hate to bring this up as it could be viewed as spam. But still want to spread the word.
Birdys are rolling on 18 inch wheels ... and I know a few folks who would like to switch to 20 inchers due to better tire selection and all ....
i found the solution .. V brake extenders ....
they also work on 20 inch Dahons to run 451 wheels
p.s. I also have in stock a front derailleur hanger for the Birdy :-)