Join Date: Jul 2008
Location: Austin, TX
Bikes: Giant OCR2, Bridgestone RB-T, Bike-E, Vision R-40, Novara Safari
Originally Posted by darkrider2
If you want to run a 1000 lumen light it will be like going up a 15% grade all the time.
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.)