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Old 12-30-2005, 07:58 AM   #21
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Originally Posted by dwightdi
If you want to overstress the tonque jack by picking up the car (with the potential of breaking it) be my guest, it will make the levering up the chains easier, if you are too weak or lazy.
In my case, it has nothing to do with weak or lazy. On many trailers, my former 1971 trdwind with EAZ LIFT, it is impossible to snap the tensioner over center without lifting up the back of the TV. ( Unless you have a 4-foot handle extension)
It is at the same time quite unsafe to undo the bars without having the back of the TV elevated to relieve stress on the chains somewhat. I know this from experience, got nailed by the handle snapping back at me unexpectedly in the beginning.
Why do you thing the electric tongue jacks would all have between 3000 and 3500lb capacity? I figured they are designed to lift that weight.
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Old 12-30-2005, 10:53 AM   #22
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. . . . It would be a pretty interesting engineering study, like we used to do in the late 60's when I was a mechanical engineer at GM and other OEM's with extensive engineering test facilities.
Dwight,

Did you work with R. Thomas Bundorf?
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Old 01-01-2006, 06:33 AM   #23
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I did not know this person. I was involved in high speed camera work and recording strain gage values at high speeds, along with trailer stability for Johnson/Evinrude (when they were making boats and trailers in late 60's).
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Old 01-01-2006, 08:55 AM   #24
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You should be familiar with his work. He wrote "Directional Control Dynamics of Automobile-Travel Trailer Combinations", SAE 670099, 1967. He was on engineering staff at General Motors in the late 60's.

His paper is the first real analysis of oscillatory behavior in automobile-trailer combinations, as a response to hitch load, location, and center of gravity.

If you like, I can fax you a copy.
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Old 01-04-2006, 01:10 PM   #25
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He wrote "Directional Control Dynamics of Automobile-Travel Trailer Combinations", SAE 670099, .
I purchased a copy of this paper after Markdoane referred me to it. One interesting outcome of the mathematical analysis concerns steering response to a side gust of wind. The rig is taken as an automobile and travel trailer combination traveling at 65 mph on a highway when the wind, coming from across the highway, gusts by eleven mph. The automobile swings towards the gust for 0.4 seconds, and the corrects back automatically in the other direction. The practical result is that drivers should be trained not to respond immediately to side wind gusts, but await the natural correction, and then deal with the subsequent steering issue. I suspect this confirms the practical experience of many of us. The mathematics in this paper involves the use of an analogue computer, so using a pencil and paper on the equations will not take you far.
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Old 01-04-2006, 05:59 PM   #26
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"What I was saying is taken from the Reese handbook on the use of their dual cam system." - I know. There is a lot of brag about 'self centering' but such behavior, if taken as stated, would tend to have adverse consequences. So I got dissonance ... there's something I don't gronk.

Very interested to hear about SAE 670099, 1967. I have seen some reference to a Hamiltonian model that was too much of a spherical chicken for me. The SAE analysis sounds as if it at least has the derived behavior getting fairly close to observed. I am curious as to the cause of the "corrects back automatically" in the SAE model. (steering? inertia? external input? suspension? tires?)

Emulating an analog computer is nearly a trivial thing these days with some of the modern toolsets (e.g. mathematica). But in the late 60's and 70's it was a really nifty way to see partial differential equations turned into dynamic models. Op Amps then were just beginning to be cheap and consistent IC's.
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Old 01-05-2006, 01:40 PM   #27
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. . . I am curious as to the cause of the "corrects back automatically" in the SAE model. (steering? inertia? external input? suspension? tires?)
Bryan,

May have been a less than optimal word choice. The analysis assumes no steering input.

The initial response is the vehicle turns 'into the wind', which is the opposite of the expected reaction. This is due to the effect of the crosswind on the trailer, pushing on the hitch pin, causing the tow vehicle to pivot. Reversal of this lateral acceleration is part of the normal oscillatory behavior.

Regarding your earlier post (#16) about self-centering causing aggravated sway: the Bundorf paper analyzed this and found that at very high settings it would be possible to contribute to more oscillatory behavior. He did not think it would occur at normal settings. I think this is intuitive. If you make an anti-sway extremely rigid you could get behavior more like a very long straight truck, with lots of oscillation.
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Old 01-06-2006, 06:13 AM   #28
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Sway vs Antisway forces

It does seem intuitive that any forces that act to keep the combination of the TV and trailer headed down the road straight are good and any forces that act 90 degrees to destabilize the unit would be bad. Low drag coeffiecent ( in direction down the road or side load) would be good. Airstream would be better than square box from an areodynamic loads. Self centering forces would be good to restore straight line position of trailer and TV. This is what (in most cases) you want. The trailer keeps directly behind the TV traveling down the road. If anything, the restoring forces would be a greater than proportional to angle of deflection. (The old Dual cam system seemed to a fairly sharp ram.) It would also be desirable to rapidly damp out any forces and energies that are disturbing the stablity of the TV/trailer combination. That is what the frictional dampeners or steering shock absorbers do. The old twin cam system used steel on steel friction which lead to galling and excessive wear. The galling action greatly changes the effective Coeffiecent of friction creating the dampening effect. This gave eratic results. I am not sure how well the new Reese/Drawtite solves this problem.
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Old 01-06-2006, 09:47 AM   #29
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Please pardon my being dense. I don't mean to cause any upset but I am having a problem trying to synthesize concepts. I can't put all the concepts of this 'return to center thing' into something that makes sense to me.

A return to center force is the defining force for oscillation. Think spings without shocks. You do not want back and forth sway any more than you want your truck to act like a bouncy baby buggy.

I am confused about the cause of the force: "Reversal of this lateral acceleration is part of the normal oscillatory behavior." I do not see oscillation as normal behavior nor do I think the source of the "lateral acceleration" that would cause such behavior is very clear.

Another issue is the confusion between a self centering force and the straight rig objective. Adding rigidity to the rig articulation ability would only change the frequency. The way to eliminate oscillation is to dampen the movement like shock absorbers do.

What really gets interesting to me in this conundrum is the stability consideration. Catastrophic 'oscillating' sway that seems to be the big concern is not a stable situation. It starts minor and builds. This is not your typical oscillating system which is a slightly perturbed stable system trying to return to its steady state. To build an oscillation you have to add energy at just the right times, enough to overcome damping forces and friction. How is this done in a trailer 'oscillating' sway disaster?

I do know that there are unstable rigs. Driviers figure this out right away as the lack of stability becomes obvious as speed increases and they usually stop and fix the problem before it causes loss of control.

Then there is the handling comfort involving driver fatigue going down the road. The rig needs constant attention as it is very responsive to even minor stimuli on the road.

Then there is the dreaded catastrophic oscillating sway. This is usually due to an inability to control a maneauver. Most common are downhill and/or hard braking turns followed by, perhaps, response to unexpected winds and gusts. The energy input here is obviously kinetic and the control mechanism is the driver.

I guess the big question is how all these considerations help move towards safer towing. In my mind, it is a good idea not to be too confident in what one thinks one knows. On this topic, it may be that confidence in knowledge is a bit out of line with the quality of that knowledge. And knowing that may encourage safer driving.
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