BHOFM

I am thinking about building a single wheel trailer, seat post
mounted. I have an old Spalding ATB, 12 speed that I want
to use for store runs.

I want a real quick hook up and remove thing.

I have the capability of fabricating any and all parts, welding
and such.

Give me some pic's and thoughts.

What about something like this with a seat post mount?

FunkyStickman

I like the monowheel pannier design... I'd be willing to try building one like that. It wouldn't give you more functionality than an Xtracycle extension, but it would be removable, so if you only had 1 bike (and didn't want a longtail) it would work fine.

Gareth

Can I assume you have a tube/pipe bending facility for 3/4 inch 16 swg steel tubing?

If so I would go for something like the Oxtail trailer, but a little more advanced in design. https://www.oxtailbicycletrailers.com/videos.html with the trailer frame made from round tubing: 3/4 X 16 gauge ERW grade mild steel, and the drawbar made from 1 inch X 16 gauge. I would also incorporate fold out cargo side rails that lay flat within trailer's profile for storage, fold to 90 degrees for normal loads, and then fold out to a second position of 180 degrees to create a larger semi skeletal flat bed area for bigger loads.

I am actually considering making a trailer like this within the next month for the Bicycle Polo team that I belong to. We don't have a fixed home court, and sometimes have to cycle several miles around Norwich until we can find a court not already being used. The trailer would haul the spare Mallets, goal posts, tap out gong, and a few tools and spares to keep the team mobile on the court and in play. With a folded out to enlarge flat deck area, I could also haul away any bikes that get damaged during a game; buckled wheels, bent forks, twisted frames. etc. (each of the six of us has trashed at least one polo bike a piece this season). When the game is over and I have returned home, the trailer would be flat packed and along with the team kit, could then all be stored in a holdall ready for the next practice session or match.

Part of my business is the manufacturing of bespoke special purpose bicycle trailers, and for 80-90% of the trailers I make I use this hitch arrangement, using a quick attach industrial ball joint:

Rockfish

I'm no expert, but I'm not sure that seatpost-mount and single-wheel really go together. Sure, the Oxtail shows it can be done, but it seems like the mount is really far from the wheel, with lots of long lever ams and bends to connect them, and you'd "spend" a lot of weight just connecting the two points. Maybe weight isn't a concern. The Oxtails do seem durable enough, at least from the photos on their site.

The Oxtail/Bob/Goeland/Talyor type single-wheel trailers seem more versatile to me as they can carry any size or shape of load. Gareth's idea above (who obviously IS an expert!) about adding folding rails to a design like this is really great!

To me, it seems like a Burley Travoy type 2-wheel rig would carry as much as a pannier/wheel setup and be much easier to DIY. You could even use a cheap hand truck as the basic structure.