Here’s The Physics Behind Tongue Weight And Why Some Trailers Sway Out Of Control

My fellow Autopians. Welcome to another edition of Ask An Engineer. This time we are going to look into a question sent to me by one of you astute readers relating to towing a trailer. The question is: Why do some trailers sway back and forth as they are being towed while most others track straight behind their tow vehicle? A lot has been written and many videos have been posted online trying to explain this phenomenon, but in my opinion, none really explain it fully. Let’s see if we can’t put it all together and get to the bottom of it.

Most of the time, when you see someone towing a trailer down the road, everything looks fine. The tow vehicle and the trailer are driving nicely straight down the road. But every now and then, something goes horribly wrong, with the end result all too often being a trailer and tow vehicle laying on its side, or worse.

What’s happening here is called trailer sway, and it can be triggered by many different things. A cross wind can start it, a bump in the road can start it — even just passing a truck can set it off. Anything that causes the trailer to be temporarily pushed to one side can start the sway motion and then the dynamics of the particular trailer and tow vehicle take over from there.

I personally experienced this about 10 years ago when I was moving a lot of heavy tools in a rental trailer. While driving on the highway on a long downhill, the trailer started to sway back and forth. It got progressively worse, and fortunately, on this particular stretch of road, the left side emergency lane was exceptionally wide so I had plenty of room to allow the trailer to move back and forth without hitting other traffic.

My only concern then was to keep it from jack-knifing and causing a massive wreck. Let’s just say that my arms got a work-out counter-steering with each sway of the trailer. As it turned out, I made it to the bottom of the hill, and once the road flattened out again, the sway stopped, and I was able to keep going. Let me tell you: It was one of the most frightening moments I have ever had on the road.

So, what was it that caused the trailer to not only sway but also get progressively worse? The answer is all about the trailer weight and the weight distribution. This video does a great job of explaining how the way the weight of a trailer is distributed affects its stability:

Here is another one that comes to the same conclusion:

The problem with both these videos is that neither explains what is really going on. Why does a trailer sway when too much weight is located towards the rear? Why does having weight pushed forward keep the trailer from swaying? And why does keeping the weight close together help?

To understand the answers to these questions we need to look at the configuration of an imaginary trailer, and do a little math. Let’s look at a tow vehicle and trailer from above. I have included a box representing the load the trailer is carrying and a symbol showing where the center of gravity of the load and trailer combination would be.

Centered Load

In the first case, we’ve loaded the trailer in such a way that the center of gravity is directly over the axle:

Let’s say we are pulling this trailer down the road, and it encounters a cross wind or a bump in the road that shoves the trailer to the left. The situation would look like this:

The shove that the trailer got has caused it to rotate to the left. Once the trailer is moving to the left, it has inertia that wants to keep it moving left. At the same time, the rotation means the tires are no longer facing in the direction of travel and they are generating a side force in the direction opposite to the rotation. The force picture looks like this, with the inertia force and the tire forces inline with each other:

Since the lateral inertia and tire forces are inline, they will partially cancel each other out, depending on how large each one is.

Let’s look at the sequence of events that occur when a trailer starts to sway. At first, the trailer is traveling straight down the road. The lateral inertia force is zero and the tire reaction forces are also zero since the trailer is not yet moving sideways.

Once some force, like a cross wind, is encountered, the trailer moves to the side. With this lateral movement, the inertial force is no longer zero, but as the trailer moves sideways, the tire forces start to grow. At some point, the angle of the trailer will be enough that the tire forces become equal to and then exceed the inertia force, which is the point at which the trailer starts to swing in the opposite direction. Once it starts moving in the opposite direction, the inertia force grows and once the trailer moves past the straight ahead position and swings to the right, tire forces start to build in the opposite direction until they are high enough to overcome the inertia force and the whole cycle repeats itself. The trailer will act very much like a pendulum and just swing back and forth.

Rear Load

But what would happen if we moved that big box to the rear of the trailer?

Looking at this from the side, we can see that the location of the weight means the trailer will be pulling up on the hitch, giving us a negative tongue load:

Let’s start the trailer sway the same way we did before:

Looking at the inertia and tire forces under this condition we can see that they are no longer inline with each other but are offset:

This offset means the two forces act together to create a moment, or torque:

Notice that this torque and the rotation of the trailer are both in the same direction. The torque created by having the center of gravity behind the axle is in the same direction as the rotation of the trailer and helps to increase, or exaggerate, the motion of the trailer. Now, in order for the tire force to stop the rotation of the trailer, it not only has to overcome the lateral inertia force, but also the moment created by the force offset, which means the angle of the trailer has to be greater before the tire force is large enough to overcome both.

Front Load

Now let’s look at what happens when we move the box forward in the trailer:

Looking at it from the side, we can see that moving the weight forward causes a downward force on the hitch:

In this case, if we start our sway the same was as before, we can see the lateral inertia force and the tire forces are again not inline with each other, but now they create a moment in the opposite direction as the motion of the trailer:

With the moment in the opposite direction to the motion of the trailer, it has the effect of counteracting the sway and stopping or reducing it once it gets started. This is why you should always have a downward load on the hitch of a trailer. The downward load on the hitch means the center of gravity of the trailer and load combination is forward of the axle which will help to create a moment that stops or reduces trailer sway. Of course, the farther forward the load is placed, the bigger the moment will be and the more stability the trailer will have.

But that’s not what the videos above say. They say you need to centralize the load as much as possible. Why? The reason is that you not only want a downward tongue load, you also want the right amount of tongue load.

The Right Tongue Load

The rule of thumb for trailer tongue load seems to be that you want between 10% and 15% of the trailers weight pushing down on the hitch. Less than that, and you run the risk that the moment created by the inertial force and the tire forces will not be enough to stop sway, and too much puts a strain on the towing vehicle.

Since standard trailers are towed with a hitch attached at or near the bumper of the towing vehicle, the downward load on the hitch pushes the back of the vehicle down and lifts the front. Too much downward force on the hitch can create excessive lift at the front. Let’s suppose we designed our trailer to have the axle all the way at the back. That way, no matter how we load the trailer, the center of gravity will always be ahead of the axle:

You can imagine what would happen to the steering and controllability of the tow vehicle in a situation like this (there’s so little traction up front, your steering inputs won’t do much to actually turn the vehicle). There are of course ways of dealing with this, such as using a weight distribution hitch, but those can only do so much. In addition to the issue of front end lift, the tow vehicle also has to be designed to handle all that extra load. Most vehicles are only designed to handle the usual 10-15% tongue load and any more would put an undue strain on the structure and could cause damage to the vehicle.

Here is an example showing how trailer tow affects vehicle loading. We’ll use a 2023 Lexus RX 350 as our example vehicle. This car can tow a trailer up to 3500 lbs. and has a max tongue load of 350 lbs. which is exactly 10% of the max trailer weight. The maximum allowable weight on the rear axle (RGAWR) is 3260 lbs. The curb weight is 4155 lbs. and I will assume the weight distribution for the FWD version has 60% weight on the front axle. The front and rear axle weights are then:

If we now add a 3500 lbs. trailer with 10% tongue weight, we will get:

The rear axle weight is still well below the 3,260 lbs. maximum, but keep in mind that these numbers do not include any occupants or luggage. You can see how even a 350 lbs tongue load adds significantly to the weight on the rear axle and makes the front axle lighter.

If on the other hand we used our trailer with the axle way at the back, then the load situation might look like this:

Now, however, we have exceeded the maximum allowable rear axle load and we have reduced the front axle load by over 700 lbs. While this trailer configuration may be more stable and resistant to sway, we have put an enormous strain on our tow vehicle, and it is completely unnecessary. 10-15% tongue load is enough to prevent sway and anything more just adds extra load to our vehicle.

Load Distribution On The Trailer

Now that we understand the need for a downward load on the hitch, we need to understand the impact of how the load in the trailer is distributed. So far, we’ve looked at a load concentrated in one place. But what if the load was actually two large boxes and we placed them apart like this:

The overall center of gravity is still in front of the axle so we shouldn’t have any sway issues, right? But what would be the effect of placing part of the weight forward and the other part rearward? To understand this, we need to introduce a concept we haven’t talked about yet, which is rotational inertia.

Rotational Inertia

When a trailer is swaying, it isn’t just moving side to side, it is also rotating. An object that is rotating has, like all other objects that are in motion, an inertia that wants to keep it moving. Isaac Newton’s first law of motion states that an object in motion remains in motion at constant speed unless acted upon by some external force. And that motion can be either in a straight line, like the truck going down the road, or rotational like the trailer swaying. In addition, an object that is in motion has a certain amount of energy, called kinetic energy which is related to the speed at which it is traveling and its mass. For an object moving in a straight line, this is equal to:

E = ½ x Mass x Velocity²

For an object that is spinning, however, it is a little more complicated. The energy of a spinning object is:

E = ½ x I x Omega²

Where Omega is the speed at which the object is spinning and I is the moment of inertia of the object. The moment of inertia is a characteristic of any object and depends on its shape, its mass, and how that mass is distributed.

 

View this post on Instagram

 

A post shared by The Autopian (@theautopian)

A perfect example of this principle at work is a flywheel. A flywheel is meant to store rotational energy so that it can be used to power some machine, or to fill in the power gaps between cylinder firings of an engine. All ICE engines have a flywheel of some sort but a classic example are the flywheels used on steam engines.

Notice how this flywheel is shaped. It has a large heavy ring supported by relatively thin spokes. Most of its mass is concentrated as far from the pivot point as possible.

To better illustrate my point, I have created two rotating objects of equal mass but different shapes to see how their rotational energies compare. The first is a simple cylinder and the second is a flywheel like the one in the picture above. They both weigh exactly the same.

First we will look at a round cylinder weighing 0.210 Kg. It’s diameter is 41 mm:

The formula for moment of inertia here is:

I = ½ x Mass x (Inner Radius + Outer Radius)

For this object, it would be ½ x 0.210 x (0 + 41/2) = 2.1525. Don’t worry about what this number actually means, we just need it to compare with the flywheel.

Now let’s look at the flywheel:

In this case, we have an outer diameter of 80 mm and an inner diameter of 77 mm so the moment of inertia is now:

I = ½ x 0.210 x (77 + 80) = 16.485. Notice that even though the weight of both objects is the same, the moment of inertia for the flywheel is almost 8 times bigger simply by virtue of its shape.

Putting these numbers back into our formula for rotational energy, we can see that for any rotational speed, the flywheel will have a much higher rotational energy than the cylinder.

The same is true for a trailer. If we divide the load between the front and rear of the trailer, it will act much more like the flywheel than if we concentrate the load towards the middle. And, like the flywheel, once a trailer loaded like this gets spinning in a sway event, it will take a lot more energy from the tires to stop it.

Stopping Sway

So far, we’ve talked about the causes of trailer sway and how you can prevent it with proper loading. But what happens if you make a mistake and suddenly you are faced with a swaying trailer, like I was all those years ago. In that situation you can’t very well get out and repack your trailer. You have to deal with the event right then and there.

Looking at a trailer that is swaying, you can see that it is following a sinusoidal path while the tow vehicle is essentially going in a straight line:

You can see that the path the trailer is following is longer than the tow vehicle path. But since both arrive at the end of the path at the same time, it means the trailer must have been going faster than the tow vehicle. And that is the crux of the problem. The trailer is traveling faster than the tow vehicle. It is trying to overtake the tow vehicle, but because it is attached at the hitch, it can’t. All it can do it travel back and forth to “use up” its speed. What we need to do is bring the trailer and tow vehicle speeds back in sync with each other.

There are two ways we can do this: we can speed the tow vehicle up, or we can slow the trailer down. Both will work, but only one is desirable. If we speed up, we can bring the tow vehicle and trailer speeds together again, but if you’re on a downhill road, like I was, the trailer just wants to go faster and faster and you will end up going way too fast in the end. Also, everything just happens quicker at higher speeds. The sway happens quicker so the rotational inertia of the trailer gets higher and higher and eventually you still end up upside down on the side of the road. There is also very little time available to react when a sway condition starts before catastrophe sets in and that time is not enough for meaningfully speed up the tow vehicle.

Slowing down

The only really viable option is to slow the trailer down, and this is where trailer brakes come in. If you have a tow vehicle with a trailer brake controller, you can quickly activate the trailer brakes independently of the tow vehicle and this will snap the trailer back into line. Of course, not all tow vehicles have a trailer brake controller and most rental trailers don’t have externally controllable brakes anyway so in those cases, sway may not be stoppable. All you can do is try your best to slow everything down by lightly tapping the brakes.

All this means it is critical that you load your trailers properly and make sure there is a downward force on the hitch when you’re done.

Anyway, that’s a bit of the physics behind why some trailers sway and some don’t. If you have any interesting vehicle-dynamics questions, please email me at askanengineer@theautopian.com!

Top graphic image credit: New Jersey 101.5/YouTube