this is the finished bicycle trailer. I built it principally because i had all the components lying around and needed a bicycle trailer. many substitutions for the different parts are available, so this is more of an overview and general conceptual how-to on building bicycle trailers.
Step 1: Plastic Bin and Crossbeam
I had this large plastic bin that comes with it's own top so the trailer could be enclosed and waterproof. The wooden beam accross the top was because the sides of the plastic weren't strong enough to really hold their shape. conveniently I was able to put rope cleats tapped through the side of the box into this wooden beam so I also end up with a convenient point for tying things on top of the lid of the trailer as well as securing the lid.
Step 2: Chassis Underbelly
the dirty underbelly. this is a good detail shot of the chassis of the trailer. the chassis is made from 8020 extrusions of aluminum. this stuff is robust and re-useable, kind of adult lego, but it is expensive. you could similarly make the same chassis with pieces of timber, or even an old aluminum ladder might work well. nuts inside the extrusion are used to affix things. you can see the cross member plate is affixed in this manner, as are the axles at either side. The axles are 1/2in steel that fit to cheap wheels also bought from mcmaster. they were originally for a front loading tricycle I was building, but i got distracted...
Step 3: Detail of the Axle Wheel Attachment.
this is the hardest bit of a trailer. you need really strong axles if they are going to be cantilevered like this. hardened steel axles greater than 10mm in diameter are best. you can potentially use two old front wheels from bicycles, the difficulty with this option is making the axles work in such a way that you can fasten them either side of your bike. In Guyana i saw a lot that used two old front forks welded to the sides of a shopping cart, and similar such ideas. Anyway, in this one I used the half inch steel pipe through the centre of a half inch bearing 24inch wheel, and merely put shaft collars with locking allen key thingies either side of the hub. the axle was then convenient and easy to stick to the 8020.
Step 4: Chassis to Axle Attachment
here is the detail of the axle to the 8020 attachment. two drilled holes through the axle were simple enough to do a friction fit to the 8020. nice thing about the 8020 is that the extrusion has the slot in it which makes the axles aligned on both sides.
Step 5: Chassis to Bike Connection.
this is how i built the trailer handle. So that the trailer wouldn't sit at a funny angle I needed an s-bend type thingy to raise the height of where it would attach to the bike. I had some funny pieces of wooden hand-railing that i drilled, screwed together, and then bound with waxed string. later this was to prove the weak point. the actual attachment to the bicycle is a pair of old crutches, these can be found anywhere.
Step 6: Handle to Seatpost Connection.
one of the critical points of a bike trailer is a sufficiently flexible attachment to the bicycle. I have always found a bicycle inner tube lashing works well, but here is another technique. I had this old rubber tubing. It fits neatly over the crutches on both sides. there are drilled holes in the crutches so i can remove the tube from one side, wrap around seatpost, then put tube back over crutches. tightening and tying the line that passes through the rubber tube makes a very fast, and very flexible connection that works well accelerating or braking and coreners well with rotational freedom in all axes. the line takes the high tensile loads when starting the bike.
Step 7: Attaching to Seatpost
this is a good detail of how the trailer attaches at the seatpost.