I just had a quick look at the Birdy V-brake; the pads are mounted about 1/3 of the length of the caliper. The V-brake cantilever (or whatever it's called) is 100mm on the Birdy and the pads are mounted 30mm from the pivot. So this translates to a 3:1 force exerted by the pads on the rims.
With 406mm rims, the extra distance required looks to be about 25mm, so that places the pad at roughly 2/3 distance from the brake pivot. This translates to a 3:2 force, close to 50% of the original value.
To me this implies that road brake pull is needed which are about 2:1 ratio compared to V-brakes. I wonder if there is mention of this at the original web site?
Last edited by jur; 08-25-10 at 08:04 PM.