BB mount adjustable chain hook for derailleur conversions originally designed for the Xplorer bike, but works perfectly for the G line in combination with a modified Saint derailleur. (only available as a full kit with dropouts)
This chain hook will also be helpful to take up the chain slack (when folded) on a 16" or 18" Brompton, but only in combination with a short cage derailleur with an adjustable chain hook attached to the lower jockey wheel and some kind of B tension to get the derailleur to swing back when folded.
The old derailleurs from around the ninetees and zeros had a B tension spring which is ideal in this case. For the bigger range cassettes of these days like a 11-42 you need more modern derailleurs though. My favourite for these conversions is the Zee if it has to be affordable and the Saint when you want a really well built derailleur with better shifting performance and longevity. These Shadow style derailleurs don't have a B tension spring so I always add an external B tension spring.
Note: I don't give tech support to make these conversions work as there are too much variables (different cassetes and derailleurs etc) So you have to figure it out on your own.
Material: 304 stainless steel
Weight: 54 gram