I knew that one day I'd want to move our water tank, a 2200 litre plastic drum. It weighs under 100kg, but it's 2.5m high and 1.1m in diameter. Kind of annoying to move, however you do it.
I have a big set of shelves I made a long time ago that normally live in the garage with lots of stuff on them. They're 2.4m long and 1m wide, based on some random need I had at the time and the length of a sheet of plywood. But the useful thing is that the top of that is a rectangular metal frame that side. By adding a couple of crossmembers in the middle I could use that as the frame of my trailer. All I needed was some bolt on wheels.
Given the size, and knowing my habits, I decided that I should also put reaction brakes on it. It's all very well having four disk brakes on the quad but if there's 500kg on the trailer pushing 200kg of quad... trailer wins.
Here's a couple of photos of the tank actually on the trailer. The rest are from when I was building it.
Attaching the headsets (chopped out of the kids bikes that I got the wheels from). Bikes came from Ceres bike workshop who get an awful lot of them donated.
rear wheel and rear triangle ready to weld on to the frame. Well, sub-frame, it's a separate U-shaped frame that bolts onto the shelving unit.
Wheels on, all done!
How the hitch was expected to work. Drilling the brake cable holes in the T, using a dremel and a full-size drill vise (my pedestal drill has a couple of millimeters of slop in it, which is not kind to 2mm drill bits. So I used the dremel).
More towing arm. The bolt on part that the T above pokes through - this is what takes the towing forces! And the hinge at the trailer end of the towing arm.
The "towbar" attachment for the quad. Which has a square tube frame under the bin, and this bolts on to that. The bolt pointing up is a 10mm bolt. The cross piece underneath is 100mm wide and designed so you can attach a bike by removing the front wheel and using the quick release skewer through this tube to attach the fork at the dropouts. It works surprisingly well and makes carrying a bike very easy. The towing arm has a 10mm rod end bearing on it.
Towing arm complete and painted. The cable adjusters are actually surprisingly useful, just to roughly balance the brakes.
Steering is relatively simple, a length of aluminium rod (10mm od, holes tapped to 6mm each end) from the towing pivot to each wheel linked with 6mm rod end bearings. Rough ackermann geometry so that it doesn't destroy the front wheels when turning with a load on it.
First test run, with random stuff out of the shed piled on to provide a bit of weight. Also, once that worked, some housemates. Because who doesn't want to grind up and down the street on the back of a bike trailer?