You have all see those caravans with 2 large spare wheels hanging off the back of the rear bumper. You may have thought, “that looks safe and protects the rear”. It may well do but there is a far higher probability this caravan will be less stable because of the weight distribution.
The worst weight distribution scenario for a caravan is a high ball weight and 2 spare wheels hanging off the back. To be more specific, there is a model that publishes 250kgs ball weight and 2 spare wheels (80kgs+ with support frame) hanging off the back. Why is this a potential problem? The swinging movement of a caravan about its vertical axis is called “yawing”. The most contributing factor to caravan instability towing at speed is from “yawing”.
Just imagine taking a pencil and adding 2 big blobs of plasticine either end then twist it clockwise and anti clockwise. The more weight either end, the more “spinning inertia” and it can be hard to stop. Then put the identical weight just slight forward of the centre of the pencil and twist this. It is far easier to stop the twisting.
The ideal scenario for low yawing and maximum caravan stability is to have as much weight distributed close to the centre as possible. There are 2 off road brands that are absolutely adamant on this: Kimberley and Bushtracker.
Note that you don't want a “very low” ball weight, there has to be some weight here for stability, at least 7% of the GTM of the caravan in our opinion.
So why do caravan manufacturers risk stability and place high loads at the extremes? Firstly it could be because the ball weight is too high for some vehicles. To offset this, the manufacturer decides to place a large load on the rear. The simple test is to ask what the ball weight is with no spare wheel on the rear!
The second reason is because there is insufficient room underneath the caravan for the spare wheels.
The third reason is some caravan dealers like putting the advertisement for their business as part of the rear spare wheel cover!
There is a another important reason to not put the spare wheel up on a rear carrier. Try lifting that wheel (35kgs) up and into position. Watch your hernia as you do this, it is hard work. It is far easier to drive back over a spare after it has been changed and wind it up from the road underneath the van.
Is there anywhere else to carry a spare? Well anywhere around the front “A frame” is a good location as this keeps the weight forward; providing the drawbar weight is not over the rating of the vehicle. Alternatively, add a second spare to the vehicle rather than place on the back of the caravan.
A study by Darling, J., Tilley, D., Gao, B., 2009. ‘An experimental investiation of car-trailer high-speed stability’ published in the Proceedings of the Institution of Mechanical Engineers, Part D: Journal of Automobile Engineering, 223 (4), pp. 471-484) indicates that dominant factors affecting stability are trailer yaw inertia, low nose mass, and trailer axle position.
It has good reading on the theory behind increased stability at high speed. This does not address off-road issues, as there is no application for this in the UK.
Trailer Yaw Inertia
It was seen that, when the trailer yaw inertia increases, the damping of the combined car–trailer system decreases dramatically and as a result degrades stability significantly. The inertia effect suggests that, when a driver is loading a caravan, the mass should be placed as close to the centre of gravity as possible in order to minimize the resulting increase in inertia.
Outside of this report, it has been suggested that beam axles with leaf springs are more stable than independent suspension. This depends on the type of independent suspension and the pivot position of the "swing arm". It would also only apply to those caravans with a high centre of gravity (generally fully size, not the compact folding caravans). It would not apply to camper trailers because their centre of gravity is so low anyway.
However, adding air suspension to a caravan needs an anti-sway bar in the event that the air suspension slowly leaks and “collapses” at severe cornering. Without an anti-sway bar in this configuration, the swaying effect could be lethal. Kimberley add anti-sway bars to their caravans with air suspension as standard.
Trailer Drawbar weight (referred to as “nose weight” in the report).
It was found that an increased nose mass improved the system stability, although the improvement becomes less when the nose mass rises above 6–7 per cent of the total weight. This effect is thought to be caused by the increase in the car rear axle load associated with nose mass that increases the ability of the tyres to generate side forces that damp out the oscillation. In addition, the increased nose mass raises the tow ball friction, which in turn helps to damp out the oscillation. Caravan users often prefer a light nose mass as it is easier to manoeuvre the caravan and to couple it to.
Tow Ball Friction
Increasing the tow ball friction is an easy way to damp out oscillation. The oscillation best dampened is the horizontal axis perpendicular to the direction of travel. (Kimberley note: we only offer a hitch with a rubber insert that dampens horizontal movement. Pintle hitches that offer no friction are not fitted to any Kimberley off road RV)
Trailer Axle Position
It was found that, a longer distance from the tow hitch to the tyre contact patch, increases stability with the caravan. This can be attributed to the fact that, when the towing length increases, the trailer lateral tyre forces act on a larger lever arm with respect to the tow hitch and this helps to stabilize the trailer oscillation.
(Kimberley Note: All Kimberley RV’s have a long length in distance from hitch to axle. A drawbar extension was introduced to allow for rear “barn doors” to swing on 4WD’s. These improve this ratio even better.)