Originally Posted by
jit5
I switched from a 172.5mm to a 170mm for a year.
The shorter crank lessens the leverage and promotes spinning.
The difference between those two is miniscule. Go get a ruler and measure out 2.5mm. Think that makes a difference? That's 25 sheets of paper. That's a 1.5% change. It's in-freaking-significant. If I put you on two bikes where you couldn't see the cranks you couldn't tell the difference between them. I guarantee it.
Sorry for the rant. I'm on a mission to drum out this idiotic myth that people need to spend a few hundred dollars to get a new crank.