Bicycle gears work by moving the bike's chain on varying chainring and cog combinations to match the desired drive wheel speed, cadence and pedaling speed of the rider. The gears are part of the drivetrain of multi-speed bicycles and the the gearing system includes the crankset, which include the chainrings, cogs, chain, gear shifter and derailleurs.
The number of chainrings and the cogs determines the number of gear or speed that a particular multi-speed bicycle have. Most multi-speed bicycles come with either double or triple chainrings on the crankset and the cluster of cogs on the rear wheel, which are called cassettes usually contain eight to 12 cogs.
To determine how many gear speed a bike has, one only needs to multiply the number of cogs to the number of chainrings. For example, a bike with 10 cogs and two chainrings will give the rider 20 gear speed options.
Experienced bikers often refer to their chainring and cassette combinations using the number of teeth that smallest and largest chainrings and cogs has. Therefore, a 39/53 double chainring has a smaller chainring with 39 teeth and the larger one has 59. Same goes with a 12/23 cassette, which means that the smallest cog has 12 teeth and the largest having 23 teeth.
To change gears, the rider pulls on or pushes the gear shifter to shorten or lengthen the gear cable. This moves the derailleur and guides the chain to the desired chainring or cog. The chainring and cog combination is called the gear ratio and it correlates to the effort required to pedal.