A towing stabiliser is a sway controlling device that helps or alters the lateral and vertical articulation between the towing vehicle and caravan or trailer.
Vertical instability can be caused by an incorrect load, a too high centre of gravity or even worn rear suspension.
You will notice when going over things like pot holes,drain covers or dips in the road, anything like this.
The other way your tow can be affected is by side winds and large lorries passing your vehicle which sort of sucks your vehicle as it comes next to you and then pushes you away as it goes past. This term is also known as snaking and higher the load the more its affected.
This can be a frightening experience the feeling of your caravan or trailer out of your control swaying or snaking behind your tow vehicle. There have been many accidents and "roll-overs" as a result of this type of loss of control of the tow vehicle.
Recommendations for helping stability
It has been proved beyond doubt that increasing the nose load (static vertical load on the towball) reduces the susceptibility of a trailer/towing vehicle to instability. Therefore, the recommendation is to increase the nose load up to the maximum allowable on the vehicle/trailer specification (whichever is lower) and also ensure that the outfit is balanced and well matched.
The further the centre of mass from the axle, the greater the susceptibility to instability. In other words, centralise the weight above the axle both in the lateral, longitude and vertical planes to balance the outfit and achieve an acceptable nose load. An unbalanced outfit is an unstable outfit. Ideally the centre of gravity for the caravan/trailer should be low down and slightly forward of the axle.
Having the mass in the trailer as low as possible tends to reduce the susceptibility to instability initialisation. Caravan and vehicle tyre pressures must be checked every time to the manufacturer's specifications.
There are things to adjust on the towing vehicle that may help like:
Tyres: A stiffer walled tyre running at the high end of the recommended pressure range will be better at damping instability.
Springs: Stiffer springs help reduce susceptibility to instability.
Dampers: Stiffer dampers (shock absorbers) also tend to reduce susceptibility to trailer instability, check these and make any are not worn.
Usually, adjustment of one or more of the above can bring significant improvements in resistance to trailer instability.
However, you cannot assume this will make the load 100% stable because you cannot assume its the vehicle that is causing the problem.
So in conclusion, there is the ability to set up your towing load to keep stable without the aid of a stabiliser however having a stabiliser gives the driver peace of mind that any instability with be decreased by using one.