Paragraph 43




  Stabiliser Test




……………………………….Centre of Mass


 …………………      A = axle of caravan/trailer


w hitch = tow hitch







Peter W Jones  AMInstP

I was begining to think that spam had permanently sunk, but they came on line again about 2200, 26-02-08.

Today they are off line again so I am adding a further two essential items, the first one being on friction based stabilisers


18a A Caravan/Trailer Stabiliser Test.To carry out this test fix a suitable length of tubular steel or wood to the stabiliser when it is fixed to the tow car, but not the caravan/trailer. Support the end away from the car on wheels as its weight will be considerable (I used furniture castors).Grasp the length of steel or wood at the position which is the same distance from the tow car as the centre of mass of the caravan. Move the wood/steel with one hand and judge whether the amount of force exerted on the hand is likely to have any retarding effect on a caravan snaking at 50mph.Following the caravan clubs rules ( and those of Dr Darling of Bath University, according to his web site), all heavy objects should be loaded as close to the axle/axles as possible commensurate with keeping to the tow car manufacturer’s figure for the maximum allowed weight on the tow hitch. This will result in the centre of mass being a short distance in front of the axle/axles.The position of the centre of mass can therefore be estimated with some degree of precision.For those with experience of calculations I add the following:-Calculation of the distance of the centre of mass of a caravan/ trailer from the tow hitch.

F= maximum allowed weight on tow hitch ( see tow cars’ handbook for drivers)
d= distance of tow hitch from axle ( or the mid point between the axles)
x= distance of centre of mass from axle (or mid point between axles)

W= weight of caravan/trailer

Using the Principle of Moments, for equilibrium :-

Fd = Wx

x = Fd/W

ie x = F multiplied by d divided by W

It is then necessary to construct a lever of length = d- x

For my small 4m body length caravan this came out to be 3.33 m

To make quite certain that it is feasible to carry out this test, I performed it myself.

The lever (made of wood) is in two pieces, to make it easily transportable in my Land Rover.

It can be clamped to a stabiliser by means of bolts and nuts which only pass through

the wood.

To test a stabiliser fixed permanently to the caravan draw bar, simply remove same,

(usually only held by two bolts), and fix some tubular steel in place of the draw bar.

In the case of my caravan, the tubular steel would have to be 3.33m long.

Whether using wood or steel, the weight is considerable and the end away from

the stabiliser needs supporting on wheels (I used furniture castors).

Fix the stabiliser to the car in the normal way. In my case I then found that I could

easily move the stabiliser from side to side by gripping the long lever between

finger and thumb. Even this test over estimates the effectiveness of the stabiliser

as there is also the friction between the ground and the castor wheels.

However, it can be seen that my stabiliser, that I have fixed to my caravan and boat

trailers every time I was on the road with them for the last 30 years, is completely

ineffective. If it were not for my special interest in this matter, I would have confined the item of scrap metal to the recycling bin.

Before carrying out the test I checked that the force of friction that my stabiliser

could exert was the same as when I purchased it, and as stated in the Haynes

Caravan manual.

I await with interest reports of tests on other stabilisers.

See also  





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