The Shocking Truth, Part 5: Bottle Generators

“This battle with the bottle is nothing so novel.”–Elvis Costello

 Elvis was right. There’s nothing really novel about bottle generators. They are so well-known that a non-cyclist will instantly know what one is, (thanks in part to the Simpsons),

Nor is it novel that they can work surprisingly well. In fact, the first thing I did when I got this idea about comparing dynamos was to check out the specifications on a series of bottle dynamos. On discovering that the Dymotec 6 was one of the best-rated, I did a quick check on prices and leapt to eBay–finding one that was in the US and was being sold for less than $50.


I also noted, in passing, that it came without a mount (bottle dynamos traditionally mount to bicycles that are intended to take them, but many or most modern bikes, certainly those sold in the USA, lack appropriate mounts, so you need to get your hands on the appropriate hardware.

Many bottle mounts look like bits of old Erector Sets (NB: I live down the hill from where the Gilbert family, of Erector Set fame, once lived). I didn’t want one of those, so I dug around until I found a mount that looked both minimal and strong. It came from England (there’ll always be an England!). But before it arrived, my patience had run out. The dynamo I had picked up turned out to be a left-handed unit…

Discursion: yes, dynamos are handed. A left-handed dynamo is intended to ride forward of the left-hand seat stay or fork; a right-handed dynamo runs on the other side. This has nothing to do with the internals of the dynamo, but rather has to do with the spring-loaded pressure system, which is designed to allow you to easily move the dynamo into contact with the wheel. If you look straight down through the roller, with the mount at the back, a left-handed dynamo will swing right to contact the wheel (and the reverse for the right-handed dynamo). This is not all that important. Most of the time!

…but, lacking an appropriate mount, my mind wandered to my workbench, and I suddenly remembered why I liked rack-mounting hardware so very much. I was able to quickly fashion a mount from a bit of steel rack strap. held down on one end by the front-rack mounting bolt, and tensioned across the fork (see illustration below). I protected the fork with a bit of helicopter tape.


Please pardon the messy shop in the background!!

This meant that the Dymotec 6 was mounted backwards–normally, for safety reasons, you’d want it installed ahead of the fork (so in case it came loose, you wouldn’t experience a sudden stopping event) but I was (1) in a hurry and (2) pretty danged sure of the mount, so I went ahead and installed it backwards anyway. Big Deal.

So. Did it work? Did it work?

In short, yes. I had good solid light front and rear.

What I also had was noise!

The tires I was using, Pasela blackwall 28mm tires, non-TG, wirebead, seemed to act like amplifiers. Remember when you were a kid and played with inflatable pool toys (if you did) and tapping one would make a kind of boom? Well, it felt like that’s what my tires were doing.

What’s worse, because (in spite of manufacturers’ best efforts) tires do not have a uniform shape, the sound varied a bit in frequency as the bottle (under pressure from its built-in spring) followed the varying shape of the tire sidewall.

Consequently, I tried something a little different, and moved the contact point from the sidewall to the rim.

Sidenote: This works because the roller on the Dymotec 6 is made of hard rubber, which provides some grip on a metal sidewall. There are other examples of this type of roller, but almost all low-priced dynamos have metallic rollers, which will slip on (and scrape up) alloy rims. You can (I am informed by others) put rubber grommets or O-rings on metal rollers to make them suitable for sidewall use, but that might take some experimenting.

Running the Dymotec on my rim seemed to improve the noise situation in terms of both the noise level and noise variation aspects.  

Having the dynamo mounted in front was also nice because it meant it was trivially easy to turn my lights on and off—the rubber-covered spring release was just inches away from my down tube shift levers. However, turning the dynamo off meant grabbing the body and moving it away until it clicked into its “off” position. Still and all, not too bad.

Costs Associated with Bottle Dynamos


There was a time when you could talk into any Target in the country and pick up a dynamo light set–and certainly any bike shop would have one. They might not have been good sets, but they would have them. It looks like that’s no longer the case, but you can go to Amazon and type in “dynamo” and find something pretty quick.

My research indicated that the best bottle dynamos was the Dymotec 6. This unit is fairly widely available–it can be obtained from Peter White Cycles, the Dutch Bike Co., Harris, and probably others, for less than $60.


While you’ll get the dynamo for $60 +/-, depending on where you go, you’ll also need to spend a little money on a mount, unless your bike comes with one built-in. Mine didn’t, so in addition to the mount I improvised from a rack mount, I spent $5 to get a seatstay mount from England. Now, $65 is not a trivial amount, but all told, it’s not bad for a light source. And…you don’t need to build or rebuild a wheel. That’s all to the good.


There’s a wide range of weights in bottle dynamos. The Dymotec 6, which has a plastic body, weights in at under 210 grams, mount not included (but not much more). Older dynamos, especially the beautiful and beautifully made Soubitez units, weigh a good deal more.



Here’s where I found things getting tricky. Remember that you have to take “handedness” into account when mounting, because you’re going to want to the dynamo to be able to move toward the wheel on its spring, once you release it from its standby position.

This required mounting the dynamo on the right side of my bike, behind the fork, using my improvised mount. In that position it worked well. I was able to reach the release button without endangering my hand, and turning off the dynamo meant grabbing the body of the dyno and moving it back to the locked position.

However, before you get there, you need to align the dynamo (this is discussed in the Dymotec manual). Essentially, you want to position the roller of the dynamo perpendicular to a line drawn out radially from the center of the hub. This minimizes additional mechanical resistance, and keeps the dynamo from “scrubbing” your tire or rim. Getting the dynamo properly aligned and the mount properly tightened to keep it that way can mean that you need an extra hand or two, keep things in the right position and to hand you 5- and 6-mm Allen wrenches, screwdrivers, and so forth. It’s not extremely difficult, and if you’re careful you’ll only need to do it once, but you should be aware. Also be aware that most dynamo mounts are designed to puncture your frame’s paint job, so that the dynamo is grounded to the frame–many dynamos are built this way and grounded through the case. The Dymotec 6 needs one additional cable to ground to the frame (see the instructions). Many dynamo mounts, such as the very fancy Dynamohalter, have a screw designed to punch through the paint for electrical purposes. The screw also helps keep the mount from rotating, so if you’re committing to this kind of lighting, it’s worth biting the bullet and going through the paint. It’s painful, but, as I said, you only need to do it once. The Dymotec can be grounded to its mounting tab via an included cable, so there’s no need for additional wiring to take care of that. A final note here–to ensure proper running of the dynamo on the wheel, whether rim or tire, the manufacturer tells you to set the dynamo so that its roller is about 10mm away from its running surface when it is not running. The Dymotec includes a knob for fine tension adjustment as well.


Although I purchased a seatstay mount for the dynamo, I was unable to use it. My foot struck the dynamo as I pedaled. This is a situation created by a combination of factors: (1) I use a crank with a very narrow “Q” factor, or tread, so my feet are pretty close to the bike at all times; (2) I have size 13 feet, which means they project back quite a bit while I’m pedaling; (3) Traditional bottle dynamos, such as this one, occupy a space partially outside the triangle formed by the seatstays. Consequently, without contorting my body, there was no way for me to ride with the Dymotec rear-mounted. My bike doesn’t have particularly short stays, but it’s possible that this would not have been an issue on a full-on touring bike. I could also have tried mounting the dynamo on the right side (as I had done in front) , but that would have interfered with mounting panniers, and so I decided not to try it. Because of this, most of my comments below will be based on the forward-reverse mounted Dymotec.


The Dymotec provides two hot and two ground connections on the “bottom” of the bottle, and comes with a nice two-conductor wire, prepped with lugs, so it’s easy to connect. In fact, it comes with pretty much all of the wiring options you need.


Overall Performance

Comparing the bottle to a dynohub, I did not notice much difference in lighting performance. The Dymotec, which meets the StVZO standard, did an excellent job of supplying power to the lights on my bike, and of charging the standlights’ capacitors. Because of mechanical coupling issues, any bottle dynamo will be less efficient than a hub dynamo, but I would have no concern using this system to ride at night.

I experimented with the dynamo running on both the tire sidewall and the rim. In general, I think running it on the rim works better for three reasons. First, pneumatic tires act like echo chambers, and so it was considerably noisier to run the dyno on the tire than on the rim. Second, the rim is more likely to be consistent in diameter than is the tire: Any variation in the rim (dents, etc.) will be reflected in the tire diameter, but any variation in the tire (and let’s fact it, unless you’re buying very pricey tires and tubes, there tends to be some variation caused by tubes bunching, etc., inside the tire). Consequently, running the dynamo on the tire will cause more variation in sound (which I personally find a little irritating) than running on the rim. Finally, you will see some wear from the roller on your tire–much less on the trim.   While some tires have dynamo tracks that deal with this problem and that are designed to reduce wet-weather slippage (e.g., the Vittoria Randonneur), using the tracks may be difficult if you also use fenders.

Wet Performance

My great fear was that moisture would cause serious slippage of the rubber roller. Fortunately, I was able to test this because the Connecticut weather cooperated. We had several days where there was heavy rain at night followed by light showers during the day. I tested wet weather running by running through deep puddles and riding in moderate rain (note that my particular arrangement of handlebar bag and fenders protected the dynamo from rain coming down, leaving it most vulnerable to road moisture). I found that with position and tension properly adjusted, I experienced no slippage running on either the tire or the rim. This was true of riding in the rain and riding through 2″ and deeper puddles.

It’s worth noting that I was not riding in a downpour, but in light to moderate rain. Nevertheless, I was very favorably impressed. There is a special roller available for the Dymotec 6 that uses a wire brush instead of a rubber roller. Supposedly, this will ensure operation of the dynamo in all weather conditions; I did not test it. My understanding from reading up on the matter is that most riders find they don’t need this roller in wet conditions, and it is better-suited to riding in snow and ice.

Charging performance

Bottle dynamos come in two flavors–voltage-limited (to protect the bulbs of incandescent headlamps) and unlimited, which presume voltage regulation at the lights. The Dymotec is of the latter type, and, consequently, can produce plenty of power for charging batteries or electronic devices.


Note that switching a bottle dynamo on or off requires physically moving the bottle into or out of contact with the wheel. If you’re using a front-mounted system, this is easily done while riding. Bart Simpson to the contrary, I suspect that most riders who use rear-mounted dynos will want to stop and engage/disengage the unit. This is not a big deal, as you’re unlikely to be turning your lights on and off. I will say that I have on occasion been on rides that pass in and out of forested areas, and it’s nice to be able to switch on your lights as you enter one of these darker patches. That’s one advantage that dynamo hubs have, since they can be switched electrically. It’s worth noting that some French builders (in particular) used to add a fitting for a shift lever onthe seat tube that could be used with a cable to engage/disengage a dynamo while riding.


Bottle dynamos are noisy. I’ve mentioned this above and I’ll reiterate it here. Running a rubber-rollered dynamo on the rim reduces this noise, and it’s likely that a read-mounted system would have seemed quieter still–my dynamo was in the perfect position for me to hear it! But recognize that the mechanical interface of roller and wheel is inefficient, and so there’s going to be resulting noise and heat. The sound is, as some have pointed out, similar to that of a small electrical motor (which is not surprising since dynamos and electric motors are, in many respects, similar devices–differing mainly as to whether motion is generating electricity or vice-versa). Some people find the sound comforting at night, since it gives other cyclists and pedestrians a little warning. I don’t like noise from my bike, and I found it a bit irritating.


Because the dynamo is mounted at the rim, I found that it was primarily noise rather than vibration that was an issue. Certainly there is some vibration, but it was for the most part lost in the road noise of Connecticut chip-seal roads. Vibration is unlikely to be a major issue.

Subjective Resistance

Again, there are two questions: Does the dynamo cause resistance, and can you feel it?

Of course the bottle causes resistance, and more than a dynohub. In addition to the electrical resistance of the device itself, which is significantly lower than that of a dynohub (see note), there is mechanical resistance, because the wheel now has an additional load connected to it at the rim.

NOTE:   Why is the electrical resistance of a bottle dynamo lower than that of a dynohub? The reason is that the small wheel of the dynamo roller spins many times for each rotation of the wheel, which means in essence that it can use smaller coils and magnets to generate the same amount of power as a dynohub. Much more motion is going into the bottle than into the hub (one consequence is that hub dynamos generally have more magnets, so more weight).

Can you feel it? Yes. The resistance is not large (again, pace Bart Simpson) but it’s there. And, in all honestly, it probably seems like more than it is, simply because you can hear it.

On the other hand–and it’s an important hand–you can completely disengage the dynamo when you’re not using it. No resistance, no noise, no vibration. Your wheel is then subject only to the vicissitudes to which any wheel is subject.



The Dymotec is built to last (I have seen 10-year evaluations on the web). One of the reasons it can last is that, in spite of, or perhaps because of, its use of a rubber roller, it’s easy to replace the roller. Mine came with a replacement roller kit and, as I’ve noted above, you can get a special roller for winter conditions. Replacing the roller consists of (1) snapping off a protective top cap with a flat-bladed screwdriver, (2) pushing a c-clip off from a plastic axle, probably with the same screwdriver, through better tools are to be had (this requires a little care–you don’t want the clip to escape!), and (3) lifting off the roller. Assembly is, as they say, the reverse of disassembly. Replacement rollers are generally around $5-$10, a trivial amount in cycling (grin!).


You can probably smash a Dymotec without any trouble. That being said, it’s unlikely that this would happen in practice without seriously damaging the bicycle to which it’s attached. So long as you don’t run into the problem that I had with the dynamo interfering with pedaling, it’s likely to keep quite safe.

Attractive Nuisance?

The risk here is that the dynamo is one more obvious thing bolted onto your bike. A casually malicious person may grab it and give it a twist while you’re parked, necessitating readjustment. But it’s unlikely to be a target for thieves. Our culture has assigned bottle dynamos a place of worthlessness that operates to protect them. All things being equal, I would be concerned for my saddle, handlebars, and wheels long before I would be concerned that someone might walk off with my Dymotec.

So, what’s not to like?

There’s very little not to like about the bottle. Other than the rear position problem I had, and that not everyone will have, my main reservation is the noise factor. I simply do not like the sound that the thing makes. If you have a particularly sleek bicycle, anything that hangs onto the frame is going to break up the line, and that can be an aesthetic consideration. If you have carbon rims, you’ll probably find slippage to be more of a problem than if you have alloy rims, and I suspect you’d want to run on the tire in that case. I’m not a fan of running anything on a tire sidewall that can leave a mark. It may be psychology, it may be the tires that I use, but the idea of a tire failing from sidewall wear (especially in front) scares the dickens out of me. This may be a lesser factor if you use heavy-duty tires.

Tires become much less of an issue if you run your dynamo on the tread surface itself. That was the philosophy behind the bottle’s cousin, the bottom bracket generator. That’s the next chapter in our electrifying saga!

This entry was posted in Uncategorized and tagged , , , , , , , . Bookmark the permalink.

2 Responses to The Shocking Truth, Part 5: Bottle Generators

  1. DavidBee says:

    Some tire brands and models — Schwalbe Marathons, for instance — actually have a sidewall tread just for dynamos. It wouldn’t surprise me if the tread pattern matched the rubber wheel on the Dymotec. I wouldn’t be afraid to run a dynamo on the sidewall of such a tire. On a cheapie asian tire, yeah, I probably would be.

    • The track is nice, if you can use it. I’ve noticed in photos of some older French bicycles that the builders sometimes provided a “hood” on the rear fender so that the dynamo had better access to the track.

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s