I've hit "post reply" so hopefully this is the right way to use the forum.
I have attached a load of photos of the build so far.
I MIG welded the chasis with 1mm wire in my friends workshop (his workshop is better than mine and his MIG welder is a dream). The steel place cut the steel to length for me, I only had to grind the angle cuts at the tip of the A frame tounge.
It is mostly made from 80mm x 40mm box section, the axel and tounge straight piece is 50mm x 50mm box section. The thickness is 3mm. It is standard mild steel, nothing fancy.
I did the calculations myself (I am an Engineer) to verify this is strong enough. It is about 5x stonger than it needs to be, but heck, I wanted it to look strong
The two long beams have open ends. I have bungs to fit in the open ends to stop water getting in. I'll drill a couple of holes on the under side so any water getting in can get out.
I sprayed it with a red oxide paint then a black enamel paint over that. The enamel paint scracthes, it's a temporary coating. I plan to sand back and spray Raptor Bed Liner onto it when I put the camper section on, I've read good things about raptor and it's quite economical.
The coupler, jocky wheel, suspension and 13" wheels all came as a "750kg unbraked trailer kit" it came with the 8 hole mounting plates for the suspension which I welded onto the axel. After than everything bolted on easy. Had to drill a few holes into my new chasis though
This is a 3m x 1.5m trailer (from wheel to wheel its actually 2m wide). The tounge is 0.9m (it has to be atleast half the length of the towing vehicle). Total length with the tongue is 3.9m
So far I estimate the weight to be 100kg or less.
It actually tows like a dream and I had to reverse it at the timber merchant and back into my driveway and it handled really well. The towing height is spot on too, the back end is up ever so slightly but once the weight of the camper is on it I'm sure it will be almost perfectly level.
I put temporary lighting on it (the lights came with the kit and will be the final ones I mount to the camper) so I could use it on the road for now. I was easy enough to solder some 12V wiring onto each bulb connector. and trace back to a wiring block (not really waterproof at this point!) then down a 7-core tailer cable to a 7-pin plug. Luckily I had wire, soldering stuff from an old project. I used an old peice of wood to screw the lights onto and I cable tied the wiring on. I used a couple of ratchet straps to attach the light board as I don't want to drill holes into my chassis yet.
That's it so far.
If you have any quesitons about the materials I used or how I designed this trailer just ask!