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Old 09-09-2012, 10:10 AM   #1
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Sway - Causes?

I've been towing my rig over all kinds of Interstates, highways, byways and roads for almost a year. Rain, wind, sun. I've towed up and down long grades, flat runs, and through wind-swept prairies. I have had triple trailer rigs go roaring by. And through all this, I can't say that I have ever had any sway of any appreciable amount (such as where I would have to adjust the steering wheel). I have a simple WD hitch with friction sway control.

I am interested to look at what contributes to sway?

- Tires on the TV and Trailer (side wall roll or flex)
- Overhang on the TV - distance from rear axle to ball compared to wheel base
- Improper weight distribution on the TV
- Improper balance of the trailer

Of course we have various hitches to eliminate sway, but my question here is what can be done to stop sway from happening in the first place?

If sway is reduced at the source, how much sway control needs to be in the hitch?

Do different TV types perform better than others in sway? Van vs. PU? SUV vs. sedan?

Can you just change tires and reduce sway?
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Old 09-09-2012, 10:54 AM   #2
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Your assumptions are correct.
Stability with respect to the snaking motion depends on the parameters of both the
towing vehicle and the trailer.
Specifically, the system becomes unstable beyond a certain speed. This speed decreases, thus rendering the system less stable, as:
1) the mass of the trailer (relative to the vehicle’s mass) increases,
2) the center of gravity of the trailer moves rearward,
3) the moment of inertia of the trailer increases,
4) cornering stiffness of trailer tires decreases,
5) cornering stiffness of the vehicle’s rear tires decreases,
6) the distance from the vehicle rear axle to the hitch point
increases,
7) vehicle wheelbase decreases.

This list is from SAE Paper 2008-01-1228
"Stability and Control Considerations of
Vehicle-Trailer Combination"
Aleksander Hac, Daniel Fulk and Hsien Chen
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Old 09-09-2012, 11:00 AM   #3
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I didn't know there was an SAE paper on it. I am reading it now - - thanks for that information.
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Old 09-09-2012, 11:00 AM   #4
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Originally Posted by mstephens View Post
I've been towing my rig over all kinds of Interstates, highways, byways and roads for almost a year. Rain, wind, sun. I've towed up and down long grades, flat runs, and through wind-swept prairies. I have had triple trailer rigs go roaring by. And through all this, I can't say that I have ever had any sway of any appreciable amount (such as where I would have to adjust the steering wheel). I have a simple WD hitch with friction sway control.

I am interested to look at what contributes to sway?

- Tires on the TV and Trailer (side wall roll or flex)
- Overhang on the TV - distance from rear axle to ball compared to wheel base
- Improper weight distribution on the TV
- Improper balance of the trailer

Of course we have various hitches to eliminate sway, but my question here is what can be done to stop sway from happening in the first place?

If sway is reduced at the source, how much sway control needs to be in the hitch?

Do different TV types perform better than others in sway? Van vs. PU? SUV vs. sedan?

Can you just change tires and reduce sway?
I think this may not be the best place to ask these questions, because you are going to get a lot of opinions for answers and some fact. Separating the two may be difficult. Here are some links to a articles about causes of sway.

What Are the Causes of Trailer Towing Sway? | eHow.com
Trailer Sway Causes
Understanding trailer sway
Hitching Up - Trailer - Sway

I found these using google search. There are many more.

Ken

PS I was composing this when Mark posted his info. My generalizations are not intended to apply to his post.
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Old 09-09-2012, 12:33 PM   #5
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If anyone is interested in the SAE paper aforementioned, it can be read here for no charge: download/delphi__com pdf techpapers 2008 01 1228.pdf download article pdf. There may be other sites too, but this is the one I found.
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Old 09-10-2012, 11:38 AM   #6
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As always, numbers can be generated and predictions made based on those. It is the place to start. Thus the discussions on hitch rigging, and trailer tire pressure questions (for example) are derivative as they all relate to a baseline for comparative purposes (when the thread is worth reading for bankable information).

Generally -- as your question is complicated to answer -- if one acquires the numbers generated by your particular combination and if they are in accord with best practice (industry or SAE recommendations or vehicle manufacturer requirements & recommendations) then the more specific questions per your use (peculiarities) are more easily approached (as comparisons to other rigs are quantifiable).

It's best to try and eliminate dumb luck, in other words. The real value of Internet RV forums and towing discussions is in using numbers as a beginning to sort out what lash-up works best and why. The details are of interest for continued reading.

One might say, for example, that it all comes down to the TV rear axle tire traction (so questions from this angle concern themselves with tire characteristics of design and use). Or, from another angle, particulars about wear points of a hitch design (discussions of details ensuing). Or, trailer balance fore-and-aft. Etc.

You may have noticed resistance to the acquistion of numbers -- even their discussion -- as being a shoot the messenger dilemma on RV boards. I'd write it up as being belief in the efficacy of the pseudo-idea of lifestyle (that all decisions can be defended by personal preference alone) as familarity with abstractions (what they are and their uses) seem to be more abstract as time goes on. As you know, the question pre-supposes the answer, and it is shameful behavior that on technical questions that some would try this tactic of diversion (name calling, for instance) in disallowing questions to avoid standard defense.

There is an ensemble of players on RV.net in the Towing sub-forum who relish playing with the numbers. I recommend a perusal of threads and posts by JBarca as introduction (trailer suspension & brake improvements, first, and contributions to tech threads as central, second; leading to other players in this realm). Contributor Ron Gratz posts there extensively, and here, occasionally, and has a gift for making very good statements which can clarify issues raised in this type of thread (that you've started). Disagreements are part & parcel of the exchanges, and are sometimes sorted out.

It -- this range of subjects -- is open to change as information becomes available. I've certainly had my mind changed for me, and by me on more than one occasion.

The caveat is that, as always, the working assumptions need to be examined. The progenitor of those discussions tends to be the work and writings by Andrew_T of CAN AM RV who has codified the experience more than ten thousand lash-ups (and more similarly to the way things were done in the 1960's & '70's when cars were the main TV).

One can, as with SAE J2807, let the assumptions stand (the information or parameters rejected by that working group) or read more extensively to try and understand the widest possible set of approaches to best performance while towing. One is always cautioned that parameters exist for reasons (quantifications), but open-endedness pays dividends as well, IMO.

I work from an ideal, that, the fewest deviations from being lane-centered under all conditions is the starting point of a very good to best tow combination.

What constitutes that is the point of the search.

Enjoy!!

.
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Old 09-10-2012, 11:57 AM   #7
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Originally Posted by REDNAX View Post

You may have noticed resistance to the acquistion of numbers -- even their discussion -- as being a shoot the messenger dilemma on RV boards. I'd write it up as being belief in the efficacy of the pseudo-idea of lifestyle (that all decisions can be defended by personal preference alone) as familarity with abstractions (what they are and their uses) seem to be more abstract as time goes on.
That's one of the major effects of post modernism. Yes, I run into that a lot in all aspects of life today.

One of the reasons I had for the thread was to wonder out loud, how much thinking about sway goes into the selection of a TV? Here's an example: It could be argued that a long wheelbase van might offer less sway than a typical PU truck. And further, that such a van with say, super low profile tires would be even better yet.

Usually I see power rise more to the top as a criterion for a TV, but maybe sway control should be at the top of the list?

(Again, just for discussion. Don't shoot the messenger!)
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Old 09-10-2012, 12:25 PM   #8
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You may see that combination vehicle stability: steering & braking are cited by Thomson (and plenty of the rest of us, even if not exactly in these words).

In general (to work this approach), the TV with the best solo performance is the open-ended approach to looking at TV choice criteria. This is where aero resistance trumps weight as the starting point (as it should . . which is where the "discussion" deteriorates).


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Old 09-10-2012, 12:30 PM   #9
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Originally Posted by mstephens View Post
That's one of the major effects of post modernism. Yes, I run into that a lot in all aspects of life today.

One of the reasons I had for the thread was to wonder out loud, how much thinking about sway goes into the selection of a TV? Here's an example: It could be argued that a long wheelbase van might offer less sway than a typical PU truck. And further, that such a van with say, super low profile tires would be even better yet.

Usually I see power rise more to the top as a criterion for a TV, but maybe sway control should be at the top of the list?

(Again, just for discussion. Don't shoot the messenger!)
It is my opinion, that there are so many aspects to towing, that it is going to be up to each individual to define the relative importance of each factor as it applies to them. You can engineer the numbers and come up with parameters to define each category. However as of yet, we are not able to engineer each human's preferences and priorities (thank goodness). You may be able to design the near perfect Tow vehicle for yourself, but no matter what it turns out to be, it will only satisfy a portion of the other users.

Am I missing the point of your quest?

Ken
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Old 09-10-2012, 12:44 PM   #10
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I'm a missing the point of your quest?

Tends to come down to TV payload for a given trailer. The rest of the post is dependent from this.

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Old 09-10-2012, 09:45 PM   #11
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Originally Posted by w7ts View Post
It is my opinion, that there are so many aspects to towing, that it is going to be up to each individual to define the relative importance of each factor as it applies to them. You can engineer the numbers and come up with parameters to define each category. However as of yet, we are not able to engineer each human's preferences and priorities (thank goodness). You may be able to design the near perfect Tow vehicle for yourself, but no matter what it turns out to be, it will only satisfy a portion of the other users.

Am I missing the point of your quest?

Ken
I was mostly just wondering if people consider "sway" much when considering their TV. Wondering where it falls in the priority list for people. Personally, I didn't give it much consideration, as an example.
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Old 09-10-2012, 10:13 PM   #12
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I was mostly just wondering if people consider "sway" much when considering their TV. Wondering where it falls in the priority list for people. Personally, I didn't give it much consideration, as an example.
I didn't consider it either. When I bought our tow vehicle I was towing a 21 Bigfoot trailer with a Jeep Grand Cherokee and a Equal-I-zer hitch. I had no significant trouble with sway so I did not expect any with a 3/4 ton quad cab truck. I was right. However before buying our Airstream, I did extensive reading here and elsewhere about towing a longer and heavier trailer. That is when I decided to think about sway. I satisfied my concerns by buying a ProPride hitch and installing it as soon as I got the trailer home. The comfortable relaxed ride has been worth every penny. I would never go back to a friction based sway control system.

The reasons we bought the truck were many., including:
1. I was tired of refueling every 150 miles or so.
2. We wanted to take more stuff with us.
3. We had uses for a truck at home.
4. The Jeep was demonstrating unpredictable reliability.

Ken
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Old 09-10-2012, 10:25 PM   #13
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I didn't either. I would be surprised if someone chose their TV on sway alone. It doesn't matter what TV you have. Sway happens when towing a TT. Some more than others, and it even happens to semi trucks. Is this just out of curiosity or are you looking for a TV that can control the sway of an As the most efficiently, If so, a size and weight of AS should be stated so everyone can be on the same page.
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Old 09-11-2012, 08:34 AM   #14
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As has been excellently described by others here, sway is the result of many factors. So, in selecting a tow vehicle, one should be looking to address as many of those factors as possible. The Airstream Trailer is a pretty stable beast and tows well, so it should follow that you'll want to try to compliment those positive characteristics with a stable, low and aerodynamic tow vehicle, something that would mitigate the causes of sway, amongst other things.

As Rednax says, though, people get hung up on numbers and start being very selective in which numbers they use. Priorities tend to move away from stability to power and weight and attempts to manage sway start to be compromised in favour of those bigger numbers. I'm not saying that a huge truck isn't any good for towing an Airstream, just that in the pursuit of big numbers, things like center of gravity and tow vehicle stability tend to get sidelined and countering sway becomes a secondary consideration.

For the record, my 7,300lb 28' International is towed by a Toyota Sienna minivan. Outright weight, power and torque may not be "big numbers" but the stability of the minivan contributes massively to sway reduction and, even with a pair of old-tech friction anti-sway bars, I don't have an issue with sway at all.

From a personal point of view, based on stuff I've read about sway generation, one of the biggest factors to induce sway is speed. Certainly people towing should address all of the technical sway reduction measures but keeping driving speed down is, in my view, one of the biggest sway prevention methods that there is.
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