Do you need to carry large loads that wouldn't fit on your racks or in a pack? A bike trailer might be the answer for you. Basically these can be as big and strong as you want to make them. I have heard tell of folks hauling couches! There are a few ways to get a bike trailer, you can buy a bike-specific trailer, convert a child-carrier, or build your own from scratch. I built mine from scratch and am going to tell you what I learned and how to do it below.
Figure Out Your Plan
There are a few different designs out there. One-wheel designs are nice because they are narrow and glide well behind the bike. The problem is that they can be unstable, needing two mount points on the bicycle to keep from tipping over. Two-wheel designs are more stable and have a tighter turning radius than the one-wheelers. I originally made a one wheel design but could not get it stable enough and modified it to a two-wheel design.
Here is a schematic of the design I settled on:
A few components you will need to figure out in your design: wheels, frame, box, and bike mount.
Is bigger better? Well, I think larger wheels will be smoother and will give you a higher ground clearance. Also, the trailer will be more stable if it is mounted lower on the wheel, so a smaller wheel will put it underneath the trailer and thus less stable.
I made my frame out of wood. If I make another one, it will be out of 1" square metal pipe. It is lighter and more weather resistant. With wood, I used two screws and "L" or "T" brackets on each corner and join.
The box can be whatever you like. Mine is a wood frame with rope webbing making up the sides. I used plywood cut out into smooth corners to create the curves of my box. I laminated them together, three thick.
You can go even easier though, buy a stout plastic box with tightly-attaching lid. Really, anything that will fit between the wheels, will be just fine.
Connecting your trailer to your bike safely gave me the most logistical difficulty as my track bike does not have good mount points on the rear drop out area. First, make sure your connecting arm is long enough that your back wheel will never reach the front of your box. Also, the bent-arm design keeps your wheel off of the connecting arm when you turn right (which kicks the end of the back wheel out to the left. A good way to make the angle strong is use a big hinge and plate to keep it steady. To connect the end of the arm to the back of your bike, you should get creative. I used 8" metal plates to make a strong eye-bolt mount point off my back left side. I mounted the plates using "P" mounts, which are basically a piece of thin metal bent into a "P" shape and covered in rubber. I used lock-nuts to attach the eye-bolt so that it is not 100% tight, but can swivel in its mount. Then I used another metal plate off the top of the connecting arm. I drop a bolt down through the top plate, then the eye-bolt, and a large washer on the other side, before tightening it down. Again, I used a locking nut to leave the front mount slightly loose so that it can swivel left and right.
There are a few things that you should keep in mind with a bike trailer. The materials used should be weatherproof. If you use wood, varnish it with marine spar varnish, for instance. Another problem is that trailers are low and sometimes hard for cars to see. You might want to get an orange flag on a 2-m flexible pole, as they have on kiddie trailers. Reflectors on the back couldn't hurt either. It also takes getting used to. Ride around a safe area before you take it on busy roads. Give yourself plenty of room to make corners and not clip cars.
Be creative! Look around, there are a number of different designs out there. Here are a few. Enjoy!
Woah, lots of designs here!
A blog dedicated to bike trailers!
Another design, here, and six here, too.
An entry about dog trailers.
More basic info at Wikipedia.