astronomerroyal

I made this in my 1bedroom apartment, so no welding. Haven't a clue how to do that anyway. Many bolts, the frame is aluminium so it's not as heavy as it might look.

It seems universal joints like this aren't used as often as I had assumed, so I'll take some photos for you tomorrow.

In words, the u-joint I found (apparently from a golf cart) is female on both ends and takes splined 1/2inch shafts. To hook everything up I did the almost same thing on both bike and trailer ends.

1). On the trailer I drilled a hole 1/2 an inch in diameter. From the back I inserted a short 1/2inch bolt.
2). There's also rubber and metal washers in there. The rubber washer sticks to the trailer surface, the metal washer sticks to the rubber washer and provides a hard smooth surface for the 1/2inch bolt head to rest/rotate on.
3). The shaft of the bolt now protrudes out of the front of the trailer. I smothered the threads in 'cold weld' (amazing stuff) and as it hardened I rammed that sucker into the u-joint, thus leaving the perfect imprint of the u-joint's spline on the 'cold welded' shaft.
4). Now the u-joint fits perfectly on this DIY splined shaft; I clamp it tight using the u-joint's own tightening bolt. The coupling now has three orthogonal axes of rotation (u-joint=2, rotatable bolt=1) and the trailer can adopt any orientation.
5). At the bike end the same thing is done, but the analogous bolt is fixed so it doesn't rotate, it doesn't need to. Also, instead of clamping the u-joint to the DIY shaft I drilled a hole through the u-joint+shaft and inserted a pin. This is a quick release way of disengaging the trailer from the bike. Pull the pin, slide the trailer off. Literally takes 3 seconds.

I highly recommend the u-joint method, the trailer doesn't resist any manoeuvres and tracks like an loyal and obedient beast of burden. It's a little complicated to implement compared to the 'strips of rubber' or 'hosepipe' methods.