The bolts in both the bike hubs that I am using have a different thread (and therefore nuts) to standard metric sizes that you can buy in hardware stores. This meant that we had to attach the two hubs somehow, without just putting a threaded rod between the two with joint connector nuts. So the solution was, I found a steel tube (pictured above), and had 2 bike nuts wedged and welded in at either end. When I tested it I found that its not perfectly straight, which is problematic as it spins at an angle, so the wooden support that we make for the top hub might rip apart on a very windy day. I am now thinking of getting a threaded rod as my axle, then welding a nut that fits to that, to a nut that fits the bike bolts (x 2). Also that way it has a tiny bit of flex compared to the steel tube.