Having used both extensively, I can't see why anyone would choose a bb dynamo when such good hub dynos are available so inexpensively. Admittedly, bb dynos are an improvement bottle dynos, but that wouldn't take much, IME. But, again, when something even better is available, why bother?

Oh yes - there is the old red herring of "added resistance when the light is off", which might be meaningful if you were racing, and if the difference was actually measureable. I'm running a Shimano DH-3N71 on the front of my recumbent. This is a dyno designed for a 26-28" wheel being used in a 20" wheel, which should theoretically add even more resistance. As it is, my cruising speed is about .3mph (three tenths) slower - with the light ON! With it off? Suffice it to say that I keep track of my commute times, and I'm currently going faster than ever.

So yeah, bb dynos were pretty cool 25 years ago, but now they're just a retro curiosity. No functional reason to use one instead of a decent hub dyno.

SP
BEnd, OR