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…

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    Product recommendations are off topic here, however: niagaracycle.com/categories/wheels/rims/16 and niagaracycle.com/categories/wheels/wheels/16 – BPugh Oct 16 '15 at 12:41
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    Some recumbents use use ISO 305. Alexrims makes some rims which are 16" presta but I don't know how many spokes. – Batman Oct 16 '15 at 15:05
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    Are you sure that hub will be ok in such a small rim? Most dynamos are regulated to put out the standard voltage over a given rpm range. Your link mentions using it in a 26in wheel; a 305 etrto would spin a good deal faster. – JHCL Oct 17 '15 at 0:54
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    This site suggests it's intended for 26"-29" wheels only. Depending on the OLN of the frame, you may need something intended for Moulton/Brompton or similar. – JHCL Oct 17 '15 at 0:58
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    @BPugh Bromptons have 16" wheels and are available with dynamo hubs, so yes, you can use a hub dynamo in a 16" wheel. (Which doesn't necessarily mean you can use one in a Yak trailer with just a rear light.) – armb Dec 10 '15 at 17:12

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.

  • 50 Mm? Is that 5,000,000 meters (5,000 km)? – dlu Oct 23 '15 at 0:05
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    yep, that's how the metric system works ;) And that's 250000 furlongs for people using imperial units. – Móż Oct 23 '15 at 0:06
  • A question for the OP is: how much does the trailer slow them down? If a dynamo can deal with fast road speeds on 26" wheels, the rpm on a fully loaded touring trailer with small wheels probably won't be twice that. A decent charging circuit would be needed in any case. – Chris H Oct 23 '15 at 7:14
  • @Mσᶎ, I was pretty sure I was getting my numbers right. That seems like a very short bearing life. Any idea what went wrong? – dlu Oct 24 '15 at 4:40
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    @dlu Ooops, 50Mm is 50,000 km, you slipped a decimal on me. I'm not sure whether anything actually did go wrong, because that doesn't seem particularly short to me. The Rohloff bearings are an oil exit path so they get lubricated, where the dyno ones only have the grease they were shipped with. Either way, $100-odd for new bearings every 10 years seems reasonable. – Móż Oct 24 '15 at 21:51

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