James_Potter said:
I had a good idea, but its too good of an idea to not have been done
before.
Correct.
Quoted message said:The idea is a bike headlight or backlight that uses the bike wheel
going around as a generator for power to light it up.
Does something like this exist?
Can I get it at a local bike store or someplace online?
It's normally called a dynamo, though technically it's an alternator.
There are good ones and bad ones. Good ones are reasonable, bad ones
are more-or-less useless.
The best is a hub dynamo, built into the front (usually) hub of a
bike. They have almost imperceptible drag, can easily generate 3W
(which is a decent amount of light if you use LEDs) and at high
speeds with the appropriate circuitry can generate twice that. The
very best of these is the Schmidt SON, which is a very finely
engineered piece of german precision. For utility commuter bikes,
where you want to always have lights available, without worrying about
batteries or packing the lights, these are very good. Combine with
LEDs, and you don't need to worry about bulbs either - fit once, and
have light whenever you need it.
If you have a dynamo that runs on the tyre, that's normally called a
bottle dynamo (it's shaped like a bottle, with a roller where the
bottle cap would be). Again, nominally 3W is normal. These have
higher drag than a hub dynamo - it's distinctly noticeable. The two
best are the B&M Dymotec S6 (but not the Dymotec 6 - that's a lesser
model) and the Lightspin.
Cheaper ones have higher drag and no more power. Drag is significant
for more than simply how hard you have to pedal - when the tyre gets
wet (riding in the rain), the roller can become prone to slipping.
The higher the drag, the more likely it is that it will slip. A cheap
dynamo is almost guaranteed to slip in the wet, but my B&M S6 has
never slipped. Bottle dynamos need care setting up - if you don't get
the alignment right, you make slipping more likely, and you can
damage the tyre.
There are other more minor types of dynamo - bottom bracket dynamo
is a cylinder that runs on the running tread of the tyre (less
alignment problems, more crud problems, no hope with knobbly tyres),
there are some that engage on the spokes.
As speed increases, the dynamo generates more power, and does so by
boosting the voltage without generating much more current. Expensive
dynamos have regulators to dump the excess power somewhere safe, but
cheap dynamos won't, and may be prone to blowing bulbs.
Also, they have a habit of blowing bulbs in pairs - the dynamo wants
to generate a set amount of current, and will fluctuate the voltage
until that much current flows. If you have two bulbs in parallel, and
one blows, the dynamo will try and put all the current through the one
remaining bulb, even if that means a sudden dramatic voltage increase.
This can cause the second bulb to immediately blow. Again, decent
lights and dynamos will have some form of regulation to try and
mitigate this.
If you have clever circuity you can use the extra power a fast running
dynamo gives you - a hub dynamo that is nominally 3W will drive two 3W
lights if you're going fast enough. You can buy lights that do this
automatically (eg solidlights 1203D), or you can wire in two lights
with a bypass switch that you flick when you are going fast enough.
The other clever circuitry thing to do is build in a small power
store. Normally a big capacitor. My lights have a 1.5F (yes, one and
a half whole Farads) capacitor in the circuit. This charges up in
about the first 100yds or riding, then if the dynamo stops, the
capacitor supplies power for a minute or so. These are called
'standlights', and it means the lights don't go out when you stop at a
junction.
Finally, you can get systems that use magnets on the frame that topple
a magnet in a coil fixed to the spokes. These are fun novelties, if
you want twinkly lights on your wheel, but no-one has produced one
that generates useful power for actual lights to see by.
regards, Ian SMith
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