Driver Knowledge Tests

What is trailer sway and how do you stop it?

Trailer sway, or fishtailing, is when a trailer begins to move from side-to-side on its own, eventually resulting in flipping and (sometimes) turning the towing vehicle over, too. You can see an example in the video below.

It doesn’t happen on vehicles with a fifth wheel attachment, such as an articulated truck, only on vehicles where the towing connection (hitch) is well behind the rear axle.

As you can see in the video above, trailer sway is made worse when the load balance causes too much weight over the rear of the trailer and not enough weight over the trailer’s hitch. A minimum of 10-12% of the trailer’s weight should be over the trailer’s hitch. If a lot of weight as at the back, it acts as a pendulum. It’s important not to overload the hitch, though. Check out our guide to towing.

It’s also important not to exceed the maximum towing weight specified by the manufacturer, and for people who have no experience towing to avoid towing a trailer that’s heavier than the towing vehicle.

What causes trailer sway?

Trailer sway is usually set off by a strong gust of wind, either from a side wind in exposed areas or from a passing truck, and quick changes of direction such as on winding mountain roads. It’s made worse by incorrect air pressure in the trailer’s tyres, and the driver trying to correct it but not reacting fast enough.

The risks can be dramatically reduced by:

  • Loading your trailer keeping heavy items down low and near the centre of gravity
  • Pumping your tyres to the correct pressure
  • Anticipating adverse conditions and slowing down in time (large trucks coming towards you or overtaking you, particularly road trains), exposed bridges with strong crosswinds, and coming out of the lee (wind shadow ) of a building or geographical feature.

Stopping trailer sway with trailer sway mitigation

This video shows how trailer sway mitigation works.

Trailer sway mitigation automatically controls the braking of the tow vehicle’s wheels independently to counter the forces of the fishtailing trailer.

Stopping trailer sway without electronic intervention

If you can apply the trailer brakes separately, do this immediately as that will drag the trailer back into line.

Take your foot off the accelerator but do not apply the brakes unless you’re at risk of hitting someone.

Steer gently against the motion of the trailer. As you slow down, the trailer will stop swinging so violently.

The risk of you losing control will depend on the overall weight of the trailer in relation to the towing vehicle. A light trailer will have less impact on how the towing vehicle moves, and vice versa.

There are special towing hitches with more friction that can help reduce the risk of trailer sway.

How should a trailer handle?

Strong crosswinds will push the vehicle sideways, even out of the lane if the driver isn’t quick enough, but they shouldn’t start the trailer swaying, even though up to 1500kg of force could be applied to the side of something like a caravan!

If you are doing everything right but your trailer is still swaying then you need to get it checked out as there may be an underlying problem.

Darren is an expert on driving and transport, and is a member of the Institute of Advanced Motorists

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