Hub dynamo
From Wikipedia, the free encyclopedia
A hub dynamo is a small electrical generator built into the hub of a bicycle wheel to power lights. Modern hub dynamos are regulated to 6V 3W, although the best will drive 0.5A at up to 12V.
The market was largely pioneered by Sturmey-Archer with their Dynohub product of the 1950s-1970s, which competed effectively with contemporary sidewall (bottle) and (later) bottom-bracket units. The Sturmey product was heavy, having a steel case, and like all bicycle dynamos of the time tended to overdrive the lamps at high speeds. Modern hub dynamos are greatly more sophisticated. Their electrical design is largely self-regulating, and lamp failure due to overvolting is now rare.
The Schmidt Original Nabendynamo (SON) is considered the best hub dynamo on the market. The SON can be used to power two 6V lamps in series at speeds above about 12mph, and Schmidt manufactures lamps designed to facilitate this. These lamps have optics based on the Bisy FL, a very highly regarded light for road use.
The efficiency of the SON is quoted by the manufacturers at 65% (so just over 5W of the rider's output is diverted to produce 3W of electrical power) but this applies at only 15 km/h (10mph). At higher speeds the efficiency, while still superior to most bottle dynamos, falls. Motor car alternators, in contrast, achieve efficiencies over 90% by using electromagnets to adjust the field current. Bicycle dynamos instead use permanent magnets to elminate the need for a battery to "excite" the field and initiate electrical generation.
Shimano makes a variety of hub dynamos under the Nexus brand. Their latest hub dynamo is the DH-3N70 (also called DH-3N71), which has significantly less drag than the Nexus NX-30. These are regarded as sound value, being considerably cheaper than the SON. However, the Shimano hub dynamos generate more drag than the SON. The Shimano units come with an automatic light-sensitive switch.
