New BIRDY spec 52T chainring, 11-34T freewheel (11, 13, 15, 17, 19, 21, 23, 26, 30, 34)
"running the numbers" through a gear inch calculator, I compute a range of 25-80.
The New BIRDY 3D forged stem has a function which not many people knows. The upper stem can rotate 180 to change the 21degree to 19.5, not sure if this info will help you but I wouldn't know this if I didn't visit Pacific museum. However Pacific also do offer the 10/21 degree stem as an optional accessory!
This I did not know...but, I think the 1.5 degree net change would be negligible. Its possible that the 21 (or 19.5) degree stem would be my preference, but its nice to know if I would have the option of change.
Pacific museum staffs also told me the reason they spec mechanical disc brakes is to prevent problems from folding & unfolding.
That makes sense to me. The origin of my uncertainty is that somewhere (noting that I've spent a lot of time "researching" Birdy details and options online) I read that the disk model was hydraulic, a statement that contradicted most of what I had read.
I have the BIRDY standard 9sp model and the original components are good for me!
Hope these helps
