Trailer sway, snaking, tail swing, fish tailing, shimmy, wobble...call it whatever you like, is very unpleasant, dangerous, has the potential to become deadly and, in most cases, is able to be avoided.

WHAT IS TRAILER SWAY?

Trailer sway is when your trailer starts to move from side to side behind the tow vehicle. If not brought under control the trailer can start to swing increasingly more violently, causing the back of the tow vehicle to move uncontrollably from side to side with the trailer (shown in the image below).

Poorly loaded trailers with excessive weight rearward of the axle(s), or with too much weight on one side of the trailer are the most common reasons for trailer sway and the most avoidable.

Trailers with a load having a high centre of gravity or high sided, enclosed trailers (concession trailers, caravans, stock trailers, and covered furniture/hire type trailers) are more likely to experience sway, especially when there are strong or gusty cross winds, or when passing or being overtaken by buses or large trucks.

Large vehicles drive a wave of air ahead of them, pushing a large amount of air to the side of your tow vehicle and trailer, as the vehicle passes, there is an area of negative pressure which will do the opposite and suck your tow vehicle and trailer towards the passing vehicle. Under normal circumstances a well-balanced trailer will settle back to normal towing straight away, but an unbalanced trailer can quickly be sent into a progressive sway, especially if the driver of the towing vehicle overcorrects in an attempt to bring the swaying of the trailer under control.

Low or uneven tyre pressures can cause trailer sway when the tyre sidewalls flex excessively under load. If the trailer gets into a sway momentum, the tyres will exaggerate the movement as if the trailer was sitting on a couple of lumps of jelly.

Tyres of different overall diameters and using tyres of different construction on the same trailer can also cause and accentuate sway.

Bear in mind that sway is more likely to occur when traveling downhill. This is due to the the trailer trying to go faster than the tow vehicle (pushing it along).  Normal braking will hold back the vehicle, but with the additional weight of a trailer, it becomes very difficult as the trailer continually tries to overtake. This is less on an issue on trailers which have brakes fitted, but should still always be kept in mind when towing.

So, how do I stop trailer sway if it happens?

If you are unfortunate to experience a swaying, snaking trailer, the normal reaction would be to apply the tow vehicles brakes and try to steer the trailer out of the sway. Doing this invariably makes the situation worse and increases your chances of causing a serious accident.

 If your trailer starts to sway: 

• Do not apply the tow vehicles brakes (unless you are at risk of hitting something) but take your foot off the accelerator
• Apply the trailer brakes if you have a brake controller fitted.
• Try to steer the tow vehicle in a straight line.
• Gently & slowly apply the tow vehicles brakes until the trailer is under control
• Pull over safely and determine the reason for the sway
• Check your load distribution, drawbar weight/angle and tyre pressures and remedy.

The video below demonstrates the importance of correct loading and how incorrect loading can cause trailer sway.

Originally seen on trailersauce.co.nz