anyone evry tried using a peugeot rear torsion bar for the basis of a trailer? easy 4 bolt mounting strikes me as making it nice and easy, and they're dead cheap from the scrappy...
I think you'll find it's the towbar that goes on the towing vehicle thats covered by this law. Any towbar fited after a certain year (2002 I think) has to be manufactured to a standard and have a rating/manufactures spec plate on it
Car suspension tends to be a bit soft for the job. The car trailer I borrow from next door weighs about half a tonne even before the weight of the Renault on top is added in. Unlaiden it bounces along with no suspension at all - it's quite stiff. Loading weight in a car tends to include weight that's also distributed on the front axles so isn't necessarily a good guide unless it's stated per axle.
Of course the laws are different here, especially in Texas where I live, but I am just finishing up a utility trailer. I used a rear axle from a Dodge Caravan, very simple to bolt spring pads to bottom of frame. Made frame from 2 x 3 inch 14 gauge rectangular tubing. Used 1 inch square tubing for vertical members, and 1.25 inch round for top rails. This was a learnin' to weld project, also a way to keep my thirsty Chevrolet half-ton parked and use my Kia Shortage for Home Depot runs. I live way out in the country so I'm always needing to haul something. My next one will be a 1940's style teardrop camper trailer, I'll use the Caravan axle for it, too, as they are cheap, plentiful, and already have brakes.