dyna-ti wrote:
Why then not cut up one of those million cheap bikes you have and use 18mm plywood from B&Q ?
I could talk you through ways to mount the BB and headtube, dropouts and seat tube, but maybe stick to a proper fork, for the sake of safety.
No way! Now we have a proper challenge. Bodgers versus people who know what they're talking about.

Just remember it's only got to survive one lap, and then possibly be recycled into thermal energy if the night is cold.
Some details like seat tubes are redundant. The seat can be set at the right height so won't need adjustment.
There are other ways to mount a BB (I've already got the parts).
Headsets are only a problem if you think traditional diamond frame bike.
Dropouts are also not a problem, but if I say too much that clever Velo will pick up on my genius idea....

My big problem is trying to come up with a way of avoiding using a chain. The simplest is front wheel drive, eg this recumbent penny farthing I bodged together 10 years ago using a folding bike frame

I prefer the idea of rear wheel drive though and I'm playing around with that.
The possibilities are endless if you are restricted by UCI ideas of a bike.