Is there a theory/method here? Understand the basic mechanics > riding 167.5 now and the new cranks I'm looking at come in 170, 75 and 80. Anyone clue a new old guy up? Obviously the 70 is closest to what I'm ridng now but with others on offer I'm wondering what the skinny is here.

ta

hey man why not try searching ?

everything you need to know: https://www.sheldonbrown.com/cranks.html

Yep, what he said.

Indeed, Sheldon has words of wisdom from the other side, thanks for the reminder