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Old 05-04-2006, 07:28 PM   #29
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I was thinking about this today before checking the replies.

It would stand to reason that tires with really stiff sidewalls and inflated to max safe pressure would go a long way toward preventing this. I guess the tradeoff is that the thing would then ride like a buck board. So I guess you have to strike a happy medium there.

I was asking my dad about this over the weekend after reading the post about the poor guy on here that got blown off I-95 around Jacksonville, FL. I asked him why it seemed that travel trailers get blown off the road and big rigs don't. Dad drove big rigs for a couple years. He told me they get blown off the road all the time, especially box trailers that are empty or hauling something with a tall CG. So I guess the problem is not related to just travel trailers.

Anyway, interesting info on here. So what can be done with suspension to help mitigate this? Has anyone done anything? Most trailers I've seen either have simple leaf springs or the duratorque axles. I've not seen one yet with a panhard rod or antisway bars like a car. Has anybody played with this? Or is it not enough of a real issue to worry about as long as you have the tires inflated right, trailer loaded properly, etc.?
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Old 05-04-2006, 09:20 PM   #30
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I liked that tire slip angle model. A nice classical Hamlitonian treatment. But trailers don't have tire slip angles by design. (slip angle is a steering tire thing) As markadone notes, the model that uses it falls apart without it. Big flaw in the model IMHO.

The helicopter blade is a resonance phenomena created by shock waves reflecting off the ends. The fact that it lasts a while says there is little damping. Now, if sway were a resonance thing it would behave a lot differently than what people describe it doing.

An oscillation is what happens when you take a stable system and perturb it little. It tries to get back to its stable state and overshoots a little and tries again, and again, and again until it gets to tired to try any more.

The first problem is that a rig going down the road isn't a stable system. Take your hands off the wheel and the first little perturbation is going to create catastrophe.

The second problem, after assuming a stability that doesn't exist, is to figure out what force is trying to return the system to its stable state. A lot of imagination has been applied to this problem. Tire slip angle is one example. Trying to use inertia in some way is another. I note that people are real proud of their inventions in this area, too!

A third problem has to do with the increasing energy in the 'oscillation' as it gets bigger and bigger. That has to come from somewhere. And it has to be timed just right, too.

Any vehicle is an inherent balance. It can't be too stable or you couldn't get it to do what you want it to do. But then, it can't be too unstable or the average driver couldn't control it. That always means that there will be conditions or circumstances where control can be insufficient and that can have disastrous results.
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Old 05-04-2006, 09:36 PM   #31
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Quote:
Originally Posted by Leipper
I liked that tire slip angle model. A nice classical Hamlitonian treatment. But trailers don't have tire slip angles by design. (slip angle is a steering tire thing) As markadone notes, the model that uses it falls apart without it. Big flaw in the model IMHO.
The oscillation model (by Bundorf) is based on the tire slip angle of the tires on the tow vehicle, not the trailer.

I think the fact that trailers don't have tire slip angles also makes the application of Panhard bars a moot point (especially for torsion axles!), but I am intrigued by the idea of sway bars on a trailer. Not so much to reduce sway oscillation, but to reduce rolling tendency.

I will need to think about it.
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Old 05-05-2006, 08:37 AM   #32
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So many models! I might have been confused. The plethora of even academic models should say something. At the very least they provide a lot of material to debate around the campfire.

If tow vehicle tire slip angle, which means steering, is the critical factor of the model, it raises questions about any oscillation the model predicts. It is essentially saying that sway is a matter of driver control as that is where the steering input comes from. It did have a fairly good set of design guidelines as quoted I think.

The stability about the long axis is not often discussed except in those threads where people wonder whether they should run with full water tanks. Occasionally shock discussions get there. I agree that it could be interesting as a discussion in its own rights.

Have you seen the airstream.com page on their EU versions? They build narrow body trailers with less tongue weight for that market. While that is needed for their roads and tow vehicles, it sure seems that the trailers would be less stable as well. And they don't seem to get into the DC.EQ or even haha debate, do they?
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Old 05-05-2006, 10:19 AM   #33
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Quote:
Originally Posted by Leipper
If tow vehicle tire slip angle, which means steering, is the critical factor of the model, it raises questions about any oscillation the model predicts. It is essentially saying that sway is a matter of driver control as that is where the steering input comes from. It did have a fairly good set of design guidelines as quoted I think.
The Bundorf paper* uses tire cornering stiffness and sideslip angles (for both the tow vehicle and the trailer) to calculate the yaw forces acting on the combination. These are used to describe the motion of the trailer and tow vehicle as a rotational inertia-spring-damper system, which leads to the frequency and damping equations.

From there he calculates the yaw velocity, sideslip angle, lateral acceleration, and towing angle responses.

What is really neat is that he can then change the tongue weight, rear overhang, tire stiffness, and 'centering force' (sway control), and determine the stability following a pertubation such as a passing truck.

This was done using both Hamiltonian methods and early computer computational methods.

If you would like me to fax you his paper, send me a PM.


*R. Thomas Bundorf, "Directional Control Dynamics of Automobile-Travel Trailer Combinations" SAE Paper 670099, Jan. 1967)
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Old 05-05-2006, 03:10 PM   #34
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Well, my take was that resonance is essentially when you hit an object's natural frequency. That is what I did with the Huey blade. And there are different order harmonics, 1st order, 2nd order, and so on, and at each of these I would get another node of zero motion while it broke the vibration modules up into shorter lengths between the nodes.

And we may be doing apples to oranges on a couple issues. Oscillation is a generic term that could apply to many things but generally is taken as a periodic and rhythmic vibration. So maybe I've just talked myself out of equating oscillation to vibration...

I've done some phugoid testing with a Cessna 206 where we would load it further and further aft CG until it became unstable. The test was you basically load the plane, get it going in level cruise, induce a vertical perturbation (bounce the elevator) and see if the plane dampens out on its own. If it did, it was called positive stability. If it went on forever but the amplitude and frequency never changed, it was neutral stability. If it got worse and the pilot had to intervene before we broke it, it was considered negative.

I find it interesting how a tail heavy trailer acts similarly to a tail heavy aircraft, only in 2-D. Most of my experience with this sort of thing is with aircraft, so I'm new to the road going stuff. Pretty neat actually.

As for slip angle, maybe that was the improper term. Perhaps gyroscopic precession would have been better, but the tire flex does play into it. A motorcycle does not turn left because you turn the bars left. Actually you turn the bars right. Gyroscopic precession, which happens 90 degrees off, causes the bike to fall left, and then it turns. Bikes steer by leaning. You can do it by weight shift, but it is much more positive under countersteering; turn right to make the bike turn left. Forks are more vertical than they are horizontal, so turning the bars to the right makes the top of the wheel want to go left. The bike leans, and you turn. Once I figured out how counter steering works, I was able to throw a Goldwing around like a 600. So anyway, if the top of the trailer tilts right, the axles are going to tilt right with the body and try to tilt the tires with it, which then gyroscopic precession would try to turn 90 degrees to that, which would flex the tires left. At least the outside more heavily loaded wheels would. If the tires were rigid, it wouldn't matter. But they are not. The tires flex, the trailer would try to steer left. I don't know if this is the real cause of sway, but I would bet it occurs.

I'm not sure who the Hamilton is you guys are talking about. My degree was in aerospace engineering and I've got a P.E.,but I don't know who Hamilton is. Not familiar with Bundorf either, although I see the work referenced. Anyway, the above is completely my homegrown theory. May be utterly wrong, but it seems logical enough. And if I duplicated some former theory, it was purely unintentional.

Yeah, I kind of agree. Most models aren't the greatest. Most of the ones I've used have these big nasty equations that are based purely off empirical evidence and curve fits so that between here and here, if the moon is just right, you can use this equation. But all bets are off if it's beyond point X.

Oh yeah, the panhard rod wouldn't apply to torsion axles. They're already very stiff in side to side flex, but it would help a bunch with leaf sprung beam axles. I've been considering building a new frame for my trailer and going back to beam axles for several reasons. If I do this, I will probably put panhard rods on it to prevent side to side sway. But I think we're talking about angular sway here.

Bryan makes a good point about the whole thing being unstable to start with.

I've personally not experienced sway on the road. I pull a 10,000lb utility trailer with just the ball and have never had a bit of trouble. The whole idea of it I find kind of scary. It would not be good to have the trailer try to pass you.

I think it was Inland Andy who mentioned an invention a guy had come up with that basically modified the steering wheel to have sensors such that if you developed a death grip on it (and I think you could set the threshold) it applied like 50% trailer brakes. The theory was that people naturally tend to tense up when bad things are happening and this was lot faster than grabbing for the manual handle down under the dash somewhere...

This is an interesting topic. I'd like to fully understand it.

One thing I was thinking about was...rather than shelling out the $3K for a Hensley, why not just build it into the tongue of the trailer? It's just a 4-bar linkage like double A-arms on a car. You could set the geometry up to project the turn center some "happy" distance ahead that would work out for most vehicles and be done with it. You'd only have a 2" square piece of steel sticking forward to slide into the receiver. Just build it as a permanent part of the trailer frame. I guess you'd have to do something for the weight distribution part of it. But if a guy was building a new frame anyway...
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Old 05-05-2006, 04:57 PM   #35
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Hamilton was a guy who developed a method for analysis of mechanical systems they teach about in classical mechanics for physics types. Bundorf looks to be an automotive engineer at GM who wrote a nice paper in 1967 (thanks to markadone I see Bundorf uses a more simple modeling approach than Hamilton).

I do note that Bundorf talks about "oscillatory behavior" rather than oscillation. That is a significant difference.

I think the last question is really to the point. For most folks most of the time in most circumstances, there is no need for any fancy hitch mechanics. Load leveling is sufficient. No need to add expense and complexity.

Andy had a good idea. I have heard of similar devices to keep drunks from starting their cars. Now put a stress analyzer in there to help trailer drivers. I note that the new built in brake controllers Ford is putting in their pickups is starting towards more intelligence in towing control somewhat on this line. Airstream also used to have a motion sensor to use selective braking to stop sway.

Then, of course, you could always go with a 5th wheel or a Class-A.
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Old 05-05-2006, 07:51 PM   #36
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Hey... I just had a thought... wasn't the Tacoma Narrows Bridge designed by engineers?

Sorry guys... going to my room now...

Roger
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Old 05-05-2006, 09:09 PM   #37
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What would you prefer, a tailor? Or maybe a stockbroker? How about a musician?
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Old 05-06-2006, 08:47 AM   #38
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What would you prefer, a tailor? Or maybe a stockbroker? How about a musician?
Hey! watch it.....



Actually, I believe that at least one of the problems with that bridge was a result of what we call "value-engineering" in my office. (architecture).
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Old 05-06-2006, 09:48 AM   #39
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Yeah. I think the original designer was an Washington State highway engineer, but the WPA wanted to save some money so they hired an architect from New York to come up with a cheaper design.

His name was Leon Moisseiff. http://en.wikipedia.org/wiki/Leon_Moisseiff
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Old 05-08-2006, 07:33 PM   #40
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OK, back on subject. You guys, and my Airstream rep, have convinced me to go Equil-l-izer. How much should I pay installed? I got what I think is a pretty good deal for this time of year (20% off). We haven't finalized a couple of options I want added (waiting on pricing for parts) but the dealer mentioned about $900 installed. That seems a little high since the web site price is $650. I know shipping and handling are extra so is $250 too much for installation and mark-up? Should I negotiate that too?
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Old 05-08-2006, 08:26 PM   #41
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RV Wholesalers sells the hitch for $400
http://rvwholesalers.com/catalog/pro...8&cat=0&page=1
add another $100 for S&H et al (inc a 10k ball)

Allow two hours shop time to set up at $80 for $160

That makes $660 and leaves a lot of room as I don't think the S&H is that high and it shouldn't take a good mechanic more than half an hour to set it up right.

But then again, you probably don't have much to bargain with unless you want to do it yourself (and that isn't that much trouble either).
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Old 05-09-2006, 04:46 AM   #42
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Minnie's Mate - We got the Equal-i-zer and the final price including installation was $825 from our Airstream dealer. The hitch itself was $635 (included $20 shipping charge); $190 for labor. They supposedly gave us a 10% discount on the hitch because we were buying it for a new trailer which we bought from them. So full price would have been about what you were quoted.

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