Trailer Lighting Using a Dynamo Hub I have a B.o.B. Yak trailer which uses a ISO 305 mm BSD rim. I'd like to build up a wheel for the trailer using a dynamo hub (perhaps a Sanyo H27) so that I could have lights on the trailer.

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…
Velocity definitely make 305 rims, and at least when they were in Australia would use their available extrusions to make semi-custom rims to order - most often so you can get the number of spokes/drilling pattern you want, but they would also make odd sizes if the extrusion was compatible. But they've moved to the US and I have no idea what they're like now as far as custom builds go.

SJS in the USA also stock 305 rims and I found quite a lot of hits just using google so decent rims in that size seem to be fairly widely available.

Schwalbe make bicycle tubes in a wide range of sizes with both Schrader and Presta valves - if you can find what you want on this page your LBS will be able to get it (although it may take a while). There are quite a few choices in 305mm presta tubes.

You may have more problems getting a dynamo that's designed for a 305 wheel. The circumference is half that of a 622 (27") wheel so it spins twice as fast at the same road speed, and unlike a childs bike your trailer is going to be used at normal road speeds. SON make dynamos for folding bikes with small wheels so if the Sanyo doesn't work I suggest trying SON. I have one and like it, although I had to replace the bearings after about 50Mm.