I recently took a trailer building class from the UC Davis Craft Center. We built a bicycle trailer out of 1/2" electical coinduit pipe. Based off the "flatsy" trailer from Community Bike Cart Design. Each of us did things a little bit differently, here is how I built mine.
You should print the pedal people plans and use mine as a suppliment. I have not included every single detail, and I modified some measurements.
I haven't brought the parts list all together yet. This is off the top of my head. See the text for more details.
The basic frame is a 10' piece of conduit bent into a square.
We were using a bender for 3/4" pipe to bend 1/2". I don't know if that makes a larger radius, but we had to experiment with the measurements to wind up with about 1.5" of overlap where the pulling arm attaches..
Use a marker and write "A" on one end and "B" on the other end.
From the "A" end and place a mark at:
From the "B" end mark at:
Begin a 90 degree bend at the 22", 54", 80" marks. The "B" end should ultimately meet up with the "A" end at around the 1.5" mark. Have a friend help you eyeball the alignment so you can keep all the bends in the same plane.
In this picture "A" is at the top and the bends proceed clockwise to the "B" end. Grind or file a fishmouth mitre into the "B" end and braze it to the "A" end.
Cut 2 pieces of pipe approximately 34" long for wheel wells and fit them approximately 4.5" in from the sides of the frame. The piece on the pulling arm side is likely 34.25". If you attach the dropouts, created in the next step, to your wheels you can test fit before brazing the wheel wells in place. I used 20" front wheels from BMX bikes. There will be room for adjustment in the placement of the dropouts later. Clamp and braze.
The dropouts were made from 3" pieces of 2"x2" angled steel. Begin by drilling a 3/8" hole and then draw 45 degree lines from the sides of the hole to the edge of the piece. This is the slot the axle will go into. The dropout will also serve as a shelf to support the cargo floor. Angle the slots so that as the trailer moves forward the axle will naturally press up and into the dropouts.
Create a "jig" from a 3' length of 3/8" threaded rod, 8 large washers, 4 nuts, and 4 wing nuts. Line up the dropouts with the frame and adjust the nuts to tightly hold the dropouts at the proper distance apart.
Position the jig approximately 14" from rear and clamp it in place and braze. Note that in my case the frame became soft and bent somewhat. It wasn't a real problem, but it might be a good idea to jig up the frame somehow to avoid this deformation.
The pulling arm will be two pieces of unequal length (29" and approximately 50") stacked upon each other.
Cut a piece 29" long and mark it at 11". This piece will be the top half of the arm. Place the frame on the ground and butt the 29" piece up to the "A" end of the frame.
Measure from the dropout to the end of this loose piece. It should be approximately 50". Cut a piece that long or a little longer, and mark it at 11".
Begin a bend of approximately 45 degrees at the 11" mark of both pieces. Stack them up and tweak things until they line up all the way from the dropout to the tip. Use a vice to flatten the end nearest the dropout so it may be brazed to it. Clamp it all carefully together, and braze.
Cut an 18" piece and mitre it to fit beween the frame and the top tube of the pulling arm at 45 degrees and braze it in.
Shape a small piece of flat metal to cover the end of the pulling arm for added strength and weather resistance. I split a 1" piece of conduit and flattened it. I then cut it to just cover the end of the pulling arm and used tape to hold in in place long enough to tack braze it on. File the edges smooth once completely brazed.
The fenders will help to protect the cargo from the wheels and will allow for oversized cargo to be stacked on top.
Cut two pieces at approximately 52.5" and mark them at:
Begin bends at the marks to create fenders which will braze at the wheel well joints. Once bent you may have to adjust the bends so that the ends meet up right at the end of the wheel well tubes. Carefully clamp them in place, keeping them vertical, and braze. They'll be about 11" to 12" high.
Cut 2 pieces to horizontally span the fenders at approximately 6" above the frame, just where the bend in the fender begins. These pieces will be approximately 18" long. Clamp and braze.
Optional horizontal bars approximately 34" long in the fenders running front to back protect the cargo from the wheels.
For my second trailer build, I've decided to add side panels so I don't have to strap down everything I carry. I'm using 3/4" x #13 expanded sheet metal from Metal Craft Warehouse in West Sacramento. I bought a 3' x 4' piece, which is more than enough, for about $23. Perhaps you could use plastic or tarp for this to save weight, but the metal is not very heavy.
The sides are basically 7" x 35" and front and back are 7" x 19". Measure and cut with an angle grinder.
Point the smoothest edges towards the top of the trailer, and be sure to use gloves. This stuff is very sharp once cut!
I use a 3/8" pneumatic coupler for quick (un)hitching. It's not the cheapest way. The female coupling is about $10, but the other parts are cheap. It makes it quick and easy to move a trailer to another bike if you have a male hitch on each.
A piece of angled metal (garage door bracket, frame bracket) is attached to the bike quick release with supplemental bolts into the frame where a bike rack would attach. The other end of the bracket has a 5/8" large hole with a 3/8" male pneumatic coupler permanently held in place by a large galvonized plumbing nut. The nut will be labled as size 3/8" but that's only the inside. The hardest part is making a 5/8" hole since normal drill bits only go up to 1/2". Get creative.
The end which stays permanently attached to the trailer is composed two 1/4" eye bolts looped together with a skateboard grommet inserted to take up any slack. Drive a flat head screwdriver into the joint where the loop meets the shaft to open up the loop. Use a vice to help squeeze the grommet onto one bolt, slip the other eye bolt on, and compress the loop closed again with the vice.
Insert the threaded end of one eye bolt through a male pneumatic coupling and tighten a 1/4" nut on the other end. This male coupling screws into the back of a female pneumatic coupling. They attach rear end to rear end. The nut you just attached will be hidden inside between the two couplings.
Drill a 1/4" vertical hole about 1" back from the tip of the pulling arm through both pieces, and insert the free eye bolt and use two nuts on the other side to help keep them tight.
I used 1/8" x 1.25" steel to make corner brackets to attach the plywood floor to. Start by marking a 45 degree angle on the corner and begin another 6" from the point. Keep going until all 4 are marked out, and make a spot in the center of each piece and drill a 1/4" hole. It is safer to drill each hole before beginning to cut the pieces apart.
Clamp the brackets such that the ends fall on center of the frame tubes and braze in place.
Drop a piece of 1/2" plywood inside the frame. It should be approximately 18" x 34". Through the holes in the brackets, mark the plywood. Then the board and drill 1/4" holes through it. These are to hold T-nuts which you'll drive into the wood from the top.
You can use simple 1/4" x 3/4" bolts or, as I have now done, you can use 1/4" x 3" eye bolts with lock washers and nylon locking nuts. The eye bolts make nice points to attach bungee cords or cargo straps.
Thread a nut all the way up the eyebolt followed by a washer, then thread the bolt into the T-nut from the top down through the corner bracket. Add a lock washer and a locking nut to the end of the bolt protuding from the bottom of the trailer.
See it in action.