Trailer sway, snaking, tail swing, fish tailing, shimmy, wobble, call it what you will, it is very unpleasant, extremely dangerous, potentially deadly and under most circumstances, totally avoidable.
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 swing increasingly more violently causing the back of the tow vehicle to swing uncontrollably with the trailer.
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. See the “Level Towing” section below for more information.
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 prone to sway particularly when there are strong or gusty cross winds, or when passing or being overtaken by buses or large trucks.
Large vehicles push a bow 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.
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.
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 aggravates the situation and puts you at more risk of a serious accident.
If your trailer starts to sway -
Careful consideration needs to be given to ensuring a level trailer when towing. Trailer stability can be seriously affected by how the trailer sits behind your trailer and can be influenced by how the trailer is loaded.
A trailer that sits with its nose up at the front can cause its loads centre of gravity to move rearwards during normal use, causing even further weight to be added to the rear of the trailer. Apart from putting additional stress on your trailer, it takes any weight off the drawbar and off the tow vehicles rear wheels. Any bump in the road or a truck or large vehicle passing by, a gust of wind or a slight change in your direction, can cause the trailer to become a very large and very heavy swinging pendulum of metal (including anything in or on the trailer) and can take over control of the towing vehicle (see Trailer Sway above)
If you are unlucky enough not to get the situation under control, the trailer can easily roll over and take the tow vehicle with it, potentially taking out you and your passengers, as well as any poor soul on your section of road at the time.
A similar result can occur it the load is too heavy on the front of the trailer. Excessive weight on the tow ball will lift the front of the tow vehicle reducing its ability to steer and control the vehicle. This situation is even worse when travelling downhill, especially under braking, as the loads centre of gravity moves forward.
Any load on your trailer, whether it is securely tied down or not will pitch forward adding additional mass to the already pitching trailer.
Any sudden braking of the tow vehicle will cause the trailer to nose dive, lifting the front of the tow vehicle, rendering any steering control almost useless.
Once the tow vehicle loses steering control, the end result can be catastrophic.
Before loading your trailer, check that the empty trailer has at least 10% of the trailers tare weight resting on the coupling (25kg for a 250kg trailer, 50kg for a 500kg trailer etc..)
A convenient way of checking this is to lay a board over a set of bathroom scales, zero the scales, level the trailer with a block of timber between the coupling and scales, and then measure the trailer drawbar weight.
Loading a trailer can be a bit of guessing game until you know your trailer intimately and it pays to check your drawbar weight after loading your trailer especially when the load has a broad range of weights. Again, the drawbar weight should be around 10% of the total trailer/load weight.
Any heavy items should be loaded slightly forward of the axle and centre of the axles on a tandem trailer. They ideally should be loaded first and as low as possible.
If you are loading loose material like firewood or sand, level the load with a bias slightly forward of centre. Remember that the load may move around while towing particularly on bumpy roads or where the roads are undulating, so check your loads position on a regular basis.
Your empty trailer needs to sit level behind your tow vehicle and if not, you will need to make some modifications to your coupling mount or the tow ball height.
If you have a tow ball that slides into a square receiver on the tow bar, you can purchase differing setups to allow the tow ball to be set at heights to level your trailer. This is by far the quickest and easiest method of correcting the trailer level.
If you have a fixed tow bar/tow ball arrangement, you will need to get some adjustments made to your trailer.
A relatively easy method of remedying a trailer that sits nose up at the front is to find the height difference between where the trailer is sitting and the level height.
Fit a packer under the coupling that corrects the height difference, making sure that the packer and coupling are independently mounted and secure. It is normal to use heavy wall RHS that is drilled for both the coupling and coupling mount. Depending on the height of the packer, you may need to fit strengthening gussets within the RHS to prevent deformation and twist.
If your trailer sits nose down, then some serious modifications need to be done to lower your coupling. These modifications could involve changing your axle arrangement, tyre diameters or modifying your drawbar and these should be carried out with professional advice. Talk to your local trailer manufacturer or engineering shop for assistance.