I've got a cheapo generator light, the kind that uses a "bottle" drive that attaches to your frame and is turned by contact with the tire sidewall. I use only the headlight, although I don't think that matters to my problem. A few weeks ago, I used it in a pouring rain, and it did not perform well. It would sometimes not light at all, but more often it was just a lot dimmer than it should be. Sometimes it would start out getting brighter as I accelerated from a stop, but then drop back to a dimmer level once I passed a certain speed. Weird. At stops I would reach back and fiddle with the bottle to make sure it was making good contact with the tire, but that didn't improve it for long.

It has worked fine since then, so my theory is the rain was bothering it. Maybe it made the tire sidewall too slippery to maintain good contact? Or something electrical, since the frame is the electrical ground? Anyone else ever experience this? (The tires are 700Cx23, so pretty thin to start with. Usually not a problem, but maybe marginal enough to be sensitive to being wet.)

And yes, I know these lights are pretty worthless as a primary lighting system. I actually did have a small battery-driven light that night as well, so it didn't worry me too much. I just like the idea of having a battery-free backup available all the time, in case I'm caught unprepared in the summer, as sometimes happens.

My experience with generator lights 20+ years ago was that they didn't always work well in wet conditions. I thought it was a slippery issue but it may well have been electrical.
A modest LED headlight with alkaline batteries might meet your need for a backup light. One set of batteries could well last hundreds of hours if used in flash mode.

If you want a dynamo that works well in the rain, you'll need to get yourself one of those fancy Busch and Muller ones. They have a head you can put onto the bottle with bits of wire which keep the head spinning in all weather. If you're okay with your cheapo dynamo, I'm sure you could make somthing to imitate theirs.

The first problem may be that your tires lack a dynamo track to better grip the roller. Usually, European touring tires have these - Schwalbe, for example. The second is that it is a cheap unit and not suitable for primary lighting. You can fiddle with roller position and pressure and get the thing to work. My solution was to toss the thing and get a front dynohub. I've never regretted doing that - noce, bright silent light, even at low speeds, unaffected by rain, mud, and snow.

B&M makers a quality bottle generator that commuters in Germany like a lot, but I have not tried it.

Paul

That's normal John. You are right about slipping in the rain. There are other types of dynamo wheels that have better traction on the tire, but they chew up the tire quicky. There are even tires that have a dynamo track for better traction. I have only seen a few. Maybe Schwalbe?
Possibly not sold in the states. They still don't work well in the rain anyway. Make sure your tire pressure is not low. I did not even know a dynamo would work with a 23 mm tire.

Don't forget to carry an extra bulb. The bulb life is short, If it gets more yellow than normal replace the bulb. They typically blow from high voltage when you are going fast. Just when don't want to lose your light.

Bottle generators are known for slipping on wet or greasy tires, which is part of why hub generators are becoming more popular. There are tires with a ribbed sidewall to reduce this problem, and proper adjustment of the generator position will help reduce this problem, but bottle genertors are vulnerable to this issue, and I would expect a cheap bottle generator to be worse than a higher quality generator.

A good hub generator like the Shimano 3N-71 or the SON (Schmidt Original Nabendynamo) combined with a decent quality headlight and taillight from Busch & Mueller or Schmidt is an excellent way to go for commuting. No batteries to worry about, no slipping generators in the rain, just flip a switch when you want light. The Schmidt hub generator is a beautifully made work of art and priced accordingly, the Shimano generator is a very well-made, functional unit that will give very good service and excellent value for the money. There are cheaper Shimano hub generators, but they offer more drag and apparently won't last as long. Look at the Peter White Cycles website for more about good generator systems.

I've read of the "slippage" problem with bottle generators, but never experienced it on my daily commute and our NW Winters are long, dark & wet. I think the slippage is a tension adjustment problem.

I used several bottle type generators on different bikes & recommend the Union brand. It produces fairly good light at a low speed & there appears to be some kind of over voltage protection, since I never burned out a bulb on fast descents. These generators do produce sidewall wear on most tires.

For years, I rode a Centurion Pro Tour in the wet season & had built a front wheel with a SA Dynohub powering a halogen light. My backup Winter bike was a Miyata Seven Ten with a Union bottle generator powering a halogen head & tail light. Both produced fair light on back country roads. My other touring bike had a Soubitez bottom bracket generator(also no slippage problems). I recently replaced it with a NOS Sanyo BB generator. All of these systems only produce 3 watts, which is probably the bare minimum required for night riding.

For a while, I used a rechargeable system: 6 Volt battery powering 10 W & 15 W headlights. Much better light especially with both lights on, but the nightly recharge is a drag & the batteries seldom last beyond one season of daily use. Don

A lot of the cheaper dynamos use the frame as the ground rather than using a two wire system. This means the frame of the bike and the frame of the generator need to make contact and that the contact needs to be a good one. Rain might interfere with this by creating a large wet connection to mother earth itself. Thus diverting valuable electrons from the lightbulb. I don't have any proof of this and I'm no EE graduate, but I think this may be a reason for poor performance in the rain.

I've not used a bottle generator, but with my dynamo hubs I've had no problems in the wet.

Both are available in the states. I'd start with Peter White.

I have had much better luck running a bottle dynamo with a standard head on the tire tread rather than on the tire sidewall...even if the tire sidewalk is patterned for a dynamo. you can only do this on a relatively smooth or inverted tire tread. IMO, if you've got adequate spring tension, then it's mostly about contact area, the less of the dynamo head in contact with your tire, the more likely that there will be slippage. also when the dynamo head stops turning in the rain, it starts wearing through your tire, and I've had a B&M rubber dynamo drive head develop flat spots too. So I always turn off the dynamo as soon as it starts slipping and I always carry a backup battery light in my bag, 'cause my dynamo usually only quits in the worst rain, once or twice a year, just when I need lights most...otherwise, I love my dynamo lights, mostly I use old Union or Soubitez bottles, and the old Union lamps produce very good focused beams as well, you can get thread in replacement halogen bulbs for the older Union lamps from Reflectalight

I'm shopping around 'cause I'm unhappy w/ decay in performance of my 15W halo system. Through 2 seasons of daily use, the battery still works (charge life down to ~60 min from >2 hrs on the package at least to begin with). I charge it at work during the day, so can still make it home and then back to work the next day. Actually my commute lately is longer, >1hr/day w/ kid shuttling from ~45 min/day w/o, so I'm going to be pushing the limit that the battery is falling back to. It's probably too much to hope for LED performance to increase and price to decrease at the rate I need to replace that 15W halo for <$200. Maybe winter '08-'09...

Dynamo hub would be cool too but then it's harder to swap rims to move between snow and regular tires.