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Old 12-28-2005, 12:47 PM   #15
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Self centering force

The old dual cam systems from Reese generate a self centering force when properly adjusted and transfering signifigant amount of weight. Frictional dampening is a secondary effect with these systems. The new ones also look like they would also have quite a bit of self centering but I have not done the calculations to quantify it. If the new ones would have a roller follower and a separate frictional dampener resisting rotation that would be ideal as the old ones lose their configuration when they wear and lose their ability to generate the self centering force. Proper adjustment of the backward tilt of the ball has nothing to do with self centering. It allows the customer to fine tune the amount of tension existant in the bars for the in between link settings. The tilt of the ball should allow you to easily insert the bars without contacting the ground when the unit is in nuetral position with the trailer on the ball. I have also seen bolts put into links to slightly shorten them to accomplish the same thing.
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Old 12-28-2005, 03:02 PM   #16
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Quote:
Originally Posted by dwightdi
self centering force ... Frictional dampening is a secondary effect ... have not done the calculations to quantify it.
...
The tilt of the ball should allow you to easily insert the bars without contacting the ground
One thing I am curious about is the relative size of the lateral force on the trailer tongue and the torque provided by sway control mechanisms. I am also curious about the DC applied torque as a function of angle, especially over the very small angles usually involved in most sway issues. If there was significant self centering compared to damping, you would have a recipe for aggravated sway.

The best advice I have heard for ball angle is to get the spring bars parallel to the trailer frame at proper loading. Chains should only be used for minor spring bar tension adjustment and should never pull the bars too far from parallel with the trailer frame.

Spring bar insertion can be facilitated by lifting the coupled hitch about 5" because at that point, the spring bar should be at near zero loading which means insertion into the ball mount should be well clear of anything and coupling to the trailer frame should be tension free (enhancing safety).
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Old 12-28-2005, 08:06 PM   #17
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Sway all begins at home. If your dancing with the wife a small amount of adjustment may be the correct thing to do. If you have one to many beers and are swaying toward the door, don't drive. As far as towing, have'nt a clue.
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Old 12-28-2005, 08:34 PM   #18
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Reese manual instruction

What I was saying is taken from the Reese handbook on the use of their dual cam system. If the cams are new and the bars properly adjusted, the load transferred is enough to cause a centering action from the cams. Properly adjusted cam followers have almost no slack in the middle. As soon as small angle of movement occurs, the followers starts up the two cams (in oposite directions) generating a centering force. I usually adjust the tilt back on the ball so that I operating with three links hanging, give the correct amount of weight transfer to level the system. I then loosen the cam follower links attachement to the A frame (with the rig straight) and adust their position to nuetral. The bars are usually trailing down a little at that point, if you have selected the right bars. This give full movement to the bars with proper side to side stability and not overstressing the bars. It usually results in the bars being able to be assembled without picking up the back of the tow vechile with the tongue jack. 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.

It is really a simple system and easy to understand but I can not figure out why they did not make the cams out of a brake friction material to control the amount of frictional dampening. I do not know of any modern engineer who would consider making a frictional brake by contacting ungreased steel on steel. Old train brakes at least used cast iron on steel wheels with different hardnesses. Reese cams usually wear out due to galling.
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Old 12-30-2005, 07:59 AM   #19
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Reese/drawtite design and testing

The new Reese/Drawtite twin cam design looks alot beefier than the old one. With the increased engineering capacity of the combined companies, I wonder whether they have done a better job of designing and testing the new hitches? The Hensely folks have published some impressive videos of the performance of their system. I wonder if Reese has some also. Seems like they should have some within engineering, if they have done a good engineering job. It would take a 8 strain gage set up (plus two temp. compensators) to record the forces in the bars during manuevers and be able to analyse the frictional and self centering characteristics. 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.

Old Reese instructions say to keep the front to back weight distribution unchanged for the tow vechicle (as a goal) when setting up the hitch. I suspect that is in error for heavier weight trucks that do not handle that well when they do not have any load in the box. OEM's design the optimal handling in these vehicles assuming that they are carring about half of their load capacity. Of course the conventional trailer load comes from the hitch rather than the center of the box so some compromise is in order.
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Old 12-30-2005, 08:50 AM   #20
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Quote:
Originally Posted by dwightdi
Proper adjustment of the backward tilt of the ball has nothing to do with self centering.
I noticed a definite improvement in handling after adjusting the hitch ball angle. My TradeWind/Dodge, and then TradeWind/Suburban rig felt way better both times. I used 750lb bars, and by the feel of them they were maxed out, or close to it.
Of course it does not have the same effect as a dedicated sway control system, like what I have now ( reese dual cam hp). With the tension on the WD bars being the same, both rigs towed way better, with less sway, with the hitch ball angle adjusted properly. I do not know the physical reason for this, but my back-o-meter is quite reliable....
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Old 12-30-2005, 08:58 AM   #21
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Quote:
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, 11:53 AM   #22
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Originally Posted by dwightdi
. . . . 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, 07:33 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, 09:55 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, 02:10 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.
Nick.
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Old 01-04-2006, 06:59 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, 02:40 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, 07:13 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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