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Old 04-08-2010, 06:11 AM   #57
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Originally Posted by barts View Post
Here's an interesting link; I took several classes during graduate school from the author, and was the teaching assistant one spring in his class in vehicle stability back in the 80s.... Equation 5.44 on page 95 is relevant...

- Bart
Thanks for the link, I've bookmarked it for future perusal.
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Old 04-08-2010, 08:58 AM   #58
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The link about the single degree of freedom model is a common misdirection in these discussions. It's failure is that it assumes an infinite mass tow vehicle with rotation around the hitch point.

Trailer sway is most often rotation about the trailer's center of mass pushing a limited weight tow vehicle around.

There are two key points about trailer sway that need emphasis IMHO. One is that it cannot be eliminated but only reduced to something comfortable. The other is that it is the driver response that is often the most significant factor in escalating episodes. That is due to the fact that most hitches tend to promote oversteer and many drivers don't compensate properly.

As for 'wheels on the corner' trailers, I saw a bunch of them heading up a canyon while at a rally last year -- sheepherders on their way to shear a flock. For most of us, the simplicity of a centered axle trailer is good enough.

As for size, some of the best sway handling advice I have seen is from tent trailer owners. Smaller trailers can be quite active but they don't have the weight to really push around the tow vehicle.
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Old 04-08-2010, 09:56 AM   #59
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Quote:
Originally Posted by bryanl View Post
The link about the single degree of freedom model is a common misdirection in these discussions. It's failure is that it assumes an infinite mass tow vehicle with rotation around the hitch point.

Trailer sway is most often rotation about the trailer's center of mass pushing a limited weight tow vehicle around.

There are two key points about trailer sway that need emphasis IMHO. One is that it cannot be eliminated but only reduced to something comfortable. The other is that it is the driver response that is often the most significant factor in escalating episodes. That is due to the fact that most hitches tend to promote oversteer and many drivers don't compensate properly.

As for 'wheels on the corner' trailers, I saw a bunch of them heading up a canyon while at a rally last year -- sheepherders on their way to shear a flock. For most of us, the simplicity of a centered axle trailer is good enough.

As for size, some of the best sway handling advice I have seen is from tent trailer owners. Smaller trailers can be quite active but they don't have the weight to really push around the tow vehicle.
You'll note that the text starts with a single DOF model; the final analysis I referred on on page 95 is a three DOF approach, and includes the effects of tongue length, CG position, speed, polar moment and damping on stability.

I completely concur with your comments regarding driver input; I've driven rigs that demonstrated complete phase reversal on input in that counter-steering produced more pronounced sway rather than reducing the problem; if one steered with the sway it went away.

Of course, a more complete analysis would include any roll steer due to trailing axles; I've not seen such in the literature in casual search, however. It's clear that the dynamics of trailer sway are complex, but that some sway behaviors can be predicted with relatively simple models.

One thing I'm curious about - where is the dissipative force coming from in the Reese twin cam hitches? It seems as if those bars act more as springs to generate a force proportional to hitch angle than as a traditional damper.

- Bart
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Old 04-08-2010, 09:19 PM   #60
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>>hampstead=This is why larger tanks (water, fuel, etc.) often have baffles.<<

Baffles do not stop the water from moving,, it just divides the large mass it into a smaller moving masses. Still creates the same problem but just minimize others such as water slide/Tip.

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Old 04-08-2010, 09:40 PM   #61
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Here is another point y'all are missing in the discussion on sway, it is called the focal point. In an AS as well as any pull-behind trailer is that point is the hitch & ball.
You have a TV that is so many feet wide X so many feet long putting force on another object with the contact point only being 4 inches wide, and that point alone is loose at its best which adds to the movement of both objects at that center point.
Before anyone goes off what I mean by loose at best is no reciever hitch is completely tight and unmovable, same with the Ball inside the tong cup which also has movement.
This is why no can completely stop sway is because of all the varibles that add movement and can not be done away with.

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Old 04-08-2010, 10:10 PM   #62
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>>I always wondered why trailers were not built with two axles far apart with the front axle built on pivot (like a car). It would reduce the sway and lighten the tongue, no?<<

In answer to your question is; YES there have been trailers built both these ways and are still being built to this day! yes it reduces sway, no it does not lighten the tongue.
If you want to know why front and rear axles is not an excepted practice in general go up on the interstate and follow and watch a semi pulling a set of doubles and how they react and you will know real quick why. The rear trailer with the tag/floater axle is the one you want to watch. The amount of movement will astound you, not to mention to pull something like this you have to have training for it and how many of you would pay for this training.
As far as the axles spread apart= these are called spread axles and you see them every day and don't realize it on semi trailers, they are used for the spreading of weight over the length of the trailer, and yes it does reduce sway because of the spread of weight, but when turning is another story because it is like dragging a boat anchor, the spread axles actually fight you in turning and wear out tires rather fast from dragging them sideways. Again find a semi that is pulling a loaded spread axle trailer and follow him until he makes some turns (actual turns not gradual) and if he does not drop the air from the axle you will see the tire bend and the trailer tip because that one point is not wanting to give. We in the trucking industry have found that with those types of trailers we put air valves on the rear axle so in slow turns we can take the air out and raise the axle slightly and taking the weight off that axle, allowing the trailer to pivot on the front axle for turns, again unstable because you are placing most all the weight to that 1 axle and you are still dragging another axle just with not as much force on it.
By the way the air is taken off and returned to the preset level using a gage & switch inside the truck and its compressor that supplies air for the truck system and brakes.

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Old 04-08-2010, 10:46 PM   #63
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I'll attest to the fact a drivers corrction can make sway worse. Recently I was towing our trailer through some high winds, and noticed my steering corrections were making the movement worse. I finally ended up grimly holding the steering wheel steady, and the wind-induced swaying was drastically reduced.
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Old 04-09-2010, 10:22 AM   #64
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Appropo of the discussion on lengthening the tongue, the Hensley hitch uses an uneven length parallelogram linkage to move the effective point of trailer rotation ahead of the hitch (and perhaps ahead of the axle). This has the effect of making the tongue longer, and will help prevent trailer sway. Note that it also should help quell driver-induced oscillations since the tow vehicle orientation will be less disturbed with the side loads acting forward of the rear axle.

Classic simple trailer sway will occur with a tow vehicle of infinite mass, and regardless of the hitch point location. This would indicate that the Hensley won't help if you don't have enough tongue weight and you trailer too fast.
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Old 04-09-2010, 12:10 PM   #65
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Haven't gone to the scales but I have tried different loads and setups. All have minimum load rear of the trailer axles. When the 50 gal gray and 50 gal black tank over the axles are full and the front 50 gallon fresh and 35 gal fresh side tank are full and the WD and sway are hooked up any sway has little force behind it. I often fill the fresh even if there will be hook ups and forget the rest when the holding tanks are empty, not going far (ie. 55mph) and not windy. The only combination that causes an issue is all empty and no WD or Sway arm. Then she wants to swing. Doesn't effect the truck much as the trailer is still front heavy, but you can get a little nervous if you are following me.. Full holding and no fresh or WD ect is not good with wind. Lots of weight and it wants to just keep dancin. Know your ride and setup. I think alot of the problem is driving too fast, ie over 55 when towing. I do it when there are miles to be done but I front load and hook everything up right, by the book, no second guessing. Which reminds me I need a new sway bar...
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Old 10-25-2015, 12:32 PM   #66
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Weight distribution

Be sure to raise your trailer above center after you have secured hitch. Then the secure chains and distribution bars for the trailer lower it back down your trailer should be level for travel and will not have as much sagging in the tow vehicle. Then put on your way bar and I just.
This doesn't work consider beefing up rear springs or air shocks or airbags whatever's appropriate. Otherwise considering other tow vehicle.
I prefer the three-quarter to the vehicle mainly because of the heavy duty brakes. I have two tow vehicles, one is a Duramax diesel pickup and the other is a suburban 2500 when more campers are going with me. The diesel gets better fuel mileage. Good luck
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Old 10-25-2015, 02:54 PM   #67
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Way late, as in 5 years, but to answer your question gamana, about you being crazy to suggest using water as a rear balast - yes, crazier'n a june bug. Plumb loco. Heehheheheeeee...

Boys, we simply CANNOT compromise on safety. There are NO safety shortcuts. Use proper tow truck, hitch, brakes, good tires, etc. Done.
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Old 10-28-2015, 03:34 PM   #68
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A friend in our local Airstream Club has 3 decades more practice towing than I. He recommends full fresh water tank to help lower C of G. At first I wondered if 300 pounds of water would make much difference and was too lazy to ask Barts or BryanL or Sarge/AF, all of whom could tell me off the top of their head how important it is on my 6800 pound Airstream. I tow with full fresh water tank when I can. I'll take any good advantage for safe towing.
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Old 10-28-2015, 07:03 PM   #69
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"I thought trailers were like boats, the more weight you added to them the better they ride." Where have I heard that? Something to do with rocks, I think. Pat
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