Originally Posted by
Jipe
Since Birdy III there is a totally new concept of tensioner dedicated to the tensioning of the chain while the bike is folded, the derailleur or a Shimano or a Rohloff chain tensioner being used to tension the chain while riding.
This new concept is a parallelogram type arm attached around the bottom bracket box of the frame. It works perfectly.
The same concept could work on a Brompton with rear derailleur.
The Birdy III tensioner came in and it fails to mount over the Brompton BB shell by a wide margin - not enough space. I thought that there could be interference of the mounting clamp with the brazing material flowing out, but, if nothing special is done, there is even interference with the bridge to the triangle. I suspect that there could be some options there, such as to attach the parallelogram arms to some BB mount adapter, of the type that got popular now, rather than to the clamp. Overall this would be turning itself into a longer-term project, nothing quick, and maybe not worth the hassle relative to the expected outcome.