The hub dynamo in the test are hub dynamo for big wheels that rotate slower than the small wheels of the Birdy (and most folding bikes).

The Shutter Precision dynamo of the Birdy disc is a special type of SD-8, the results of the tests in the link aren't valid for small wheels hub dynamo that have other magnets. Its the only commercially available hub dynamo compatible with the Birdy disc since the SON XS for Birdy disc was discontinued (its actually the same model that was mounted on the Brompton in non disc version, now replaced by the smaller SON XS New).



On my Ti Birdy, I mounted a Velogical rim dynamo on the front wheel. It weight less than any hub dynamo but the power output vs. rotational speed characteristics is very different from a hub dynamo (less power at low speed, up to much more at very high speed).