The phenomenon is not limited to caravans! It occurs in almost anything on wheels, aircraft, sailing boats, economic systems, and a whole slew of unexpected things. Its cure has been known for countless centuries (at least with sailing boats) and that is to inbuild a mechanism that (preferably) automatically reduces or cancels the effect of the disturbing force.

A run-away economy, for example, is slowed by increasing interest rates, and thus reducing spending. An 'over-steering' car (i.e. one that attempts to tighten a turn) can be stabilised by increasing the slip angle of the front tyres relative to the back tyres, thus causing the vehicle to widen the turn and thus reduce the forces acting upon it.

A swaying/pitching caravan is damped by placing some part of its weight onto the towing vehicle and the letters pages of caravan magazines are full of generally good advice on the amount of weight that is so needed. As by so doing, weight is transferred from the front to the rear wheels of the towing vehicle, a so-called stabilising hitch (in effect a semi-flexible beam) is added between the towing and the towed vehicles to transfer part of the drawbar weight onto the towing vehicle's front wheels. This hitch does not reduce the weight on the towbar, but does increase the load between the towing ball and its receiver.

The general advice, in Australia, is for the towball weight to be 10% of the trailer weight carried by the towed vehicle (curiously it's only 7% in Europe!). This is safe advice, but  because there's a lot more to it than this, results in many caravans having far higher towball loadings than is necessary: and/or the exclusion from consideration of otherwise adequate towing vehicles on the basis of their maximum towbar rating.

Imagine (or have) a heavy dumbbell, and something heavy of the same length, but with the weight evenly distributed. Hold the former a bit away from the middle and rock it to simulate a pitching (and also swaying) caravan. Now try the same thing with the other evenly loaded bar. You will find that the latter is faster to respond and much easier to start and stop.

A loaded caravan behaves in a similar way. The effect is described technically as the 'polar moment of inertia' and explains why a caravan with a centre kitchen is likely to behave quite differently from a caravan with an end kitchen. And why one may pitch deeply and slowly, whilst another may pitch less but at a high rate.

The effect of the weight distribution is much greater than many people (including one suspects, many caravan designers) may realise and is probably due to a confusion between weight and mass.

Weight is more or less what you think it is (technically the gravitational force with which the earth attracts a body). Mass is the resistance of that body to being accelerated. So 150 kg on the towball of a violently pitching caravan results in far higher forces than 150 kg on the same 'van at rest.

The location of weight along the length of a caravan has a huge effect on its mass. The forces are this: the so-called polar moment of inertia (i.e. that which causes the pitching) is directly proportional to the imposed weight, but proportional to the square of the distance from the centre-line.

In other words, a 200 kg kitchen at the rear of a 6 metre van has many times the effective mass of that same kitchen centrally positioned. In some instances this mass will be counterbalanced by a similar mass at the other end (but this still results in the dumbbell effect. And, as shown later in this piece, setting the axle/s further back aids stability in still air, but may detract from it in side air movement (including those caused by passing road trains).

A few sketches drawn to scale with weights placed at various locations will show the magnitude of the effect of weight on a 'van dynamic towbar loading - and why towbar weight may, in some circumstances, be safely reduced way below the generally recommended 10%.

Determining the ideal weight distribution is hugely complex as it also involves dynamic relationships between the spring periodicity and damper settings of the towing vehicle, but as a rough guide having major weights as close as possible to the axle/s (or at worst centrally distributed along the 'van's length) is far better than making a dumbbell on wheels.

The above should really be addressed in the design stage, but a quick walk around any caravan show indicates that it is rarely considered (or possibly understood) by most caravan builders. The only palliative is for owners to shift as much weight as possible away from the ends of their caravans.

The second problem is that little no cognisance appears to be taken of the effect of side wind forces on stability.

These twisting forces are imposed (and must be resisted by) the towing vehicle, but because the towing attachment is way behind that vehicle's rear axle, the forces cause that vehicle to sway.

It's a bit more complicated than that - because caravans are rarely rectangular in side section but one can readily estimate the position of the so-called 'centre of pressure' (where the wind forces can be perceived as acting) and see if this is in front of or behind the axle/s. It's also a matter of the position of the lateral centre of gravity but hopefully one gets the general idea.

What the above amounts to, is that a caravan, should not really be balanced by moving the axle/s fore or aft to get the desired towball weight, without considering aerodynamic side wind loading.

If there has to be an unbalance the centre of pressure should in front of the axle not behind it (i.e. the axle further back - not forward). The latter situation can result in so-called positive feedback, which may cause the outfit to snake uncontrollably.

Where snaking in side winds is a problem with twin axle vehicles, stability can be enhanced by increasing the pressure of the rear pair of tyres. This reduces their slip angle and partially negates the disturbing forces. It works to a point - but is a band aid fix for a fundamental design problem.

Despite their axle placement, fifth-wheel caravans are less affected by this problem because their attachment point to the towing vehicle is above the centre line of the towing vehicle.

Collyn Rivers