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DIY Bike Cargo Trailer

Build a Practically Free Cargo Trailer For Your Bike:

Our family bikes all the time. I commute to work year round. I bike around town on short errands. We bike to the library. We do this partly for health and partly for economic reasons.

Usually we get by with using either panniers or backpacks. However, those can only carry so much. I decided to try and build a cargo trailer.

If you start googling around, there are tons of web pages on building bike trailers. My own design, in particular, owes a lot to this web page. I also think this one and this one are pretty good ones. (No guarantee, of course, that those links will stick around...)

However, I quickly discovered, that the trailer is not the hard part. Building a cargo trailer is easy. There are umpteen different ways to do it, just pick one you like. The hard part, is the hitch.

Read on...

First, don't do ANYTHING... until you have some wheels. You need your wheels first, as they affect all your other measurements.

I was fortunate to have a pair of 12" wheels from a long-outgrown scooter that used to belong to my son. I recommend keeping an eye out on trash day, as people are always throwing out old/broken bikes. Another place to find wheels is the local police auction of found/stolen/unclaimed bikes, or a local thrift store.

Next, is a means of mounting those wheels. Again, I was fortunate to have some angle iron in my scrap pile, from some old shelving. (Another option I have read about is to buy electrical box cover plates, as they bend well and are easy to work with, but I have no personal experience with that.)

You need a notch in your plate, to receive the axle of the bike. Drill a hole and then use a hacksaw to cut the notch part.

The little bit of wood in the photo is a shim, about 1/8" thick. It goes under ONE of the angle irons on each side of the trailer. Specifically, it goes under the outside supports. In my reading, I learnt that your trailer will ride with more stability if there is a slight inward lean (at the top) of your trailer wheels. This shim is supposed to give that. I think it works, I don't know for sure, as I have not tried my trailer without it!

My wheels came with these little washers that have tabs on them. They fit into a little hole on the scooter frame. I believe that this is a safety feature to help keep the wheels on kids bikes from accidentally slipping out of the frame if they become loose. Not a bad safety feature for a trailer either! I was able to use these to fit through some of the existing holes on my angle iron mounts.
Okay, NOW you can start building the trailer part. As I mentioned above, I found inspiration from other trailer-building web pages on the net. Your trailer will need to fit your material and your needs.

My trailer is meant to be used with one of those large plastic storage bins on it. So I measured the bin, added a couple inches for extra room, and that was the cargo area dimensions that I needed. Then I measured the wheel axles, so I also knew how much space I needed to allow for them. A bit of math and I was ready to go.

I started with measuring and cutting six 1x2's to length, and then screwing them together. I next cut out a piece of 3/8" plywood to cover the entire base, and then cut out notches in that to accommodate the wheels. The diagram at left illustrates this. I suppose the base plate could have been cut to just cover the centre section. However, building it like this should give extra strength and rigidity to the 1x2 frame members.

To start with, I only used a few screws to fasten on the base, in case I would need to disassemble things later. Once it was built and tested on a few rides, I added more screws for strength.

Here's a shot of it, before I painted it, to give you an idea of how it looked.

The angle iron wheel mounts are screwed to the frame, and the wheels bolted into place. This results in a very low riding trailer. I had read that having the axle of the wheels situated above the bottom of your cargo area would give a more stable ride, and it seems to do so. With the angle irons I had options to mount the wheels a few different ways, but this seems to work well.

Note, on the larger photo, you can just see the shim located under the outside piece of angle iron.

Okay, let's talk about the hitch.

I came across roughly three types of bike trailer hitches. One would hook up high, onto the seat post. I actually tried that one. It seemed to work not too badly with my jury-rigged hitch attachment. No photos, sorry, I attached it to the pannier rack. But the result was a very large and wobbly trailer hitch arm, and more importantly, a very LARGE AND HARD TO STORE trailer.

The other two types involve either hooking it onto the axle -- which several resources did NOT recommend as the axle is not really engineered for that sort of a load -- or hooking it onto the rear seat stays.

My kickstand kind of makes it difficult to hook things onto the rear seat stays.

Here is the other problem with those sorts of attachments. Sure, my size twelve feet are on the large size, but I'd bet that even a size nine foot would have a problem with heel strikes on a trailer hitch. By heel strikes, I mean striking the heel on the hitch as you pedal. Any hitch that hooks onto the stays or the axle is right in that area where your heel goes as you pedal.

(Come to think of it, 10+ years ago I used to have a child-carrying trailer, and I definitely did have heel strikes.)

The other main problem, in my opinion, was that a lot of the DIY bike trailer hitches, and some of the commercial ones as well, required you to permanently mount some sort of clip/hook/thingy to your bike. I did NOT want that at all. Aesthetics aside, it means you can't easily move the trailer to another bike.

Instead, I hit on this design.

Rather than hook or clamp something to the bike seat stays, I would instead clamp something to the upright stays that support my rear rack. We have racks on 3 of our bikes, so that means the trailer should be moveable between bikes. As well, it is higher and further out of the way to eliminate the chance of heel strikes.

I took a piece of good hardwood and positioned it behind the stays and then traced them. I use white oak, which is very water resistant and very strong. I would NOT use a softwood, as you really really really REALLY don't want this breaking and plunging into your spokes. No Knots!

I then cut out two grooves, which matched the seat stay positions. This allows the piece to lock into place on the stays.

... continued ...

A hole was drilled through that piece, and a t-nut embedded in the wood. In this photo that is the rear piece of wood.

A matching piece of wood was cut (again from white oak) to fit over the front of the stays. I also cut some shallow grooves here (shallower than on the rear one) so it would lock in place against the stays. A matching hole was drilled through it. I then could use the black knob to to lock the two together.

As a woodworker, I have a lot of bits and pieces in my shop. That knob is a standard "jig hardware knob", that I originally got from Lee Valley Hardware. (Here is a link to the 1/4-20 Jig/Fixture hardware page on their website. You should be able to come up with something similar from your local hardware store.

When the pieces are together it looks something like this. The other part to this, is the trailer connection. Now that I have something to clamp to the bike. What I came up with there is a simple U-bolt arrangement. A U-bolt is attached to the wood, and then some heavy cable is looped through this and bolted firmly to the forward arm of the trailer. I used some old surplus power cord from a dead computer for this. This is maybe a bit less than elegant, but it works so far. This allows for a loose connection, for making turns, and when you lean the bike, and yet still holds the trailer quite firmly.

Note that many trailers often have some sort of backup hitch arrangement, like some webbing or something, to "catch" the trailer in case the main hitch somehow fails or comes loose. I have not implemented that yet, but I might still think about it. As a cargo trailer, I'm a bit less concerned than I would be if this were a child trailer. Right now, it all depends on the main knob not coming loose.

Another view of the hitch items, loose on the bench.



Here we have the hitch attached to the bike. The wood mount is securely clamped to the rack stays. The trailer is itself securely but loosely looped through the U-bolt.

One thing I have not discussed yet is the "arm" that connects the trailer to the bike. I simply eyeballed the lengthy and height of it, to match the location of the hitch on the bike. It is visible in a number of the photos, but not this one. Here we see the bit of angle iron which juts out from the front of the arm. This I also just sort of guessed at the length. You need to have a bit of a right-angle piece here, in order to support cornering. If you just had a straight arm from the trailer, then you could only turn left. Each time you turned right, the wheel of the bike would strike the arm. By having this right-angled bit here, you give some room for the wheel of the bike to turn right as well as left.

The finished and painted trailer. A spare red reflector is mounted on the back, for some all important safety. I do use this trailer in the evening, so a reflector is mandatory.
Here I have the cargo bin mounted. I drilled a few holes in various strategic places around the edge of the trailer to accept hooks from this bungee cord that I had.
And here is my final, and possibly most important shot. We have a large family, with only an average sized garage, so storage space is a big deal. This trailer is small, and even better -- it is light. I can easily pick it up and hang it on a few hooks on the wall. It is up and out of the way, but still easy to access, and I can even leave the bin in place on the trailer.

My total cost? $0. That's a zero. Nothing. I built this entirely from materials on hand. If you had to buy the parts, the most expensive bit would likely be the wheels.


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