The project electronics have been very straightforward so far. I used two DC-DC converters shown below with the treadmill, and both worked perfectly. However, when I switched the same electronics to the bike generator, both stopped working abruptly. I think this is because the bike is a lot more efficient at converting mechanical power to electricity and overpowered the converters. 

This is a good problem to have! I have a few possible solutions for this, the easiest of which is of course, to go with higher power converters, but this can get expensive. Another way might be to introduce a battery stage before the DC-DC conversion, with a battery charge controller. However, I don't want the project to be limited by needing a battery to work, since batteries are expensive and a bad investment as they decay rather quickly. So, I might burn off the excess power with a passive load, like LED strips or a heater block. Anyway, happy biking!