Originally Posted by
cplager
Hi,
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.
Incorrect.
Your arguement is actually self defeating as if you state the the hub is spinning faster on a smaller wheel, then the smaller wheel is also moving at a higher rpm for a given speed than a larger wheel.
Comparing actual wheel sizes here...
A 406 mm wheel turns at 168 rpm at 10 miles per hour, a 622 turns at 123 rpm, at 30mph the 406 turns at 504 rpm and the 622 wheel turns at 368 rpm.
So if a bottle generator on a 700c wheel needs to have the bike rolling at 10 mph to bring it up to full power, the smaller wheel only has to be turning at 6.5 mph to spin the bottle generator at the same speed as this is where the rotational speeds of the wheels match.
Whether it is a hub generator or bottle dynamo, a smaller wheel does not have to turn as fast to deliver the same output as a larger wheel