Originally Posted by jschen
For what it's worth, Sheldon Brown advocates calculating a
"gain ratio" that takes into account crank length, and based on the numbers, he seems to advocate the belief that a longer crank equals slightly lower gearing.
According to Sheldon's gain ratio calculator, 170mm cranks in 39/29 yields 2.6. The same gears with a 190mm crank yields 2.4. With 175's it's 2.6.
Originally Posted by terrymorse
The difference between 170 and 175 is 3%. That's 3% more torque into the cranks.
Since the crank length is the radius of the circle, and not the circumference, you are actually getting less of a % benefit than the straight percent increase in crank length.
i.e. pi (170^2) < 1.03(pi(175^2))
or less than 1% advantage.