This trailer was built over time, with a couple of iterations. A few times in this Instructable, I'll use the phrase, "Ask me how I know..." Most of these lessons were discovered during failures of the trailer on the road... these were cases when "the wheels coming off," is more than a figurative phrase.
Also, I make no claim that my solutions to these problems are the best or only way of solving them. However, in the spirit of Instructables being "open-source hardware," feel free to choose, mix, and match these solutions to your favor.
Step 1: Overview & Anatomy
Main body: the body of the trailer is a plastic bin with a lid; I used an Akro-Mils Nest & Stack Tote (NST) Model 35300 (29 1/2 x 19 1/2 x15)--I bought mine at Grainger. I like it because it is pretty intrinsically strong and stiff. However, I think this plan can be adapted to most plastic bins.
Wheel mounts: the wheels are mounted to metal brackets on the sides of the container. They are adapted from brackets from Home Depot, with some cutting and drilling.
Internal Structure: the plastic bin is not strong enough to support the wheels (ask me how I know), so an internal frame is needed to hold everything together. It connects the wheels to the bin, and the bin to the trailer connection.
Hitch (trailer side): to connect to the rear wheel hub, the hitch needs to take a right angle and come in at roughly 45 degrees to the axis of travel. Therefore, when you make a turn in one direction, the hitch comes in roughly parallel to the rear wheel. There is a flexible coupling (rubber tubing) to allow turning. The connection to the rear wheel is made using an air hose quick-connect.
Hitch (bicycle side): the connection to the rear wheel to the trailer is made out of a metal bracket with an angle that matches the hitch, which remains semi-permanently in place on the bicycle (you can remove it, but you will need a wrench). It doesn't add much weight, so I leave it on all the time.
Step 2: Wheel Mounts
The wheels are mountain bike wheels and tires that I got from a University Bike Purge--basically, the abandoned partial bikes all get thrown in a dumpster. These were two roughly matched front wheels that were not completely taco'd.
These mounts are well suited because they raise the wheel axle relative to the tub, and thus lower its center of gravity. If you can get smaller wheels, it is probably not as big of a deal. Even with this setup, if you take a corner too quickly/sharply, you can flip the trailer. Ask me how I know (although I managed not to break the bottle of single malt I had inside).
The mounts are made out of a Stanley "Door Stop" (see photo of bin). From the original state, you cut off the upper "wing", and drill a hole to match the axle size. You might ask why I didn't tap this hole as well--the reason is that bicycle axles seem to be some oddball thread that I couldn't find a tap for.
When you mount the wheel to the hole, you will need to add a pair of lock washers (I used split lock washers; I guess tooth lock washers should work too). Since these wheels are right-hand threaded, when you're rolling down the road, one side is going to try to loosen and fall off the trailer. Ask me how I know. Nylon lock nuts (nylocs) are not an option--again, the odd threads of the axle.
Also, you'll want to tighten these wheels down with a fair amount of torque, given the cantilever. You might need to hold the cone nut (using a cone wrench--a bicycle tool) to avoid overtightening the bearings (freezing up the wheel). And if you get a cone wrench, you might want to go ahead and rebuild/regrease the bearings. Note that I only used a thin hex nut before attaching the wheel to the bracket, slightly reducing the cantilever.
Step 3: Internal Frame
My original solution was to use a 2x4 to connect the wheels, with the wheel brackets lag bolted into the end grain of the wood. This might result in the brackets loosening, and the wheels rubbing against the top of the tub and/or falling off the trailer. Ask me how I know. FYI, putting screws into the end grain of wood is a very weak connection. I tried improving it by gluing in dowels at right angles to the screws, to give a stronger connection, but that didn't work either.
So the working solution was to use EMT (electrical metallic thinwall) electrical conduit and threaded rods, to form a "sandwich" with the wheel brackets at the ends. I used 1" nominal conduit. To match the angled sides of the plastic tub, I needed to cut angled ends to the tubing. I used a pipe cutter and angle grinder, but there are other options for sure (metal cutting bandsaw, hacksaw if you want a workout, etc.) In the future, I would change the angles slightly, so that the wheels are canted inwards at the top (like racing wheelchairs) slightly, to increase stability.
The lower EMT conduit is bolted to the plywood, through the plastic floor of the tub. The upper EMT is connected to the lower one using hose clamps. This results in a strong structure--probably not the lightest solution, but it seems to work so far.
Step 4: Hitch (trailer Side)
The basic structure of the hitch is 1/2" EMT electrical conduit; you need to make three bends. Find or borrow an EMT bender to do this--I have doubts whether you could make these bends work by hand. First a right angle at the bottom, to "hook" the connection into the structure. Then a 45 degree bend, to bring it off at an angle, and another 90, to come back to the wheel.
Then there's the complication of the flexible coupling connection. Solution #1 was to use a flexible electrical conduit with plastic threaded ends. However, this solution might result in your trailer continuing down the street in your original travel direction after you make a turn. Ask me how I know. So solution #2, from the conduit to the wheel, is made out of:
- 1/2" EMT weathertight coupling (it's a compression fitting, so when you crunch it down, it will stay attached better than a standard setscrew EMT coupling)
- 1/2" NPT galvanized coupling
- 1/2" NPT male thread x hose barb fitting
- rubber tubing, with fiber reinforcement; attached with hose clamps
- 1/2" NPT male thread x hose barb fitting
- 1/2" NPT galvanized coupling
- 1/2" NPT x 3/8" NPT bushing (size reducer)
- 3/8" NPT air hose quick connect (the heavy part, with the hardware)
However, after taking this out for a ride, I found a few problems. The hose is flexible enough that if the trailer is loaded, the connection sags at the front. Also, you get a lot of push-pull action when you're pedaling down the street--it surges back and forth, giving you the feeling of being a puppet and puppetmaster at the same time.
To fix this, I added a coil spring wrapped around the rubber tube, to stiffen it up. This required some disassembly, gloves, and an ample supply of foul language. It was a tight fit, so I ended up sticking the tube in the freezer, to make it easier to ram the pieces together. It's not a great solution, but it definitely reduces the two problems mentioned above.
Step 5: Hitch (bicycle Side)
The first connection strategy was to use the existing hole, and connect it to the quick-connect on the rear wheel. However, that didn't work: the weight of the trailer pulls down on the hitch, and it rotates downward. Grr.
Therefore, I added a bolt and additional hole to stop the rotation; had to drill another hole for the quick release skewer at the axle. You have to line everything up first to make it, and then drill the hole. A quick release skewer only needs a small hole (~1/4" or less).
Step 6: Finishing Touches / Using the Trailer
I'd recommend a few safety measures--such as a blinking red LED, or reflectors, or both. Incidentally, it seems like most bike stores have a surplus of reflectors (they are usually removed if you're installing a rear light), so just go in and ask--I got a few of them for free.
If I wanted to be extra safe, I'd get a trailer flag (so motorists can see there's something behind you), but I haven't bothered yet.
Pulling a trailer behind you feels a bit weird--it still does "surge" (push and pull, with your pedaling), but you get used to it. This would be solved by a more rigid connection (e.g., a universal joint). Also, you can't take corners nearly as fast, and going uphill is more of a pain (depending on your load).
Also, it doesn't track terribly well--i.e., it "waggles" as I head down the road a little bit. I'm not sure if this could be helped by truing up the wheels or something. Based on random web reading, it seems like the longer the distance between the axle and the tow point, the more stable it will be... but I'm not planning on changing that.
One nice effect of the air hose quick connect is that there's a rotational degree of freedom. Therefore, you can set your bicycle down on its side, or lean it up against something, without flipping the trailer. Also, I think it makes it corner a little more nicely.
With this trailer, I've managed to reduce my car use a fair amount--grocery runs for moderately large things aren't a problem. However, Home Depot lumber runs probably aren't in the works. It's also neat because (at least here in People's Republic of Cambridge) people sometimes smile and wave when they see a homemade trailer like this passing them.
One last step I'll need to complete--putting an Instructable sticker on it, so people know how to make these things!