I had been wanting to make a cargo trailer, and the time was ripe. On several occasions I had popped down to the shops on my bike and either struggled back heavily laden or had to return in the car to pick up a bulky item. I found no shortage of usable designs on the internet but the raw materials turned up on the side of the street. In my area there is an annual trash amnesty when people leave out large pieces of junk to be collected. This year, amongst other things, I trashpicked a kid's bed frame and a mini BMX-a-like with 20" wheels.
The bed frame had ideal dimensions, about 100 cm long and 60 cm wide. Better yet, it was constructed of steel angle whose flat surfaces would be easy to weld, and came with two additional braces of exactly the same length as the frame that would make ideal internal struts for the wheel dropouts.
First I pried the metal hooks from the corners of the frame and removed the springs and lattice. Then I removed the brackets on the ends of the struts by grinding off the rivets that held them on, and put them to one side for later use.
Using an angle grinder with a cut off disc, I chopped off the rear dropouts from two donor bikes. Both were trashed kid's bikes with 20" wheels. I kept the wheels off the first bike because they were in better shape. (I also kept the rear reflector in case I wanted to attach it to the back of the trailer.)
I cut the dropouts to equal size by bolting them together then cutting their tops off parallel to the slot. At the time I thought this rather ingenious, but If I'd been clever I would have cut one set longer by the thickness of the angle iron in the struts, which sit slightly lower than the rest of the frame. By not doing so the inner dropouts would be slightly lower than the outer ones and the wheels on the trailer would slope inwards. It didn't prove disastrous, but it was something that I could have avoided with a bit of forethought.
Very laboriously, being a novice welder, I stick welded the dropouts halfway along the long side of the frame. I used 1/8" E6013 electrodes which were probably not ideal for the job — 3/32" might have been better. As it was, I blew a few of divots out of the edges of the joins. I tack welded the corners of the dropouts, then welded over the edges using AC at about 125 A. After grinding off rather a lot of low-quality and porous welds, I finally ended up with some decent looking joins, including this one, my first "peeler", which shed its slag without me having to chip it with a welding hammer.
I welded dropouts on the struts equally badly, if not worse. After repeatedly grinding off porous welds, one of the struts had distorted so I ran a bead on the other side of the strut to help straighten things up. To my surprise this fix actually worked rather well.
Fortunately I had lined up the dropouts sufficiently well that I could fit the wheels in. I used both front and back wheels from the same bike. The back wheel was wider so I used this as the template for setting the distance of both struts from the edge of the frame. I clamped the ends of the strut to the frame, tack welded the corners, then welded over the edges. After the practice I'd had welding the dropouts, this step proceeded rather faster and with a bit less grinding. I smoothed up these welds by removing most of the excess with a grinding disc and smoothing off with a coarse flap wheel.
With the struts and wheels in place, this was now starting to resemble a trailer.
The trailer connected to the axle of the rear wheel by means of a curved towing arm connected to the hitch. The curvage allows the rear wheel to turn when taking a corner without bumping into the arm. It was constructed using bits of a kid's trampoline from which I recovered quite a bit of steel tubing. Using flux core welding wire in a MIG welder, I joined scrap tubing to make an elbow which I then attached to the corner of the trailer frame. Here is what the towing arm looked like after grinding the welds flat.
To attach the ball joint, I bashed a ½"-20 fine grade nut into a narrower bit of steel tube from the same trampoline, then welded the tube inside the towing arm good and ugly. (To avoid breathing zinc fumes from the galvanized nut, I used a respirator with particulate filters.) By this stage the neighbours were getting fed up with all the noise I was making with the grinder so I had to finish up.
And I could think of no better way to tart up the trailer body than a "Mexican chrome" job. First I cleaned up any exposed metal with a wire brush drill attachment and applied cold galvanizing spray. Then, after sanding any shiny surfaces, I sprayed quick coats of primer and metallic finish topcoat.
The hitch is the only remotely tricky mechanical part on the trailer. I borrowed the idea of using a quick release ball joint for the hitch, although I used one with ½" thread because for some reason it was cheaper than 3/8". The ball joint can rotate up to 15° in any direction, allowing some wobble, but larger movements cause a disconnection. With a bit of luck, if the trailer should hit the kerb and tip over it won't unseat the rider.
I attached the hitch to the rear axle of the bike by taking an angle-shaped bracket removed from the bed frame, carving a slot to attach to the bike axle, then drilling a hole for the ½"-20 bolt that affixed the ball socket. This looked fine at first, but the bracket was too thin and bendy.
After reinforcing the bracket with a chunky bit of scrap I could no longer tighten the hex nut using a socket wrench, but it was still possible to tighten it using an adjustable spanner, and it no longer deformed when pulling a load.
Now, how to attach the cargo? I had considered building side rails using more angle iron, along the lines of another trailer I had seen, but I wasn't sure whether they were worth the extra weight. The dimensions of the trailer might have been suitable for attaching the basket of shopping chariot — but that might have been a little too trashy for my neighbourhood. Zip tied milk crates, on the other hand, were a tried and trusted local favourite.
In the end I decided just to put the bedsprings and lattice back on the base. The unused spring holes at the sides would serve as useful anchor points for bungee ropes to tie down the load. So what if the trailer looks like a bed frame on wheels. Just call it suspension.
- Scrap metal for trailer frame and tow bar
- Scrap kid's bikes for wheels and dropouts
- ½"-20 quick disconnect ball joint
- ½"-20 bolt and fine grade nut
- Cold galvanizing spray
- Metallic finish topcoat
You will also need
- Welder and electrodes or filler wire
- Welding helmet and gloves
- Chipping hammer
- Wire brush
- Angle grinder
- Metal cutting wheels
- Metal grinding wheel
- Coarse flap wheel
- Ear muffs and protective goggles
- Respirator and particulate filters
Welding can be Dangerous and appropriate safety measures should be taken.
- Protect yourself when welding by using good quality welding helmet and gauntlets. Cover exposed skin with cotton clothing.
- Ear protection is essential when grinding. Protect your eyes from sparks using a face shield or wraparound goggles without vents.
- Remove flammable materials from the vicinity when welding and grinding. Keep a fire extinguisher handy.
Lastly, a reminder that this trailer is not suitable for human cargo.
Bike Shop Hub