The Double Pendulum Principle

Why caravans sway and roll over - and how to prevent it - a book on-sale soon

Updated January 2019

Any caravan towed by an overhung hitch (as virtually all are) and its tow vehicle is prone to sway and may even jackknife and overturn. Well over 200 did so in 2018.

The major cause is that tow hitch at some distance behind the tow vehicle's rear axle. If the tow vehicle sways in one direction, that overhung hitch not just allows the caravan sway in the opposite direction. It cause it too. Likewise if the tow vehicle sways in one direction, that overhung hitch causes the caravan to sway in the other direction. If that sway exceeds more than a few degrees (at speed) the rig may become uncontrollable, 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.

The major problem is that the sway behaviour is only predictable and controllable over a limited range of sway that becomes less at speed. Above a critical speed-  that is specific to each rig and its loading - the caravan and tow vehicle are 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 effect when towing is almost always due to the combined interaction of many factors. The major ones 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.

The energy needed for chaotic action to occur here is the kinetic energy of the moving rig at a speed unique to that rig, its manner of loading (and particularly the tow ball mass).

That not generally understood by caravan owners is that a rig may still appear to be stable at or above that critical speed in almost all normal driving. Most owners may never encounter a critical speed situation.

 If however there is no choice but if there is no choice but to make a sudden and strong emergency swerve (e.g, to avoid a head-on collision) whilst above that critical speed, that rig is likely to be triggered into a non-controllable and rapidly  escalating snaking. Within a typical few seconds that rig jack-knifes and usually rolls over.

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

The critical speed is related to many issues. The major ones are the ratio of tow vehicle to caravan weight, the length of that hitch overhang, tow ball mass - and very much so - excess speed. If you are towing a caravan that is appreciably 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 above explanation and the several main ways of assisting prevent it have long been explained in my Caravan & Motorhome Book and also my Camper Trailer Book. Both are totally up to date.

RV Books will, however, shortly publish a digital mini-book (by Collyn Rivers) that fully explains caravan and tow vehicle dynamics in plain English.

It shows how you can assess your own rig's probable stability, and how to improve that 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 all but complete and is hoped to be on sale (downloadable from this site) in mid-February 2019.

 

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