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Small Shopping Trailer

the completed trailer
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Completed: July 2001. Added rigid hitch Sept 2005
In July 2006 I got 10 of these built as "MassTrailers".
This page contains guidelines for building a simple two wheel bike trailer. The one shown is brazed except for the joint between the trailer and the towing arm, which is welded. For simplicity it's better to cut one long piece that extends forward and bends to make the arm, as a single bit of metal. As I did with megatrailer. After I took these photos I cut the arm off and added two bits of sleeving with a removable pin to hold the arm in place, which lets me remove the arm and replace it pointing up and forward to make it easier to tow the trailer by hand. Some time later I put the strips in to hold a bin or crate in place, making it even more a shopping trailer.

Read these instructions the whole way through before you start. In fact, if you weren't going to do that anyway, I suggest not trying to build the trailer. Too much of what gets done requires thinking about later steps to let you plod through one bit at a time. That's the price you pay for generic instructions. Obligatory disclaimer: I am not qualified to instruct you in anything like this, and don't even know enough to know whether this is a sensible design. And I don't know you at all. So you can't blame me if you damage something while trying to follow this guide.

Materials needed:

  • about 6m of metal tube, ideally square section to make everything easier. Galvanised tube makes rust less likely but is irritating to clean before brazing (or toxic if you don't clean it off). I'd rather clean the gal off than clean and paint the whole trailer.
  • A 5-10cm long compression spring and a slightly longer length of chain that fits inside it, as well as inside the tube above. A small bolt to hold the chain inside the tube.
  • A small metal plate, about 40x20mm and 3-5mm thick.
  • Two bike wheels, ideally 12-16 inches in diameter.
  • Dropouts to fit these.
  • Metal strip or rod, 2-3mm thick, to be the "bed" of the trailer.
I used an old child's bike, as they're easy to find and usually cheap or free. You'll also need some sort of metal joining equipment - brazing or welding will be easiest, but you could probably bolt or rivet this together with a bit of extra hassle.
parts before starting
The bits, laid out as they will go together.
Start by cutting the metal up. You need two pieces the width of the trailer, three pieces the length of the trailer bed, and one that is about 500mm longer than that (this becomes the left hand side of the trailer and the arm, instead of the two bits that I show). Notch the long piece 300mm from one end and bend it to about 45 degrees, as in the picture. The front of the bent part needs to be 300mm from the cross piece at the front of the trailer, and about 100mm left of centre (to allow for the width of the bike and the spring). It helps if you also bend the arm up slightly to compensate for the difference in wheel size (the trailer will sit about 200mm off the ground with 12" wheels, but the axle of a 26" MTB is about 300mm from the ground)
main frame
The main frame
The first bit to be assembled is the outside frame. I clamp the shorter piece (not the bent arm) in the vise and get the front and back ends square, then put the arm on top. It's not essential that the trailer be square, or even that the sides be parallel, but it's nicer that way. If you're a long way out it might be impossible to get the wheels parallel later, or to get a snug-fitting box to sit in the trailer.
adding wheels
Adding wheels
I start by attaching two dropouts (not matching ones) in the middle of the frame that is in place, then putting the wheels in them to set the position of the inner frame pieces. This ensures that the wheels will actually go into the trailer once it is built.

Attach the dropouts so that you have one set with a horizontal slot (normally the ones off the rear wheel), and the opening of the slot at the front end. This means that the wheel will not fall out even if the nuts come undone or the axle breaks. The other dropouts can have the slot vertical, and if there is a wheel retention device (lawyer lugs) leave it in place unless you intend to take the wheels off the trailer frequently. Remember to leave enough of the dropout in place to allow you to tighten the nuts once the wheels are in place. I try to get at least 15mm between the trailer and the axle, and 25mm would be as much as I would be happy with as there will be sideways forces on the dropout.

adding wheels
Wheels 2
Once the wheels are in, put the inside dropouts in place then add the inside pieces and adjust them until all the joints can be made easily. Getting the two pieces on each side parallel is easy, just measure the gap at each end. This is important for the horizontal slot dropouts, as you will get the wheels parallel by moving the wheel in them.
the bike hitch
The hitch
The bike hitch is simple: a plate with a hole in it, which goes under the bike axle nut next to the drop-out. Cut the plate to approx the diameter of the spring plus 5mm one way, and 30mm the other. For a 25mm diameter spring this means a plate about 30mm by 55mm. Cut one link of the chain in half, and drill two small holes in the one end of the plate to fit the legs of the cut link through. Attach the chain to the plate as shown.
hitch again
The hitch 2
Drill a 10mm hole 10mm from the other end of the plate. Now thread the chain through the spring as shown. The next step is easier if you use tie wire to hold the spring compressed. Tie the wire from the loose end of the spring, round the plate and to the other side of the spring, so it is held compressed quite hard (as hard as you can, basically).

You need to drill a hole in the end of the arm and put a bolt through it to hold the spring onto the arm. The position of the bolt is important - it needs to hold the spring tight enough to stop the trailer slopping around, but loosely enough that the trailer can turn relative to the bike. I guess about 10-15mm of slack should be enough. Drill hole, insert chain, add bolt and nut, cut tie wire off.

completed hitch
Complete hitch
This is how it should look when it's together. The spring always pulls sideways, and if you're lucky it will pull the hitch so it looks like this. Don't panic if it doesn't, it's supposed to be flexible enough to work anyway.
welding done
All welded up
This is the complete trailer ready for painting. I suggest putting the wheels on and testing it first, just in case something has gone wrong.
Painting the joints is essential, it stops rust. Painting the whole thing just makes it look nicer.
bin holding strips added
Bin holder
Later I added these bin holding strips that let me just drop a bin or crate onto the trailer and not need to tie it in place. Note the pin holding the arm in, and the vertical socket that lets me tow the trailer by hand.
Completed trailer
The trailer as it is now, with two Sydney standard milk crates in place. You can see the way that the arm slopes up to the bike. The black cable is shock cord that has a hook on the end (in Australia they're Ocky Straps, in New Zealand Bungee Cords). I collect these from the roadside (or other people give me broken ones) and put new shock cord in them. One hook and tying the other end to the bike/trailer means they don't get lost (or "borrowed") as easily.
This is my most recent version (number 12). The main difference is that I now use two supports up the centre of the load area made from the same steel as the rest of the trailer. It means that I cut 5 bits the length of the load area, and need 4 short bits about 50mm long to put the new bits below the trailer. It's worth getting the steel supplier to cut them, especially since there's only three lengths involved - the sides x5, the cross members x2, and the towing arm is "the rest" Uses 1 6m length of steel, about $20 in gal square tube in Australia I think.
I also tried a rigid hitch rather than the spring one. It's a little nicer to ride with, but more expensive and more fragile. The spring hitch just bends and because it's based on a bit of chain it's extremely strong. This hitch broke while being abused by a cop at a protest. It's based on a 10mm stainless steel rod end bearing, which together with an M10x75mm stainless steel bolt, 4 washers and a nylock nut cost me about $AUS60. The rest of the trailer cost less than that. Note that the trailer is drilled to accept the chain if I decide to go back.
I found a bit of steel for the bike mount that already had 20mm square tube attached so I cut it to length, otherwise I would have just bent something up.
It's nice and rigid to ride with, but heavily loaded (as here) it's also hard on the bike for the same reason. It's also got limited flexibility, so the pivot must line up with the bike axle to stop it twisting under load (it will break). The trailer also needs to be fairly balanced, as the night will not resist vertical forces very well. But with less than, say, 50kg on it should be fine.

I've now given this trailer to Phuong, and she is pretty happy with it. I've also done another Greens promo ride and was surprised at how difficult people found it to attach the trailers. I'm sure there is no sufficiently simple way - many people I'm sure can't fix punctures, let alone remove a wheel from their bike, so attaching the trailer is another step up.

The latest model is more mass produced and is powder coated!