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Old 05-19-2018, 08:15 AM   #1
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The physics of trailer sway

After reading this thread: http://www.irv2.com/forums/f45/cant-...ph-391458.html and many like it over the years, it struck me that people just don't know what causes the problem and how to address the issue - too many responses based on emotion and not physics or data.

This is a very complex problem and I'm surprised a good mechanical engineer hasn't stepped in to provide a better explanation. I'm not a good mechanical engineer, but I'm going to take a shot and hopefully a good one will chime in.

Despite all the opinions to the contrary, there's no reason why a properly designed tow vehicle/trailer combination can't go 200 or 300 mph. There are practicalities involved, but it's not even particularly difficult – but this is an aerodynamics problem first and a mechanical problem second.

A truck/trailer combination doesn't just oscillate for some unknown reason – there are forces involved. In this case, as a trailer with a flat stern goes through a fluid (air) it creates vortices that build on each rear edge. All things being perfect, they would occur at the same time, but this is the real world, so they alternate from side to side. These exert a drag force that pulls the trailer to one side until the vortex sheds. The response to this is a movement of the trailer toward the vortex and a movement back to center when it sheds. Then a vortex builds on the other side and the process repeats.

You can see this by following a semi in a light rain/mist. The one shedding off the top will make the trailer bounce. There's too much turbulent flow off the bottom to get a strong vortex there.

The airspeed at which the movement becomes noticeable has a lot to do with CG location, suspension design and condition, and tire sidewalls. It is 100% velocity dependent. The vortex forces increase with airspeed.

A number of things resist this movement. The force acts through a moment arm from the center of gravity of the trailer back to the stern, so the trailer is attempting to rotate around the center of gravity. This is resisted by the the force exerted by the tow vehicle through the hitch (including anti-sway devices there), and by it's suspension through the road. I know that the center of aerodynamic pressure moves as speed increases and as it moves it can have a de-stabilizing effect, but I'm unable to explain that aspect well, but it can be significant. Aerodynamic lift can also reduce the load on the steer axle, especially if the combination starts with a nose-up attitude.

Obviously stiffer sidewalls are better. Torsion axles are better than springs because there is less capability for lateral movement – no bushings, no shackles, etc. Since there will be motion induced in the vertical as well as in the horizontal, shocks would help to control movement in the springs.

Don't forget that on a flat-backed tow vehicle like an SUV – the same things are happening back there but at a lower level – and when the vortexes shed off it causes the tow vehicle to rotate and wag the trailer. Those aerodynamic forces then buffet the trailer. If all that gets in sync, you are really in trouble.

At this point something has to change to maintain stability, and you can slow down or push through it, but slowing down is more predictable.

The same things that allow motion on the trailer allow motion on the tow vehicle, so reducing those has an effect – stiffer shocks to resist the roll and the sideways push on the rear axle (maybe a watts link or similar device to locate the rear axle side-to-side!), urethane bushings, stiff sidewalls, dual rear wheels increase stability by both the increase in track width and the increase in sidewall spring rate because there's twice as many of them.

The ability to control this mess and resist the movement has to do with the relative masses between the vehicles and the relative lengths between the steer axle to the drive axle and the drive axle to the hitch ball. There's also the need to have sufficient weight on the front axle for proper alignment both static and dynamic – hitting a big bump that lifts the front at the wrong time could create a moment that drives a controllable situation into an uncontrollable one – thus the need for a WD hitch to put weight back on the steer axle.

Obviously once you buy a particular trailer/tow vehicle combination there's not a lot you can change, but you need to start with controlling those things that produce the primary effects first:

1) reduce the aerodynamic forces at the rear of the trailer – about the only thing you can do here is add airtabs or some other trip strip to induce turbulent flow at the back.

2) Improve tow vehicle/trailer controllability – move mass between the trailer and tow vehicle, and in the trailer (can use water as ballast) to produce the proper tongue weight with the ball height and weight distribution to create level to no more than about a half-inch nose down attitude on the trailer while getting the nose of the tow vehicle as close to normal ride height as possible. This might require additional spring rate at the rear, and moving cargo into the tow vehicle to improve the ratio of the masses between the vehicles. Remember this will change as you burn fuel and with your passenger load, and has a big effect on half-ton vehicles and smaller SUVs. Add a cam-type sway control system to the hitch.

3) Reduce the uncontrollable mechanical movement in the suspensions, starting with the trailer – stiffest sidewalls you can get (maybe go to bias tires, which removes the speed rating concerns) go to urethane bushings, lower the trailer, and add shocks. In the tow vehicle – urethane bushings and stiffer shocks. It sounds like there's nothing to gain in the tow vehicle tires. If you have a GM product, the idler arms are known weak points and will wear quickly towing heavy loads and put slop in the system. I'd cut to the chase here and get a supersteer idler arm support, a cognito motorsports idler arm than pittman arm support kit, (I used to go though an idler arm every 20K miles and these items put a stop to that). Might also consider a PPE idler arm.

4) Consider changing the aerodynamic interaction between the tow vehicle and trailer – most likely with a small roof wing. You don't need a huge one, and small airtabs on the side of the tow vehicle might help, but that's probably not visually acceptable. The other problem with this approach is that it's just not predictable and wing angle and placement would just be trial and error.

5) Add a lateral axle location device to the rear.

If those changes don't allow you to tow at the desired speed, then you need to change up the tow vehicle (mass, length, track, wheelbase) or the trailer (airstream type, axle location, CG location through a different type or layout), because there are some things you just can't overcome technically or economically.
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Old 05-19-2018, 02:01 PM   #2
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Proper loading of the trailer has a lot to do with its stability in towing. This video shows what happens when at least 10% tongue weight is maintained.

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Old 05-19-2018, 02:16 PM   #3
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I think it has more to do with where the center of gravity is. I can tow my boat at any speed I want but only after I moved the tandem axels back 6 inches. I've moved quite a few back for individuals who complain about swaying. This isnt really practicle on a TT due to shacles being welded in place, so all you can do is load weight forward. 10~15 percent tongue weight may not be enough on particular TT.
Just my thoughts...
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Old 05-19-2018, 02:23 PM   #4
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Quote:
Originally Posted by GBB View Post
I think it has more to do with where the center of gravity is. I can tow my boat at any speed I want but only after I moved the tandem axels back 6 inches. I've moved quite a few back for individuals who complain about swaying. This isnt really practicle on a TT due to shacles being welded in place, so all you can do is load weight forward. 10~15 percent tongue weight may not be enough on particular TT.
Just my thoughts...
Moving the axles back puts more weight on the tongue and less on the rear of the trailer, so it's the same as moving the weight from the rear and transferring it to the tongue.
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Old 05-19-2018, 02:30 PM   #5
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Moving the axles back puts more weight on the tongue and less on the rear of the trailer, so it's the same as moving the weight from the rear and transferring it to the tongue.
What I just said...you may not be able to just shift enough weight.
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Old 05-19-2018, 02:34 PM   #6
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Let's not forget that it is usually the tires that limit the speed.
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Old 05-19-2018, 03:39 PM   #7
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Let's not forget that it is usually the tires that limit the speed.
That was addressed in the OPs thesis.

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Old 05-19-2018, 03:51 PM   #8
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This post was well written and factual. I think we did hit the big picture though in the other post. We mentioned suspension being soft, tires holding the load (though not specifically sidewall stiffness), enough truck (speaks to the mass ratio point), WDH to better counter some of those forces, etc... We did not talk about wind and aerodynamics though, but the WDH hitch helps with that.

Still, I like this post and appreciate the OP for taking the time. I think its one everyone should read. Can we have things stickied on these boards?

One of the reasons semis, or even fifth wheel RVs for that matter, don't have near the troubles we have is because they go to extremes on a few of these points. Front-to-rear weight ratios on the trailers are 100s:1 or more (well, not 5er RVs). Mass of the tow vehicle is quite high. Long wheelbases and long trailers with hitch points over the truck axles help to prevent wind from rotating the trailer. And those guys can really move with few problems from sway.

Anyway, great post, thanks!

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Old 05-19-2018, 03:51 PM   #9
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That was addressed in the OPs thesis.

Ken
Where?
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Old 05-19-2018, 06:14 PM   #10
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Originally Posted by khuffmanjr View Post
One of the reasons semis, or even fifth wheel RVs for that matter, don't have near the troubles we have is because they go to extremes on a few of these points.
Exactly. Boat trailers also need the axles farther to the rear because the fuel tanks and engines represent the majority of the weight. Also makes 'em much easier to back up! I don't know much about fivers, but I wonder why they don't put the axles way at the back? might be that the trucks don't have the ability to handle that much pin weight? Or is it to shorten the right turn radius and reduce driver training to handle one?

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Proper loading of the trailer has a lot to do with its stability in towing. This video shows what happens when at least 10% tongue weight is maintained.
The video illustrates a great deal of what I wrote. The push the guy gives is what the vortices do. Note that the combination is stable even without enough tongue weight. It can't recover from a large destabilizing force though.

If that test track was in a wind tunnel you could see the whole thing.

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Where?
in the "3" section.
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Old 05-19-2018, 07:26 PM   #11
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in the "3" section.
The only thing he says in section 3 about tires is:

"It sounds like there's nothing to gain in the tow vehicle tires. "
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Old 05-19-2018, 07:48 PM   #12
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Originally Posted by dexters View Post
The only thing he says in section 3 about tires is:

"It sounds like there's nothing to gain in the tow vehicle tires. "
"3) Reduce the uncontrollable mechanical movement in the suspensions, starting with the trailer – stiffest sidewalls you can get (maybe go to bias tires,"
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Old 05-19-2018, 07:58 PM   #13
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Stability with respect to sway depends on the parameters of both the towing vehicle and the trailer. The system will become unstable beyond a certain speed. This speed at which the system becomes unstable decreases, 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.

If you really want to know at what speed a particular combo will become unstable you will need to calculate the sway damping ratio for it.
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Old 05-19-2018, 09:02 PM   #14
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"3) Reduce the uncontrollable mechanical movement in the suspensions, starting with the trailer – stiffest sidewalls you can get (maybe go to bias tires,"

OK thanks but the speed is still limited by the speed rating of the the tire.

Also, there are road speed limits.

Where is the speed limit 80?
Portions of the Idaho, Montana, Nevada, South Dakota, Texas, Utah, and Wyoming road networks have 80 mph (129 km/h) posted limits. The highest posted speed limit in the entire country can be found on the Texas State Highway 130, and it is 85 mph (137 km/h

And for travel trailers, even Texas has limits, a lot have 65 mph.

http://www.doityourselfrv.com/state-towing-speeds/

Texas

Towing speed limit: 60 mph during the day; 55 mph at night
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