This is part of an old triple wheel Semcycle stack that I had. The unicycle that attached to the top has since been given away so the parts are pretty useless to me.
The middle wheel is 16" and bottom wheel is 20". You’ll need a 20" Semcycle to attach to the top to make it work.
I’m passing this on for free, but will have the recipient pay shipping. I’m in the Seattle area if you want to pick it up.
Let me know if you have any questions.