Originally Posted by Jipe

The chain tensioner I am talking about is the one mounted on the bottom bracket, all recent Birdy are equipped with this tensioner that works very well. But it limits the chanring size to 55t or maybe 56t.



Standard Shimano cassettes are indeed cheap but start with a 11t cog that combined with the limited size of the chainring produce a relatively short gearing even with 20" wheels that are limited in total diameter to about 462mm (ETRTO 406 with 28mm wide tires, 28-30mm is the max with 20" wheel for the front fork).