Have multiple applications you want a single project to serve? Here is an example of a single project designed to accommodate two uses.
I had many sources for inspiration including one of the instructables' examples that uses a plastic bin, hose, spring and a quick connector. The first trailer I built went for minimal parts count using a piece of one half inch thick sheet of scrape plywood u-bolted to a seven sixteenths diameter axle and tires from a stroller. The hitch was made from a piece of bicycle tire wrapped around the rear basket handle and bolted onto the top and bottom of the end of a piece of conduit, that was u-bolted to the plywood. (see last picture below)
For the second trailer I also used a one half inch thick piece of plywood (three quarter inch next time) u-bolted to a one half inch axle and tires from a wood carrier cart, and a two inch caster wheel.
My original plan was to bolt the conduit to the metal frame running perpendicular to the axle (see first picture of last step) but I decided that I wanted only the trailer axle to carry 95% of the load. Put another way... I decided not to build a semi-trailer where a large portion of the load is carried by the rear tire of the bicycle. I used the half-inch plywood (new) to start with because it was handy, only needed a trim and was light.
The u-bolt arrangement permits the plywood to be leveled without bending the conduit permanently. The first piece I used had enough contact area together with a slight offset angle to keep it from slipping when the u-bolts were tight. I put a carriage bolt through the second piece of conduit (which had a straight end to fit through the PVC pipe) because it was too short to provide enough contact area with the plywood. Even though attached in a small area I expect the conduit will bend before the plywood will break, especially when I replace it with three quarter inch sheet. If the conduit bends or the plywood breaks from too much weight then I have not done my job in balancing or securing the load.
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Step 1: Two Out of Three Ain't Bad...
I try to avoid using the dropout with any carrier/trailer project since wheel realignment can sometimes be a pain. Instead I use the fender mount bolt holes above the dropout to keep axle and carrier adjustments and maintenance separate.
In most cases even If I use the dropout hole an adapter is required to match the frame bolt holes with the holes needed for the carrier. In this trailer project I decided to use a 2 inch diameter caster wheel to keep the cost very low since I found a good one at the flea market for a dollar and a caster wheel provides two of the three required axis of rotation.
To mount the caster wheel to the frame I thought I would need to make an adapter but then the idea hit me that all I really needed was to cut parallel flats on the caster wheel and drill holes to match the fender mounts.
This initial design resulted in a hitch design that worked only if I did not stray from level ground, rest the bike on the kick stand or let it fall over. It soon dawned on me that a good trailer hitch design required provision for all three axis consisting of yaw, pitch and roll. Since I live in a hilly area with many bumps, gulleys and inclines this setup, which lacks roll, was not good enough to get me past the driveway so I started thinking about a way to add roll at very low cost..
Step 2: Adding Roll
The first trailer hitch redesign borrowed from the original hitch design by again using a folded piece of tire bolted to the caster wheel base plate to form a loop in which to insert and hold the conduit tubing. This worked very well to provide the trailer with roll and I did not need to go further, but went ahead and replaced the piece of tire with a piece of PVC. Note that the one inch PVC has to be notched for the three quarter inch u-bolt to fit. Either configuration depends upon the materials at hand. Even other materials like upholstery strap might do or a washer with a large enough hole for the conduit to slide through, just the u-bolts. To be honest I liked the feel and fit of the piece of tire much better and will probably use it again. If I do I'll add it to this instructable.
I used a couple of u-bolts to secure a piece of one inch PVC pipe to the caster wheel base plate through which I ran the end of the three quarter inch piece of conduit tubing connected to the trailer. Bolts are run through the conduit at each end of the PVC (or loop of a piece of tire) to keep the conduit from sliding back and forth in the tire or PVC. I used wing nuts and washers to secure the bolts which did not have full threads. To keep the bolts and washers from rattling I added a piece of bungee cord.
Update: Since my next project incorporated a hose quick connector I decided to modify this project (at the expense of loosing my fishing rod holder) to incorporate a quick connector too. If you need more pictures and description than the picture shown to make the modification just ask for more pictures and I will provide more pictures and a full description too.
Step 3: Other Uses...
Sometimes I like to head off without the trailer and go fishing since I live a short distance from the river. The PVC makes a very nice holder for my fishing rod and may have some other uses.
Step 4: Total Cost:
1- 2" caster wheel ($1.00)
2- 3/4" u-bolts ($0.88)
1- 3/4" x 10' conduit ($1.49)
1- 1" x 3" PVC (Free, had some left over from galvanized water supply pipe replacement.)
(optional: use piece of bicycle tire from six inches to nine inches long folded and bolted to the caster wheel base plate with a one inch loop to accept the conduit like the PVC - also free from a discarded tire that had a bad bead.)
2- fender mount machine screws and nuts 1/4"-32 x 1.5" ($0.37)
2- 1/4"-20 x 2" hex head bolts, washers and wing nuts ($0.76)
Brain power and mechanical engineering and assembly skills, 1 can of sweetened condensed milk ($0.93)
Grand total: $6.31
(This does not include the conduit-to-plywood mounting hardware which consisted of 2- 3/4" u-bolts and a 1/4"-20 x 2" hex head bolt not shown. Note: this u-bolt arrangement is used to level the trailer after which a hole for a hex head or carriage bolt can be drilled through the conduit and plywood. )