Sway - Causes?

[mstephens]

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?

[markdoane]

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

[mstephens]

I didn't know there was an SAE paper on it. I am reading it now - - thanks for that information.

[Ag&Au]

Originally Posted by mstephens

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.

[mstephens]

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.

[slowmover]

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!!

.

[mstephens]

Originally Posted by REDNAX

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!)

[slowmover]

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).


.

[Ag&Au]

Originally Posted by mstephens

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

[slowmover]

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.

.

[mstephens]

Originally Posted by w7ts

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.

[Ag&Au]

Originally Posted by mstephens

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

[purman]

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.

[MrUKToad]

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.

[SteveH]

Originally Posted by MrUKToad

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.

I definately agree with that statement, and I'll stick my neck out and say any travel trailer will sway, if it's towed fast enough.

Additionally, I'll state that not all travel trailers tow the same, and what's more, not all Airstreams tow the same.

As an example, we had a 23' Airstream that we towed a lot, and NEVER had a swaying event. But, later had a 25' that was a bear to tow, wanted to sway all the time, and only stopped after I switched to a ProPride hitch. Interesting also this same trailer wore tires very rapidly, so I suspect that axle alignment contributed to the tendency to sway.

These statements are from my expereince towing trailers, not from an engineering viewpoint as I am not a mechanical engineer.

[mstephens]

Originally Posted by MrUKToad

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.

That's an interesting TV to be sure. Now, what are the characteristics that make the Sienna well suited? Is there short overhang? Low CG? Stiff tires?

Have you ever towed without any anti-sway bars attached? What do you think would happen?

So, there are kind of two schools: First school is solve all sway issues with a hitch. Second school is solve them with the vehicle and maybe only a little bit with the hitch.

Thanks also for mentioning speed. In my case in California the max is 55MPH with trailer, so I generally hold it to 55-60 no matter where I am. I hear of people going 65-70 though.

[MrUKToad]

Originally Posted by mstephens

That's an interesting TV to be sure. Now, what are the characteristics that make the Sienna well suited? Is there short overhang? Low CG? Stiff tires?

Have you ever towed without any anti-sway bars attached? What do you think would happen?

So, there are kind of two schools: First school is solve all sway issues with a hitch. Second school is solve them with the vehicle and maybe only a little bit with the hitch.

Thanks also for mentioning speed. In my case in California the max is 55MPH with trailer, so I generally hold it to 55-60 no matter where I am. I hear of people going 65-70 though.

The Sienna is wide and relatively low and has quite a good overhang to wheelbase ratio, all of which help. It also has semi-independent coil spring rear suspension, which isn't as good as full independent but much better than a live axle with leaf springs - for stability that is. I have stock tires on there which are not bad, but could improve things by using low profile tires; something I may consider when it comes time to replace. The secret really is the hitch receiver, an OEM "Hidden Hitch" model but strengthened with the addition of two pieces of steel welded behind the receiver box and that extend to a point close to the rear axles where they are bolted to the body. This not only adds strength but helps transfer the weight on the hitch forward when the weight distribution system is fitted. I think the Sienna has good payload figures, too, as I don't exceed the axle ratings or the Gross Vehicle Weight.

I haven't driven without the sway bars and neither would I. The trailer is 28 feet long and will be prone to swaying, even with the most stable tow vehicle available. To give you an example, and to show it's not necessarily the TV that will or won't prevent sway, I was caught in an interesting situation last year on the I59, northbound, between Birmingham and Chattanooga. In a 15 mile stretch of construction work, the pavement had been temporarily widened by a foot or two on the right to allow all traffic onto one side of the road. The trouble was that the extended piece was a couple of inches lower than the rest of the pavement and there was a step there which, naturally, the Airstream's wheels found. They kept dropping down the step then back up it (at 50mph) and that making driving quite difficult. What didn't happen, though, was the trailer giving into the sway. The two sway bars stiffened the thing sufficiently to prevent more than just a minor wiggle as the wheels went up and down the step. Being a sensible fellow, I noticed that the Semi drivers all went to the left lane, close to the concrete barrier but not hopping up and down the step, so that's what I did. I was glad when the 15 mile stretch was over, though.

I do think a stable tow vehicle will go some way to helping with sway, but so will a good hitch set up. We only use a basic Eaz-Lift set up but a Hensley or ProPride would, I'm sure, improve things further - another thought for the future.

Speed is quite an issue, particularly in the US where not only are the limits higher than in Canada (mostly) but driving in excess of those limits seems to be almost a right. People on this forum will regularly tell you that they tow at 75mph, because that's what the semis do, but for me, that's way too fast to pull a trailer. Still, that's up to them; I shall continue at 60-65mph safe in the knowledge that I have a stable TV and trailer set up.

[SteveH]

Originally Posted by MrUKToad

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.

What year model of Toyota Sienna do you have?

Just wondering because I checked and a 2010 has a manufacturer's maximum towing rating of 3500 pounds, which is not quite half of the weight you are actually towing, so surely you have a different model?

[mstephens]

Originally Posted by MrUKToad

Speed is quite an issue, particularly in the US where not only are the limits higher than in Canada (mostly) but driving in excess of those limits seems to be almost a right. People on this forum will regularly tell you that they tow at 75mph, because that's what the semis do, but for me, that's way too fast to pull a trailer. Still, that's up to them; I shall continue at 60-65mph safe in the knowledge that I have a stable TV and trailer set up.

I agree, and I go even slower. I tow at about 58 - 62 MPH average speed. I have had my rig up to 75 MPH just to see what it feels like. It was every bit as stable feeling at 75 as 55, but I had no interest to tow at that speed.

Thanks for your Sienna details!

[MrUKToad]

Originally Posted by SteveH

What year model of Toyota Sienna do you have?

Just wondering because I checked and a 2010 has a manufacturer's maximum towing rating of 3500 pounds, which is not quite half of the weight you are actually towing, so surely you have a different model?

I have the 2011 v6, 3 litre which yes, has a manufacturer's tow rating of 3,500lbs.

I can hear you catching your breath from here!

Contrary to the opinions of many, the manufacturer's tow rating is neither a scientifically derived figure, nor one that is in any way binding. It's more the province of the marketing departments who appear to pluck such figures out of the air in an effort to get you to buy a vehicle. That's not to say there's no testing done but the figures have been set first, then the testing begins. Here's an article from Truck Trend magazine that bears out what I'm saying:

The Numbers Game: Current Practice & The Ratings - Consumer Feature - Truck Trend

In the case of a minivan, if you do your research you'll see that all makes and models of minivans on the market today have a tow rating of 3,500lb and have done for the past 30 years, despite some very definite advances in design and technology over that period. In reality, manufacturers don't want you to tow with a minivan because that's a "mom" car and they want dad to go out and buy a pick up truck as well. It's also interesting to note that the profit margin on a pick up truck is much greater than on a minivan - need I say more?

So, understanding the dynamics of towing and the need for a stable tow vehicle, my towing guru at Can-Am RV tells me that the Sienna, along with other similarly specified minivans, make excellent TVs. He modifies or fabricates the hitch receiver, improves the transmission oil cooling and adds a brake controller along with a good quality WD system and sway control (back to the original point of the thread!) and rolls out what proves to be a very good tow vehicle, despite the manufacturers arbitrarily set tow rating. They do test these combinations extensively at Can-Am but the real proof is the thousands of satisfied customers who have been using cars, minivans and SUVs as Airstream TVs safely and effectively for years.

I rambled on a bit there but essentially the Sienna, suitably set up, works well for me and plenty of other Can-Am customers.