I wanted a bike trailer that I can leave out in the weather without rotting, and without trapping water to breed mosquitoes. Figured a shopping cart would make a good body of the trailer. There are many shopping carts around, but just taking one that is in good shape would seem like stealing, and I never got around to investigating where to legally acquire an old one. I finally found this one all smashed up next to a dumpster - looks like a car just t-boned it.

Step 1: Parts

Parts
- one busted up shopping cart
- two old bicycle forks and fitting wheels
mine were mountain bike ones, BMX ones would have been better because they are smaller and more solid. I found them in the garbage somewhere.
- primer and paint
- matching male/female air hose fittings (there are different types)
- material for the tongue and bike hitch - I used an old metal pipe that came from a tent or something, wire, scrap metal

Tools
- welder
- angle grinder with cutting wheel and wire brush (you can use a manual or drill-type brush, but BOY are the grinder ones faster and more powerful!)
- bottle jack and waste lumber to straighen out cart
- several clamps or locking pliers for set-up

Step 2: Straighen Out the Cart

Use lumber to fill the space, and use the jack to bend the cart. Careful not to get yourself in the way if this slips. That said, the jack is very controllable, so if you hold it in place you should be able to feel if anything starts shifting.
Some bending worked with the jack, but to get anything that was too far out back in, the only thing I could figure was a maul. Good to get out the frustrations.
Thing wouldn't bounce but not bend when I just jumped on it.

Step 3: Attach the Wheels and Front Leg

Temporarily clamp on the forks with the wheels installed to figure out where the cart would be balanced so that some weight remains on the front end where it will hook up.

I made a front leg for the cart so it can stand on its own, e.g. if you want to take it into the gocery store (an exercise in social engineering I have not tried yet). The leg has to be in the right place to balance, yet to not interfere with the bike. I initially wanted to attach one of the swivel wheels salvaged from the original undecarriage, but it didn't work very well, so I scrapped the idea and am keeping them wheels for future projects.

Keep in mind you need a strong part of the cart to weld to, so you might be constrained by that. If this is a problem you could weld a different piece of scrap metal to a strong piece of the cart and then to the fork.
Mark the spot, weld on the forks.

Fix welds on the cart that may have come loose in the straightening process.

Step 4: Make the "tongue"

I used a metal pipe with an air hose fitting (some other instructable inspired that) to hook the trailer to the bike.
The other end of the air hose is attached to the bike near the rear wheel dropout, but not on it. I didn't want to put that kind of force on the axle and risk bending it and having my wheel no longer spin smoothly. On one bike I just used wire to attach it (holding so far), on the other I made a more elaborate bracket out of scrap metal.

I initially figured it would be easier to hook the trailer up to the back of the seat post, but this led to a really bouncy uncontrolled ride when I tried it. So, I went with the arm-around-the-side most bike trailers use now. Luckily I had just clamped it for the test.

Test the setup as much as possible while it is only clamped on, especially any part you are going to weld.

Once you are pretty sure all is lined up the way you want it, weld it.

Once all is welded, test it with some stuff in the cart. Make it heavy, go over some potholes. It's better to have welds fail now (yours or pre-existing ones on the cart) than when you are out there hauling real loot, after you have done all the painting. Does that sound like I am speaking from experience? You betcha...

Step 5: Paint

Since I want this thing to be sitting outside, it will need some rust protection. The cart itself will probably be OK, but I wanted to protect the welds and at least some of the thinner parts.

Clean off the flux and oxide from the weld using the wire brush on the grinder. Make sure you wear goggles or a face shield and nicely covering clothes - things WILL spark and stuff WILL get flung off fast.

Prime - I used sprayed primer, it seems to dry faster.

Paint - I used hammer paint, it nicely hides dings and imperfections. Not that it matters so much here... this vehicle will not be pretty.

Step 6: Go Out for Garbage Eve and Bring Home Lotsa Loot

Or help a friend move.

Or do whatever you would with a trailer.

My largest haul so far was a particularly successful garbage eve trip that netted an old electric lawn mower plus a regular and an olympic weight lifting bar (apparently the latter is 50lb). The biggest problem was the length of the olympic bar with the weight near the ends, but it worked not too badly, and the cart survived.