Bottle Dynamos

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Mr Tom
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Re: Bottle Dynamos

Post by Mr Tom »

I've used two different B+M dynamos for short trips for years and they've always worked fine, even in the rain. One was the cheap one (S6?), and the other was a lot more expensive with less drag, not that I noticed it.

I always wondered if anyone's ever had a hub dynamo and bottle dynamo running at the same time on a wheel. Not sure what it would achieve except maybe running lots of lights or charging a battery and running lights at the same time maybe.

For my main bike I now have a SON dynohub which I really shouldn't have spent out on, but over the years I've grown to love it.
rogerzilla
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Re: Bottle Dynamos

Post by rogerzilla »

The S6 was the better one. The plain 6 was the normal one.

Bottles are (in the real world) less efficient than hub dynamos because of the frictional losses at the roller. Hub dynamos don't have any moving parts that make contact, except the usual hub ball bearings, so their losses are due to electrical/magnetic causes only.

There is scope to improve efficiency much further but you would probably add weight as well as cost. Car alternators, which don't use permanent magnets, can be quite a bit more efficient but they need an external power source (battery) to initially excite the rotor coils.
Carlton green
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Re: Bottle Dynamos

Post by Carlton green »

I wouldn’t like to call whether it was better to have rolling element or plain bearings in dynamos. Rolling element bearings are capable of giving the lower drag but get a bit of muck in them and they can give resistance or even lock-up; plain bearings are, I believe, a bit more tolerant and - well made ones - even do long and sterling service in some high load applications (many are used in car engines).

Yep, bottle dynamos do drag more than hub dynamos but what drag there is is still basically a non-issue / negligible. Bottles only drag when they’re engaged, whereas some hubs always (slightly) drag.

There is scope for more efficient power generation but at what cost(s) and at what (wattage) ‘saving’? If I wanted to power electronic gadgets or charge battery packs as I rode then ideally I’d use a good hub dynamo; on the other hand a bottle dynamo can work fine for just powering lights for night time riding - bottles have a long history of doing just that - so if that’s all you require then IMHO they’re near enough perfect.
Don’t fret, it’s OK to: ride a simple old bike; ride slowly, walk, rest and admire the view; ride off-road; ride in your raincoat; ride by yourself; ride in the dark; and ride one hundred yards or one hundred miles. Your bike and your choices to suit you.
2_i
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Re: Bottle Dynamos

Post by 2_i »

Carlton green wrote: 2 Jul 2022, 8:52pm Yep, bottle dynamos do drag more than hub dynamos but what drag there is is still basically a non-issue / negligible. Bottles only drag when they’re engaged, whereas some hubs always (slightly) drag.
You sound like someone having no practical experience using bicycle dynamos whatsoever.
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andrew_s
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Re: Bottle Dynamos

Post by andrew_s »

A hub dynamo is around 60% efficiency; a bottle dynamo is 25-30% efficient.
To generate 3W of electrical power, your legs have to provide 5W of push for hub dynamo, and ~10.5W of push for a bottle. Lights-off drag is 1-1.5W for a hub, and 0 for a bottle.

However, the big difference between hub and bottle dynamos isn't dynamo drag, but tyre drag.
The rolling resistance of a fast tyre is around 10W (Conti GP5000), the rolling resistance of a tyre with a bottle dynamo track (Marathon Greenguard) is around 20W, so using a bottle dynamo costs you a an extra 20W at all times (there are 2 tyres), which is enough to be quite noticeable.
Back before hub dynamos, and being into Audax events, I used to use a roller/bottom bracket dynamo, so I could use faster tyres.
If you're going to use M+ for puncture protection anyway, tyre drag doesn't matter.

[edited in light of updated bottle dynamo efficiencies]
Last edited by andrew_s on 3 Jul 2022, 7:23pm, edited 1 time in total.
iandusud
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Re: Bottle Dynamos

Post by iandusud »

Carlton green wrote: 2 Jul 2022, 8:52pm I wouldn’t like to call whether it was better to have rolling element or plain bearings in dynamos. Rolling element bearings are capable of giving the lower drag but get a bit of muck in them and they can give resistance or even lock-up; plain bearings are, I believe, a bit more tolerant and - well made ones - even do long and sterling service in some high load applications (many are used in car engines).
It is worth pointing out that where plain bearings are used in high load applications in cars they are fed with oil under high pressure so that the actual bearing surface is the oil itself (which is why such plain bearing are so well suited to high loads). Obviously in a bottle dynamo this isn't the case. That is not to say that plain bearings are well suited to use in a bottle dynamo. In fact I would have thought that if they are of an appropriate length and properly aligned they would be ideal. Some means of getting a drop of oil on them every now and then would be good things though.
iandusud
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Re: Bottle Dynamos

Post by iandusud »

andrew_s wrote: 3 Jul 2022, 6:15am However, the big difference between hub and bottle dynamos isn't dynamo drag, but tyre drag.
The rolling resistance of a fast tyre is around 10W (Conti GP5000), the rolling resistance of a tyre with a bottle dynamo track (Marathon Greenguard) is around 20W, so using a bottle dynamo costs you a an extra 20W at all times (there are 2 tyres), which is enough to be quite noticeable.
This is a very good point. I've always said that the best way to improve the performance of any bike is to fit better tyres. Our good tandem is fitted with Supremes and has a hub dynamo.

However for my purposes the tandem in question is fitted with Marathons (with a dynamo track) and will continue to be. As stated previously lights will be used infrequently but when needed will need to be good. I think a bottle dynamo is definitely worth a shot in my case.
Ross K
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Re: Bottle Dynamos

Post by Ross K »

Am I missing something here?

If the "daily hack" tandem is to be used infrequently and only for short trips, why not use a self-contained good quality battery light such as an Exposure, with huge light output, long runtimes and fast charging rather than utilising a bottle dynamo?

Even just opening up the ability to use fast tyres (Supremes or GP5000) would easily sway it for me.

By the way we use Supreme 37s on our tandem, they're excellent!
Carlton green
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Re: Bottle Dynamos

Post by Carlton green »

andrew_s wrote: 3 Jul 2022, 6:15am A hub dynamo is around 60% efficiency; a bottle dynamo is 35-40% efficient.
To generate 3W of electrical power, your legs have to provide 5W of push for hub dynamo, and 8W of push for a bottle. Lights-off drag is 1-1.5W for a hub, and 0 for a bottle.

However, the big difference between hub and bottle dynamos isn't dynamo drag, but tyre drag.
The rolling resistance of a fast tyre is around 10W (Conti GP5000), the rolling resistance of a tyre with a bottle dynamo track (Marathon Greenguard) is around 20W, so using a bottle dynamo costs you a an extra 20W at all times (there are 2 tyres), which is enough to be quite noticeable.
Back before hub dynamos, and being into Audax events, I used to use a roller/bottom bracket dynamo, so I could use faster tyres.
If you're going to use M+ for puncture protection anyway, tyre drag doesn't matter.
Thanks, that’s one of the best posts I can recall reading on this topic. Informative and I think context is very important to decision making.

For what it’s worth I’ve pretty much always used relatively common everyday or basic go anywhere tyres that were intended to give a puncture free and comfortable ride, low rolling resistance is nice but it’s not at the top of my priority list. Different folks make different choices to match their own needs.

iandusud wrote: 3 Jul 2022, 6:27am
Carlton green wrote: 2 Jul 2022, 8:52pm I wouldn’t like to call whether it was better to have rolling element or plain bearings in dynamos. Rolling element bearings are capable of giving the lower drag but get a bit of muck in them and they can give resistance or even lock-up; plain bearings are, I believe, a bit more tolerant and - well made ones - even do long and sterling service in some high load applications (many are used in car engines).
It is worth pointing out that where plain bearings are used in high load applications in cars they are fed with oil under high pressure so that the actual bearing surface is the oil itself (which is why such plain bearing are so well suited to high loads). Obviously in a bottle dynamo this isn't the case. That is not to say that plain bearings are well suited to use in a bottle dynamo. In fact I would have thought that if they are of an appropriate length and properly aligned they would be ideal. Some means of getting a drop of oil on them every now and then would be good things though.
Ah, it’s true that car engines have pressure fed lubrication so perhaps the analogy wasn’t strong enough. A lot of plain bearings use low friction coatings and others use a material (like oilite) that holds oil within it. It’s not uncommon for electric motors to use plain bearings that are not oiled under pressure. I’ve managed to get oil to a bottle dynamo’s main bearing, on one type I removed the pulley wheel and dribbled light engine oil along the shaft and on another with fixed pulley I did something like immersing the pulley and shaft housing in light engine oil - a crude solution but it worked, you only need a few drops of oil to somehow or other reach the bearing.
Last edited by Carlton green on 3 Jul 2022, 11:05am, edited 1 time in total.
Don’t fret, it’s OK to: ride a simple old bike; ride slowly, walk, rest and admire the view; ride off-road; ride in your raincoat; ride by yourself; ride in the dark; and ride one hundred yards or one hundred miles. Your bike and your choices to suit you.
cycle tramp
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Re: Bottle Dynamos

Post by cycle tramp »

2_i wrote: 3 Jul 2022, 3:11am
Carlton green wrote: 2 Jul 2022, 8:52pm Yep, bottle dynamos do drag more than hub dynamos but what drag there is is still basically a non-issue / negligible. Bottles only drag when they’re engaged, whereas some hubs always (slightly) drag.
You sound like someone having no practical experience using bicycle dynamos whatsoever.
I think that's a little unfair. Personally I'd be hard pushed to tell if a dynamo had bearings or bushes by the drag, simply because in this thread, we're talking about drag in a theory. Riding a bike in the real world is subject to more variables which have a greater influence over the speed at which I travel rather than just drag from a dynamo. These factors are;

(I) wind speed and direction (relative to my journey)
(II) how I feel on that day (good/ abit tired/ completely knackered)
(III) how long my bicycle journey is
(IV) when I last checked my tyre pressured
(V) when I last bothered to oil my chain

All the above factors have an influence on the speed at which (and I suspect when it comes to magazine bike tests (II) plays more of a role than any bicycle journalist would care to admit)
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2_i
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Re: Bottle Dynamos

Post by 2_i »

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.
iandusud
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Re: Bottle Dynamos

Post by iandusud »

Ross K wrote: 3 Jul 2022, 9:36am Am I missing something here?

If the "daily hack" tandem is to be used infrequently and only for short trips, why not use a self-contained good quality battery light such as an Exposure, with huge light output, long runtimes and fast charging rather than utilising a bottle dynamo?

Even just opening up the ability to use fast tyres (Supremes or GP5000) would easily sway it for me.

By the way we use Supreme 37s on our tandem, they're excellent!
The tandem will be used frequently but not the lights. It will be left locked up outside when not at home and I don't want to have to take the lights off every time I park up or concern myself with whether or not they need charging. As a "daily hack" I'm not at all interested in fitting fast tyres as I would rather have the extra puncture protection of a Marathon.

I agree with you about Supremes BTW, which to be fair have proven to very puncture resistant on our other tandem (only 2 punctures in over 8000 miles) but any gains of a faster rolling tyre on local journeys are insignificant and certainly don't warrant the extra cost.
2_i
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Re: Bottle Dynamos

Post by 2_i »

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.
Here are actual data: http://www.myra-simon.com/bike/dynotest.html. FER 2002 is actually a spoke dynamo and Pioneer s-thing was a rear hub dynamo.
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RickH
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Re: Bottle Dynamos

Post by RickH »

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.
Former member of the Cult of the Polystyrene Head Carbuncle.
slowster
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Re: Bottle Dynamos

Post by slowster »

iandusud wrote: 30 Jun 2022, 6:37pm I fitted a Union 35 lux front light on my cargo bike which cost less than £20 and is impressively good, so I would probably opt for one of those.
I have one of these 50 lux lights on my utility bike: https://www.bikester.co.uk/busch-muller ... gid=300956. The main difference that I have noted between it and a B&M 80 lux light, is the latter throws more light to either side a short distance in front of the bike, and I do like that extra illumination when making a 90 degree turn into a poorly lit side road. Otherwise, however, I find the 50 lux light to be excellent.

Your reference to a Union front light reminded me of this thread of Brucey's - viewtopic.php?t=143190. It looks like the 50 lux UN-4276 is still not available in the UK, but one of the google results from searching for retailers was this:

https://www.bikeparts.de/Fahrradteile/D ... 41519.html

€11.15!!!!!!!!!!!!. And that is including german VAT.
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