What Are Bike Dynamos And How Do They Work?

What Are Bike Dynamos And How Do They Work?


What exactly are dynamos and magnetos, and how do they work? Plus, why might you need one? Ollie took a closer look to answer this and more with hub based dynamo systems.

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If you’ve always wondered what hub dynamos are and why they might actually be useful, this video is for you. Ollie took an in depth look specifically at Hub based dynamos to answer many common questions.

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99 Comments on "What Are Bike Dynamos And How Do They Work?"


  1. Do you have a dynamo on your bike? If not, would you get one now? Let us know your thoughts in the comments below 👇

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  2. Don't have dynamo……yet but after this brilliant video i'm one step closer to get one:-) Very good work one of the best from Ollie brilliant.

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  3. '…that's nearly four times more.' Um, it's actually 4.5 times more than 96g. Great video nonetheless Ollie as been wanting to know about the benefits and drawbacks of dynamo hubs for a while.

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  4. So the most important question, how is the power transferred from a spinning wheel to a wire to charge stuff?

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  5. Is there a battery in the hub so you charge devices when off the bike? or somehow you use a wire while the wheel is spinning?

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  6. Poor Ollie; seems he has been struck down with a terminal form of Richardson’s disease…the compulsive need to put on glasses merely to appear geeky

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  7. For what its worth there is big differences between a Magneto and a Dynamo . A Magneto produces AC voltage with permanent magnets and a Dynamo produces DC voltage using a commutator ..

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  8. "Surprised how normal" the front hub looks??
    It looks like a pregnant rear hub without the freewheel….

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  9. You say it costs 6 watts at 25kph turned on, how many watts does it produce at 5 volts? A normal smart phone charger needs around 1.2 amps at 5 volts to charge at its intended charging speed (actually ranges from 1 to 2 amps, depending on the phone), which is exactly 6 watts. If these dynamo/magnetos are producing 1.2 amps (or 6 watts at 5 volts), then technically there is no loss, just redirected power. If they produce MORE than the 1.2 amps (more than 6 watts), they are more efficient than the DT swiss hubs, as not only are they redirecting power, they are producing additional power.

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  10. You're wasting power charging up a powerbank to charge the phone. If the charging circuitry is well designed, there will be no problem charging the phone directly

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  11. When I was a kid I used love my old bottle dynamo(s) I had this awesome front lamp on the right hand fork that would light up tree canopy dark roads in Bedfordshire. My mates had these crappy Ever Ready battery lights that used a pair of AAs that were just no comparison. The last time I used one was around 2000 but was experiencing problems with the contact wheel slipping on the tyre in the wet and without a proper mounting boss on the fork used a hacked solution that utilised a bracket that used the cantilever boss. Just use modern LEDs now but when I spec up a new touring bike, probably a Condor, I'll have one of the Son hubs. As others are saying though it is necessary to add somekind of circuitry to maintain steady current.

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  12. Ollie, is there really such a thing as a chargeable electric blanket?! That would be a tingly sleep….

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  13. I have a Son28 dyno hub, hooked up to a Supernova e3 triple heads light and a e3 tail light. I also have  a sinewave Revolution usb charger. This setup was expensive but its worth it knowing that I have lights any time I need them, and I can charge a powerbank witch I can then use to charge my phone, ipod, gopro and gps while I'm out touring.

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  14. Dynamos are awesome! I've been using a Shimano Alfine dynamo hub with B&M lights(front and rear) on my commuter bike for almost 2 years and the lights are great to have whenever I want. They are also pretty bright compared to alot of the midrange bike lights I've used in the past.

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  15. Maybe they aren't called Magnetos because you need a Sony Pictures license for it. Maybe a Marvel license too.

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  16. As always, great vid. One question though. I tent to buy cheap power banks because after some time they don't work as they should, e.g. they take super long to charge and I just get a new one. I'd be willing to buy a good quality one if I had it recommended. Would you or Josh recommend any specific power bank for bikepacking with a dynamo?
    Thanks

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  17. can someone explain why a dynamo that is switched on causes more drag than when its not when the size and weight of the dynamo hasn't changed???

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  18. Nice review. Used to have one on my bike back in the 70’s to run my lights. May well consider one again after watching this👍🇦🇺

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  19. An AC-DC "converter" is called a rectifier. It's not something anyone needs to know, but when you do know it, you can never help but correct those who aren't aware of it.

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  20. If someone is intrested, there are some nice information on some dynamos on: https://fahrradzukunft.de/14/neue-nabendynamos-im-test/
    Its german but you should be able to comprehend the graphs in the lower part of the article:
    (1): mechanical input lights off
    (2): electric output on 12 Ω
    (3): mechanical input on 12 Ω
    (4): electric output on 24 Ω
    (5): mechanical input on 24 Ω

    There is actually quite some difference in quality.

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  21. Someone should develop a dynamo hub that uses regenerative braking so it charges more when you are slowing/freewheeling

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  22. I love how Ollie puts on a Midwest American accent when he wants to sound like a scientist. Those of us across the pond typically don an RP British accent when we want to sound smart. Hilarious!

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  23. I thought about going dynamo on my adventure (gravel) bike set up but for all the extra cost and effort it just seemed easier to get a solar rechargeable battery with 25k mAh which lasts 3-4 days. If you intend to cycle through the night or go a very long time in crap weather and no access to electric then I can see why you would usea dynamo.

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  24. Maybe its because I was much younger, or maybe its because it was ancient 80's technology but last time I rode with a bottle dynamo it was a hell of a lot more than 6 watts of drag and was like cycling through treacle. But then it was powering a bulb not a nice modern LED.

    Oh the torture of hand me downs……

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  25. A fantastic video for people who already know everything suggested by the title. For others wanting to learn, not so great…. no description of exactly what it is to a cyclist, no view of how it connects to the bike or how it's actually used in practice. It's said that you can use them to charge equipment like phones and computers and heated blankets(?), but not even the slightest hint at the charging capacity (e.g. a 10-minute ride will charge your heated blanket for a toasty night's sleep?).

    Not up to GCN standards.

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  26. Was it said not to charge a phone directly because of the Direct Current from a dynamo, then later we're told that they're not dynamos but magnetos and produce AC?

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  27. Do e-bikes use dynamos? I would think that front & rear hubs generating a constant 12 watts would significantly extend battery life. They could probably rig an on/off switch so that the dynamo was only engaged when the motor wasn’t. Also, why not develop a braking system like hybrid cars use to add a little more boost to the battery? How about solar panels on a helmet or backpack? Add all of that up and the re-charge might peak as high as 60 watts. Or is that crazy? (It’s 2am here and I have insomnia!)

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  28. What I'm curious about is if it is a brushless design or not, which would be WAY superior to anything else out there and much more efficient. I looked at one of Hunt's videos and it showed the guts of one and it does look brushless, hence no commutator. It that is the case, it would actually be an syncronous alternator and use a rectifier internally. That would be slick indeed and be practically maintenance free… except for the bearings.

    But somehow, "syncronous rectifer" just doesn't roll off the tongue, does it!?!? It's the antithisis of a sexy name. Yeah, stick to Dynamo.

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  29. Well, Ollie definitely did a better job then Dan or Si ever did with the science glasses gag. That was entertaining, I laughed hard. No offense to Dan or Simon it's still funny when they do it.

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  30. Always so many myths about electronics, how much current flows into your phone has nothing to do with the current that the dynamo produces, the voltage provided to the phone will cause the phone to draw current, they all have voltage regulators, so unless you supply way too much voltage they wont draw any more current than usual. Electricity works on the principle that if you apply a voltage across a circuit a current will flow, it does not work by supplying current, none of these devices supply current, they produce a voltage and the circuit you attach them to will draw a current. You could have a dynamo that can supply 10amps of current but if you put it on a device that only draws 1amp, it will only draw 1amp. The only way to push more current through is by increasing the voltage.

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  31. Informative video, but more to the point, i'm quite impressed with how Ollie has managed to fit in so well with GCN. Chapeau!

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  32. Every extra Watt of drag is because an extra Watt of power from the dynamo is being used by a gadget.

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  33. Do you need a dynamo specific wheel?… so do you generally buy them separately and then take them to you LBS to build???

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  34. Title of video: "What Are Bike Dynamos And How Do They Work?" And yet, you never told us WHAT a bike dynamo is. At about 1:40, if you're paying attention, you can finally figure out what a dynamo is. So, for those of us that thought, "what's a dynamo?" and came here to find out. Well, eventually, you can divine it. But, you might consider starting a "what is" video by saying, well, what it is.

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  35. I use a SON28 with a sinewave revolution,just connect anything with a USB socket to the sinewave and Bob's your uncle

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  36. 6 Watts of drag at how much electrical output power? It depends on the load you're drawing. I can't afford the SON kit so I use the Shimano ones, they're fine as long as you stay away from the "DH-32" model that's nfg.
    You can get phone adapters with rectumfryer and surge protector that let you charge from the dynohub without intermediate powerbank, LiPo batteries are pretty efficient but you always incur at least 10-20% loss by storing the energy in a battery and then getting it out again.

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  37. I really hope you're going to do a follow up video to this one. There's a lot of questions and confusion down there !

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  38. A standard phone battery has about 13 Wh (watt-hour) capacity. You're at a loss of 13 Watts if you charge your phone fully in an hour. I think phone batteries can charge at 13 Watts.

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  39. Drawbacks for hub dynamos: weight, drag, and……price, low output and maintenance. I believe both SP and SON hubs have to go back to the manufacturer for bearing replacement, which, if you ride a lot of miles, could be every 1-2 years. And the hub has to be removed from the wheel for that…..so then you have to have the wheel rebuilt. And the amount of light you get with a hub dynamo light is pretty sad compared to a battery light system you can put together for $30, as I have. And I can move that battery system to any of my bikes, even though they have different wheel sizes/brake systems. I really want to like dynamo lights, but I just can't get past the hundreds of dollars extra cost for far less performance.

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  40. If you're going to say "Reverse the polarity," you need to pull out a Sonic Screwdriver, not nerd glasses.

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  41. thanks but would've liked some info on output and what sort of limitations on the amperage you can pull from these, as well as different models and specs. on to the next vid on these…

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  42. I love my hunt dynamo wheelset. The drag is negligible, and it's a nice piece of mind to know that your lights won't run out of batteries.

    Oh, and one minor correction: the dynamo hub generates AC current, not DC. This is why the lights tend to flicker at slower speeds. In fact, if you freeze the frame at 3:18 you'll see a neat little sine wave on the flange letting you know that you need an AC to DC converter for your USB devices.

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  43. Just a few nitpicks from a geek… take this with a pinch of salt.

    > DC current from your dynamo can damage your phones battery

    As you later divulge, these things output AC, so I can't say I'd recommend charging any DC device using them (not even a battery)… unless you like explosions? You would need a bridge rectifying diode.

    The USB connector that is commonly used to charge mobile phones uses DC 5V. 6V isn't far off; in fact, by the time it gets through 3 feet or so worth of cable it's probably down to 5.5V (which is within 10%). A lot of phones will happily accept nearly 20V anyway. 3 watts is a bit on the high side, but I think that's more likely to burn up the cable. Intermittent connections might damage some batteries, but mobile phones usually detect that and refuse to charge from what they consider to be a faulty charger.

    In terms of computers, I would recommend not charging one of those. Your average PSU for a laptop is rated somewhere between the 45-60 watt mark. You'll be riding for quite some time to charge that! Additionally, they draw 115V in the states and 230-250V everywhere else, so I think they're unlikely to work on 6V to begin with; on top of the bridge rectifying diode, you would need a transformer, which will probably make the diode seem light-weight.

    > loads of additional battery packs

    I happen to have a solar cell that would be suitable for charging a mobile phone; it occupies about the space of your hand and outputs 6V, 1 watt.. all that's needed is a soldering iron and a sacrificial USB cable. The weight is probably less, and of course the power source doesn't come from pedalling… plus there's the benefit that it actually does output DC. Maybe have a look for something like this in a mobile phone shop, since I'm pretty sure I've seen them for somewhere around the $25-30 mark.

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  44. Why not show a dynamo hub with disc brakes? And why not slowly charge a battery then use that for lights???

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  45. "The DC current coming directly from the dynamo can damage your phones battery" – nope, current is irrelevant. My 5v 40amp power supply is fine for charging a phone, my 12v 1A power supply would kill it. Besides, how is anyone going to wire up a Lightning connector directly to a bike dynamo… You don't have to use a USB battery pack, you could use the same DC-DC convertors that dash cams use, both of them are bucking the 6/12v input to a 5v USB output.

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  46. My friend is adamant that if you put a dynamo on the front and an electric motor on the back, and use the dynamo to power the electric hub motor on the rear, you will have an infinite supply of power. He genuinely believes he has discovered this and believes I'm being ignorant for dismissing his idea… He also thinks Boris Johnson and Donald Trump are bold & competent leaders… So you've got some idea of what Im dealing with.

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  47. Late to the party, but it seems no one answered the question "Why are magnetos on bikes actually called dynamos?". The same reason photocopier is called "xerox". First energy source for bike was actually a bottle dynamo. The name just stuck.

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  48. I have a son dynamo hub on my gravel bike with an edelux light and a Shimano Nexus with a non branded plastic light on my commuter, I love dynamo lights, not worrying about batteries is excellent. I want to upgrade my mountain bikes to dynamo lights too.

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  49. "Dynamos are for charging IT devices, or charging lights. Or driving lights directly!"
    Sign of the times, I guess…

    Also, complaining about the drag of a switched on dynamo: It's physics, dummy! Also known as the law of energy conservation.

    On the longest tour I have ridden up to now I had a be on dynamo harvester charging device (now available from NC-17) that actually extracted up to 12 Watts out of my SON. And if you carry a lot of electronics, as we tend to do these days, you are happy about every single one of these watts.

    That's why you don't want the delux version of the SON on a tour. Yes it's a bit lighter, but it also extracts less power. Go with the 28.

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  50. Usually I didn't understand many foreign people videos but you are understandable and I liked your video👌

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  51. Too bad no links for all that were mentioned. Are these brushless or brushed in transfer? If these dynamos had enough output, they could help charge a battery on an electric bike to extend range.

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  52. I have a dynamo "F12W-PRO", it can charge my iPhone directly. At the same time, it can provide the power of the lights.

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  53. I saw that someone below mentioned installation and use video. Did you ever make one ? Can I have one on the front and the back ? If you have time, that would be great. I need to figure this out, and instead of me dinging around and messing up, it would be great to just watch a video instead.

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  54. Hi, I have discovered that, if I connect my dynamo to my light system, the chip controls the output voltage from the dynamo and limits it from maximum 24V to less than 5V, at this voltage, my voltage regulator won't work, means, I can´t charge things when the lights is on.

    I don't understand how my light system works. If it drains the voltage down to 5V, it might be, taking all the extra energy which my dynamo generates and the light itself doesn't need, away and make it even harder to ride.

    Or, it could be, it sync with the output from the dynamo and acts as a motor driver and put back the extra energy into the dynamo? Could be, but I don't think so. Because the output is drop down to 5V, otherwise the voltage would be higher.

    And a interesting fact, my front light works always, but my back light works only when the input to light system is AC current. And if it is at a low speed 3km/h, the blinking between front and back doesn't sync either. Front light twice or more and back light only once.

    Do you guys have any idea?

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  55. Take a look at velogical bottle Dyno. Modern coin size bottle dyno. Lighter set up than battery lights. Zero drag when off wheel. Rated at 1.5 watts not three. Pair it with an iqx light. I run a son hub too but prefer overall the velogical for my situation. Modern dynamos should be called generators output ac which is then rectified at headlight electronics etc .old dyno used brushes and were a labour of love. You cannot be self sufficient apart from lighting with dyno and accessories and will need to grid charge at some point.

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