It happens sometimes that for bad luck or different causes you cannot see the completing of an almost finished project... anyway you will be happy to know that in my case I decided to comment the pictures and publish the steps anyway.
Step 1: The Bed Frame
A friend of mine obtained a nice bed frame with small wood boards. Cut to be shorted, the bed was perfect to make a beautiful bicycle trailer. As you can see in the pictures, some metal plates are already soldered to the frame, and I decided to maintain and use them as supports for the wheels. The wood boards will be a beautiful and trendy plane to place the load.
Step 2: Remove the Unused Parts
Some metal bars are not useful in my design, so we decided to remove them with a grinder. Pay attention to not cut the frame pipes, and remove any trace of welding, so to have a flat surface.
Step 3: Cut the Bars
From two pieces of the bed frame we cut the bars which will keep the wheels. Those bars already have two plates soldered, so it's easy to weld them to the main frame.
Step 4: Assembling
As you see from the picture, the project slowly is taking shape. Place all the pieces one near each other, so to mark the positions with a marker. Measure that there is enough space for the wheel hubs, and enough space for bigger wheels in case you wish to change them, I used 16" wheels in this case.
Step 5: Welding
To weld the frame, if you are not expert, ask to someone which can teach you. Since the pipes are quite thin you have to pay attention to not make holes in them. Then you can use the grinder to remove part of the welding material (but you haven't if you are good in welding technique).
Step 6: Welding Dropouts
Cut the dropouts from a thick steel par. At first cut four holes at the same distances (about 4 - 5 cm) then cut at half distance between them, and finally with the grinder cutting blade remove the metal pieces to open the little forks.
After the cutting process, place all the dropouts together in a big vise, and refine them so that they all have the same shape.
Then you can use a threaded bar with eight nuts, placing all the four dropouts on the frame, and welding them to it.
Step 7: The Connection Bar
In my opinion an hinge connected just under the seat works very well.
So I decided to make a connection bar with right dimensions and angles to reach that point. I cut the profiles and welded them together.
Step 8: The Hinge
The hinge needs to be designed so that it leaves the three movements free. First movement is the one around the seat post, second movement is the rotation around the horizontal axis, last movement is the one which sets free the torsional force.
To obtain all these joints I managed to use a cheap little wheel for furniture. Simply remove the wheel and add a new pivot. Linking the pivot to the metal brace you will obtain the wanted movements. The first movement is obtained attaching the joint to the seat post with two threaded bars, not tight, so that the hinge can rotate around it.
Step 9: (almost) Finished
The trailer is almost finished. I should had only insert the wood slats and lock them pushing a wood profile in the split under them, adding the tires and paint the frame.
Unfortunately the man which asked me to make the trailer wished to reduce the width placing the wheels inside the frame, so he cut some bars and the project will be completely revolutionized.
You need a bit of imagination to see the finished trailer in my last pictures ;-)