Guide to trailer sway – what it is and how to fix it

If there’s one thing you want to avoid in a caravan or trailer, it’s swaying! But what causes trailer sway, what can you do about it, and how do you avoid it in the first place? You’ll want to read on – this advice might just save your life!

This is what trailer sway looks like

The dashcam video above shows a sequence of a fast overtaking caravan and tow vehicle whose minor trailer sway escalates until seconds after, the combination jack-knifes.

How did this happen?

The overtaking manoeuvre is likely to have caused it, but a fast-moving truck travelling toward a fast-moving rig on a two-lane road could also be a likely culprit.

A big truck, particularly if flat-nosed, generates a diagonal bow wave of considerable force. It also creates a vortex (a sort of air-like whirlpool) along its immediate side. This vortex causes an overtaking rig to be drawn toward the truck when it’s some metres behind its cab. But, just as the tow vehicle nears the front of that truck, the strong bow wave pushes the front of the tow vehicle strongly away from it. Meanwhile, the vortex sucks the front of the caravan towards the truck – a movement enabled by the overhung hitch causing it to sway strongly.

How to stop the trailer sway once it’s started

If you find yourself in this situation, stay calm and maintain a steady course. Do not attempt to steer out of the sway. Ease off the accelerator, and avoid braking the tow vehicle. Instead, apply the trailer’s electric brakes using the dashboard controller.

How does speed affect trailer sway?

At low-ish speeds, a strong side force, as with that truck-affected rig, may just cause the rig to sway around a bit and then settle down. The effect, however, is very much speed-related. If you are attempting to overtake a truck travelling at approximately 100 km/h, your rig is likely to be travelling at 110-120 km/h. Physicists call what may happen next chaos theory.

Given a sufficient nudge (here, the truck’s bow wave) and an adequate energy source (momentum), a fast towed caravan may exceed a certain critical speed (unique to each rig). If that occurs, the rig is triggered into a sequence whereby the behaviour is not random but not realistically predictable either; it is now totally out of the driver’s control. The swaying caravan generates such side forces on the tow vehicle’s rear tyres that they cause that vehicle to fishtail violently. That overhung hitch, in turn, causes the caravan to do likewise – but in the opposite manner. After one or two more increasingly violent swings, the rig jack-knifes.

At speed, any strong enough side force can do this: cornering too fast, sudden changes in camber, inner wheels running onto a soft surface. The risk can be minimised but never totally eliminated in any rig with that overhung hitch. A fifth-wheeler (that has its hitch over the tow vehicle’s rear axle) will, in this scenario, sway a few times. But that’s all.

How to prevent trailer sway

Where the weight in your trailer or caravan is located is very important to preventing trailer sway. The ideal is a compact mass just to the front of a well-set-back axle. That’s impracticable, but the closer to that ideal, the better.

Sometimes, these things are better understood visually. In Episode Two of our television/YouTube series, Are You rvSafe?, we were able to show the effect on caravan sway by changing where the weight is located in the trailer/caravan. Check it out below!

Unfortunately, too many caravans are virtually the opposite. Long and heavy, with batteries, gas bottles and spare fuel upfront and a heavy washing machine, spare wheel, bicycle rack and sometimes a toolbox at the extreme rear. These are virtually recipes for jack-knifing because the effect of weight at the end of a beam (mass) is far greater than that weight centralised.

Two 40-kilogram spare wheels at the extreme rear of a long rig have an effective total mass of more than 300 kilograms! The issue only worsens if the tow vehicle is not substantially heavier (many are much lighter). As with horse-drawn carriages, a towed caravan ultimately relies on the mass and stability of whatever pulls it. This is basic physics.

Five rules to help prevent trailer sway

• Do not even think about exceeding 100 km/h when towing.
• When loading, place everything heavy close to the axles – never at the rear.
• Consider the relative weight of the towcar to the trailer. Imagine a large adult walking a chihuahua.  The little dog can try all it wants but can’t pull the person off balance.  Now consider a small child walking a Great Dane.  That dog goes where it wants – it’s a mass thing. 
• Consider increasing the tyre pressure in your tow vehicle’s rear tyres to account for the extra weight.
• Make sure your tyres are in good nick. Any tyre older than six years is suspect.

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