Trailer sway and sway control

[RockyMtnRay]

This article accompanies the following article on Choosing a Weight Distributing Hitch.

Trailer sway is a complex mechanical phenomenon, but basically it means that the trailer is trying to rotate left and right on its axle, at the same time that it's moving down the road. If the front of the trailer starts to move in one direction for any reason, of course it is up to the tow vehicle to resist that motion and push it back toward the center. If everything is arranged correctly, the motion dies out and a stable towing situation is re-established. But in the wrong circumstances, the push causes the front of the trailer to go past center and even farther in the other direction, where the tow vehicle has to push even harder to get it back in line. If the cycle grows and repeats, the uncontrolled sway can throw the tow vehicle out of control. And even if the driver is able to regain control, the phenomenon is distinctly unsettling and dangerous.

The tendency of a trailer to sway is minimized by locating its center of gravity well forward of the axle. Traditional travel trailers are very sway prone, because they can’t do this very well. The reason is that a traditional travel trailer is so heavy that the manufacturer has to put the axle very near the center of gravity, in order to reduce the percentage of trailer weight that's on the tongue (usually only about 10%). By doing so, the manufacturer limits the tongue weight to something that's manageable by typical half-ton rated pickups and SUVs - something in the 700 to 900 lb range – but the center of gravity is so close to the axle that the trailer tries to sway.

By contrast, TrailManors are designed to be inherently very sway resistant because the axle(s) are much further aft of the trailer's C.G. As a result, about 15% of the trailer's weight is on the tongue, but because TMs are so much lighter than a standard trailer of the same "open" length, the tongue weight is still quite reasonable. As long as you properly load a TrailManor, there is absolutely no need to use any sway control. In fact, the factory has told members of this board NOT to use sway control because (1) sway control should never be needed and (2) if used, it will mask any loading problems, leaving you vulnerable to a sway-induced accident.

Types of sway-control mechanisms - friction vs dual-cam

A TM is built specifically to be very sway-resistant, and under any normal circumstances, a TM doesn’t need additional sway control. You do not need to buy an anti-sway device for the hitch to safely tow a correctly loaded TrailManor trailer. But if you modify or load your TM in such a way as to move the center of gravity toward the rear, you might consider conducting a simple sway test to see if a sway control device might be good.

Certainly hanging a bike rack and several bikes off the rear of a TM will move the center of gravity to the rear. Adding this load so far back has the potential to reduce the TM’s inherent sway resistance and could make it somewhat sway prone. The operative word is "could". Whether or you not you actually get sway after you load several bikes on the back of your TM depends on many variables - how many bikes you load on the rack; how heavy they are; how the trailer is loaded (e.g. how much heavy stuff, including water, is loaded behind the axle rather than in front of it); and how long and heavy your tow vehicle is, particularly the ratio of wheelbase to rear overhang.

Before you go through the hassles and expense of buying/installing anti-sway, I'd do a test trip with your tow vehicle and TM loaded normally with the typical number/weight of bikes mounted on the rack. Be sure to do at least a little driving at higher speed (over 70 mph) as the potential for sway is always higher at higher speed. If you have no detectable sway even at higher speed, then you probably will be fine.

If you load up your TM and conduct a test drive, and decide that you want to get sway control, you will find that there are two types of sway control devices. They are known as “friction” and “dual-cam”. In a friction device, one end of a flat metal bar is attached to the ball mount on the tow vehicle’s hitch. A second, similar bar is attached to the trailer’s frame behind the coupler. The bars overlap, with a pad of friction material squeezed between them. When the trailer turns with respect to the tow vehicle (as it does when sway starts), the bars slide across each other, and the friction resists the turning action. Very simple.

A dual cam device is a bit more complicated, but it is built into the tips of the spring bars. When the trailer starts to turn away from a straight line, the mechanism lifts the tip of one of the springbars. The force required to lift it actually resists the turn and pushes the trailer back toward a straight-behind position. Quite elegant.

If a test drive shows that your rig does have some sway tendency, then I'd definitely recommend stepping up to the full Dual Cam anti-sway. By contrast to simple (and less expensive) friction anti-sway, the cam action actually causes a centering force that forestalls the onset of sway, instead of resisting through mere drag after it's already started. Furthermore, the centering force of the cams will continuously fight the sway by trying to push the trailer back to a centered position behind the tow vehicle. By comparison, the friction device will actually resist (instead of help) recentering of the trailer.

Finally, while friction devices should be disconnected for sharp turning (e.g. backing into a campsite), the cam devices don't particularly need to be. Furthermore, the cam devices are not effected by rain or wet roads, while the water reduces the friction of a friction device. Cam anti-sway is just plain a whole lot better than a friction sway resistance.