A belt-drive system can now be ordered with any Kinetics rear gearing – belt drives need no lubrication and are in theory very durable. There’s one caveat – to get the Brompton to fold, this upgrade does things to the belt that aren’t approved by Gates, it folds back on itself. But this is done with low tension and around the largest radius possible, so I think that belt durability will still be very good.
This is still a bit experimental, belts are much fussier than chains. Because of the complexities of the Brompton fold, there’s only two belt ratios I know to work:
- 22/55 for Alfine, a bit lower geared than usual, giving gearing of 21.5″ to 87.5″ with the 11-speed.
- 19/70 for Rohloff, this is lower geared than usual, giving gearing of 17″ to 89″.
21/60 might also work for Alfine, giving 25″ to 100″ for the 11-speed, but I’ve not tried that combination on a real bike yet.