The Double Pendulum Principle
Updated December 2018
This article explains how and why a caravan and its tow vehicle is prone to sway and may even jackknife and overturn. Over 200 have done so this year.
The major cause is that a conventional caravan's tow hitch is at some distance behind the tow vehicle's rear axle. If the caravan sways in one direction, that overhung causes (not just allows) the tow vehicle to sway in the opposite 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.
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 overhand, 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.
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