Hi Stevegor.

The bike has an aluminium chain tensioner. The copied Brompton design requires a tensioner. The rear hinge used in folding is behind the bottom bracket which would mean that the chain length was too great as you fold it under. The chain would fall off without. I suppose you could use either derailleur or tensioner on your R20 to accomodate the different sized chain rings. Maybe a cheap derailleur set with the adjusting screws into the right plane would work out. Can you lock out the sideways movement with the adjusting screws on those things? I'm not sure.