If you have driven a trailer of any size you know first hand what trailer sway is about, if you have not, it is something you should understand before you do. I’ve read it is the leading cause of trailer accidents, but in trying to find any actual evidence of that, I’ve come up empty. None the less, I’ve experienced a good bit of it first hand and it’s easy to see how it can cause serious accidents. Let’s take a look at what it is, how it operates, and how to cope with it.
What is sway and how does it work?
In a nutshell, trailer sway is when a force pushes on your trailer from the side causing it to pivot, which in turn pushes your tow vehicle in the opposite direction which can cause you to veer off course. The natural reaction is to correct for this which if not done correctly can exacerbate the sway and lead to even greater instability and loss of control.
Wind is the most common cause of trailer sway. This can come from wind blowing at you from the side or from passing vehicles which create a push-pull vortex as they pass you. You may ask, why does the wind push the trailer harder than the tow vehicle? The answer is that the trailer has a much greater surface area for the wind to push on. Wind pressure is by the square inch and travel trailers have a lot more surface area than your tow vehicle does, thus, they are pushed much harder. Sway can also come from uneven road surfaces which rock the trailer unevenly though this is less common.
In both this case and that of the wind, the way most trailer hitches are constructed exacerbates sway. Think of the trailer hitch as a lever attached to your car. Pushing the end of the lever has a stronger impact than pushing on the side of the car at its center of mass. The farther the point of attachment, the greater the lateral torque exerted. Trailers are also easier to pivot by design. The have one set of wheels near their center point as where cars and trucks have four spread out points of contact.
So to sum it up: trailers are pretty easy to push sideways and when that happens they exert strong counter forces on your tow vehicle potentially causing you to swerve and lose control of your rig.
How to limit sway: Hitch Selection
The first approach to sway is to have less of it. This is done through special hitches designed to control sway. There are many manufacturers and a few different designs that each address sway in their own way. For a primer on hitches, read this article on selecting a hitch.
Friction Based Hitches: These stiffen the connection between the hitch ball and coupler. Typically you set the stiffness to your liking before heading out. You want it stiff enough to limit sway, but loose enough to allow the trailer to turn.
Hydraulic Cam bars: These are hydraulics that go between the trailer and vehicle hitch. Like friction hitches, they stiffen up the connection. Some of them can be electronically controlled so that they loosen up in a given direction when you sue the turn signal, thus, they can be fairly stiff while going straight, and loose when turning.
Weight Distributing Hitches: These hitches have steel tension springs that ensure your trailer stays vertically level with your tow vehicle. This keeps even weight on all your tow vehicle’s wheels which increase its resistance to trailer sway and allowing you to better control both tow vehicle and trailer by proper steering.
Rear Axle Hitches: 5th wheel hitches place the point of contact between the trialer and the tow vehicle above the rear axle of the tow vehicle. This effectively removes the lever effect I mentioned earlier making it much harder for the trailer to push the tow vehicle around. Of all the hitch strategies this is the most effective against sway. A few manufacturers make traditional undercarriage hitches that accomplish the same thing. Hensley is one of the most well known.
Equally important to having a good hitch is making sure it is used properly and maintained in good condition. Follow the manufacturer’s recommendations for service and configuration.
How to deal with sway: Safe Driving
The second strategy for dealing with sway is through knowing how to drive a trailer well, both in avoiding sway and properly correcting for it when it happens.
Speed: The most important advice here is not to drive too fast. The faster you drive, the more potential instability you will encounter. Wind turbulence is stronger and correcting for sway is more difficult the faster you are traveling. Exactly what speed your rig handles best at takes some experience to discover. If it starts to feel unstable, then you are probably going too fast and should ease up. The windier it gets, the more you should slow down to retain full control.
Down Hill: When going downhill, especially when engine braking, your trailer will be pushing on your tow vehicle. If it starts to sway while going down hill it will exacerbate the amount of sway as it is not only pushing to the side but straight at you as well. Couple going down hill with high speed and you have a recipe for a disaster. Keep your speed under control, and use your actual brakes if you feel sway starting to happen. You may want to increase your trailer brake resistance a bit when going downhill to give you better control.
Correcting Sway: If you are swaying gently, the best thing to do is keep driving straight and slow down a bit to get things stable. A trailer moving forward will naturally straighten out due to the forces involved so the key is to hold’er steady and try to keep moving forward. If sway gets more dramatic or you think you are loosing control the best counter is to manually apply the trailer’s brakes. As the trailer brakes and the tow vehicle moves forward it will pull them into a straight line. Most trailer brake controllers will have a manual button or lever to engage the trailer brakes.
Don’t slam on the tow vehicle’s brakes, especially if you have the brake tension on the low side on your brake controller. What happens is the trailer pushes at the tow vehicle at an angle which will greatly exaggerate the sway force, quite possibly jackknifing and wiping out your whole rig.
Don’t steer against the sway. Many drivers will steer right if they feel the tow vehicle starting to point to the right. What can happen is you then start to turn strongly to the right, the trailer whips around the other way, and now you are swaying to the right worse than you were left, correct like that again and you set up increasing instability leading to disaster. Keep it steady. If you are right next to a big rig you may need to nudge the wheel just a bit but do so as little as possible, your goal is to keep going straight ahead on the curve of the road as you were before the sway started.