I had modded a 20 to be a sort of chopper.
I cut the top of the headstock off the frame, as the 20 uses a long steerer tube. I modded the BB to take alloy cranks. I removed the bridges from the rear stays. But I never got further than that.
I tried various stuff on this frame. Even got a 26 X 1.5 mountain bike wheel in there.
This gave me the idea to fit 24" wheels.
That is one of my favourite size wheels. I have a couple of Raleigh Phaser bikes. They were kids roadster bikes that came with 1 or 3 speed gears.
But I modded one into a MTB and the other into a BMX.
I tried 24" 451 wheels and forks on the 20.
But since I had cut the headstock above the frame tube, instead of below. The bike was pretty high, the steering would be slower than it was as stock.
I was toying with the idea of cutting the rear stays off and slipping in a rear triangle from a 24" wheeled mountain bike. This would give more gears, better brakes.
I was messing with the frame. Its got damaged chainstays and down tubes. So I cut off the BB and rear stays.
The 20 takes a 28.6mm seat pin. This is reasonably common these days. The seat tubes tapered at the top to hold the seat pin. 28.6mm is a common outside diameter for the seat tube of bikes. So I cut up a 24" wheeled MTB. I cut the top tube in front of the seat tube. I cut the downtube in front of the BB shell. This frame has no hole in the front of the seat tube or the bb shell. Those are for letting gasses out during welding.
heres the donnor frame before cutting up.
My tape measure is to see if its a 24" wheel frame. They have 13" from wheel drop out slot to brake bridge.
I smooth off the rough metal and stripped the paint off the seat tube. Heres it with the 20 font end sitting on top.
I have cut more off the 20s frame. So it can use a 28.6mm front mech. I have cut the seat stays from the MTBs seattube. Then slipped the 20 seat tube down. Then welded around the seat tube just under above the seat stays. And below the 20s top tube. I welded around the seat stays, to join them onto the 20s seat tube. The frame is 18". I reangled the down tubes, got to extend them. Thinking of welding a flat peice of metal to them. To take cable adjusters. So the gear cables can go through the frame and then under the BB shell.