Step 1: Start at the Inner (wheel) Supports
Obtain two 8' lengths of 1x3 and 3 8' lengths of 1x2 pine (extra is okay - it's cheap). Optional: stain them with polyurethane (clear or stain) before even making the first cut (seal cuts at the end or as you go). Note from the intro pic that I also included an aluminum rectangle in the frame - near the lip of the tub - it was probably unnecessary, but I'll include it in the instructions (later).
Note that I'll show the materials used as I go along, discussing the materials in the same step as they are used.
Find the two wheels at the local used bike shop - or use some wheels from your kids bikes -that they never use.
Find a big, solid, rectangular plastic tub, about 10" deep, about 2' wide and about 2.5' long. Exact dimensions dont' matter. If it has a stiff outer lip, then you can make it easily removable, like mine is.
Find a Simpson Strongtie-brand (common) 90 degree angle bracket for 2x4s (3.5" wide, 1" depths), a 1/8th inch thick length of steel (about 1" wide, 4"long). I never used the shelf brackets in the pic - but would be helpful to stiffen a floppy plastic tub.
I generally only use stainless steel, #6-32 size bolts and nylon lock nuts - and washers of course. 1-1/4" length bolts worked great for attaching brackets to 1x2 and 1x3 wood. 2"+length bolts are needed to attaceh two pieces of wood together. I think I got them from boltdepot online. Various woodscrews are/can be used in place of bolts. I like bolts since the wood doesn't tend to split.
The idea in the first step is to make these two wood pieces longer than necessary, based on the wheel size you use. I used the weaker/narrower 1x2 wood for the inner supports - because I reinforced it with my (overkill aliuminum rectangle). The wheel well will be about 28" long (for 26" mtb wheels), so cut the inner supports to be 36" long. 2" of length will overlap two 1x3 cross pieces to form a joint in a future step. The remaining 2" (36"-28" - 4" = 4" total) or so on each end sticks out, but looks cool, because it looks like a log cabin. The pictures here don't show the cut wood, just two random lengths of both 1x2 and 1x3. In the middle of these supports will be metal brackets where the wheel attaches. Make these parts next below.
My tub isn't as long as the wheel well needs to be, so I put the center of the tub a bit forward relative to the axles - which is best for bike trailers in terms of weight placement. The amount that the tub is off center isn't important. If your tub is the same, then just be careful to center the axles on the supports, not in the center of the tub (as is done this step below).
Cut about a 3" length of 1" wide, 1/8" steel and bolt it to a Simpson strong tie 90 degree bracket as in the picture. The bracket provides the angle strength and the thick steel is a strong mount surface, i.e. a beefier flat to which to clamp the wheel (with quick release). Leave room for the quick release hardware on the wheel. Drill a 23/64" hole, into which fits the typical mtb axle. This hole is roughly centered in the bracket, centered about 3/8" from the edge (keeping it exactly 3/8" is not critical, but keeing it consistant is helpful for step 4). Cut slots from the hole to the edge (so you can get the wheel on/off). More pictures show various stages. I used a hacksaw to cut the slots. I added epoxy at all joints again. I also painted the final parts. Attach this bracket to the center of the support. Repeat the process for the other support to get left and right completed inner supports.
Bolt this beefy bracket onto the center of the 1x2 wood (36" long). The slot should open towards the ground. I also attached it such that the 2" of the 1x2 was horizontal. If you do it differently, just be sure to line up (correctly) the height of the outer support later.
Step 2: Optional Aluminum Rectangular Support
Step 3: Outer Supports
Regardless of whether you made the aluminum support, you can now make the outer supports. Cut two 1x3s at 36"long (same as inner supports). These supports will run such that the 3" runs vertically, like the beam in a house.
Get a 10 to 12" long, 1" wide, 1/8" thick flat steel Simpson strong tie. Cut it roughly in half, using each half as a bracket for the outer wheels (lever side of the wheel quick releases)). No need for 90 degree brackets here. The picture here shows the cut lines. As with the inner support bracket, drill a 23/64" hole and hacksaw slots for the axles. Drill the center of this hole 3/8" from the edget of the steel (keeping it exactly 3/8" is not critical, but keeing it consistant is helpful for step 4). Temporarily locate these pieces on the center of the outer support wood pieces. Locate the bracket bottom parallel with the bottom of the wood beam. This means the axle will have to go through the wood. So mark the location of the axle and then hollow out a notch big enough for the quick release lever to still operate. The bracket will be long enough to support the wood from any notch you cut in the wood so dont' worry about the wood snapping. You should only have to cut up to a 1" notch into the 3" width of the wood.
Step 4: Cross Supports and Wood Frame Finish
Note that if you didn't make an aluminum rectangle, you should now attach the inner supports to the tub with either temporary or permanent woodscrews. Drill them in from the inside of the tube out into the inner supports. Keep the innner supports level with respect to the tub. Note the the tube won't be easily reomveable (of course) if you permanently screw it on now. This isn't necessary with the aluminum rectangle, since it is already a tight fit to keep the inner supports in place (quick clamps were used to hold the frame to the tub).
Put a wheel on as in the picture here. Measure how far away from the inner bracket that you need to locate the outer bracket. This distance should be just the width of the wheel hub. I think I got the final product to within 1/16" of an inch or less, but the 1x3 outer support should flex a little if you are a little off. Cut a new piece of 1x3 to be the front crosspiece. Cut is to such a length that is extends past the outer bracket (on both L and R sides) by at least an inch or two (room for error). Center this cross piece exactly with the centerline of the tub and then mark the location of the intersection of the inner support with the crosspiece as in the picture here (clamps help).
Attach the front crosspiece to the two inner supports using 2" angle brackets as in the pictures. Either bolts or screws will work. Epoxy or polyurethane in every hole (after predrilling, before screwing/bolting) keeps the wood from rotting.
Repeat this for a rear crosspiece. The rear crosspiece will not be against the side of your tub if your tub is shorter than your wheel well length. The aluminum rectangle provides extra support in this area.
Now to attach the outer support to the crosspieces. Use your earlier measurements (earlier this step) to locate the exact position of the outer support along the crosspieces, such that the wheel will fit nicely onth othe brackets. Keep in mind that the outer bracket itself has about 1/8" thickness, which should be accounted for here, since it rests on the surface of the outer support. Alternately you can just put the wheel on the outer support and inner bracket and then mark the location where the outer support should intersect the cross pieces.
Whatever method used, mark the location where they intersect - with an rectangular "x". Next, a notch is cut into the outer support. The crosspieces are left intact. The depth of this notch is important. Set the depth so that the inner and outer brackets line up with each other. That is, so that the 23/64" hole/slot line up and the wheel sits on the frame vertically - not skewed or cambered. This number will no doubt vary, depending on how consistant the other holes were drilled in the previous steps. With the wood I used, the slot was about 1-1/4" deep (with a width of the wood that fits into it).
Use a drill to take away wood in the deep part of the slot. Use hand saw or miter box to cut the slots. Use epoxy to glue the joints - or polyurethane glue. Reinforce the joints with some angle brackets, L-brackets and screws and/or bolts. The picture here shows the completed frem. In addition, it shows the tow arm connected. I wanted the metal (white) hitch arm to be low, so I added another 1x3 vertically to the fraem as in this picture and connected the bar to it. This 1x3 would not be needed for a larger bike, such as a 26" bike. Also, I tend to make the hitches come off my bicycles low, so the hitch arms tend to be low on my trailers.
Step 5: Hitch Arm and Hitch on Bicycle
I located the moving parts of the hitch back on the bike so that it was near the back of the rear wheel. This way, less severe bend is needed on the hitch arm.
I extended the trailer arm so that it's end was about 12-14" in front of the trailer. This allows the trailer to turn without bumping ont he bike - with plenty of slack for carrying some slightly long things in the trailer.
The aluminum square of the hitch arm of the trailer attaches to a 1/4" thick piece of weldable steel (via a pin) that was bent into a 90 little stub (3-4" long by 1-2" long). This "stub" can move up and down on a steel hinge (brass coated only). The stub is loosely connected to the hinge, providing rotational freedom. With two pins and hinge, this acts as a sort of U-joint, but, I feel, just a strong or stronger and less prone to binding up than a real U-joint. ihave the exact system on my other (large, heavy) trailer -also an instructable.
The bike hitch attches to the bike frame via a collection of brackets which are connected to two long, flat pieces of steel. One was another simpson strong tie. I used thick rubber tape to protect the frame and then used four hose clamps to secure it to the bike. I had to cut out a few holes to make way for existing bolts for the rear rack. I drilled slots in the steel flate pieces into which I wove the hose clamps - this was done to keep them in place and provide a better grip with the frame.
One pin acts as a quick release for the trailer. When no trailer is attached, the bike can keep the hitch on without problems. The unloaded hitch only has moving parts past the brass-coated hinge (so only the stub moves around). An improvement would be to put the quick release at the hinge/stub junction - a shorter hitch pin.
Step 6: Use It
You're now ready to take your (willing?) dog for spin in your new bike trailer. Poor Snowy (wink)... I mean Ebony.