I have noticed that trailers – e.g., U-Haul types or those carrying boats –tend, at highway speeds, to sway back and forth behind the cars towing them. Surely the swaying is obvious to the driver. What causes this phenomenon? Why don’t the trailers just track steadily behind the towing vehicle?
Often it’s the weight of the trailer is too far aft leaving not enough weight on the hitch. I can understand this with the U-Haul, but not the boats.
Overloaded trailer and/or underinflated tires come to mind.
Drunk towbars.
Yep, this is the major cause, although too much wt. forward can also cause it, and either can easily lead to disaster. It’s usually also accompanied by overloading of the trailer.
So, we have:
Plain overloading
Too much tongue weight
Too little tongue weight
Improper tire inflation
Drunk towbars 
Not mentioned yet:
Excessive speed
Tow vehicle is inappropriate
How can a vehicle be inappropriate? Wheelbase and overall geometry are every bit as important as engine power. We used to have a Jeep Liberty that did OK at towing a popup trailer, but while it had a factory tow package and more than enough towing weight capacity, it could not handle our longer hardside, even with a load-distributing hitch. The Jeep just couldn’t keep its power on the road, and we had essentially no control of it. A pickup truck, with its longer wheelbase, tows the trailer effortlessly with no swaying or wagging.
We did have one moment of “will the trailer get to the bottom of the hill before we do?” but just enough pressure on the brake pedal to activate the trailer brake controller but not actually activate the truck’s brakes was all it took to keep things sane.
In theory, a trailer sways because it is trying to go faster than the tow vehicle. I don’t know what causes the condition - I just try to prevent or control it.
One thing that might exacerbate trailer swaying is the fact that most trailers don’t use shock absorbers in their suspension. This means that any oscillation in which the trailer springs participate is not damped by shock absorbers, as in automobiles. Proper loading and equipment (weight distributing hitch or stabilzer) is probably the best solution to the problem, though.
I find that on our pop up trailer the anti-sway bar works very well. I haven’t noticed any swaying with it, but we also use a truck to pull it. The anti-sway bar is just a friction device that seems to dampen any sway. Of course it makes a god awful noise first thing in the morning when it is damp!
Could it be the same thing that causes it to “rock”?
Am I mistaken, or don’t most states have laws (that dictate max. speed while towing a trailer)? I thought that you were limited to less than 50 MPH. I’ve been passed by bozos hauling humongous boast, doing over 75 MPH!
I would suggest a high center of gravity and weak springs would do it.
Humongous boast being nothing to brag about…
A glace at the geometry shows that the trailer will obviously tend to steer itself to stay behind the tug…what is not obvious is that when the trailer is accelerating from side to side, there is a steering force generated on the trailer due to the offset between the trailer wheels, and the trailer center of gravity. If the center of gravity is ahead of the axle, then this force will tend to center the trailer, and improve stability. If it is behind the axle, it will tend to steer the trailer farther from center, and be de-stabilizing. Improper balance (tongue too light) is probably the most common reason for trailer instability.
The tow vehicle’s mass acts as a stabilizing influance. The heavier the tow vehicle in relation to the trailer, the more sins of stability can be tolerated.
The dynamics of the tug-trailer combination are fairly complex and difficult to analyze. One thing that definitely occurs, is that when the trailer is off center, it attempts to steer the tug, and much more so if the trailer is pushing forward on the ball. (downhill or braking). The direction of this steering is in a direction which tends to increase the angle between the tug’s direction and that of the trailer. This is destabilizing in all cases. The force of this steering increases with speed, and the accelerations it produces (which interact due to trailer balance) also increase with speed. Thus a trailer/tug combo can be stable at low speeds, and become unstable at higher speeds.
The farther the ball is from the rear axle of the tug, the more effective this steering is. This is why 5th wheel/Goos-neck setups are desirable…they place the tow connection directly over the rear axle, which eliminates the trailers steering lever on the tug. The longer/heavier the trailer, and the shorter/lighter the tug, the more the tail is able to wag the dog.
The friction devices mentioned by other posters work mostly by applying a steering force to the tug, which tends to keep the trailer and tug pointed in the same direction…it is opposite in direction to the existing mass*acceleration forces. (there is also some friction damping of the oscillation) As noted above, trailer-pushing-tug makes for the worst stability. Trailer brakes eliminate, or minimize this situation, so are very desirable from a stability, as well as a stopping standpoint.
I put a fair amount of thought and effort into fixing a trailer that had a bad sway problem. In this case, the problems were, in order of worst to least problematic:
1- Axel too far forward (not enough tongue weight)
2-Inappropriately soft springs (allowing excessive rolling motion of the trailer)
3-Worn out rubber bushings on ends of those springs, allowing steering motion of the Axel.
4-An under-designed tongue which allowed flex.
In addition to fixing the above problems, brakes and a friction device were added. This turned an evil handling trailer that was unsafe at any speed into a trailer that has been safely towed at speeds I dare not mention.