If you have modest power requirements for gear on your bike (e.g. lights), then I agree with Adrian and omnibot's recommendations, below. Most of those generators use a dynamo (a wheel that contacts the rear wheel of the bike, drawing some power through friction.)The problem with these is that they can be heavy, make noise, add moving parts, and add friction to your ride. A much smoother, quieter solution is simply to use magnetic induction. This will work well if all you need is a little power for LED lights. I just finished an instructable showing how I did this on my bike (it uses a commercial product, but I also link to DIY versions in step 2):https://www.instructables.com/id/Magnetic_Induction_Bike_Lights/Good luck!
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To be honest, while I love DIY and power generation, Lion rechargeables are really the way to go for bicycle lighting. Your average dynamo (generator) won't generate enough voltage nor continue to supply power while stopped (unless a capacitor is used) to be effective for safety lighting. When it comes to bicycle lighting and safety it's "the brighter the better" and you certainly won't get that from a dynamo.
There's plenty of power in a moving bicycle to shine even super-bright LEDs. The question is just how much drag you want to feel while biking. The Reelights in my instructable, above, are just safety lights. That is they make me very visible to cars, but don't really illuminate the road in front of me. At this level of power draw (and the efficiency of induction rather than a friction-powered dynamo), the drag on my bike is pretty much unnoticeable.
use an old motor- connect the spindle to somthing round like a bottle top and then connect that to the tyre!
To generate power for lights? To use your bike to generate power for other devices? You'll need to clarify your question.
I suggest using stepper-motors as AC-generators with rectifiers if you need dc. Here are some instructables :BikeGenBike GeneratorAlso this should have some useful curcits.Here are some links to other such things :Electricity with Stepper MotorsStepper Motor Voltage Doubler Circuit