Bike Forums > Determining gear ratio?What's the formula?
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 12-15-03, 07:10 PM #1 Limba Senior Member Thread Starter   Join Date: Oct 2003 Bikes: Posts: 151 Mentioned: 0 Post(s) Tagged: 0 Thread(s) Quoted: 0 Post(s) Determining gear ratio?What's the formula? Is it divide the chainring by the cog then multiply by 27?
12-15-03, 07:35 PM   #2
Stubacca
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Quote:
 Originally Posted by Limba Is it divide the chainring by the cog then multiply by 27?
Sort of.

Gear Inches = (Chain Ring / Free Wheel) * Wheel Diameter (including tire) in inches
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Last edited by booyah; 12-16-03 at 09:36 AM.

 12-15-03, 09:50 PM #3 fastedddie Member     Join Date: Jan 2003 Location: Champaign, IL Bikes: Trek 720, Trek 7700FX Posts: 35 Mentioned: 0 Post(s) Tagged: 0 Thread(s) Quoted: 0 Post(s)
12-16-03, 01:36 PM   #4
Don Cook
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Quote:
 Originally Posted by Limba Is it divide the chainring by the cog then multiply by 27?
The gear ratio, is literally the number of teeth on one gear arithmetically compared to the number of teeth on a different gear. The most common method of making the comparison is to divide one of the "teeth" numbers by the other.
Usually, the larger number of teeth on a chain ring is divided by the smaller number of teeth on the rear cog. Thus, you might say for a ratio of 2.34, that for every one tooth on the rear cog there are 2.34 teeth on the chain ring. Though in mathematics, you could divide the smaller by the larger and still be correct. The "gear inch" method isn't the gear ratio. But it's still a pretty neat way of showing the relationship between the gears of both the chainring, the rear cog, and the wheel size. There's another one that Sheldon Brown likes where you introduce the length of the crank arm and arithmetically compensate for its affect.

12-16-03, 08:59 PM   #5
DieselDan
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Quote:
 Originally Posted by booyah Sort of. Gear Inches = (Chain Ring / Free Wheel) * Wheel Diameter (including tire) in inches
multiply by pi and you'll find out how far you traval with each pedal rotation in inches.