Originally Posted by
steelbikeguy
can you elaborate on the shortcomings of hub dynamos regarding rpm?
The German standard requires the dynamo to bring the light up to full power at a fairly low speed.... 10kph or so, IIRC.
Of course, there are some dynamos being sold that don't meet the standard, or are designed for smaller diameter wheels, but anyone worried about low output at low speed wouldn't buy one of those.
Steve in Peoria
Unless you are climbing steep hills, the hub runs at higher rpm than the crank. And on steep hills you likely are OK giving fewer Watts to the dynamo. So a crank dynamo would have to be physically larger for a given power output.
That chain dynamo from above can be small due to higher rpm. But as pointed out, has its own issues. Especially on FD drivetrains I assume.
A tire friction dynamo also runs at high rpm. But has tire wear, noise, friction and other issues.
In the end the hub dynamo still is the best of all bad options.