How to Use Bike Gear Ratios

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Chainrings on a crankset and cogs on a rear cassette are designated by the number of teeth on each. The larger the chainring on the crankset, the further the bike will travel per pedal stroke, which will require you to apply more force. This is opposite when considering the rear cassette. The smaller the cog on the rear cassette, the further the bike will travel. Chainrings and rear cassettes are often changed depending on your terrain. Understanding gear ratios is useful when considering crankset and rear cassette choice. Utilizing a formula, you can determine speed based off of gear ratio and pedal revolutions per minute.

Determine the number of teeth on the chainrings of the crankset and cogs of the rear cassette. The number is typically stamped into the metal. If the numbers are not readily visible, count the teeth on each.

Determine your average pedal revolutions per minute when cycling on flat terrain. This will typically be between 90 and 100 rpm for most riders.

Determine wheel size in inches. A typical road bike has 26.5-inch wheels.

Use the following formula to determine miles per hour traveled for each possible gear combination on your bike:

  1. Gear development (in) = (chainring ÷ cog) X wheel size (in) X 3.14

Example: (39 ÷ 15) X 26.5 X 3.14 = 216.34 in

  1. Gear development (ft) = gear development (in) ÷ 12

Example: 216.34 in ÷ 12 = 18.03 ft

  1. Speed (mph) = gear development (ft.) X rpm X .0114

Example: 18.03 ft X 90 rpm X .0114 = 18.5 mph