Bicycle gear ratio.....
Nominal gear ratio is Front chainwheel number of teeth divided by rear sprocket number of teeth which will just give you a ratio. This number is typically multiplied by the bikes nominal tire diameter. That number is typically used for comparison for that bike. If you have a bike with several gear combinations then repeat the process again and again.......
For instance a typical road bike of several years ago would have a high gear with 52 teeth on the largest sprocket up front and the smallest sprocket on the wheel would be 13 teeth. So 52 divided by 13 equals an even 4 to 1 ratio. The 4.0 ratio is multiplied by 27 inches (for the tire size) giving you a high gear of 108 inches. A typical low gear for this bike might be 42 teeth on the crank with the large cog on the wheel having 28 teeth. This low gear ratio is 1.5 X 27" nets 40.5 inches. So for this example you would have a gear ratio range from 40.5 to 108 inches.

I might add that ridein's results (which are commonly accepted numbers) also give you "equivalent wheel size". Using his numbers, a high gear of 108 inches means that if you had a direct-drive wheel that was 108 inches in diameter, one turn of the pedals would advance you the same distance along the ground as your 4:1 ratio system with 27" diameter wheels.

You probably don't want to try and straddle a 108" wheel, for a number of reasons!

sb..
Meaning that would result in a travel of 28.3 feet? That seems to be a very long travel for one turn of the pedals.

"That seems to be a very long travel for one turn of the pedals"

Not really, considering that 52/13 was abou the tallest standard gearing available a few years ago. Now 53/11 is not uncommon, but many would be hard pressed to use that gear ratio to start up from a dead stop.

yes, bikes are great.
They are so effecient you would travel 28.3 feet with one turn of the pedals.....that is why they call it the high gear. For comparison the low gear ratio of 1.5 multiplied by wheel size (27) and multiplied again by Pi would give you a distance traveled of 10.6 feet per crank revolution.

http://www.sheldonbrown.com/gears/

very neat tool for calculating gear ratios is several units:
gain ratios
gear inches (us standard)
development in meters
m/kph at various cadences

input your chainring and cassette sizes and it'll give you info on every combination.

Then taking ridein 10.6 feet per revolution of the crank one more step.  Lets say you candence is 90 revs/min then you would have a speed of 954 ft/min.  

I have a spreadsheet that has all my ratios for all my bikes so I know my gear inches for each gear combination and all the duplicates.  It's a bike addition

Best Regards,

Heckler
Sr. Mechanical Engineer
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