save you some bucks and hair loss(not to mention an ulcer)...having said all that, I will repeat what I said before...it is yours and one of the great beauties of building your own TD is the ability to build it your way...if this is what you want...GOFERIT...it would certainly be ubercool and totally unique since in the history of TD's going back to the 30's nothing of the type has ever been built(that I know of) and I would like to see it done just for grins...if nothing else................
madjack

[oldtamiyaphile]

I know your design is a bit different, but it seems that while in isolation a TD will produce quite a lot of lift (will be weightless at about 160km/h), the wake of the TV will never allow actual take off.

It seems that once the wake is accounted for, the TD's centre of lift is quite a long way back, behind the axle, so tongue weight should increase with speed. I also found that adding a plate or box to the tongue reduced aerodynamic lift at the front considerably.

In summary, I've found that this is about the optimum shape for towing with a small SUV:

[TD Beej]

Thanks Madjack and everyone for their input here, but I am un-swayed.

Straight-line stability really doesn’t count for anything, if you try the simulator at http://www.towingstabilitystudies.co.uk/towing/caravansnake.html you will see that you can set the simulator to hundreds of miles an hour without a problem, however if you swerve the vehicles will not restore itself. In fact if you play with it a bit it shows you how much dampening to a snacking action you have at a given speed (going down with speed and in fact going negative if you go fast enough). Every time you make a design choice that can increase stability (or snake dampening) the higher you performance limits and the faster you can bring things back under control when the trailer is upset. It is about getting the best handling in adverse circumstances so you prevent problems or get yourself out of trouble.

Another words, just because you can cruise at 75 with you trailer behind you in your TV doesn’t mean that another TV will be able to do that, or that that side wind won’t mess up your tranquility, or that the slight undulation in the road going around that easy bend with a wet road won’t be a disaster, or that the next owner will have as good of a feel for what is happening.

On our last trip, we were even more heavily loaded then the last, not a problem for the “mini” van, or for me, but when my wife drove (only once) she swerved back and forth, not enough for her to notice, but I did. To her she was driving normally and just making those slight adjustments you do when you drive, but it was a uniform, rhythmic, sway back and forth, needless to say I didn’t get the rest I was hoping for. What I am trying to get at here is if there were a trailer on there to it could pull perfectly fine for me, but with her things could quickly get out of control.

It isn’t about to much or doing just enough, it is about getting the most safety and performance with your design limitations (time, money, skills, size,…).

Oldtamiyaphile, thanks for the flow illustrator picture. For comparison you might like to look at the one I made. http://www.youtube.com/watch?v=jvqZ2TaDZlY It is surprisingly different then you results, If I could capture a frame I would put it up.

[oldtamiyaphile]

Beej, I think your parameters are not representative of airflow at highway speed. I can't remember what I used for mine but yours looks a bit like it's traveling slowly through oil.

Have a play with the settings

Regarding stability, you're going with a dual axle, right? I imagine having two sets of tyres that will scrub when asked to yaw will help out a lot, no?

[Nobody]

I THINK there's been waaay too much 'thinking' going on here . The dynamics/principles of trailer towing have been 'studied' to death by various agencies, individual, corporate, & government, for longer than I've been alive (I'll be 70 in a few days). They've all ended up coming to basically the same conclusion - design the trailer essentially as Andrew shows, 'balance' the load so there's more weight in front of the axle than behind, match the trailer to the TV (weight & power wise), remain alert & aware of your 'tow' even when the TV 'doesn't even know it's there', & mainly use a little uncommon 'common' sense. Trailers have been towed behind motorized vehicles for a hundred years, & with other 'power' sources long before that; the principles were learned early on, & they STILL apply! No need to reinvent the wheel.