Bike Cargo Trailer

I ride to school every day on my bike, and while it's great to get through traffic as well as a whole lot of fun, it's not great for carrying cargo. Every now and then I have to haul some wood, a robot, or some other build for a science project that simply does not fit in a backpack. A cargo trailer would solve that issue, but the ones on the market are either upwards of $300, or cheap junk that would probably quit as soon as I started putting any real load on it. This DIY bike cargo trailer is the answer to the problem, and it does it with very little drawbacks. It's cheap, at $30 in materials, it's fairly lightweight and easy to pull, and it can tow a LOT of weight. It's also fairly easy to build, with no welding or brazing required unlike other cargo trailers.

If you have a set of basic hand tools and a power drill, you should have most of the tools needed for this project.

1. Power Drill
2. Drill bits - anything that will work with steel is fine, I used a 10$ titanium coated set from Harbor Freight. If this is your first time drilling steel and/or pipe DO NOT splurge on a nice set because there is a good chance you will break a few.
3. Threadlocker/Loctite Blue - Keeps bolts from getting loose from vibration. Make sure to get blue so that you can take it apart if you need to.
4. Hacksaw - Again, nothing fancy, I used an $8 one from HF with the blade that came with it.
5. Conduit bender - This is the annoying tool that costs a ton. You can get one for ~$30 bucks, or you can ask around and see if somebody will let you borrow one - the one I used was from my school. If you splurge and get the fancy one like the klein tools one I used, it might come with an angle guide which is pretty handy to have, although you can definitely do the project without it.
6. Hammer
7. Adjustable wrenches - at least two if you have hex head bolts and nuts so that you can hold one side while tightening the other
8. Screwdriver
9. Hex Wrenches
10. Channel-locks
11. Level
12. Speed/Combination square

And materials:

1. 2x 10ft 3/4'' EMT Conduit ($21.96) - EMT, or electrical metallic tubing, is great for building things like this because it is insanely strong, doesn't need to be painted, and is basically rust-proof because it's galvanized.
2. 4x Galvanized tie plates ($3.92) - These are dirt cheap and galvanized so you don't have to worry about rust.
3. Bike Trailer Hitch ($6) - I was able to get this from Aliexpress for a little less than 6$, which you could do too, but since links to aliexpress don't last long here's one from Amazon that's only a little more at 9$.
4. EMT conduit couplings ($3.13) - As far as I can tell they only sell these in packs of 5.
5. Bike wheels (Free) - I had an old bike that I salvaged these off of, but if you don't have one just find one off of facebook marketplace for a couple bucks. I used 26 inch wheels because that's what I had, but they can be any size.
6. Bolts ($5) - Unfortunately Home Depot doesn't put all of their hardware selection on the site so I can't drop any links. However, it cost me around $5 for 2x 1/4''x 2 1/2'', 12x 1/4''x2'' and the nuts to go with them.

Total: $34.01

I think this embodies the spirit of "On a Budget" because not only is it 1/10th the cost of a commercially available trailer, but I used basic tools that most people would have, and it gives a pair of decent wheels from a dead bike new life.

The first step is to make a rough sketch of the trailer. I used Fusion 360 because the sketch workflow works great for designing something like this. To model it, I created a sketch, laid out lines based on a design I found on the internet (while I found it on a couple of sites, I think it originated at Mark Rehder's drumbent.com), and finally made it a pipe.

At this point is also when you have to decide what size to make the trailer - in terms of length, it has to be at least enough to clear your wheels, but beyond that it's up to you. I made it so that it can comfortably carry a 27 gallon bin yet still be less wide than my handlebars, so I don't have to worry about it hitting anything.

When designing, keep in mind that 3/4'' EMT conduit isn't actually 3/4'' - that number is for electricians as a guide on how much wire can be pulled through - and the outer diameter is actually .824". Also, set the radiuses of each bend equal to 6 inches if you are using 3/4" conduit (the bender's takeup).

I am attaching my fusion file, but use it at your own risk - I didn't fully model out some of the parts, like the wheel mounts, and some parts may have different dimensions to yours, so double check before making cuts or bends.

Credit to RAHUL CHAURASIYA and Henk Kommers for the wheel and bike models respectively.

This step is all about following the design from the last step - picture #2 from step 1 basically is a blueprint for all the cuts and bends needed.

First up is a 45˚ bend at 1 foot down the pipe. The complicated part of conduit bending is the calculations, as despite being simple math it's easy to get confused. There are videos on youtube that explain the concept really well.

With a 45˚ bend, there's a 3 inch takeup on the bender. This means that you should subtract 3 inches from the length you wanted the bend to be at and make the mark on the conduit (pencil works great). Then line up the bender's arrow, snap in the angle guide if you have one, and bend. If you have an angle guide, bend all the way until it stops - if you don't, put a level on the handle and go to 45˚.

Next is a 90˚ bend. To make this easier, we'll do this from the other side, so flip around the bender and use the star instead of the arrow. When bending from the star, there's no need to calculate take-up - just mark the distance you need and bend. Check that the bend is 90˚ using the square.

Finally, you should be left with a couple feet extra. Cut it off with the hacksaw. When cutting, use a little oil to reduce wear on the blade, and only put force on the forward motion for the same reason, as hacksaws only cut on the forward stroke. You can clean up the burrs by clamping channel-locks on the edge of the pipe and twisting.

The second part is more of the same, except for the last bend. Make sure you start from where you left off on the last pipe, at the back of the trailer. This ensures you have enough material to be able to bend the last little bend before you cut it. Since this bend is going the opposite direction than the others, this has to be an "up-in-the-air" bend. This means that you place the bender vertically on the handle, with the pipe in the air, and pull down with your hands to bend it (see pictures). It takes substantially more effort because you don't have as much weight on it so grab a friend if you need the extra muscle. Once you're done, once again cut the excess.

The holes on the tie plate make it pretty simple to drill into. Drill 1/4'' holes (or of the diameter bolt you chose) in the three holes on the third row, as in the picture.

Then, measure the size of your wheel's mounting bolt. Mine was around a 5/8'', so I used that drill bit to drill into the middle hole on the 7th row. Then, draw a line down from both sides of the hole and cut inwards from the edge with a hacksaw. It doesn't need to be perfect, but make sure you error on the side of making the hole slightly too wide, not too small, otherwise the wheel won't fit.

Do this step 4 times for 4 wheel mounts.

Place the two excess pipes over the frame from the last step as in the picture, and they should be longer than they need to be (the picture was taken after the cut and so does not show it). Mark and cut the pipes so that they are of equal length and as long as the frame. Make sure the pipe overlaps the frame, as the two will be bolted together.

Then, use a wheel mount as a guide to drill three holes down the pipe.

Now, hammer both ends of both pipes so that the ends are completely flat. I did this on a concrete parking block outside my house which worked pretty well. Hammer them so that the holes you drilled are horizontal as in the second picture. Make sure you do this step after drilling the holes so that the metal shavings don't get trapped inside the pipe.

I went with 1/4'' bolts for everything, so I drilled 1/4'' holes. Click on the first image to see the explanatory notes on where to drill the holes.

When drilling into steel, I like to drill a pilot hole first as it is really hard to stop it from walking. The key is to never change direction while drilling, even if it looks like it is not going straight, because it will break the bit. None of the holes have to be super precise here, just enough that it fits together, which is why you can get away with not using a drill guide. If you have trouble, just switch to a smaller bit and work your way up.

Now, bolt everything together. I started with the main frame, putting the EMT coupling at the back to join the two pipes, then tightening two 2 1/2'' bolts at the front. Add a drop of thread-locker before tightening each nut.

Then, it was time to bolt each wheel mount onto the rails, and bolt the rails onto the frame. Finally, attach the hitch and the wheels. Add a little camber to the wheels so that the tops point inwards by making the inside bolt lower than the outside bolt. This helps for stability by making the wheelbase a little wider.

I capped off the open end of the conduit with a 3d printed part I designed to avoid water or other contamination getting inside the frame - if you don't have access to a 3d printer just use some tape to seal it.

Insert the hitch into the open end of the conduit, send a bolt through, and tighten it up.

Then, detach the bike mount (black piece), and attach it to your bike. The installation may vary depending on how your wheels are fitted - for me, I had to remove the quick-release skewer completely, slide on the mount, and put it back on. Tighten it up more than you normally would, so that the trailer's tongue weight doesn't rotate the bracket.

Finally, line up the two and connect it with the hitch pin. If you have a safety strap, loop it around the frame and back onto the hitch.

Rather than adding a floor or platform, I chose to leave it as is for a couple of reasons. Adding anything would add weight, cost, and depending on the material could rust or degrade over time, since I store the trailer outdoors. However, you could certainly do it, perhaps using one-hole conduit straps and a sheet of plywood.

I chose to simply loop a ratchet strap around the bottom. It's light, cheap, and leaves me with options in case I want to modify it in the future. Two ratchet straps on top hold the box (or any other cargo) in place and that's it.

After using this for a couple of weeks, it has definitely solved the problem it needed to. I have tested this with up to 250 pounds, and it holds up quite well. Because it uses MTB wheels, it rides on pretty much anything with ease - you could probably take it to the trails if you wanted to. The only thing that has developed a little bit of rust with the heavy rains is the hitch, but it is minor surface rust and doesn't affect anything. The actual trailer and any load-bearing component is completely rust-free.

The only thing I would think about changing is the hitch. The reason I went with a premade one was because I wanted it to have a full range of motion as well as attach and detach to the bike quickly and easily, which it does - however, the spring part of it goes back and forth while accelerating which is a little annoying as you can feel it on the bike, especially with higher weights. Once you get up to speed it's perfectly fine though, which is why I am keeping it for now. In the future I may try to design my own that has less play yet is still as clean.