Super tiny and super powerful Velogical dynamo looks very similar to a brushless motor for RC models (see photo in the middle of this page), so I wondered if such a motor could do the same job for a fraction of the price. The result is one explored dead end of scientific progress.
Tactical data: three-phase outrunner brushless motor, coils of 10 turns, internal resistance 62 mΩ, max. load 30 A for 1 minute, 1400 rpm/V. There is almost no magnetic cogging or resistance when the three leads are not connected to anything, and very strong resistance when they are shorted together.
Wooden drive wheel was made with a sharpened end of a file, using a table drill as a lathe. A section of old inner tube made a decent antislip surface. Outside diameter was 25 mm. All vents on the motor were covered with sticky tape and a strip of plastic was rolled around the perimeter to reduce dirt ingress. The jig holding the motor on rear seat stay is also made of wood, with rubber bands providing contact force (no slippage occurred). The rectifier was well insulated after taking the last picture, so there were no shorts. This was the test circuit:
Short-circuit current was 3 A at 30 km/h, but open-circuit voltage at the same speed was only 1 V. In the usual working range, the values oscillated around 0.5 V and 100 mA. Therefore the motor is not suitable as a generator for lights.
Some voltage was eaten by the rectifying diodes, but even without them it wouldn't be high enough to light up a LED anyway. Theoretically the voltage could be increased by rewinding the coils, but a much thinner wire (with higher resistance) would be needed so that more turns would fit inside. Or one could add a three-phase step-up transformer between the motor and rectifier, but I don't know how efficient or heavy it would be for this frequency range.
P.S.: One beautiful brushless motor for sale ;-).