I would down scale the generator a little from a gas verson.
Most gasoline versions are intended to output 110 or 220 volts AC current at something approximating 60 cycles.
A bicycle charger for charging batteries you would be much better off with permanent magnet generator, or maybe even a car alternator rewired for the slower speeds you would produce with a bicycle.
Lets not get ahead of ourselves here,
You would probably be better off with a 'Stationary Bike' that had a fairly heavy flywheel rather than an actual bicycle tire.
Flywheels store and use energy much more effectively than a light weight bike tire.
The generator should be one that either puts out directly in the voltage/amperage range you need to charge your battery (like a car alternator for charging automotive type batteries)
OR,
A generator that outputs in a standard voltage so you can use adapters/transformers to get the current you need for charging your batteries.
Battery chargers come in all shapes and sizes, everything from cigarette lighter plug in kind for smaller batteries (flashlights, radios, lap tops, ect.) to chargers that take 440 volts in 3 phase!
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As for the actual building, that's pretty straight forward...
A smaller generator mounted so the shaft is turned by a bike tire is the most simple, and pretty effective if you only have small batteries to charge.
This kind attaches to your bike frame or forks, and will charge your batteries when you are riding someplace.
I can't guarantee the current output or the sustainability, but it's cheap!
These little dynamo generators usually fall apart.
If you can find a good one, they do OK, but most of the time they are made in China and they are JUNK...
You would be better off finding a small permanent magnet motor in something you discarded and making your own generator with that....
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This rig is for converting your bike to a stationary rig.
This lifts the rear wheel by the axle, and the tire turns the generator.
Personally, I'd probably find a way to mount the generator on the bike and charge my stuff while I was going down the road...
http://www.econvergence.net/electro.htm
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Here are some links you might want to check out for ideas if you want to build your own,
http://www.mattshaver.com/bikegen/index.htm
http://www.amphibike.org/index.cgi?page=pages/3_bikes/monocog/genOverview
http://www.scienceshareware.com/pedal-power-build-your-own.htm