I've built a few pedal generators and repaired a few. Most are based on exercise bikes. One thing I know for sure is that it is better to have a direct drive from the pedals to the generator using chains and sprockets as if they are set up correctly they are the most efficient versus belts or a two stage approach or running a generator from friction on the tire. You need as good an efficiency as it is really hard work to make a useful amount of power. The average person can do 200 watts for a short time and maybe 50 for a more sustained period.
One easy approach from the mechanical perspective is to use an electric bike wheel as a generator as it already has the necessary gears. Regulating the voltage is a bit more of a challenge and could be helped in some instances by rewiring the stator. I managed to run some power tools directly from the gennies which were either bike wheels or washing machine motors. Cheap power tools like drills tend to smoke where as Makita's had no problems. Same with juicers, some smoke, some don't. Easy to charge a battery though and that will also regulate to some degree.
Greg
