How to make a DIY tow bar for a collapsible cargo wagon I am a technology education major who has to carry heaps of tools and materials with me quite often. I live off campus and ride my bike as I don't drive. I was thinking of jury-rigging one of those collapsible cargo wagons to my bike for transporting my 40 pound toolbox and my regular school bag. Has anyone had experience with doing something like this?? Any help is appreciated.
Years ago I made a side car for my BMX bike. The side car clamped to the bike at three points with a rigid connection. Three wheel skids were cool.
Not the same thing as pulling a wagon but that's all I have :)

As Ced commented, it would be good to have a picture of the wagon you want to tow.  

The first place I go to get ideas is Google images for bicycle tow bar. 

I was surprised at how many results there were.  

Here are some options I saw for connecting a tow bar to a bicycle


Seat post (this is the easiest to make and install - if it works)
Chain stays (solid, low center of gravity connection)
Fender mounting point(s) (seems like a lot of stress on a fender mount)
Axle (bolting to a solid axle, if the axle is long enough might be OK. I may be over cautious but using a quick release skewer makes me nervous)


After looking at the options (more detailed descriptions below) here's what I'd do:
For very little money make a prototype similar to The Dumb Stick (linked below).


Buy a three foot long two inch diameter PVC pipe
Drill a hole in one end large enough for your seat tube
Drill a hole in the other end for an eye bolt with a carabiner
Install the tow bar, hook up your wagon and experiment  


My biggest concern is with stopping, especially if you are going down hill. Start small and work your way up.

If the seat post connection fails my fall back would be a chain stay connection.
I had a child trailer with a chain stay clamping system that worked OK. Pulling 50 pounds of kids is a lot of work and put a lot of stress on brakes.

Seat Post
The Dumb Stick (not an endorsement, just as an example)

Is it just me or is his seat too low?  

Something like this would be easy enough to make. I'd go with steel or aluminum over "poly" if this solution went past the prototype stage.  

Thoughts:


At rest all the weight would be resting on the wagon wheels (assuming it has four wheels, or even three). It's important that the tow bar only have horizontal stress, not vertical.  
I get how something like the Dumb Stick pulls a kayak or wagon.  
But what happens when it stops? Won't the wagon want to keep going and make the stick swing out to the right or left?
I found a video of a guy who built something like the Dumb Stick and pulled his kayak around Florida. The video was painful to watch (it was too long and badly shot) but the tow bar seemed to work fine. He didn't go very fast and the roads were very, very flat.


Many of the images that come up in search look a lot like the Dumb Stick. There seems to be a lot of kayak and canoe towing going on.

Chain Stay
Another option for attaching a tow bar is to connect to the non chain side of the chain stay. This is a more complicated method of attaching to the bicycle because it will require some kind of firm yet flexible attachment.

They are clamping to the frame and the axle. Some rigs only clamp to the frame. This looks solid but I hate clamping things to my frame.

Fender Mounting Point
Some DIYers attach the tow bar to a fender mounting point. I'm very uncomfortable with this option. I don't think bicycle fender mounting points are designed to handle the kind of stress we are talking about.

Rear Axle
Another option I see out there is to attach a hitch to the rear axle.

Call me paranoid but I would not do this to my bike.
Think this wheel should have a few more spokes to pull a load?

I'm looking forward to seeing other suggestions.