JohnDThompson 

The acceleration of gravity is independent of mass...

Looigi

Yes, but terminal speed in air depends on density, other things being equal.

Joe_Hoffmann

Everyone in this thread misses the cool factor of having a car alternator on a bicycle. My main point in starting this thread was for more of an "art" bicycle setup, not AN ACTUAL REPLACEMENT FOR A DYNOHUB!
But I should have clarified that. The purpose would be to build a "mad max" style bicycle.


Obviously a car alternator is not mean for a bicycle, due to the weight and friction. I think if you only desire 6 watts, then the wattage required to spin the alternator would be less than 30 watts, which I think would be do-able for the length of a parade or art bike festival. You could vary the current and voltage into the field coils to produce any power that you wanted. I don't think it would draw 200 watts no-load, but then again I could be wrong. I'm not an electrical engineer.

njkayaker

"Cool" is arguable. There's an "alt bike" forum that might be a better audience for this.

Yes, if you have a point, don't keep it a secret.

DBA

If you can find one, the old VW bugs used to use a 6v system. I'm not sure how heavy the alternators were, but I'd bet they are easier to spin than a modern 12v alternator designed to produce enough juice to run all the electronics in a modern car.

Personally, I'd just use a bike specific dynamo for lighting purposes, but if you wanted to make a trainer/generator to charge batteries or run a tv during a power outage.

Joe_Hoffmann

apparently I started this thread in the wrong category.

RPK79

Hello! Spoiler Alert!!

darkrider2

Sturmey-Archer's hub puts out about 3 watts if I remember correctly. That's enough to run a 2-300 lumen led light. Even that level of output puts considerable drag on your pedaling. If you want to run a 1000 lumen light it will be like going up a 15% grade all the time.

dougmc

Your idea is right, but you've exaggerated the effect considerably.

1000 lumens is probably around 10 watts for a good LED. Assume that your generator is typical and 50% efficient, and that's 20 watts of drag. (That said, I doubt a retrofitted alternator would be even that efficient.)

Let's say a typical recreational cyclist can sustain 100 watts, so that's 20% of their power gone. And yes, that's huge. So on level ground, looking only at air resistance, that would slow them down by about 7% (1-cube root(0.8)). Going up a steep hill, the light would slow them down by 20%. (Though the percentage would reduce if they produced more power (i.e. worked harder) going up the hill, like most cyclists do.)

But where a 100 watt cyclist could do perhaps 15 mph on level ground, or 14 mph with this generator going ... a 15% grade (and without this light) would slow them down to around 2 mph. (And no, that's not a typo -- I calculated it with 180 lbs of rider+bicycle.)