If you need to transport heavy cargo with your bike you can build a tough trailer like the one I helped to build in Piubici for the Bricheco girl. Unlike the first trailer I built, this one is bigger and heavier, but let you carry whatever you wish.

You only need a pair of small bicycle wheels and some steel profiles, and of course a basic welding know-how.

Step 1: Frame Material

To obtain cool and easy to weld steel bars I suggest you to look for a used old bed frame. Since in last years people love to sleep on wood staves, you probably will not have any difficulty to find those metal frames. And of course you will keep the springs for your next project (actually you need one for the hinge)!

Step 2: A Sketch of the Project

You can see that this trailer frame has been designed to match the wood box dimensions. Anyway you can go through the inverse process, build a frame you like, then make a box with the right dimensions.

Draw a fast sketch, and note all the measures you need. This is very important to avoid mistakes. Notice that I used the bed corners to become the trailer corners, that is very useful since you will need to only weld straight pieces.

Then ,using a grinding machine, cut the bed frame in the pieces you need. Try to make straight cuts and to keep the blade orthogonal to the bar, to do that help yourself with paper tape.. After cut a piece mark it with paper tape to recognize it. place crossed pieces of tape to mark remains.

Step 3: Refining

Having all the pieces cut, place them all together, referring to your project. All pieces have to touch one each-other almost perfectly. The more the lengths are precise the more will be easy to weld them. If you find out that a bar is too long change the blade of the grinder and with the thicker blade smooth and shorten the bars.

Now with the same grinder blade remove all the painting around the extremities of all the bars, and also on the surfaces corresponding to the welding points. You cannot weld on the paint.

Step 4: Welding

Place all the pieces on a flat surface, and safe to welding. Then measure all dimensions and also diagonals to be sure that the frame parts are exactly in the right position, as previously decided in the design.

Be careful since the first weld joints will pull the metal and change the angles. You can adjust the angles pulling and pushing the welded pieces, then continue the work with the edge pieces. Then you can wedge the other parts in, so to weld them very handy.

To make a perfect welding the best way is to have a good teacher ;-) anyway there are a few rules to follow.

Try to not use a thick electrode, usually these tubes have a very thin surface and it's very common make an hole while welding with 2 mm or bigger electrodes.
Another way to avoid holes is to interrupt the arc every about half second, wait the same time, and make contact again, in that way you can regulate the temperature of the fluid metal.
If an hole appears you can close it, but it's not easy and it takes a lot of time (and material) so be careful to avoid holes.
I found very difficult starting to weld with a new electrode, to avoid mistakes start a new electrode on a test surface.
If you have to weld thick metal pieces, like dropouts, increase the amperage, compared to welding two thin layers.
Avoid looking at the spark, always use good welding glasses or helmet, there are very handy Auto Darkening Solar Welding Helmets.
Wear appropriate dresses: non-flammable gloves and coverall.

ALWAYS ASK TO SOMEONE EXPERT TO LEARN HOW TO WELDING, HIGH CURRENTS ARE VERY DANGEROUS!

Step 5: Dropouts

You can make the dropouts from a thick steel par. Follow the schematic to understand the process steps.

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.

Step 6: Place the Dropouts

The dropouts have to stay aligned together and with the frame orientation, while soldering. The best way to do that is to squeeze them between nuts along a threaded metal bar, possibly with the same diameter of the wheels bar.

The distance between the dropouts depends on the wheels you wish to use. In my case is abut 9-9.5 cm. Use 10 cm if you want to use 26" wheels.

Step 7: Weld Dropouts

Also in this occasion the first welding joints will pull the metal, so be careful to keep the bar strongly connected to the frame. Weld the dropouts and complete all the welding process. Now you can remove the threaded bar and check that the wheels fit well.

Don't follow my example about the dress, remember, non-flammable gloves and coverall!

Step 8: Wood Box

The wood box is not difficult to build. In this case we added metal profiles to protect the wood edges.

Step 9: Mini-wheels

Four mini-wheels will be very useful in two ways. They act as locker to keep the box steady on the metal frame, and they will let you moving the box when you take it off from the trailer.

It's very important that you secure the wheels in the right position with strong screws. To determine the exact positions flip the box upside-down, flip the frame too, and place it on the box. As you can see in red in the drawing the four corners are very good places where to screw the mini-wheels.

Step 10: The Hinge

The hinge has to be very strong, to avoid to lose the cargo on the street. A good way is to use two chain rings, gripped in two pieces of metal pipe.

The long bar soldered at the pipe extremity will keep the hinge firm when attached to the bike axle.

To avoid that chain makes a bothersome clacking with the bike movement you can add a spring (one from the bed frame) which keeps the two rings in contact.

Step 11: Load It Up!

Now place the wood box on the frame and load it to test the strength and stability. As you can see in the cover picture you can also carry a person if you wish! Enjoy!

Step 12: UPDATES

I still have a few updates to do. As you can see I drilled some holes in the wood panels, so to use them as handles.

Then I glued and screwed a red wood profile all around the edge, the wood container now is more nice-looking in my opinion.

Add also a bicycle tube around the bar soldered to the hinge to avoid ruin the bicycle frame paint.

The metal frame has been covered with a special paint to protect it from rust, and we have to paint it with a red colour. Next thing to make is also a good fastening handy to lock and unlock, for the wood box, and rubber foot to avoid clanking against the trailer. So... stay tuned!

Step 13: Trunk Clasp

To avoid that the trunk beats the frame clanking and making bothersome noise, you have to add at least two clasps. You can choose to add four of them, maybe one for each side, anyway for now I tried to add one on the front and one on the rear edge.

Make some experiment referring the distance between clasp and hook, so that when closed the clasp remains very tight. If it's very tight you probably don't need rubber foot between wood box and metal frame.

Remember to add two red reflectors on the frame sides, since the trail has to be well-visible in the night.