Originally Posted by
CEBEP
Perfect drivetrain for Birdy would probably be rear hub + belt. Belt for Birdy will be just perfect as it doesn’t have to go through the rear triangle. The biggest challenge would probably be how to figure out a tensioner for belt while folding. As far as I know belts are not supposed to be bend in the opposite direction.
This used to be the gospel and many still believe it. But several belt bikes are now being offered with variable chainstay length and a belt tensioner, with "back-bending" of the belt. R&M makes them, and there are also some offroad bikes. I'd definitely like a belt for my Alfine 11 Birdy. (Now I use a chain and chain wax for cleanliness, which works, but must be applied frequently.)
Some factors I think are preventing a belt Birdy:
- R&M don't seem interested in developing the Birdy further, with their massive focus on mid-drive ebikes. Pacific focuses more on lower price models, and have licensing limitations.
- Belts require a straight beltline. The Birdy suspension pivot is slanted, so as the suspension moves the beltline changes, and the rear & crank axles change from parallell to non-parallell. The deviations are small, but perhaps enough to either cause problems or keep R&M from wanting to experiment with it.
- As you say, a belt tensioner that works when folding, and also when riding, can be difficult to design. I guess it needs to be quite strong and stiff.