Originally Posted by
steelbikeguy
There is no change in the hub dynamo. If the wheel is turning, there is voltage at the terminals.
There are small losses when the dynamo turns... a few watts, if memory serves.
There is additional drag when the light is turned on and the current flows, but I think it is about 9 or 10 watts total for the SON. Other hub dynamos have different losses, but the SON usually has the lowest losses (this is based on old data... maybe other dynamos have improved since then??).
With the light turned on, spinning the front wheel will only result in a few rotations (roughly). 9 or 10 watts isn't much compared to what it takes to push a bike down the road, but it is a lot compared to the tiny amount of power required to keep a well lubed wheel spinning.
Steve in Peoria
Thanks for the info - I’m contemplating building a wheel with SON dynamo.
The last set of wheels I built, I remember soon after finishing when I spun the front wheel on the truing stand, it went on for quite a while - I’m thinking may be 2 min but I may be wrong on that. A small drag to avoid charging and using batteries seems like a good idea at this point.