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Old 06-18-19, 03:49 PM
  #22  
Mikefule
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Originally Posted by beach_cycle
I don't understand gear inches; the concept is sound, but the formula is not logical. I'll use my Meridian for an example: I have a 60t chainring driving a 22t cog on the axle that spins a 26" wheel. So 60 / 22 ~= 2.73 (so every revolution of the crankset rotates the tire 2.73 times [2.73:1]). How far does my tire travel per rotation? Using basic geometry, the circumference of any circle is the diameter * PI (~3.14). So 26 x 3.14 = 81.64; therefore, that tells me my wheel travels 81.64" per revolution. When my drive wheel rotates 2.73 times per crankset revolution, I travel 222.88 inches per a single crank revolution.

Gear inches 70.91"?
60 t divided by 22 t = 2.72
2.72 x 26 inch wheel diameter = 71 inches. Approx.

That means that the distance you travel per single revolution of the cranks is the same as the distance you would travel with one revolution of the cranks on a penny farthing with a 71 inch wheel.

If you want to know how far you travel per revolution, it's (72 x pi) inches.

The history is that the directly driven wheel (cranks attached to the hub) pre-dated the chain driven bike. The penny farthing was known as the "ordinary bicycle". On an ordinary, the bigger the wheel, the faster you went, for a given cadence. Big wheels were impressive, fast, macho, etc.

When they invented bikes with smaller wheels, geared up, they were called "safety bicycles". In order to market the safety bicycles, they made the comparison directly relevant to the diameter of the equivalent ordinary.

The convention has persisted long after the safety bike has become the new "ordinary".
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