This was the most difficult part of the project. I scratched my head for a good week or so in trying to figure out how to connect the two before I even started this project. While I had a general idea on how to connect them, I didn't have a specific solution. I knew I would use the rear triangle of a mountain bike as I needed it to fit around the rear tire of my mountain bike.
With that in mind, here's how I made what I've come to call...The Hitch.
I originally settled on using a rear axle used for the B.O.B. Yak and Ibex trailers
(what I used as inspiration) and ordered a couple online. This would allow us to switch the trailer between bikes on the trail and share the load. After they arrived, however, I realized this wouldn't be feasible as the dropouts were too small and, even after enlarging them, they still got in the way of the rear derailleur.
I also tried opening up the angle of the arms of the hitch, but that caused a reduction in the length and the wheel ended up rubbing against the frame. It's never good when that happens.
I've since packaged the axles back up and sent them back to the store.
It finally came to me when I realized I could use the braze-ons
to connect the two.
As I said earlier, I cut off the rear triangle of the junker bike as close to the seat-tube as possible and used the pipe bender to bend the bottom portion of the frame so that it was similar in length to the top of the frame. A picture is worth a thousand words here. Please see below.
As you may recall, I originally had three vertical supports for the trailer. I came to the realization that it would be easier to attach the hitch to the trailer if I used the center post as the pivot point. (This appears to be similar to how the B.O.B. trailers are connected as well.)
With that in mind, I cut a length of the seat post (from the junker bike) so that it would fit between the top and bottom frames of the trailer. I then cut two notches on either end of the seat post so that it would be cradled by the top and bottom frames.
I brazed the head tube to the hitch and then slid the seat post into the seat tube. I placed the washers on top of the seat post and fit the assembly between the top and bottom frame. (The washers served as to keep debris out of the down tube as well as to take a little pressure off of the joint between the seat post and the top frame.)
Before attaching the hitch to the trailer, I finalized how to attach it to the bike. (I figured it would be easier to do this before it was attached to the trailer as grinding the hitch by itself would be much easier than grinding it with the trailer attached!) I ended up having to cut off one of the dropouts so that it didn't interfere with my derailleur. I also had to drill the braze-on hole a little larger so that the screws I was using could fit into both the hitch and my bike.
After finishing the hitch/bike connection, I attached the hitch to the trailer by tilting it and sliding the seat post so that the notches were in the center of the trailer (a hammer came in handy here). I used a pair of vise-grips to flatten the notches (again, to increase the surface area with which the brazing material can form a bond) so that they were somewhat snug against the top and bottom frames. I then brazed the top and bottom frames to the seat post (allowing the down tube to pivot around the seat post).
NOTE: Make sure you use strong screws to attach the trailer to the bike. The braze-ons aren't very large so having a screw made of zinc or copper (or other soft metal) might cause failure if the stress from the trailer causes them to sheer off. I splurged on some stainless steel screws and they worked like champs.