Originally Posted by ddeadserious

I may be wrong, but I'm fairly confident it's mostly preference. Shorter cranks are going to be better for spinning, but you won't have as much leverage since the arms aren't as long. Longer cranks are going to have more leverage and thus easier to get going but maybe a bit more awkward to be spinning since there will be a larger range of motion. On some bikes(like fixed gears, or bikes with particularly low bottom brackets), it's more crucial to select the proper crank length to avoid pedal strike on turns, but I believe that's not as much of an issue on modern road bikes.

Originally Posted by datlas

Ride more.

Worry less.

Originally Posted by prathmann

My road bike has 175mm, my touring and folder bikes have 170mm, and my tandem has 165mm cranks. I constantly switch between 170mm and 175mm and never notice the difference. The shorter cranks on the tandem are slightly noticeable, but I just tend to spin a bit faster in a lower gear to compensate. Can't really say that any length is better or worse than another.

Originally Posted by carpediemracing

Personal preference.

If you're sensitive to 5mm of saddle height change (just change your saddle height arbitrarily up or down by 5mm and go out for a ride and see if it feels weird) then you'll probably notice 5mm of crank length difference (total of 10mm difference in the diameter of the circle the pedal makes).

Since my personal preference is for longer cranks, I'll explain a bit about how I ride. If you're similar then you may like the longer cranks. If not then not.

I tend to push when under pressure, I rely on short bursts of power (while drafting or in races), and I don't do well on long climbs. Longer cranks work for me - with a 29" inseam (and saddle to BB height of about 67 cm, 175mm crank, Look Keo pedals) I use 175mm cranks. I tried to go back to 170s in 2008, unsuccessfully. I'm still thinking about it now (will build up my backup bike with 170s).

I find that the longer cranks work really well on power hills, meaning shorter, out of saddle hills that you can roll over using higher power. For me that means little roller hills about 20-60 seconds long, where normally I'd use a 39x15 or 39x17 and instead I'm rolling a 53x15 or similar.

Longer cranks do NOT help me on sustained climbs, meaning more than a minute or two. In fact I think they hurt me once I'm back to a sustainable effort (i.e. not anaerobic, so efforts of more than a few minutes).

I don't time trial but I can get either crank length up to speed okay, seated, 35-37 mph.

Interestingly enough I sprint faster on the 175s than the 170s. I lost enough leg speed that even after a year on the 170s I couldn't sprint as fast.

When I was younger I was fastest on shorter cranks but I spun more and weighed much less. As I got slower and heavier and more powerful I've gone to longer cranks. 167.5 -> 170 -> 175. I spent about 10 years on each size, +/- 2 years. My weight went from 110 lbs to about 140 lbs to about 180 lbs (no height change). I'm hoping to get back to the middle weight range and then be able to use the 170s effectively, i.e. gain 2-4 mph in top speed (the 167.5s had me going 4-6 mph faster than I do now).

I have two good friends, former leadout teammates, good sprinters on their own, who are about 6'2" 6'3", both ride a 60 cm frame, both have about 35" inseams (they used to trade bikes on rides, they are that similar in fit). They both run 180s, and the one that's more in the bike industry is constantly talking about trying longer cranks (per Leonard Zinn). Both used to race. One does group rides still (last race was in 2001?) and the other has dabbled back in racing but really stopped in 1997. Both were using 180s back then and still use them now (albeit on newer/different bikes).