I've been eyeing long tail cargo bikes on marketplace for awhile, and they always seem to be very highly priced. On the other hand, tandem bikes are sometimes almost given away. So I had the thought of removing the seat, and cockpit from the rear of a tandem, and getting a long tail rack to mount up. Is there something I'm not thinking of?

Working backwards, as I do, I have 20 inch wheels I want to put on a bike I can ride (5'6" adult).

I cannot easily throw them on a yardsale adult bike. Derailleur and pedals will hit the ground. Single speed or IGH would help, but a yardsale bike likely doesn't have horizontal drop outs. And I can only get the crank arms so short before it sucks to ride the thing.

Many folding bike use a 20" standard, but they bring a lot of complication to the build I'd could do with out. And they seem to be maintaining a higher used-market price than anything else.

BMX bikes use 20" wheels. But, I guess (?) an adult still rides a fairly small frame (less than 22" top tube). I am uncertain this could be built up to a comfortable riding around town bike? With a tall seatpost and high bars, could I ride this like a normal bike? BMX bikes is a huge hole in my bike knowledge. Will I be able to differentiate between a 20" top tube bike made fora teen ager as opposed to made for an adult?

BMX bikes with larger wheels (24" or bigger) exist, and are adult sized. But they get back to problem one, crank arms hitting the ground if I put on 20" wheels, OR uncomfortably short cranks. Generic fixed, single speed, and cruiser frames also have this problem.

Recumbent bicycles and adult trikes (upright, recumbent, 2-1 or 1-2) do sometimes use 20" wheels, but are absolutely not the direction I want to go with this.

Is there a frame type I am missing? A solution I have not considered?

Any other insight or babble will be appreciated.

I was gonna just build my own but they’d weigh like three pounds. So imma butcher this one instead

Or whatever the fuck I was experiencing where pedaling would fully compress the suspension

Or yknow, do all that then toss the original tires back on since the new rear frame will actually fit those tires.

Move bike cable and got matched tires

It an old Trek 4900 alpha put back together with spare parts and made fixed gear