ItsJustMe

Sure. First, you need a big enough generator to act as a brake and to be able to generate 100 watts at 12 volts (typical generator hub is 6 watts at 6 volts I think). That'll probably weigh about 5 pounds.

You need 6 amps for 1 second? That's 6 coulombs of energy you need to store. At 12 volts, that's half a farad of capacitance. Since the horn will actually stop working when it gets down to 9 volts or so, and the voltage on a capacitor is proportional to the remaining charge, you need 4x as much as that, or 2 farads.

2 farads of capacitors that are fast enough to be capable of delivering 12 volts at 6 amps will be about the size of a lunchbox and will weigh another 4 or 5 pounds.

So yeah, you could do it. It would be very complex and would weigh probably about twice what a lead acid cell would, and would add a lot to the spinning mass.

EDIT: actually after doing some research I think this would be about 10x heavier and at least 20x more size to give you a 1 second burst, versus a lead acid cell which would give you 20 minutes of horn sounding per charge and take up a fraction of the space.


I was thinking more along the lines of a mechanical horn. You've already got a lot of mechanical energy, and you want to produce mechanical energy. Converting to electrical energy along the way is inefficient and a lot of junk to carry around the other 99.9% of the time.

Something like a horn diaphragm that could be linked directly to a corrugated disc on the wheel.

In the end any such thing would be ridiculously complicated and I doubt would be much (if any) lighter than a small 12V gel cell and a car horn.

Here's a battery that would be more than enough:
http://www.batteryspace.com/sealedle...12v23ahs1.aspx
It weighs 2 pounds and would fit nicely on top of a rack. Combined with the car horn, maybe 3 pounds. It has enough power to honk a horn for about 20 minutes continuously per charge. $25 for the whole deal, very simple. A cordless drill battery would also be a good option, and might be a little lighter.