The Double Pendulum Principle

Why caravans & sway roll over - how to prevent it

Updated March 2019

Caravans towed by an overhung hitch sway and may overturn. Well over 200 did so in 2018.

When the tow vehicle sways in one direction, that overhung hitch not just allows the caravan sway in the opposite direction. It causes it too do so. Likewise if the caravan sways in one direction, that overhung hitch causes the tow vehicle sway in the other direction. If that sway builds up (at speed) the rig may jack-knife and even 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 single pendulum's sway is basic and totally predictable: it's how a grandfather clock works.

A double pendulum, however, is different. It acts as a single pendulum over a very small range (only a few degrees). Above that however it is quite suddenly triggered into a non-correctable action called chaotic motion. In essence its behaviour is random-like, but not literally random - e.g. it can be predicted but needs huge computer power to do so.

The double pendulum trigger effect when towing is due to many factors. They include the relative weights of the tow vehicle versus the caravan, the distance of the tow hitch from the vehicle's rear axle, and the tow ball mass. All interact 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.

All of the above can result in the tow vehicle being triggered into a highly unstable condition called over-steer that results in that chaotic jack-knifing sequence - non-controllable and rapidly escalating snaking. Within a typical few seconds that rig jack-knifes and usually rolls over..

That rarely understood by caravan owners is that their may appear to be stable in normal driving. Most owners may never encounter this situation.

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

The major 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.  

Our all-new 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 you its margin of stability. The book has a section that explains the issues in more technical detail. That section is primarily for engineers working in the caravan industry and technically minded readers seeking detailed explanations. It includes many references.

This book is now on sale - please see our Bookshop.

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