Just how bad are bottle dynamos?

Question

My tourer has a dynamo front wheel, which is great. It's so handy* that I ended up using it for most of my commuting over the winter, despite having a hybrid that should serve as my foul-weather commuter. So I'd quite like to add a dynamo to the hybrid which does a few tens of km per week. The budget simply isn't there for a new wheel** (or rebuild, but the rim is old) plus lights, but a bottle dynamo from a reputable brand is only around £/$/€12 and my tyres (marathon plus) already have tracks.

Bottle dynamo vs. hub dynamo covers some of it (longevity, tyre wear), but I'm interested in day-to-day reliability as opposed to longevity, i.e. can I just jump on and the lights work assuming I don't clobber it? In all weathers? I can with the hub system and this isn't addressed at the linked question.

---

* Clip-on lights (the vast majority of battery lights) get stolen where I park (and I can't change that) so have to be removed.

** I will downvote any answers that mainly suggest a hub dynamo. I know they're better in many ways, I know you can get a wheel for ~£100 delivered, but that's too much. Unless there's an option that's similar in total price to a bottle, forget it.

Answer 1

Bottle dynamos aren't actually as bad as often claimed. They are usually cheap, lightweight, have less resistance than people think and zero resistance when not in use. There are some bad parts, though:

• Most of available dynamos are complete rubbish and won't last in use. High quality dynamos have been superseded by hub dynamos and battery lights.
• Resistance is still higher than with decent hub dynamo
• They are noisy and the pitch depends on speed, making any slowing down obvious
• The cable connections are often very unreliable (see "complete rubbish")
• The clamps damage finish on the fork. Some systems use fork and frame for ground and depend on puncturing the finish, preventing putting any protection between clamp and fork. This also doesn't work with carbon forks.
• The dynamo is in perfect position to fall into spokes and can cause serious accidents if not properly installed. It is also easily knocked into spokes, so you have to check every time if you leave your bike in public place.

Answer 2

The truth is, that the vast majority of bottle-dynamos are utter crap. They are built to be attached to BSOs (BSO = Bike Shaped Object, a bike that's built so cheap that you can hardly call it a bike), allowing the BSO to pass the legal checks (needs to have working lights compliant to law), but are simply not designed to cope with low temperatures and/or rain/snow. As such, they work fine on a mild, dry summer's night, but fail abysmally when used in snow-rain.

That said, I have come across two designs that stand out from the rest. Both of these two modify the driving wheel of the dynamo:

• The Nordlicht Dynamo replaces the plastic driving wheel with a soft ring made out of rubber. This increases traction a lot, but in my experience it's just not enough to fight snow-rain conditions.

  When I still used a Nordlicht Dynamo, I always used a knife to cut some vertical slits into the rubber ring. This did increase traction further, yet it still failed in wet conditions near freezing.

• The AXA HR Traction uses a driving wheel that's much larger than the typical bottle-dynamo's driving wheel, and which provides traction with vertical ribs that are significantly further apart. The larger diameter means that half the force is needed to create the same torque about the dynamo's axis. And the bigger spacing of the ribs means that there is more force available to press each rib into the tire's rubber.

  I don't recall how close I've come to the snow-rain conditions with this dynamo yet, I've not been using it as long as I did use the Nordlicht. My experience to date is, that this dynamo is significantly harder to slip than the Nordlicht.

Neither of these two models can be purchased with the 12 Euros you mention in your question.

In any case, the examples of the Nordlicht and the AXA HR Traction show you what you need to look for:

• Designs of the driving wheel that significantly increase traction compared to BSO-dynamos. Any bottle-dynamo designer that doesn't change something here, is not concerned with robust usability of the dynamo. The BSO-dynamo driving wheel design is just so bad that it is the first thing to be improved to get a dynamo that works in rain.

• Low resistance of turning the dynamo.

If these two conditions are met, good bottle dynamos (like the two I talked about above) can be used in normal rain without much problems. Snow-rain is always tricky with bottle dynamos as the sludge will get between the tire and the dynamo's driving wheel. Of course, if you avoid riding in snow-rain nights anyway, a good bottle dynamo may serve you well.

However, if you buy a classical bottle dynamo like the ones you see on BSOs (tiny driving wheel made of hard plastic or, worse, metal), it will likely fail you in any bit of rain.

Answer 3

As a grad student, I purchased a Breezer bike with a bottle dynamo for commuting, ~10 km round trip. I probably replaced the bottle dynamo 3 times within 2 years because wear on the dynamo rendered it inoperable. During that time, I came to dislike bottle dynamos because:

1. My bottle dynamos had a mechanical mechanism to disengage them from the wheel when not in use. Somehow the dynamo shifted frequently while disengaged, which meant having to readjust the position with a toolkit in the dark when you reengaged it and found it nonfunctional.
2. The bottle partially slips on the tire in the rain. This is a problem both for light as well as additional wear. I think using my bottle dynamos in bad weather greatly shortened their lifespan.
3. Bottle dynamos do not last. If you care about the reliability of having a light, this is an issue. The components wear quickly enough that you cannot expect to just hop on your bike and go with light.

Eventually, I coughed up the money to rebuild my front wheel with a hub dynamo. The improvement is amazing and I would never go back. There are occasionally problems with the light wiring, but there is nowhere near the uncertainty of a bottle dynamo.

I would recommend a bottle dynamo only for someone who rides occasionally at night only in good weather. Otherwise, you are better off with the battery-powered light you need to readjust the bottle dynamo in the dark.

Answer 4

I don't want to use battery lights because I always forget to turn them off, so they are empty by the time I really need them.

On a bikes I used in urban conditions (lots of street lights) where the main purpose was to be noticed, I used reelight hub lights and flex mount with great satisfaction. Especially as a back light, this is good enough.

If you need a strong front light, They seem to have solutions too, but I don't know whether they are strong enough

Answer 5

There is a bottom-bracket bottle dynamo - more of an barrel shape than a bottle, and would have resolved a lot of the niggles people have with bottle dynamos.

I think this may solve your problem with bottle dynamos.

• It goes in front of the rear wheel and clicks in/out in a fore/aft direction so not poking out the side. No risk to spokes at all.
• The roller presses on the tread of the tyre not the thinner sidewall.
• Larger diameter means its not turning quite as many RPMs as the tiny cap on a bottle.
• Lowers the weight closer to the ground.
• Wiring comes from the middle and goes half front and half rear, rather than one big long run from dynamo to most distant light.
• Unit is more protected from damage being right under the bike
• Less visible to miscreants and the weak-minded.

Downsides

• More exposed to water and road grit though.
• Less common == more expensive.

Answer 6

Regarding your question about bottle dynamos:

I have used bottle dynamos on two old city bikes, both at the front and the rear wheel, and it sucked. Not only do they add much more resistance (wich is not fun early in the morning when you're already late and on work clothes), I also found it difficult to adjust the force with wich they sit against the tyre properly, and and maintain it for a long period. But the worst was that one of them would often slipp around the freezing point and no amount of extra pressure against the tyre would solve that. In short, I found them unrealiable, wich is not what I want for my bycicle lights.

I never used high quality bottle dynamos (if such a thing exists) however, so it may be that with a good, new one you won't have such problems for many years. I can only give my personal experience here.

---

Regarding you real problem, about cheaply adding lighting to your bike:

When, about 8 months ago, I got tired of it, the very obvious solution to me was to buy a used wheel on ebay. After a little wait for the right item to come up I ended up getting a 26" front one in good condition, with tyre and 3W Shimano hub (including the connector) for about 25-30€ (including delivery). A quick search on ebay uk suggests you should be able to find a similar deal.

but the rim is old

Given the fact that your bike also seems to be old (meaning you are more likely to tolerate using 2nd hand parts on it), how much you like hub dynamos and the price difference to a "good" bottle dynamo and to the 100£ new wheel you have set, I would suggest this as a viable solution (assuming you have fairly standard 26" or 28" wheels that can be easily found 2nd hand).