How to Understand Bicycle Gear Ratios
Current bicycle gear ratios are based on the original "penny-farthing" bicycle invented in 1871, where the front wheel was 60 inches high and only suitable for a very tall person. The large front wheel enabled the rider to reach a significant speed to beat walking, but the design of the bike was unsafe. Later, the safety bike was developed with both wheels the same size, and several different size sprockets added to the rear wheel made up for the loss of the large front tire. Calculating the gear ratio in inches as an equivalent to a large front tire has been the standard way to express gear ratios ever since.
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Instructions
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1
Measure the front tire of your bicycle. Most tires range from 16 inches to 24 inches in diameter on standard bikes. Write down the size of the front tire in inches.
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2
Count the number of teeth on the pedal sprocket and write it down. On a 10- or 12-speed bicycle, there is a second pedal sprocket; count the number of teeth and write it down as well. Label the gears appropriately. For example, on a 10-speed, the smaller pedal sprocket is for gears 1 through 5; suppose it has 25 teeth. The larger pedal sprocket is for gears 6 through 10; let's suppose it has 27 teeth. You will use this information shortly to calculate the gear ratio.
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3
Count the number of teeth on each sprocket on the rear wheel and write down all the numbers. There are usually five sprockets on the rear wheel, but this can vary. Suppose our 10-speed has the following number of teeth on each sprocket: 9, 11, 13, 15 and 17. Remember, these are going to be gears 1 through 5 respectively on the smaller pedal sprocket and 6 through 10 on the larger pedal sprocket. Label them appropriately.
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4
Calculate the ratio for each gear. If the small pedal sprocket in first gear has 25 teeth, then the largest rear sprocket has 9 teeth, according to our examples listed above. The ratio of first gear will be 25/9. In second gear, the ratio will be 25/11, and in the third gear, 25/13, and so on. Continue calculating the ratio for every gear combination.
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5
Multiply the numbers in Step 4 by the diameter of the front wheel to determine the size of each gear in inches. For example, if the 10-speed has a 24-inch front tire, and in sixth gear it has a ratio of 27/9, then 3 x 24 = a 72 inch gear ratio in sixth gear. This is equivalent to a front tire that is 72 inches in diameter.
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Tips & Warnings
Don't confuse gear size in inches with distance travelled. This is an erroneous assumption some people have that each turn of the pedal causes the bike to travel the gear size in inches. Actual travel distance is a different issue from gear ratio and requires a different calculation.
References
- Photo Credit woman on a bicycle image by Wimbledon from Fotolia.com
