All About Gear Inches

According to Wikipedia "Gear inches is a system that assigns numerical measurements to bicycle gear ratios, to indicate how low or high a gear is." So the number representing any wheel, chain ring and cassette cog combination can be compared to another to help determine how well any particular bike is set up for different challenges.

An an example a rider going on tour with 250 pounds of bike, rider and gear would want a low enough gear inch option to climb the steepest hill he encounters. Whereas the racer riding a flat time trial would look for quite a different range of gear inches. Here is an example (from the Wikipedia article) of how gear inches are calculated. For a 27" tire with a 48-tooth chain ring and an 18-tooth cog the gear inches would be 72.

Here is a rule of thumb chart (courtesy of Adventure Cyclist magazine).

• 20 inches for very steep hills
• 30 inches for steep hills
• 40 inches for medium hills
• 50 inches for slight hills
• 60 inches for almost-flat
• 70 inches for flat
• 80 inches for hard riding on flat
• 90 inches for downhill
• 100 inches for steep downhill

And of course these are just one person's rule of thumb. Again the type of bike, your load and your personal capability can have a significant affect on which gear inch number is best for you under the above conditions. The nice thing about gear inches is that you can calculate them for a variety of bike components and predict what a change in chain rings or cassette cogs will do for your riding. Another point the above chart does not address is top pedaling speed. At some point in your highest gear you will "spin out", or in reality reach your max cadence in your highest gear. If your max gear inch number is 130 you can reach a higher flat-out pedaling speed than if that number is 100 assuming good riding conditions.

Some Typical Road Bike Setups

700x23 tires / 11-24 cassette	Gear Inches	Product Examples
standard double crank 53/39	45-127	SRAM Force Carbon Crankset & Bottom Bracket
compact double crank 50/34	39-120	Deda D-Power Compact Carbon Crankset
triple crank 53/39/30	34-127	FSA SLK MegaExo Triple Carbon Crankset
700x23 tires / 13-29 cassette	Gear Inches
standard double crank 53/39	35-107	Campagnolo Record Ultra-Torque Carbon Crankset
compact double crank 50/34	31-101	FSA Team Issue Carbon Compact Crankset
triple crank 53/39/30	27-107	Shimano Dura-Ace FC-7803 Triple Crankset

One conclusion you can reach by looking at the gear inch numbers in the above chart is how nice of a compromise you can get with a compact double crankset. Not quite as low or as high as a triple, but with less weight and overlapping gear options.