A simple to build trailer with easy to find parts has been a goal of mine for a few years. It had to fit various styles of bikes and carry different loads or containers. This is the result of brainstorming with many like-minded friends. It has lasted through load tests and distance trials. This design has been through two workshops with a community bike shop that I volunteer with, and people with various degrees of skill all managed this build. You will need a hacksaw and a drill with a good collection of drill bits.
Step 1: Selecting the Bike Frame
You will need to cut up a bike frame for the main part of the trailer. Choosing the best frame for the job makes this easier. Choose a steel frame. You need to find a frame with this plate on it. This plate is found where the rear brakes attach. There should also be a plate where the kickstand attaches. You will have to strip the frame of all cables and gears. The seat and pedals are also unnecessary. The part where the pedal arms(cranks) attach is called the bottom bracket, this will become the pivot point of your trailer.
Save the wheels and front fork. You can use any wheel that fits, but I recommend using the same size as your bike uses so you only need to carry one size of tube when you travel.
The front fork is removed and kept. It should fit where the seat tube came out, and the seat post clamp should tighten enough to hold it in place.
There are special tools for working on bikes, I highly recommend finding a community bike shop near you to help with stripping the frame if you have trouble. The crank arms can be especially troublesome without proper tools.
Step 2: Other Hardware
Don't be alarmed. The picture is from the workshop I ran and this shows the hardware for 10-12 trailers.
The door hinge is there to show how they are modified. I cut the pins off with a hacksaw to have access to cheap metal plates that everyone should have laying around. The holes in the hinges become helpful as drill guides in later steps.
The shoulder bolts will be the trickiest part to find. If you live in or near a large city, a bolt or industrial supplier will have them. the third picture shows a close-up of various shoulder bolts.
Step 3: Cutting Up the Frame
Cut the frame with your hacksaw along the lines shown in the first photo. A friend can help here to stabilize while you cut.
Careful when you cut the rear triangle that you cut near the plates, but don't cut the plates off from the rear triangle. These plates are important for joining to the bottom bracket.
Step 4: Bending the Rear Triangle
Once this piece is separated from the frame, it needs to be bent. The photo is the best way to describe this process.
A two by four is wedged into the triangle and held in place while foot pressure is applied on top til the top tubes are parallel to the bottom tubes.
Step 5: Attaching the Parts
This is where it starts to look like a trailer. Stick one end of the bottom bracket in the hole for the kickstand and attach with a nut. Next, take one of the hinge plates and drill it on one hole to fit the other side of the bottom bracket and bolt it on.
The hinge plate should reach the brake plate on the rear triangle. Either drill mounting holes, or match mounting holes, plate to plate. Make sure to use two bolts in the brake plate to eliminate twisting.
As I said before, stick the front fork into the other end of this pipe and you will see the main body of the trailer taking shape.
Step 6: Mounting the Trailer to Your Bike
The large washers are drilled to accept small bolts and springs that will allow you to attach and secure the trailer to the hitch plates. I found the small bolts from U-lock mounting clips to be perfect.
The springs are from brake and gear adjustment screws found on some bikes.
You may need to drill a hole to mount the spring bolt so the washer fits over the head of the shoulder bolt. This is what holds the trailer to the bike without falling off.
Check the pictures, sometimes they are worth thousands of words. The plate held in the fingers in the third picture is drilled to fit your bike's rear axle. If you have quick release, no drilling is required. You can use the existing hole. Just make sure to mount the plate tightly to your bike.
Step 7: Mounting Crossbars(handlebars)
The crossbars are mounted in two different ways. Depending on your use, this can vary. I used a steering stem from a more modern bike at the front of the trailer. Use a steel one, aluminum will break when you try to stretch it over the pipe. Spread open til it just fits over, then squeeze it closed and clamp in place using the screws it came with.
At the back is where I used the hose clamps. In the picture, the clamps go where the zip ties are holding the handlebar on.
The U-bolts are used to fasten your box/crate or platform that you decide to use. I used a bakery tray, used for delivering bread and buns to stores, on one and a storage tote for the other.
Thanks to everyone whose input helped make this build possible. The Wrench, for access to tools, and friendly advice, and Benita for the many great pictures from the workshop.