see thread: URGENT HELP WITH SON DELUX
I started that thread to find out about the Son delux dynamo hub
and if it was suitable for bike with wheels ABOVE 20"
seems the SON 20R has been replaced with the SONdelux
the old SON 20R was only legal in Germany for up to 20" wheels
now it seems its been renamed the SON delux and is OK for wheels up to 28"
even though its the same thing, so it will include your 20" wheels
So I guess the SON delux, although expensive, would be an option.
Reading tech descriptions of products if in doubt is always worth while
To buy a SON go to: https://www.starbike.com/php/suchen.php ... n&b=SON&q=
and take your pick (they charge £12 (15E) for delivery but it will arrive by courier in 3 days) or
the (mostly in german) Son dynamo pages for general infohttp://www.nabendynamo.de/english/index.html
In my investigations SON dynamos are the cats meow,
mine is currently on its way to me for use next year on my 26" wheel tourer with M&B lighting and an EWerk