First off, I apologize for not having pictures right now. I'll try to take some and upload tomorrow.
OK, so I'm making a seatpost mounted single wheel trailer, similar to a trail-a-bike. My goal is to use bike parts from the local co-op as much as possible. This goal has made creating a two axis hitch and mount at the seatpost challenging.
The body of the hitch is a thick BMX stem. A front quick-release hub is clamped in place of the handlebars. The trailer is mounted to the front hub using a front fork with the steer tube extending back to the rest of the trailer. The front hub provides freedom of movement in pitch. I enlarged the steer tube opening in the stem and pressed 1.125" headset cups into this opening. Bearings and a threaded headset race go on next for each cup. I used 1.125" headset parts so that my 27.2mm seatpost could fit down through the assembly without contacting anything.
Now comes the part I'm struggling with. How can I keep those threaded headset races tightened down with a good amount of preload while keeping them in place against the seatpost without damaging the seatpost? I'm currently using a shim made from a aluminum can wrapped around the seatpost several times so the threads of the headset races have something to dig into while the rest of the hitch is free to rotate. Unfortunately the mount tends to jostle around and spread the threaded headset races apart until the headset is really loose. I'm trying different sorts of clamps around the seatpost above and below the headset races but none have held for long. Currently the aluminum shim extends past the headset races and under the clamps. I'm thinking there might not be enough friction. To resist the forces this hitch'll see. I haven't even loaded the trailer yet!
So what sort of clamps might work? Clamp the shim or just the seatpost? Use a thicker material for the shim so the headset threads grip more? Don't clamp the seatpost and try something else? I'm trying to imagine something that would sandwhich the headset down without interfering with the freedom in yaw required for the trailer.
Thanks for reading.
edit: I've attached a picture.