Bicycle Lighting - Generators Compared
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An excellent article comparing various generators was published by Chris Juden of the CTC in 'Cycle Touring and Campaigning', February 1998. The same data was used in other magazines at around the same time, and is the basis for the following assessments.
It is clear from Chis' article that, from a randonneur's point of view, the ONLY performance characteristic of any interest is the drag factor. All the units tested were more than adequate in terms of power output at medium and high speeds.

The outstanding performer is the Schmidt Original hub generator - the weight is 633g for the front hub unit and it is available for 129UKP upwards, from the CTC shop. There are 36- and 32-hole options and various front light combinations. At randonneur speeds, the Schmidt draws about 6W with the lights on, making it equal best (with the much cheaper Shimano Inter-L, about 60UKP) but the real virtue is its remarkably low drag of only 1W with the lights off - just about negligible. The biggest drawback (perhaps even more so than the price) is the use of inaccessible sealed bearings - I know of several people already who have been put off by this.

Of course a bottle or roller generator has the virtue of zero drag during daylight hours, but the best of these (the Union Turbo and the AXA-HR) draw at least 10W at randonneur speeds - now that is NOT negligible!

Chris Juden chose to put his drag figures into 'real terms' by comparing them to slopes of various gradients. Instead, I've tried here to tabulate the sort of delay you might expect over the course of a PBP, and over a typical early summer 400km, by running these generators. This is using the figures taken from the graph on page 49 of CT&C. I've assumed an average riding speed of 20kph, which might give a total time of 75h for the PBP (15h off the bike) and 22h for the 400 (2h off the bike). I've taken a figure for a cyclists' power output under these sort of circumstances as 60W - using data from Fred DeLong's book 'Bicycling Science'. Speed is assumed to be proportional to the square root of the power - this is a compromise figure but seems appropriate to an undulating and exposed course like PBP.

Dyno          W     W    total added total added
Model       (day)(night) time  time  time  time

                           (PBP)       (400)
(No dyno)     0      0   60h00  00    20h00 00
Schmidt      0.8    5.6  61h15 1h15   20h25 25m
Shimano I-L  5.8    6.2  63h12 3h12   21h04 64m
FER 2002     0      8    61h23 1h23   20h29 29m
AXA-HR       0     10.2  61h47 1h47   20h37 37m
Union Turbo  0     10    61h45 1h45   20h36 36m
Union Roller 0     12.8  62h15 2h15   20h48 48m

As you see, the penalty for using a generator is not all that great. The Schmidt is obviously a very fine unit, but expensive and needs to be built into a wheel. The FER is a clever design which declutches for zero drag during the day, but may, to judge from Chris' report, be subject to reliability problems over long distances. A very good choice for short-distance utility purposes though. The equally efficient Shimano has aquired some notoriety since its introduction - it seems to have the unusual property that, at higher speeds, there is more drag with the lights turned off than turned on! Hence its much poorer performance in the table. It leads me to suggest, tongue in cheek, that the most efficient way to ride with this particular unit is to ride with the lights OFF at night and ON during the day!

All the above are hub-type generators, and thanks to units like these this type of generator is enjoying a new-found credibility, although they are still aimed mainly at the utility market. The Union Turbo and AXA 'bottle' generators are very closely matched and either of these relatively lightweight units would be, I suggest, an excellent choice as a backup lighting system (very necessary on PBP). However I personally have great reservations about using this type of generator as the main power source on long night rides, as there is a serious safety issue with the wear on the tyre sidewall. Again, for shorter distances or with heavier tyres, either the Union or the AXA is a good choice.

Finally, the Union roller generator is included for completeness - this unit has a very good reputation, so it is interesting to see that there are so many other options on the market which perform better. It did, however, come out top for power output at low speeds - good for hill-climbing.

Riders who prefer battery lights (as I do) might care to consider that there are also delays that must result from battery use. Either a very large and heavy lump of battery must be dragged all the way, day and night, up all those hills, or, if you travel light, you have to buy new batteries at frequent intervals. On an event like PBP, this could involve queuing, or searching for the right place to buy them - this when all you really want to do is rest up for a while.

Maximising Output

All generators have adequate output, given that they are invariably tailored to provide 3W and no more, this limit being influenced by legislation in Germany, a major market for cycle generators.
Some types have a rising output at high speeds - this means that there is a risk of bulbs being blown during descents, so it is often necessary to fit a voltage regulator (small, simple and cheap). Some generators come with a regulator fitted as standard, as do some lights intended for the generator market.

Traditionally, generators power both front and rear bulbs, a 2.5W bulb at the front and a .5W bulb at the rear. A preferable arrangement is to use a front-mounted generator to power a front light only. The bulb will then be a brighter 3W type, and there will be no messy wiring running to the other end of the bike. Rear lighting can be left to a modern battery-powered LED unit which will do the job supremely well. 3W bulbs can be hard to find but in the UK they are sold under the Reflectalite brand.

Some efficient generators (notably the Schmidt) can be made to provide 6W at speed. By using two 3W front lights wired in series to provide 6W, then adding a switched bypass to one of the lights for lower speeds, a generator system can be provided with a 'high beam' for fast sections. The drag will be greater, but that hardly matters in downhill situations.
See also Using a generator to run a 6W bulb

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