The story is in the early days of caravanning there were no limits to the size of trailer you could tow behind a car, with the result there were a number of quite serious incidents. The Gov't were disposed to start to introduce legislation to limit caravan towing, but the Caravan Club and the caravan manufacturers lobbied hard, and were able to convince ministers that by advising customers to keep their caravan's laden weights less than the tow vehicles unladen weight (along with other criteria about nose loads and speeds) this would improve the incident rate. This suggestion found favour with the Gov't, as it meant it didn't have to revisit the construction and use regulations, and rewrite what was already a complex piece of legislation.
Since this arrangement has been in place, the EU has become part of our lives (Will it still be the case in a few weeks?) and various directives now make it mandatory for car manufacturers to test and specify a cars maximum permitted towed load. These limits refer to ideal trailers and unfortunately caravans are far from ideal. So the industry recommendations still stand but have have changed value over the years. It is still suggested novices to aim for no more than 85% caravan to car ratio, and as experience is gained ratios up to 100%.
The dynamics of Cars towing caravans are very different to articulated HGV's. On a correctly loaded artic trailer the centre of mass will lie almost halfway between the fifth wheel coupling and the trailer's axles. This sets the vertical load on the coupling point to a high figure and a similar load on the trailers axles. THe position of the trailer wheels are set towards the rear of the trailer and control the yaw load very effectively resulting in generally stable towing.
By comparison the wheels of a caravan are almost central to the trailers mass, and the Centre of mass is located only just in front of the axle which is what develops the required nose load. The distribution of the mass particularly behind the axle leads to much larger yaw loads, and is one of the principal reasons why caravans are inherently far more liable to instability than artic trailers.
The industry suggestions are only guidance and cannot be enforced in law. BUT bearing in mind the inherent unstable nature of a caravan, there is good sense to keeping the towed weight ratio as small as possible.
IN relation to Ray's comment: Generally when you take out car insurance, the policy covers the vehicle and all its standard specified capacities including the manufacturers towing capacity.
I can only recall one instance reported on this forum where a contributor claimed that after a towing incident, their insurer imposed a towing limit that undercut the vehicle manufacturer's specification.
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