The Language of the Drivetrain
Before the invention of chains and derailleurs, bicycles were "Penny Farthings"—those antique bikes with one massive wheel in the front and a tiny wheel in the back. Because the pedals were bolted directly to the center of the wheel, the only way to go faster was to build a larger front wheel.
Even today, with 24-speed carbon fiber road bikes, engineers and bike mechanics still use Gear Inches to measure and compare the mechanical difficulty of a drivetrain.
Visualizing Gear Inches
When you calculate your Gear Inches, the result tells you the theoretical diameter of the wheel you are currently pedaling.
- If you are in a 100 Gear Inch combination, it feels exactly as difficult as pedaling a massive, 100-inch direct-drive antique wheel. This is a high-speed sprinting gear.
- If you are in a 30 Gear Inch combination, it feels incredibly easy, like pedaling a tiny 30-inch wheel. This is a "granny gear" used for climbing steep mountains.
The Formula
To find your current Gear Inches, you evaluate your front chainring against your rear cog, and multiply it by the physical diameter of your real wheel.
Gear Inches = (Chainring Teeth / Cog Teeth) * Wheel Diameter
Why Gear Inches Matter
If you own a road bike with 700c (27-inch) wheels, and a mountain bike with 29-inch wheels, you cannot simply look at the teeth on the gears to compare them. The larger mountain bike wheel changes the math. Converting both setups into "Gear Inches" gives you a perfectly standardized number to compare how difficult they will be to pedal.