Why Caravans Sway

Why caravans sway and roll over - how to prevent it. Updated December 2019

Updated December 2019

Caravans towed by an overhung hitch sway and may overturn. Over 250 did so in the first six months of 2018.

When the tow vehicle sways in one direction, that overhung hitch not just allows the caravan to sway in the opposite direction. It causes it to sway in the opposite direction.

Likewise if the caravan sways in one direction, that overhung hitch, not just allows but causes the tow vehicle to sway in the other direction. If that sway builds up (at speed) the rig may jack-knife and often roll over. 

In effect the tow vehicle and caravan interact as if they were a double pendulum. The upper pendulum is the tow vehicle. The caravan acts as it were a second pendulum pivoting from the bob (that overhung tow hitch) of the upper pendulum.

A fifth-wheeler's single pendulum's occasional sway is minor and totally predictable. A double pendulum, however, acts as a single pendulum over only a few degrees. Above that it can suddenly be triggered into a non-correctable action.

The double pendulum trigger effect when towing is due to many factors.

. Relative weights of laden tow vehicle and laden caravan - the greater the better

. Distance of the tow hitch from the vehicle's rear axle - the shorter the better

. Distance between the front and rear axles of the tow vehicle - the longer the better

. Tow ball mass - short centre-heavy trailers need 6%-7%, all other caravans need 10%

. Interaction with external force/influences such as gusting side winds, wind gusts from overtaking or overtaken trucks, cornering too fast, a sudden and major change in road camber, veering off the side of the road or steering overcompensation. Excess speed is always a major factor.

Any or all of the above can result in the tow vehicle being triggered into a highly unstable condition called over-steer. If that happens above a critical speed unique to each rig and its loading, that results in a chaotic jack-knifing sequence. It is non-controllable and rapidly escalating snaking. Within a few seconds that rig jack-knifes and usually rolls over.

That rarely understood by caravan owners is that their rig may feel ultra-stable in normal driving. Further, most owners never encounter this situation. It is however escalating as caravans become bigger and heavier, whilst tow vehicles become lighter.

There are some quite frightening dashcam videos available online showing the double pendulum effect in operation on towed caravans. Our Caravan Rollover page shows three of them.

Causal factors include the ratio of tow vehicle to caravan weight, the length of hitch overhang, tow ball mass and excess speed. If you are towing a caravan that is heavier than the tow vehicle you are at far greater risk than if not. This is particularly so when descending any steep gradient at speed - and even more so in there are also bends and/or side winds. The rig's critical speed is also related to tow ball mass. The lower than mass the lower the critical speed.  

Our book Why Caravans Rollover - and how to prevent it is a plain English explanation of not just how and why - but what you can do to increase its margin of stability. It include a section that enables you to assess the likely stability of your own rig. And how to fix the defects.

Friction sway reducers (ineffective at speed)

Friction sway reducers work well at low to medium speed. All however have a fundamental limitation. This is that their frictional damping is constant - but the forces they are attempting to damp increase with the square of the rig's speed. They improve ride comfort but at (say) 100 km/h they have virtually zero effect (tests have shown that that damping as then being less than 1%.  

Our book Why Caravans Rollover - and how to prevent it, explains all. The major part of it is written in totally clear English. It also has a section that explains the issues in more technical detail. This part is primarily for engineers working in the caravan industry and technically-minded readers seeking detailed explanations. It includes many references.

In essence it explains just why caravans may jack-knife and roll over, how to assess the stability of your own, and how to fix any issues.  

This book could literally save you from a rollover! The Caravan Council of Australia describes it as 'compulsory reading'.

 It is on sale now -  please see our Bookshop.

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