Originally Posted by
taras0000
These are both true. In one of the other threads around here, someone ( I think Carleton) calculated the gain ratio difference between 165 and 170 cranks, as well as the difference in foot speed given the same rpm for each crank. If I remember correctly, the gain ratio wasn't a huge difference, but the foot speed differential was greater than one would think.
If I remember correctly, for every 2.5mm step down in crank length, you have to go down 2 gear-inches to have the same torque (feeling on the pedal).
So, this is why racing 170mm cranks on a 98" gear feels like racing 167.5mm cranks on a 96" gear.
You lose leverage...but you gain more effective cadence ranges and knees don't come up as high, so you can bend over more.
Then there is "tangential velocity" of the foot spinning around the shorter or longer cranks.
It's a mess of calculations
I've done them. There is also mention of it in the Bicycle Science book. I'll see if I can dig out the formula they came up with that involved crank length, chainring, cog, and torque.