Cycling UK Forum

RickH wrote: ↑3 Jul 2022, 1:41pm There are some interesting (& expensive) modern takes on the bottle dynamo that claim higher output, & even better efficiency, than a hub dynamo.

This article looks at the Pedalcell & Velogical units:-
"Rim Dynamos Can Now Generate 70% MORE Power Than Hub Dynamos"
https://www.cyclingabout.com/rim-dynamo ... b-dynamos/

I do note that they probably aren't devices for a hack bike though.

A rim dynamo removes a lot of the power loss from a roller on a rubber tyre of unknown sidewall characteristics. It's still nowhere near as good as a hub dynamo and, given that it costs more, is doomed to failure.

rogerzilla wrote: ↑3 Jul 2022, 3:23pmA rim dynamo removes a lot of the power loss from a roller on a rubber tyre of unknown sidewall characteristics. It's still nowhere near as good as a hub dynamo and, given that it costs more, is doomed to failure.

For a hub dynamo there is relatively little difference between a bench test and common real life situations. For a rim dynamo, there is a world of difference.

2_i wrote: ↑3 Jul 2022, 12:48pm

andrew_s wrote: ↑3 Jul 2022, 6:15am a bottle dynamo is 35-40% efficient.

This is ridiculous, 20-25% was typical for bottle dynamos.

It would seem that my memory was inadequate, and that the better models (B&M Dymotec 6, Nordlicht etc) weren't as much better than the cheap and basic as I'd guessed.

Interestingly, the best bottle dynamos could be more efficient than hub dynamos, e.g. the Dynosys Lightspin at about 70%
https://fahrradzukunft.de/18/labortest- ... ferdynamos
(the B&M S6 obtained for testing turned out to be broken - apparently B&M used to claim 55% for it, and 40% for the "ordinary" Dymotec 6)
From memory, these were introduced at much the same time as the SON, cost more or less as much, and didn't do anything to resolve roller slip problems etc, and were consequently outcompeted by hub, and haven't been available for about 15 years.

The test above also includes the Velological rim dynamos.
These are interesting, in that the output power doesn't level off like a regular dynamo, but keeps on increasing with speed, so that at 30 mph the output could be 17 W, which means not far short of 30 W to drive the rim roller. I'd expect there to be roller slippage in wet weather.
The extra power may be good for charging things, but there's a warning to check your LED bike light warranty before use

andrew_s wrote: ↑3 Jul 2022, 7:56pm Interestingly, the best bottle dynamos could be more efficient than hub dynamos, e.g. the Dynosys Lightspin at about 70%
https://fahrradzukunft.de/18/labortest- ... ferdynamos
(the B&M S6 obtained for testing turned out to be broken - apparently B&M used to claim 55% for it, and 40% for the "ordinary" Dymotec 6)
From memory, these were introduced at much the same time as the SON, cost more or less as much, and didn't do anything to resolve roller slip problems etc, and were consequently outcompeted by hub, and haven't been available for about 15 years.

The test above also includes the Velological rim dynamos.
These are interesting, in that the output power doesn't level off like a regular dynamo, but keeps on increasing with speed, so that at 30 mph the output could be 17 W, which means not far short of 30 W to drive the rim roller. I'd expect there to be roller slippage in wet weather.
The extra power may be good for charging things, but there's a warning to check your LED bike light warranty before use

The bottleneck of the bottle dynamos is the coupling to the wheel needed for transmission of mechanical power. Hub dynamos take it completely out of the equation. For reliable transmission you need to press the roller harder, but that can increase losses due to greater tire deformation. You can optimize the contact for dry conditions and minimize losses, but, when water comes into play, which is lubricant, slipping starts. Other factors will be wobbly tire, knocking the dynamo out of alignment, swapping tire, etc. Before hubs, I actually rode with the spoke FER, not the best in efficiency, but with a reliable coupling - always worked. Hub dynamos have their weakness point in that the magnet motion is not very rapid. Good hub dynamos became possible with stronger magnets which, at first, were very expensive, but then became more commonplace. Much of my commute across the year is in darkness and the hub dynamos + LED lights came in as some sort of liberation.

Velogical dynamo delivers less power than standard dynamos, in fact nearly as little as half, so you can afford a weaker pressure for the roller, hence the possible gain in efficiency, but at the cost of dimmer illumination than possible with other dynamos.

I used a bottle dynamo for years, ran 2 halogen lights off it in series.
I've updated to a hub dynamo in recent years but only because I needed a new wheel anyway.
I never really had a problem with bottle dynamos, drag?....never really noticed.

iandusud wrote: ↑30 Jun 2022, 10:09am We have just acquired a second tandem with a view to using it as our everyday bike and saving the "good one" for weekend rides and touring. Part of the thinking behind this is that our "good one" owes us getting on for £5k and as we don't have a car and cycle everywhere we often need to leave our tandem locked up outside and would rather have £600 worth of bike stolen should that happen. I love the dynamo lights our best tandem (we regularly ride on unlit paths where good lights are essential), and the new one has a bracket for a bottle dynamo. I don't want to go to the expense of replacing a very good wheel (Phil Wood hub) with a dynohub. So how about a bottle dynamo. What are peoples experiences of current offerings? Your thoughts greatly appreciated.

Getting back to the practicalities of Bottle Dynamos, there's a couple of things worth remembering about your question;

a) As the dynamo bottle is going to be bolted to a tandem, its important to remember that any drag theoretical or otherwise is going to be spilt between two persons, and as it is a tandem bicycle, with the same frontage as a normal bicycle but with twice the power, this probably still means that if even with the bottle dynamo on you'll still be faster on the flats and definitely down hill than I would be on my solo without my bottle dynamo engaged.

b) Whilst there is some tyre limitation, this is nowhere nearer as important as limiting thenumber of spokes that can be used in the front wheel that a dynohub might bring.

c) As you already have a bottle mount attached to the frame, fitting the dynamo may be slightly easier than suggested, however if you do regularly hit 30 or 40 miles per hour you might want to investigate limiting the power to the front light.

However there are some downsides to bottle dynamos which haven't been talked about,

a) Mounting my bolt dynamo behind the front fork, meant re-mounting my mudguard stay lower down the mudguard blade with some m4 nuts and bolts (in order to clear the dynamo bottle when it was on). Doesn't bother me, but if you like your bicycles to look nice, it's a bit of a hamg nail

b) if you regularly change tyre sizes (perhaps running 2 inches through the winter and 1.25 inches in the summer) you are going to have to adjust your dynamo setting every time you decrease or increase your tyre widths as the dynamo band runs at different heights depending on the size of the tyre

c) Your dynamo is at risk of being knocked or bent when parked up or stacked with other bikes. Way back in the 1990s I spoke to someone who lost their expensive lite spin dynamo after it was stacked in the back of a van with other bikes.
Not only is this annoying, it can be dangerous as a knocked dynamo, or a dynamo with a bent mounting bracket or a loosened mounting bolt can enter your spinning bicycle wheel with terrible consequences.
I'm lucky, when I used dynamos in the past they were protected by my rear rack from any knocks, and now I have them behind my fork blade, I mount my front rack or low rider racks in a way the protects them. I've also always used new ny-lovk bolts when mounting dynamos and regularly check that the mounting is secure.

Dynamos, hub and bottle are current generators so don't have the issues of overvolting LEDs unlike incandescent bulbs which fail.
The Pilom site I linked to earlier describes it in more detail along with several test results at 50 kmh
So you won't need to worry about led failure or voltage limiting, if using LEDs.

http://pilom.com/BicycleElectronics/Dynamo.htm

rjb wrote: ↑4 Jul 2022, 8:38pm Dynamos, hub and bottle are current generators .... ....
http://pilom.com/BicycleElectronics/Dynamo.htm

I do wish people would stop saying that, it is not helpful in understanding how bicycle generators work when driving a resistive load.

All modern LED lamps have voltage-limiting circuitry built in. However, in niche cases like Nicelite LED lamp replacements in an old Schmidt E6, you do need a voltage regulator or the LED will get upset and put itself into "limp home" brightness. A 1W LED can't handle a hub dynamo on its own - the excess power needs to be dumped somewhere. The standalone regulator gets quite warm - what a waste of effort.

The regulator is usually two zener diodes, back to back (to handle the AC output of the dynamo), earthing through a meaty resistor.

wjhall wrote: ↑4 Jul 2022, 9:53pm

rjb wrote: ↑4 Jul 2022, 8:38pm Dynamos, hub and bottle are current generators .... ....
http://pilom.com/BicycleElectronics/Dynamo.htm

I do wish people would stop saying that, it is not helpful in understanding how bicycle generators work when driving a resistive load.

A hub or standard bottle dynamo* is indeed a current source.

It will generate a constant current through any resistive load, within reason, provided you are going fast enough, where "fast enough" depends on the effective resistance and what dynamo you are using. The voltage is whatever is required to push the standard current through the resistive load. If you are going slower than fast enough, you get a lower current & voltage, but if you go faster than fast enough you still get the same current at the same voltage.
So, with a SON hub and a 12 ohm resistance, you get a little over half an amp at 6 V, at above about 6 mph, and with a 24 ohm resistance you get a little over half an amp at 12 V, at above about 13 mph.

This was common knowledge back before LEDs, when people would run two filament lights in series, for extra light

* Lightspin and Velological dynamos are not standard dynamos.

rogerzilla wrote: ↑4 Jul 2022, 9:56pm All modern LED lamps have voltage-limiting circuitry built in.
A 1W LED can't handle a hub dynamo on its own

Commercial LED bike lights don't generally contain electronics to limit the voltage produced by a dynamo, but to increase it.

A dynamo output is limited to about 0.5 A, which, put straight into an LED, would only require about 3 V from the dynamo, giving a lighting power of about 1.5 W.
To get more light, you need to put more current through the LED, so the light electronics make the dynamo generate higher voltages to reach the 0.5 A limit of the dynamo, and the resulting high voltage/low current is converted to a higher current at about 3 V to drive the actual LED.

Back before commercial dynamo front LED lights became available, I used a DIY light for a while that consisted of a bridge rectifier, 2 or 3 Cree LEDs in series, three optics, and three switches, plugged straight in to the SON hub.
It got a bit flashy at low speed (no smoothing), but was brighter than a halogen light. I had it up to about 35 mph with no problems.

cycle tramp wrote: ↑4 Jul 2022, 7:09pm
c) As you already have a bottle mount attached to the frame, fitting the dynamo may be slightly easier than suggested, however if you do regularly hit 30 or 40 miles per hour you might want to investigate limiting the power to the front light.

However there are some downsides to bottle dynamos which haven't been talked about,

a) Mounting my bolt dynamo behind the front fork, meant re-mounting my mudguard stay lower down the mudguard blade with some m4 nuts and bolts (in order to clear the dynamo bottle when it was on). Doesn't bother me, but if you like your bicycles to look nice, it's a bit of a hamg nail

b) if you regularly change tyre sizes (perhaps running 2 inches through the winter and 1.25 inches in the summer) you are going to have to adjust your dynamo setting every time you decrease or increase your tyre widths as the dynamo band runs at different heights depending on the size of the tyre

c) Your dynamo is at risk of being knocked or bent when parked up or stacked with other bikes. Way back in the 1990s I spoke to someone who lost their expensive lite spin dynamo after it was stacked in the back of a van with other bikes.
Not only is this annoying, it can be dangerous as a knocked dynamo, or a dynamo with a bent mounting bracket or a loosened mounting bolt can enter your spinning bicycle wheel with terrible consequences.
I'm lucky, when I used dynamos in the past they were protected by my rear rack from any knocks, and now I have them behind my fork blade, I mount my front rack or low rider racks in a way the protects them. I've also always used new ny-lovk bolts when mounting dynamos and regularly check that the mounting is secure.

Thank you for your thoughts on this which I will address in order.

Whilst we hit 50mph on our "nice" tandem on Sunday the "hack" tandem is only going to be used for local runs where we're lucky to hit 20mph. The dynamo mount is on the front of the left seat stay. I won't be changing tyres in winter as this tandem will be wearing "winter tyres" all year round. Like in your case the dynamo will be protected from knocks by the rea rack.

When it comes to the point of having to replace the front rim I would almost certainly build up a new wheel with a dynohub, as I am a big fan of them, but I don't see the point in replacing perfectly good things (be it on bikes or elsewhere) unnecessarily as it only adds to excess consumption and global warning.

Greystoke wrote: ↑4 Jul 2022, 6:40pm I used a bottle dynamo for years, ran 2 halogen lights off it in series.
I've updated to a hub dynamo in recent years but only because I needed a new wheel anyway.
I never really had a problem with bottle dynamos, drag?....never really noticed.

Good to hear some positive feedback. I suspect that the perception of drag depends a lot on the usage. For example our tandems are used for both leisure and as workhorses. We regularly load up with 30kg of camping gear. In these scenarios the extra drag of a dynamo is neither here nor there. However I also ride on weekend club rides a carbon fibre road bike with no mudguards, racks or anything extraneous. It is like a whippet and lovely to ride and I would never dream of fitting a dynamo or anything else that I feel unnecessary for its usage.

I have a Nordlicht (Swiss) bottle dynamo with fittings for front fork either side, dual pivot or v-brakes. or will fit the rear frame. Complete with B&M rack fitting rear light. The small front light with it will require bulb, holder, & fittings. I have some halogen bulbs if required.

It works best on Schwalbe Marathon Greenguard tyres as they have dynamo sidewalls, and the best position is behind the front fork so the front wheel pushes the dynamo wheel not pulls it. I have no idea if it still works OK because it is some years since I used it, but there is no obvious sign of deterioration. It has done many, many reliable miles.

£5.00 for the lot, to cover the cost of sending it to you. michael.griffiths@talktalk.net

Bargain for some lucky punter and it comes with what looks like a rare zinkens dynashoe adapter.

tentman wrote: ↑5 Jul 2022, 5:45pm I have a Nordlicht (Swiss) bottle dynamo with fittings for front fork either side, dual pivot or v-brakes. or will fit the rear frame. Complete with B&M rack fitting rear light. The small front light with it will require bulb, holder, & fittings. I have some halogen bulbs if required.

It works best on Schwalbe Marathon Greenguard tyres as they have dynamo sidewalls, and the best position is behind the front fork so the front wheel pushes the dynamo wheel not pulls it. I have no idea if it still works OK because it is some years since I used it, but there is no obvious sign of deterioration. It has done many, many reliable miles.

£5.00 for the lot, to cover the cost of sending it to you. Dynamo.JPG michael.griffiths@talktalk.net

PM sent.