Author Options:

Frictionless generator for a bicycle? Answered

 Right now I have a 12 volt 5 amp Werker lead acid battery powering lights, cell chargers, gps, and a few other gadgets.  When I go on long trips I take the batter charger with me to charge the battery with a wall charger.  The wall charger is heavy and unnecessary if i could find a way to harness the power I generate when I pedal.  

i have seen plenty of generators which generate electricity by physically touching a dynamo to the tire but there must be a way to do it inductively, with no friction.  What if I covered the wheel with hard drive magnets and ran them all past a coil on the frame?  I know magnets going past coils will make electricity but not about rectifiers and capacitors and etc.  How could I make this?

Thank you



Best Answer 8 years ago

You might checkout the info here on wind-driven generators--several of these detail alternator / generator construction from scratch, which could be adapted to bicycle use.

A pedantic note about "frictionless" generators: Sure, you can roll-your-own generator from magnets and coils, but placing an electrical load on the generator creates a mechanical load when you move the coil or magnets.

I.E., you don't get anything for free. The higher the load (current draw), the harder it is to pedal...

A lot of free energy information at below link.


The power you generate when pedaling is used to move the bike, add in a generator and you have to pedal harder - remember that.
You'd do well to use this for braking though - it's regenerative.
The magnet idea is fine in principle, the tricky bit is getting a steady output to charge the battery. Less than 12V out - no good at all. Also 5A is a lot to aim for.


 are you guys saying the wheel will become harder to turn as current starts to flow through the coils?  

My spedometer works the same way, an electric pulse is generated by a magnet on the spoke moving past a coil.  This only generates enough electricity to count a revolution, but with bigger magnets and coils i should be able to generate electricity.  

If this only output  100 milliamps could I use a capaciter to smooth the current flow and then a rectifier to change the voltage to 12?

If your pedaling is generating a substantial amount of power, you will find it harder because it's your effort generating the electricity.

100mA is very little, yes you could smooth and rectify it, but it'd only be good for your lights really.


 My bike odometer creates a tiny amount of electricity by moving a magnet past a coil.  Does it cause drag because it is generating electricity?  Bigger magnets and coils should do the same thing.  Its basically free energy.  The only thing making it hard to pedal should be the added inertia from the mass of the generator.

Yes the odometer does add drag, but not enough that you'd notice it.
No free energy here - you have to put that in with your legs. If the magnets & such aren't generating power you'll only have inertia, but try much taking power out and you'll feel it.


 I still am not sure how an induction generator would slow down the bike.  A magnet moving past a coil shouldn't cause any drag, except that caused by the weight of the coil or magnet, which is negligible.  

I think he means a 5Ah battery.  The charge rate would be less than that.

That's peanuts, it's a LA charger 5A is about right for a car-battery.



8 years ago

I used this system for a while.   The only problem I had was my bike would still flash 10 minutes after I parked it.
There is also a hack to it here on instructables.


8 years ago

Hmmm, based on lemonie's idea, I wonder if there would be a way to cause the generator to be activated by the brake lever?

Method 1:  The brake lever is mechanically connected to the generator, so that when the lever is pulled it causes the generator to make contact with the wheel.  This could be used during braking or on long descents down hills.

Method 2:  The generator is always connected mechanically, but activation of the brake lever closes an electrical contact that connects the generator to the battery.  When disconnected there would be nearly no friction, but when connected the generator would slow the bike and siphon off all that energy.

but when connected the generator would slow the bike and siphon off all that energy.

..but only at a rate that the generator can extract. Wouldn't you need some sensing to extract ENOUGH energy to stop as you wish/need/scream for ?You could have a pretrigger as you touch the brakes I suppose.

You and I know hills, I see downhill regenerative-braking, but nothing more than that. Holme Moss, good downhill where you'd get a lot of charge. But if this is a car-battery-charger a big downhill is what you'd need I think.


GIven the drop, and the weight of the cyclist, and how long it takes, we could estimate how much power you could extract from a given drop, that'd be an interesting ballpark figure I think.

Maybe 200KJ to be rough, but you'd have to take out friction. It's a good downhill (if you make it to the top,,,)


Well, the generator alone could not sufficiently slow the bike, but it would work in tandem with the bike's normal brakes.

Another idea is just to hook up a basic solar panel to charge your battery.  A local hardware store often sells 12V battery "trickle charge" solar cells for $10 or so, one of them might be enough to keep your batter topped up...  as long as it's sunny!

External magnets on a bike will get covered in iron/steel crap off the road in no time, I don't think friction is a major power loss in the old fashioned dynamos, though I've never seen an efficiency spec for them. I daresay a modern motor would work a lot better. Try a stepper motor for a test.

I suspect you'll really feel even 20W going into a charging circuit, but I'm not a cyclist so I can't be sure