Originally Posted by
cplager
... No, it doesn't.
The output of a generator depends on how quickly it is being turned. If you have a hub generator and small wheels, the RPM of the wheel is higher and therefore your generator will be turning faster with a 20" wheel than a 799c wheel.
For bottle generators, it is how fast the tire is moving past the bottle that matters. So, at 10 mph, the outer edge of the tire is moving 10 MPH (relative to the fork) with 20" wheels and 700c wheels*. So, the bottle dynamo spins at the same rate regardless of the wheel size.
Cheers,
Charles
* If one wants to get nit-picky about this, since the dynamo runs on the sidewall and not the edge of the tire, it's almost certainly the case that the dynamo runs more slowly on the 20" wheel than 700c wheels as, say, 1/2" from the outer radius of a 20" wheel is a bigger change than 1/2" from the outer radius from a 700c wheel. But this is not a large effect.
+1.
To help illustrate, if you have two wheels, one with a 40" circumference and one with an 80" circumference (just for arguments sake and to simplify the explanation) the smaller wheel will have to turn twice to cover the same distance as the larger wheel, that being 80" of linear travel . A bottle dynamo will only sense a total of about 80" of travel in either case whereas a hub generator will sense two revolutions with the small wheel as opposed to one revolution with the larger wheel for the same distance traveled.