Hi, Friends:
I intend to try the configuration shown below.
It is a hub dynamo that spins against the tire’s outer rubber. This would allow the dynamo to be shared with different bikes and easily be removed when not needed.
Do you think the hub or lights would stand the ~10✕ increased spin speed?
Thanks for your insights.

Have you tried to turn a dynohub by hand when it was not attached to a rim? It takes a lot of energy to start to turn it. I doubt your tire will have sufficient grip. And even if it did, you might not be able to go very fast on the bike.

Sanyo used to make a dyno that worked like this. It was probably the best dyno of its time.
How well your dyno works depends on the windings.

On edit: okay, I missed your idea of using at dynohub. Might be okay. I would still find a sanyo, they are out there.

The Dynohub or other hub style dynamo may indeed spin too fast for proper output, but it can be limited if you build a voltage limiter into it. I will also agree with @Unterhousen that for what you are looking for the Sanyo, and there was another manufacturer with a similar design dynamo that rode on the tread of the tire, would be a better way to go. Here is one on Ebay Sanyo Dynamo going for $45.

With that said, I have a Sanyo dynamo that I only use infrequently. It creates more drag than I would like and I prefer to use a battery powered light. At the time, the late '70's, this was pretty good. Battery powered lights needed the batteries replaced often, rechargeable batteries weren't that good and the traditional bottle dynamo had issues with drag, alignment and wearing the side wall. And the DynaHub output was marginal. Very clever design though.

One more thing about riding on the tire tread, the Sanyo slipped when it got wet leaving you with no light, or intermittent light at the worst possible time.

I haven't had the chance of holding a dynohub in my hands, let alone check its inner drag.
Your suggestion seems quite reasonable to me, Tourist in MSN. After all, light output always comes at an energy cost.
It didn’t occur to me that the dynohub spinning drag must be much higher than that of a side bottle dyno and that it is only overcome due to the rim diameter leverage. According to that, I also gather that in my configuration the dynohub should be much heavier than they are or that some kind of compression spring would be needed (quite a claptrap to my mind) to force it down to hold grip against the tire tread. That weight, plus the added drag of an oversped dynohub is what would work against acceleration and speed.
Anyway, thank you very much for your enlightening reflection (unintended pun). A trashed bad idea saves time and money.
Cheers to your health.

Bizikleto: Velogical has a lightweight rim dynamo that has less friction than a dyno hub and doesn't slip like the old bottle dynamos. I considered it when I was ordering my bicycle. I went with the dyno hub instead because it is not noisy and I new the durability was very good.

I have sometimes been tempted to buy a velogical.

One issue with removable dynamos is the lights still need wires. I used to use a dyno/headlight combo. Those were the bad old days of lights, but it did get me to work and back.

At some point, dynos saturate. I don't know what happens to drag at that point. I know SoN made a different dyno for small wheeled bikes, but it was pretty popular with people who used 700c bikes and I think they may have removed the distinction.

Not sure what you mean by saturate?

I think the small wheel bike model was renamed the Delux model. That said, I heard it on the internet, so that could easily be wrong.

Yep that was my idea too. OP, here’s a pic of the Sanyo. As you see it mounts under the bottom bracket and runs off the front tire.

They worked better than bottle dynamos but not as well as a hub dynamo.

When I bought my first dynohub over a decade ago, I used that wheel on three different bikes. Eventually, I bought more dynohubs. No longer have to move that wheel to different bikes.

My point is that if you have several bikes that have the same wheel size and same type of axle, you could get by with one dynohub wheel. Then later if you like it enough, get more. If you had some bikes with rim brake and some with disc, you could have a wheel built up with a disc hub and rim brake rim, so that the wheel would work on both. I have one such wheel, although the only reason I built the wheel that way was that the hub that I wanted was out of stock so I got the hub that also worked with disk.

But the axle could be an issue if some are thru axle, some are quick release. There are ways to make a through axle wheel work on a quick release bike, but that gets more complicated with adapters.

Side note: I use a bolt on skewer on my dynohub wheels, not a quick release skewer. I suspect most thieves in my community do not know what a dynohub is, but I am sure some do. I want to make sure that the opportunist thief can't grab my wheel. Thus, that type of skewer. I keep a 5mm allen wrench for my skewer packed with my spare tube.

They operate on the REAR wheel.

I installed one on my spouse's bike. She rarely needs lights, but when she does, I can be sure she hasn't charged them, so this was a good choice. The drag annoys her, so I might eventually replace it with a hub dynamo. She doesn't want me to because of the weight, but she probably won't notice the difference.

I had the same question.
I assume that it is reference to the steel core saturating? i.e. unable to become further magnetized.
If it was an inductor and you were applying current to the winding, then saturation is an issue. For a dynamo, the level of magnetic flux in the core is determined by the magnets. It doesn't seem likely that the designer would use magnets so strong that the core would saturate.

I have a vague memory of someone running tests in a manner similar to what the OP is suggesting. The open circuit voltage did rise in proportion to the dynamo's rotational speed.
For the OP's application, there's no particular value to the extra open circuit voltage, since the winding inductance will still limit current to about a half amp, and most lights are designed to just draw a half amp and don't care about what the dynamo's open circuit voltage might be.

Personally... for a dynamo that can be swapped between bikes, I was always fond of the old block dynamos of yore. They attached to a bracket on the fork and could be removed by taking out a single bolt. Yeah, it's still a bottle dynamo with all of their issues, but it was convenient. No wires either, unless you wanted to wire up a tail light.
My old Jos block dynamo...



Steve in Peoria

Saturate isn't the right word, but I'm pretty sure they won't just increase in power output ad infinitum. Drag might increase to the level of slippage though. Open circuit voltage isn't super important unless you are switching on the light while rolling.

About a decade ago, I was a bit frustrated with the high cost of USB chargers that were powered by a dynohub. So I tried to make one. And after two or three of them quit working, I put a volt meter on my dynohub output on a shallow downhill. It was over 30 volts and I was not trying to go fast.

From that I realized that if you had a USB charger in the circuit but were not using it to charge any batteries, you had better have a circuit that could handle pretty high voltage input. That was the day I decided to give up trying to build one and to buy one that was designed by someone that knew a lot more than I did about designing those things.


I have a few of these in a box somewhere in storage, but Union brand.

I had one on my errand bike. In fall after the end of daylight savings time, I often was riding home after dark. I was riding on well light streets, the incandescent light was not very bright but it was bright enough to be a good to-be-seen type of light.

I recall adding some zener diodes to it to prevent the bulbs from blowing out, but that was a long time ago, do not recall the details.

Not sure if you can still find used ones like these, it has been a long time since these were in common use.

geeze. What was I thinking!!?

Thanks for your suggestion, Rick. I had heard of them. Actually I was looking for PedalCell, a still more efficient device among this last generation rim dynos (not tire dynos, mind the difference), but unfortunately they had to stop production and close the business altogether. Then I dismissed the idea, but looking at https://www.cyclingabout.com/rim-dyn...n-hub-dynamos/ I found your suggestion again.
By the way, the web I have linked to above is a comparison between dynohub charging sets with these last gen rim dynos commented by pro world tourer Alee Denham.
__________________

Thanks again, Tourist. The thing is that I have a through axle boost hub in one bike and a quick release one the other bikes.As a reference, Alee Denham compares here (https://www.cyclingabout.com/rim-dyn...n-hub-dynamos/) dynohub charging sets with last gen rim dynos to a great advantage of the latter.
Cheers.

I've measured 100v open-circuit on my SON28.. but it was at 50mph. There's nothing quite as fun as riding down a steep hill at 50mph and trying to read the multi-meter rubber-banded to your handlebars!

I'm going to guess that someone in Germany knows what fails first when trying to maximize output power at high speeds. Maybe the copper wire gets warm enough to produce shorts in the enamel insulation? There is probably some hysteretic core loss at high speed that would contribute to heating the wire.

As for open-circuit voltage... the headlight circuit design would determine how sensitive it is to high voltage. I took apart a dead B&M Lumotec Eyc for fun. It uses a little momentary switch to control the function, so some internal circuitry is always powered up. It has a bidirectional protection zener at the dynamo input. It appears to be a 100V transorb. That would make you think the circuitry is rated to handle 100V, but the transorb is located at the input to the full wave bridge rectifier, and at the output of the bridge rectifier is a 680uF, 10V cap. I'm not sure what the transorb is protecting if that cap is only rated for 10V. I can't decide whether there is enough current draw by the Eyc to keep the dynamo voltage pulled down to a low, safe level, ... or whether B&M assumes that no one is going very fast.

I think the question about whether open-circuit voltage matters will end up being the standard engineering answer.... "it depends".

Steve in Peoria

All the charts I have seen show power plateauing, even with a resistive load. Open circuit voltage is at zero power delivered.

Soubitez made one in roughly the same era. My 1984 Alex Moulton AM7 has a braze-on to mount one & I've got a NOS one in the box of parts when my great grandson does the restoration in 2084.

can you share that data?

My measurements of a first generation SON28 only goes up to 20mph, but doesn't suggest that the peak power doesn't increase linearly with speed.




Steve in Peoria

What load were you driving? I could always be wrong, and I could also be misinterpreting the charts I found. I found people complaining that the data was from a resistive load.
I just looked on the interwebs for dyno output charts that were driving a resistive load. I'll have to go back into my search history to find my exact search terms.

My testing was with a resistive load. Data was taken with the following resistances: 1, 5, 10, 15, 20, 30, 40, and 50 ohms.
Voltage was measured with a Fluke 179.
Speed was measured with a Cateye Mity, I believe.

The raw data:




Steve in Peoria

This data is over a decade old. Screen print below:



Above screen print from this article. Article describes testing apparatus, and includes some discussion on difference between LEDs and resistors.
https://www.cyclinguk.org/sites/defa...ub-dynamos.pdf

Reading this article was one of the pieces of info out there that convinced me to buy my first dynohub. So, although it is over a decade old, I remember the article for that reason.

That is a great article! Olaf and Andreas are great guys, and Chris Juden has been a wonderful cycling journalist.
Looking through the text, their testing was done with a 12 ohm load on the dynamo under test. That's a very reasonable way to do it.
The results show power increasing up to a certain speed, and then increasing at a much slower rate. I see this in my own data.
For instance, this is the data for a 10 ohm load....




We usually think of dynamos as being current limited, which would suggest that the output power would reach a limit once the dynamo was putting out its rated current. If you assume a half amp limit, then the output power is 3 watts into a 12 ohm load. In truth, the current isn't strictly limited, but does increase slightly with speed.

I should point out that my first graph showed the peak power as speed varied, and that assumes that the load resistance is adjusted in order to obtain the peak power. At most speeds, a load resistance of something other than 12 ohms will allow you to get more power out of the dynamo.

Steve in Peoria