Step 1: Collect Tools
Adjustable wrench (two)
Ratchet with 5/16 socket
Drill with phillips bit
If you have a circular saw, it may be easier than using the hammer and chisel. Eventually I got sick of using the hack saw and borrowed a friend's angle grinder with a cutting disk.
Step 2: Collect Materials
6 each conduit straps, two-hole kind, sized for 1/2 inch conduit
About 19 linear feet light gauge angle stock (find at hardware store or local big box store)
4 washers, 1/2 inside diameter
38 washers, 5/16 inside diameter
8 wood screws, 1 inch long
8 bumper washers that fit the wood screws
24 bolts, 5/16 diameter by 3/4 inch long, hex head
2 bolts, 5/16 diameter by 1 1/2 inch long, hex head
24 nuts, 5/16
24 washers, split ring, to fit 5/16 bolts
3 foot 1/2 inch threaded rod
2 each 20 inch diameter wheels, plastic spokes
3 feet 1x3 nominal lumber
2 feet 2x6 nominal lumber
6 each nylon washer, 1/2 inside diameter, about 1/2 inch deep
12 feet rope, about 1/4 inch diameter
4 machine screws, no. 10 diameter by 1 1/2 inch long, phillips head
4 nuts, no. 10, with nylon locking inserts
1 caster wheel with removable wheel and ball bearings at its base
1 1/4 bolt with pin, to be used as a quick release between tongue and receiver
Step 3: Cut and Assemble Chassis Frame
Cut 2 each 3 foot pieces and 4 each 2 foot pieces of the angle iron. Bolt together using the 5/16 hex bolts with 1 split ring and 1 cut washer each. The frame will have some weakness in twisting, but we'll make it more rigid in the next steps.
Step 4: Assemble the Axle
Cut the 1x3 to about 26 inches long and position in the middle of the chassis frame. There should be about a 1 inch overhang on each side.
Bolt the 1x3 to the frame with a 1 1/2 inch long 5/16 hex bolt. I used a cut washer and a nut with nylon insert, since this is a crucial connection. You'll have to counter sink the hex head so that it doesn't interfere with the axle.
Before attaching the axle, spin a 1/2 inch nut onto the threaded rod first, then two nylon washers (bushings). Spin two more nuts onto the threaded rod and position next to the conduit straps as shown.
Pre-drill through the wood and angle iron and bolt the conduit straps onto the chassis frame.
Step 5: Construct Center Support
Notch out the ends of the 2x6 to accommodate the bolts that are securing the axle assembly. You can use a hammer and chisel and cut away wood until if fits, basically trial and error. I actually used a circular saw and curf-cut the end and bashed out the loose bits.
Secure the 2x6 into place using wood screws and bumper washers. Use four screws each side, two in the ends and two on the bottom. Be careful here, I split the first 2x6 doing this, so I recommend pre-drilling and not torquing it down a lot.
After the 2x6 is secured, the frame becomes much more rigid without any twisting action. The 2x6 also supports the cargo and resists bending of the axle under heavy load.
Step 6: Assemble the Tongue
Once your satisfied with the length and bend angle, cut two pieces of angle iron to reinforce the tongue. The lower one is about 10 1/2 inches and upper one is about 12 inches. Bolt these to the tongue to form a C in section.
Cut two more piece of angle iron, each 7 1/2 inches long. Bolt these to reinforce the bend in the tongue.
Fashion a clip that will reinforce the connection between the tongue and chassis frame. Its made from three pieces, two 2 1/4 inches long and one 1 1/2 inches long. They all bolt together with one bolt, then bolt them to the tongue and frame.
In early models of this trailer, the weakest part was the tongue, which flexed so much during riding that it permanently bent and twisted and caused the trailer to sit crooked. So if there seems like a lot of reinforcing on the tongue, it for good reason.
Step 7: Build the Receiver
Cut a length of angle iron about 4 inches long. Grind or cut down one edge so that its no more than 3/4 inches. Grind or cut the corners of the short leg as well.
Bolt the caster wheel to the angle iron in such as way that will allow you to skewer the angle iron with the wheel axle skewer and tighten down.
Step 8: Build and Attach Coupler
Nest these two pieces together and bolt to the end of the tongue. Jam cut washers between tongue and coupler so that the coupler fits inside the caster wheel receiver, but not tightly, as a little play is good here. This should take about 14 cut washers total, but it depends on the side of your caster wheel and washers.
Step 9: Install Wheels
You could put some thread lock on the outside nut, but with the end cap, its probably not going to work itself off, especially if you tighten it down to the other nut.
Before putting on the end cap, cut the end off the threaded rod leaving about a 1/2 inch extension beyond the outside nut. The cap will cover this.
Don't tighten the nuts onto the washer because it binds up the bearing and the wheel doesn't rotate smoothly. Instead make it snug and tighten the bolts to each other.