[wallaby]

Glend. I love your work. I have built a similar thing to you tho the design is different. I bought/had made a heavy duty 7x4 trailer with an extended draw bar (2 m; 50 x 75mm box steel) strengthened, 15in sides, 50mm square axle and 12in power brakes, 7 leaf eye to eye springs and 16 in rims. I built a box with a rounded front face and extended it over the longer draw bar which meant that I ended up with a 9x4 sleeping box. I used 2nd hand caravan windows and built a plywood door at the front. I ended up being able to put in a double bed mattress and enough room to build 600mm long pull out drawers that come out of the back and which includes a 2 burner camp stove. I put in a ceiling that allowed me to put in a 60 litre water tank. This sits above the axle and helps reduce bounce and unbalanced load (if at the front or back of the trailer, for example. The box was made of 15mm C-D ply at the sides and floor and 7mm ply for the roof. I covered the roof and front of the ply with 1.5mm galvanised tin. All of this I bought 2nd hand from a building disposal yard. I also used a lot of framing (about every 200mm) for added (extreme) strength. I also did similar with the top and braced these against a runner at the top of the walls. This will allow in excess of a tonne of weight that can be carried on the roof. I note that you say that you are using 6mm ply. I think that without framing this will not be very strong. I suggest add some framing: just glue and screw it to the existing wall structure and to runners at the top and bottom. In this way your "torsion box") will be much much stronger and you will be able to take it anywhere!! As it stands, I would worry about too much offroad in its present construction. You only need to use lightweight pine for the framing - say 20x 20 mm will be enough. Remember it is the glueing that gives strength to the construction, not the screwing. Also, definitely add electric brakes. It is easier to stop 750kg with brakes than without and believe me, you will need to have them!! The trailer is rated at 1350 kg; the axle is rated at in xs of 1550 kg. The weight of the trailer with the box is about 600kg fully loaded. I don't think that you need shock absorbers.

Cheers