Physics of Trailer Sway?

Factual Questions
1

I know that if a towed trailer is not balanced properly, with too much weight on the rear (not enough on the tongue) the trailer will start to sway back and forth while driving and the swaying gets worse as the speed increases.

I’m curious as to what is the actual the physics that causes that to happen?

I assume something to do with mechanical / harmonic resonance, but I’m guessing. Even if so, what exactly is happening that starts the trailer swaying (does it start on its own or is it caused by the truck motion?) and then what causes it to get worse as speed increases?

2

This video demonstrates the effect: Towing a trailer can be dangerous with the wrong weight distribution - YouTube

The extra weight on the back, when pushed sideways, increases the momentum of the lateral movement. I’m not much of a physicist though, someone will be able to describe what going on here with much better than I.

3

Although that is a great demonstration, in my experience it is not correct because a trailer is never loaded that way. On a properly loaded trailer most of the weight is directly over the axle. It is not put only on the front and rear as the video shows. For race car haulers they recommend that 10% of the total weight be on the tongue. My trailer was quite low and I only needed about 5% to be stable. They make trailer tongue scales to set the car in the correct place then you adjust the tie downs so it goes there every time.

Dennis

4

When we were hauling 3 horses in a 4-horse trailer, we sometimes put 1 horse in front and 2 in back, to keep more of the weight on the trailer wheels and off the hitch. Since we had a bumper hitch, this also kept the truck steering wheels down to the ground.

5

Trailers often start to sway or snake when pushed sideways by air pressure. This can be a gust of wind or the pressure wave of an overtaking vehicle. Most of the time it self-corrects, but if the driver either tries to steer out of it or brake hard, that can multiply the effect.

Much depends on the setup: a heavy trailer behind a light tow truck (it’s recommended that the trailer weight should never exceed the tow vehicle weight and ideally be less than 80%); a badly balanced trailer and here - too much weight behind the axle(s) is the worst setup. Tyre pressures can have a marked effect too; if the trailer tyres are too soft it will be inherently unstable and the manufactures instructions should always be checked.

Three horses in a four-horse trailer is not good because there will be too much weight on one side. A sensible solution is to balance it with some bales of hay or straw.

If you are ever behind a trailer that starts to snake; drop well back until it’s under control and then get past it as soon as you safely can.

6

Uh, it wasn’t to demonstrate how trailers are loaded, all it showed was that too much weight behind the trailer’s axle(s) is what causes sway.

7

Here is an European dissertation going into details of the involved physics :http://tuprints.ulb.tu-darmstadt.de/4927/1/2015_Zhang_Dissertation.pdf

Quote from the literature review section of the thesis :

The thesis has good links to other relevant work on the OP too, if you are interested.

8

You’re right, but I think Mixdenny has a point. Specifically:

  • By putting those weights at the extremes of the trailer, the people in the video could give the trailer a fairly different polar moment of inertia than a real trailer would have. A single set of weights that can be moved along the length of the trailer would perhaps be better in this respect.

  • More importantly, the car/trailer system isn’t modeled all that well here because the car seems to be attached to the treadmill by a length of string or something similar. This adds a third “link” to the system that doesn’t exist in reality.

That second point may or may not change the results, but it seems like an easy thing to eliminate from the system.

9

The third link is the driver trying to keep driving straight down the road.

10

Maybe. But I’d like to see that assumption validated.

11

This seems like picking a nit just for the sake of picking a nit.

12

Why do you say that?

It’s good form for an experimenter to acknowledge the assumptions on which the experiment rests. Mixdenny said he thought the variable-weight scheme was unrealistic in some ways; I agreed that it was.

The third-link thing adds a degree of freedom that isn’t present in reality. Richard Pearce seems to think it won’t matter, and he may be right. His position is viable and so (IMHO) is mine. Neither of us knows for sure.

What’s your objection to that conversational arc?

13

To explain better, that video claims that is more then 40% of the load is on the back it is unstable. But you can’t put 60% of the load on the tongue as that would bottom out the rear suspension. You don’t load trailers by front and rear weight, you use total weight and tongue (front) weight and it sure as Hell does not need to be 60%.

They should have put all the weight in the center, then slowly increased the tongue weight until it is stable.

Dennis

14

Apart from weight distribution, another thing that has a big effect on stability is the distance from the hitch ball to the rear axle of the tow vehicle.

This is the “lever arm” that the swinging trailer uses to sway the tow vehicle - keeping it short is beneficial. In some cases, shortening it by a couple of inches has a noticeable effect.

The ultimate in this is a “fifth wheel” trailer, where the pivot point sits right over the rear axle (perhaps in the bed of a pickup truck). In this case the length of the lever arm is effectively zero, and the trailer thus loses its ability to sway the tow vehicle.

15

My take is that, when viewed from above, a trailer “wants” to rotate (sway) about its center of gravity but only “can” sway about its pivot point (wheels). The closer that CG and pivot point are, the more likely sway can develop.

Travel trailers are the most vulnerable to sway, since they’re mostly balanced atop the wheels to avoid the large tongue weights of 5th wheels. There can be 1000 to 1500 lbs difference in tongue/pin weight between similarly sized fivers and TTs. If you squint at most 5th wheels and view them as a uniform box, it becomes obvious the wheels are more to the rear (instead of center) than in a travel trailer. Looking at large 18 wheelers, this is even more pronounced. The hitch is at one end and the wheels at the other - with the CG in the middle and nowhere close to a pivot point.

I’ve stated this before, but there’s a growing problem due to much more powerful light trucks combined with lighter (and therefore longer) travel trailers. I believe quite a few rigs are considerably over the safe limit, and more vulnerable to the type of sway that results in dynamic instability.

Here is a video of what happens with too much trailer, too little hitch and too little truck (advance to 0:45 mark). I suspect the CG was further aft (less tongue weight) due to the loads strapped to the rear bumper of this one. Once the rig is dynamically unstable, it cannot be corrected. The only outcome (imo) is wreckage alongside the road. Imagine had this happened earlier in the video where the land drops off steeply.

There is an unusual type of hitch which can overcome this by using a trapezoidal framework instead of a hitch ball, but it’s very expensive ($4000) and most won’t buy it.

Again, this is all my opinions, not a physicist.

16

Thanks for the input so far.

I personally liked the U-Haul video that Fiendish Astronaut posted - it doesn’t explain the physics and exaggerated the problem, but when looked at from the perspective of U-Haul explaining to someone who’s never towed a trailer that sway is a very real potential problem if their trailer isn’t loaded correctly it was quite effective.

@am77494 - point made that a lot of factors affect sway, but you lost me at “trailer arm moment of inertia”:confused:. Thanks anyway:)

Xema’s comment was useful in that I can now visualize the trailer being like a pendulum with the hitch ball being the pivot point.

I’m still not clear on why, once the sway starts, the trailer (pendulum) keeps swinging? Is that due to wind buffeting or due to some other physics force? I can see with a clock pendulum, gravity is pulling the pendulum down so it gains momentum and keeps swinging, but that same type of downward gravitational force is not present here (or is it?)

@Pullin

I’m misunderstanding what you’re saying here. From what I know, the way to prevent sway is to load the trailer so the weight is directly above or in front of the trailer’s wheel axis - closer to the pivot point (the hitch). Loading weight behind the trailer wheels moves the centre of gravity further away from the hitch pivot point makes sway more likely.

17

https://m.youtube.com/watch?v=Ba3PfXgHj0A

I don’t know if this guy is a good driver or just got really lucky but here is a nice recovery from a swaying trailer that for sure looked like it would result in a crash.

18

I once tamed the sway of a long sailplane trailer which I towed with a short tug. (Bronco-II) In the process, I looked into the sway issue a great deal.

One point few consider is that with a rear bumper (vs. 5th wheel) tow point, the trailer can and does apply steering force to the tug whenever two are not aligned. This is because the the tug’s rear wheels serve as a fulcrum, and the farther the ball is behind the axle, the more leverage it has. When the ball is located over the axle (5th wheel or goose neck) then the trailer has no steering leverage on the tug, and that point of interaction is eliminated.

Another point that doesn’t get enough attention, is that when braking, the trailer is pushing the tug. The steering effect mentioned above causes the trailer to steer the tug in the direction the exacerbates the misalignment. So a heavy trailer can jack-knife a light tug when braking no matter how it is balanced, depending on how far behind the rear axle the ball is. Having brakes on the trailer is very helpful in this regard. Applying trailer brakes can even stop sway due to other reasons. Without trailer brakes, applying the tug brakes once a sway has started often results in immediate jack-knifing.

But to the OPs question regarding tail heavy vs. hitch heavy :

Consider the trailer going around a modest curve…

If the sideways force due to radial acceleration is balanced in front of, and behind the trailer axle, then it produces no steering couple.

If the front is heavy, then the trailer tend to steer toward straight ahead. You could say that the hitch-heavy trailer “wants to” go straight AKA understeer.

Now make the trailer tail heavy. There is now more lateral force behind the axle, which tends to turn the trailer into the curve. Once it starts turning, it “wants to” turn sharper.

The hitch heavy trailer exerts force on the ball that tends to steer the tug into the turn. (oversteer)

The tail heavy trailer exerts force that turns the tug the opposite way. (understeer)
If the tug is light and the hitch moment long, then the hitch-heavy trailer can still cause trouble, because even though the trailer is stable, it causes the tug to oversteer.

The tail-heavy trailer is totally unstable without the tug. If the trailer is light, and the tug quite heavy, it might not become unstable until very high speed.

The above is looking at the forces during a constant radius curve. As the radius changes or reverses, dynamic forces come into play that cause the situation to be far harder to analyse and explain.

Now if that isn’t enough, the height of the load in the trailer causes it to roll side to side. The usual leaf springs on the trailer then convert this rolling in to steering of the trailer axle.

One interesting device that helps a lot with trailer sway is the sway-brake friction device. The adds a great deal of friction to the coupling in the yaw axis. The friction forces tend to steer the tug in the opposite direction as the inertial forces that cause the sway, especially under braking.

19

I didn’t explain it well. Moving the weight forward moves the CG ahead of the wheels, and away from the pivot point. Sway can be prevented by doing this, but in some cases the hitch limitations prevent moving it very much. It’s common for 5th wheel trailers to have the CG far enough forward that the pin (hitch) weight exceeds 2000 lbs. While travel (or cargo) trailers are limited to a much smaller hitch weight, often as little as 300-500 lbs. This means the trailer CG is much closer to, although still ahead of, the wheels.

When I pulled doubles (camper + boat) behind my truck, I endeavored to move the camper CG as far forward as possible to add stability. Since my fresh water tank was well forward of the wheels, I kept it full. The extra 350 lbs. forward of the axle had a limited effect due to moment arm, but every little bit helps. I also stored tools and other heavy objects in the locker beneath the (front) bed.

I wasn’t even considering aft CG (behind the wheels) since that’s been covered as a recipe for disaster. Imagine what would happen with a CG behind the wheels if you unhooked the trailer?

20

A clarification (full disclosure: I have never driven with a trailer and know nothing about them)

They aren’t saying to put 60% of the weight on the tongue, just that 60% is supposed to be forward of the axle.
This means that the 40% from aft will balance out 40% from up front, leaving only 20% of the weight on the hitch.

That still seems a bit high, but I know nothing of realistic tongue weights.