Instructables
Parts:
1 junk Womens 24 inch 15 speed bike frame (head tube angle may be wrong on Mens bike)
1 aluminum handle from broken gardening tool (20 inches in length and slightly smaller in diameter than the inside diameter of the bike frame
2 16d  3 1/2inch nails or small diameter bolts and nuts
duct tape as needed to make garden tool handle fit snug into bike tubes
two nuts that originally held on the bike tire of the junk bike
4 washers

Tools:
adjustable wrench
drill
metal saw
hammer
misc bike tools

Steps:
1) saw off top tube and head tube from bike frame leaving head tube attached to top tube
2) slip garden tool handle 6 inches into end of down tube
3) drill hole three inches from end of down tube all the way through both tube and handle  
4) insert nail in hole and bend over end to secure or secure with nuts and bolts
5) slip other end of tool handle 6 inches into end of top tube
6) make sure head tube is perpendicular to the ground
7) drill hole three inches from end of top tube all the way through both tube and handle
8) insert nail in hole and bend over end to secure or secure with nuts and bolts
9) knock collars out of both ends of head tube with hammer
10) cut off upper half of head tube to use as a spacer on the seat post to lift connector high enough to clear the rear tire if needed
11) remove seat post of lead bike
12) slip spacer and head tube onto seat post
13) reattach seat post to bike 
14) bolt on rear fork of connector over the front tire nuts on the trailing bike using washers between all parts
15) take out for a couple of test runs

observation: The junk bike is oriented upside down in relation to the other bikes.

note: I slit the to tubes and attempted to attach to the handle with hose clamps orginally but this did not work very well.  I did not go to the trouble of removing them yet.

concern: I am not sure there is enough play between the two bikes to make this suitable for anything but very level terrain.
 
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Mosqito (author) 4 years ago
After my first test run I am planning to replace the bent over nails with nuts and bolts. The nails allowed too much twisting which led to my son falling off when I forgot to remind him to lean into a corner.
rimar20004 years ago
Very clever!