Originally Posted by
elTwitcho
Not exactly. You can compensate with a step up in a cog, but then for instance (and these numbers are pulled from the air just to explain the example) you'd be running 48-19 so that it FELT like running 48-18. But you'd only be going 48-19 speed. So really, you end up going slower for what feels like the same effort.
That said, if you're more comfortable riding 165s as an issue of bike fit, ride 165s. Personally, having given them both a spin (165s and 170s) I hate the shorter cranks. Personal preferrence is everything though, ride what makes you happy. I'm just saying it bears pointing out that 165s are going to require more torque to move the bike.
Shorter cranks don't fundamentally move you any more slowly for the same amount of effort -- they shorten the path your foot takes in rotating the cranks. You can get back the same effective ratio (Sheldon calls this the "gain ratio") by increasing the cog size. You don't end up going any more slowly for your effort, you just end up doing a little more rotation along a shorter path than with the longer crank. The radius is tighter with shorter cranks, and from a biomechanical perspective, certain lengths will work better with certain legs, and it's a matter of finding the sweet spot in the middle.