Hi Alan
Probably didn't make myself clear there .
To use a 20" wheel "single sided" you need to use custom made HT axles or BMX wheels with 14mm axle .
20mm hubs are often used for trikes and these are EXPENSIVE but If you check out the transportable thread you will see how I converted some hubs to disc brakes
A Lucky find at the local tip a few weeks ago was a BMX bike with 48 h rims 14mm axles with a standard screw on freewheel LF rear hub and a disc brake front hub ( QUANDO hubs )
They are going on the next project a tadpole tandem when the weather improves .
From the above photo you will notice the 14mm axles The LH wheel is a front BMX disc hub wheel ,centre is the rear wheel converted to disc brake with a screw on converter and the RH wheel is a standard front BMX wheel with a 14mm axle suitable for rim brakes .
The rotors are fitted the correct way around for strength another wise precaution the axle tabs are fitted to the axles so I don't lose them
48h disc brake wheels are like rocking horse droppings VERY RARE !!!!
The screw on adapter can be used on the Right hand side as braking tightens up the screw on adapter . These were a great find and at £5 for the wheels and £5 for the adapter were a SNIP !!!
32h and 36 h wheel are OK for up to around 16 stone with 14mm axles over that then double section rims and 20mm hubs are recommended .
Hope its starting to become as clear as MUD
20" 24 or 26 " rear wheels can be used
regards emma