Calculate gear inches like you give a f#\$%

Traditional gear-inch calculations divide chainring by cog, then multiply by wheel diameter. The resulting number lets you compare gearing combinations, but it has no bearing on real life.

If you’re gonna be a nerd, be an informed nerd.

The normal way to calculate gear inches:

(chainring / cog) x diameter

The awesome way:

(chainring / cog) x (diameter x 3.14)

This shows you how far your bike travels with each turn of the cranks.

Don’t forget pi!

Have fun out there,

Lee

This is an homage to Thug Kitchen. Eat like you give a f#\$%.

Know more. Have more fun!

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7 replies
1. Gary says:

I don’t see how crank arm length matters. In this calculation, Cranks are basically in units of “teeth per revolution” this should negate the crank arm length. It does change leverage, but that is a different discussion, right?

2. leelikesbikes says:

Gary, I think so.

Crank length changes leverage, which changes the amount of pressure felt (or required) at your feet. Longer cranks allow you to pull taller gears, but there’s a breaking point where the cranks are too long for a rider to turn them effectively.

When I can, I’ll dig more deeply into this. You know I love me some diagrams.

3. Arben says:

The crank arm is certainly important—when we pedal, we apply a force to a lever arm which rotates the chainring. The amount of torque generated by a given pedaling force is linearly dependent on the length of the crank arm (at least when the pedaling force is perpendicular to the length of the crank arm).

The Sheldon Brown link above seems pretty good to me.

4. leelikesbikes says:

Arben, you are correct.

Changing from a 175mm to a 165mm crank arm reduces torque by about 6%. To get the same feel at the pedals, reduce your gearing by 6%. Changing from a 30-tooth chainring to a 28 will more than do the job.