Understanding trailer sway

In this ultimate guide to towing series, Robert Pepper discusses trailer sway, including why it happens, what to do when it happens and how to prevent it.

This could be the most important article you’ll ever read on towing.

I’m not one for hyperbole, so please take that seriously. The reason this article is important is because it deals with the biggest danger in towing, one that can easily cause a fatal crash.

We’re talking here about trailer sway, or snaking. That’s when the trailer begins to oscillate behind the towcar, swaying from side to side, and it just gets rapidly worse until the whole rig is out of control and then you will crash. It can take as little as three seconds from the onset of sway to when you’re out of control.

This is what trailer sway looks like:

So, in this article, we’re going to look at why trailer sway occurs, how to prevent it, and what to do if it happens.

Why trailer sway happens

A trailer naturally follows the towcar. Sway happens when the trailer is disturbed from its path, and it happens all the time. It’s when it is undamped that it becomes a problem. Anything that pushes the trailer out from its path behind the towcar can cause sway. That might be a gust of wind, a swerve of the towcar, or a bump in the road. Once disturbed, the trailer’s movement should damp out, returning it to behind the towcar. However, sometimes it doesn’t and instead of returning to behind the towcar, it overshoots. The towcar then pulls it back into line and it overshoots the other way, and that carries on until the trailer pulls the towcar out of line and even turns it upside down.

Why so? It’s complex, and we’re focusing here on what happens more than the physics. Essentially, when a trailer deviates from behind the towcar, there is eventually a force which returns it to behind the towcar. However, the trailer has momentum so it overshoots and swings out the other way. If in the course of doing so, it has also managed to make the towcar deviate from going straight, then it’ll end up going further the other way than it might have done, and the towcar’s return-to-centre force lags a little compared to that of the trailer as the towcar-trailer coupling isn’t fixed.

What makes trailers prone to sway

Let’s be clear here. The reason is not simply “lack of towball mass”, whatever OldMate58 says, even if he’s been driving MC for four decades. Here’s a list of things to which influence sway:

Relative weight of towcar to trailer. Consider a LC300 towing a 6×4 garden trailer with a max weight of 200kg. It wouldn’t matter how you loaded that 6×4, it’ll never have enough mass to upset 3,000kg of Cruiser. Consider a male bodybuilder walking a chihuahua. Little dog can try all it wants, it can’t pull the big man off balance. Now consider a small child walking a Great Dane. That dog goes where it wants – it’s a mass thing. Consider also a small child pushing an empty shopping trolley around a supermarket. Then, fill the trolley with a month’s worth of shopping, and have the kid try and control it around corners. Simply, the more mass you have in the towcar relative to the trailer, the less sway you’ll have.
Weight distribution in trailer – should be over the axles and not at either end.
Weight distribution in towcar – ideally between the axles.
Towball mass, or more accurately, the position of the centre of gravity.
Tyre pressures and conditions. There’s more weight on the rear axle, about 1.5x the towball mass, so increase tyre pressures accordingly.
Tyre age. Old tyres don’t grip well. Any tyre older than 6 years is suspect.

Risk factors for trailer sway

Regardless of how prone your rig is to sway, there are some situations that will increase the propensity to sway, and some of these are:

Speed – the faster you go, the more likely you are to sway.
Downhills – the steeper the hill, the greater the chance of sway.
Wet weather, dirt roads – less grip for the tyres.
Windy conditions – wind can induce sway.
Bumpy roads – and so can bumpy roads.

So let’s say you’re doing 90km/h on a sunny, calm day on a nice smooth road. You turn a corner, and as you do, a storm comes in with wind and rain. The road turns rough, and there’s a steep but fast downhill, so you pick up speed to 100km/h.

You now have many sway factors to deal with, so it’s best you control the one you can, which is speed, and slow down!

Dealing with trailer sway

In Australia we mandate independent brakes for trailers over 2,000kg GTM, and that’ll include most caravans. That means dealing with sway is easy. Just activate the trailer brakes by using the manual override. As a swaying trailer is moving quicker than the towcar, all you need to do is slow the trailer relative to the towcar. See our braking article for more details.

If you don’t have independent trailer brakes then you might think you can accelerate out of sway, and indeed this has worked for some. It is however a high-risk strategy as you’re adding speed and run the risk of simply getting to the accident faster, particularly if you have a diesel 4×4 towing a heavy caravan.

Preventing trailer sway

Sway prevention is largely a case of looking at what makes the rig prone to sway, and not doing that – so for example using a heavy towcar relative to the trailer, and centralising weight in the trailer.

Electronic anti-sway systems

Pretty much every new vehicle with a tow rating now has TSC, or Trailer Stability Control. This is a variant of ESC (electronic stability control), which detects when a trailer is swaying, and independently applies brakes on individual towcar wheels to combat sway. This is automatic and standard, so you may not even notice its operation.

There is also now TSC for trailers, which uses sensors on the trailer to detect sway. It either applies all trailer brakes at the same time – equivalent to you manually applying brakes – or applies individual brakes to the trailer’s wheels. This is becoming common on new, large trailers, and can often be retrofitted to older caravans. Car-based TSC cannot be retrofitted.

Final words…

Trailer sway is a real risk to your safety. You need to understand the risk factors such as relative van-to-towcar weight, and speed. Then, know how to “press away the sway”. It could save your life.