Originally Posted by
dscheidt
I can think of half a dozen ways to do the engagement that have no moving part of the clutch rotating when disengaged, and which have zero drag. And that's without thinking hard; there are surely others. I still don't think it's a good idea, but not for the 'it doesn't work' reason. The point of a dynohub is simplicity and reliability. Adding moving parts rarely enhances either of those goals.
I don't believe you. Just because you don't consider it part of the clutch it doesn't mean your mechanism won't add drag and it doesn't mean it will be reliable or efficient.
Using your same simplistic point of view there's no reason tossing a few magnets inside an otherwise rotating hubshell should add any drag...but it does because in order to make it reliable the generator mechanism needs it's own set of bearing surfaces, etc. Likewise for mechanical engagement: in order to have efficient engagement while "on" you need tight tolerances and secure fit which means you need planetary gears, bearings, etc which will drag even when the electrical part is mechanically disengaged.
Think about it, why isn't a bottle dynamo as efficient as a hub dynamo in use? There's nothing magical going on inside the hub; it's the tradeoff between designing an efficient engagement mechanism and one that offers complete decoupling. You can't half both, although an electrically decoupled design like a SON offers the best tradeoff.