Understanding Bicycle Gear Ratios

[henry3369]

I'm confused as to how gear ratios work in bicycles. I'm given a problem which says that gear ratio = r_front/_rear. It then says that the gear ratio changes from 4 to 0.7. Does this mean that the radii of the gears are changing? I'm unfamiliar with gears on a bicycle so I'm not sure if the radius of the gear can change,

 

[Svein]

Here is a picture of a bicycle gearing system. The rear wheel has a "rosetta" of 7 cog wheels with different diameters. The pedal setup has just two wheels with different diameters. The gear ratio is then (selected diameter at the pedal setup)/(selected diameter on the rear wheel rosetta).

 

[CWatters]

See also..
http://adventure.howstuffworks.com/outdoor-activities/biking/bicycle4.htm