I am planing to make a large pancake alternator, the kind used for wind power. I think 36 to 48 inches will give me the most momentum for pedal power. what would be the rim speed at a 42 inch diameter cranking at 50 rpm? If any one can tell me about calculating the HP needed for a certain amount of wattage I would love that, but really it all depends on the charging resistance. Also can you tell me what percentage of energy is lost for each sprocket, or notched belt pulley? and time belt pulley if different

Rim speed in inches/minute Circumference* rev/min 42 inches *pi* 50= 6597 inches/minute. 1 horsepower is 746 Watts Timing belts are around 90% efficient. Chain around 85%

I was reading in book last night that the air gap alternators don't even start producing power until they reach about 150 RPM. well that means we will need a gasoline engine.