I know I could use battery lights, but I've become very fond of not having to change or even think about batteries (a lot of our biking is "car replacement" rides around town, so it is really nice to just know that you've got lights). Seems like this would also give me power to charge a phone or other small device since the draw of a rear light will be well below the capacity of the dynamo.
A wheel built on 305 mm rims is going to turn just about twice as fast as a wheel built on a 622 mm (700C) rim. My impression is that the output voltage of the hub increases with speed – and that the output of modern hubs are regulated. So I'm wondering what issues I might run into using a dynamo generator as a power source. Some that I've thought of are:
Availability of rims (the hub is available in 28, 32, and 36 hole drillings). A rim drilled for a presta valve would be nice since it would mean that a single pump would serve for bike and trailer without having to convert the head.
Availability of suitable hubs. The speed difference between the bike wheels and the trailer wheels has been pointed out, but I haven't been able to find data on the hub behavior. I would assume that dynamo hubs would be designed to tolerate a full range of biking speeds – say up to at least 65 or 80 kph (40 or 50 mph). Since the trailer is placarded with a speed limit of 40 kph I'd been thinking I was within bounds.
Finding short spokes. By my calculations I'd need a 110 mm spoke to do a radial pattern and could go up to around 155 mm doing a 4-cross. Seems like recumbents and BMX bikes would mean that spokes this short are available somewhere. Failing that could I have them custom cut from straight gauge spokes?
So it seems like the big issue is the hub? Does anybody know the limits on modern hub designs? So far e-mail queries have gone unanswered…