For those interested, I found the following discussion on dynohub drag:http://www.ecovelo.info/2009/11/05/new-dyno-hub-test/
...then followed a link from that discussion to the Schmidt web site, and translated the relevant terms for the graphs:http://www.nabendynamo.de/service/pdf/ar_01-02_2009.pdf
Basically, the SON 20R has the least drag* (coincidentally, that's the dynohub model on my bike!
), and (if I'm reading it properly? Feel free to correct me if I'm not) produces about 1.5W more drag at 10km/h, 4W more at 20km/h and 4.5W more at 30km/h.
If we compare that to the total power required for (and this is just one example of many possible permutations) a 90kg person+bike to travel at 32km/h on the flat with no wind (see here: http://en.wikipedia.org/wiki/Bicycle_performance#Power_required
), which is 175W, then the drag makes very little difference to our cycling. (Of course, there is this statement in the Wikipedia page: "Computational fluid dynamicists have looked at this bicycle modeling problem and found it hard to model well"
). And it just occurred to me that, apart from the drag, the minor extra weight of the dynohub, compared to a standard hub, might make a slight difference for performance during ascents.
I've never noticed any drag with the SON 20R (but then, I've never been without it, so perhaps there IS a noticeable drag but I'm just used to it?).
If I got any of the above wrong, please mention it, because I'm just doing my homework and edgerkatin' meself 'ere.
* At least, that's what Schmidt claim, and it IS their product.