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SON 28

Probably the best (and also the most expensive) generator you can find. It advertises low resistance when turned off, good efficiency, long life (5 year warranty) and no maintenance (no way to get inside, it's all pressed together). So far after six years of troublefree use (over 16000 km) it seems to be true.

The cartridge bearings are well sealed, there is even a pressure compensation device to prevent moisture ingress during temperature changes. The axle is hollow (solid variant doesn't exist) and the skewer comes with a quick release lever, Allen head (pictured) or a security head. Two output terminals are located at one end of the axle; connectors are included (together with heat-shrink insulation), spare ones are readily available at normal electric shops. Other side of the hub may contain a disk brake mount: ISO (six bolt flange, pictured) or Centerlock (splines). The whole hub is pretty small and lighter than a combination of standard hub and a tyre-driven dynamo.

Number 28 in the name says the generator is designed for 28" wheels. I laced it into a 20" folder where it spins faster and generates more electricity. If you prefer low resistance over high power, Delux variant is a better choice for 20" wheels.

Characteristics

Measured with 20×1,75" (47-406 mm) wheel, circumference 1546 mm was set in the speedometer. Rescaled speeds for 28" wheel are included in the plots. For wheels of different size, divide the speeds by circumference of my wheel and multiply by circumference of yours.


Open circuit voltage rises mostly linearly and reaches 50 V at 25 (34) km/h, I didn't measure it any further.


Maximum short circuit current is 540 mA regardless of speed.


With a 28" wheel and a load of standard light bulbs, nominal 3 W are produced at about 20 km/h. With lower currents the efficiency is better and you get almost half watt more. With a smaller wheel you get a mobile nuclear power plant :-).

For the best possible efficiency, equivalent resistance of the load should be 30 Ω or more. With a 20" wheel, you can raise the voltage up to 12 V and connect LEDs in long series with small resistors (except those which should work at low speeds). For 28", something around 8 V is better.

Source data to download (XLS, Excel 97).

Does it pay off?

Let me round the SON's cost to 240 € and try to calculate how much would other power sources cost. I use the generator almost all the time, even in daylight when there's traffic. It has generated cca 3.5 kWh of electricity over the 6 years since purchase. That equals something over 1000 AA batteries costing about 800 € in total - single use cells are definitely the most expensive energy source in existence. They only pay off for applications with extremely low consumption and long usage time (calculators, watches, speedometres, emergency lights for infrequent use etc.) where accumulators with their aging and self-discharge would perform worse.

Rechargeable NiMH AA cells (8 pieces in series) would keep my lights running for about 5 hours, which means one charge per week (if I only rode to work and back) and about 200..300 cycles in total. I know from experience that NiMH become unusable after two years of such cycling, so I would have used up three sets costing 80 € in total, a charger would cost about 40 € (one RC or two ordinary with four shafts), cost of electricity from the grid would be negligible. Of course, a wheel without a dynohub needs another hub: with disk brake mount and bearings of comparable quality, it would cost another 40. Total sum: 160 €. It would take another 6 years to equal the price of the generator, then it would start costing more - SON is said to work for at least 50000 km and its maintenance costs are zero.

Li-ion 18650 rechargeable cells would have similar capacity as NiMH, one set (3 pcs in series) would cost something around 24 €, charging would be somewhat faster and service life longer - let's say three years, maybe even more, so I'll guess 40 € for batteries for 6 years, plus again 80 for charger and hub, around 120 € total. That's better, it would take 20 years to equal the SON's price. Moreover, Li-ions are easy to get for free from discarded laptops and other electronics, so if I don't mind lower capacity (and therefore more charges per week or more cells in parallel), I could get to 80 € for the whole life of the bike. So Li-ions are cheaper than SON.

Of course, SON is not the only generator in the world. Shimano Nexus costs about 40 €, Sturmey-Archer about 80 (brake included). Accumulators have no chance of beating those on the economy front, so it's up to your taste if you prefer losing time by charging or by riding slightly slower.

All of this is just theory. If I designed my lights to run on batteries, they would be completely different and use much less power, so less batteries would be needed, they would last longer and price could be pushed lower.

There are also other aspects, not just economy. Ecologically, a generator is a harmless pile of metal, plastic and magnets, while battery electrolyte is not something you would like to see leaking into soil. Weight of batteries and generators of similar power rating is about the same. Mechanical drag is pure zero with a battery and about 5 W with SON (most people don't notice, but it's there). Batteries need regular charging and worrying about remaining capacity (a spare set of batteries and a second charger at the other end of your commute minimizes worries, but increases price), hub dynamo is simple: fit and forget, as long as you're moving, you're shining, there's nothing to run out. On the other hand, if you stop moving, you stop shining and sometimes it may be a problem. Anyway, this paragraph digresses too far from objective numbers and presents my subjective opinions about which power source is "better", so I'd better stop now :-).

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