I was thinking about getting shorter cranks - I am 5’6 (167CM) using 170mm cranks - and playing around with this guide: Sheldon Brown’s Derailer Gear/Internal-Gear Calculator
What the screenshots below are suggesting that if I go from a 52-36 on 170mm cranks, to 50-34 on 160mm cranks, I get the high end speed of 53-39 on 170mm cranks while still maintaining the climbing ability of the 52-36 on the 170mm cranks.
I don’t understand how that’s possible.
I went from 172.5 mm cranks to 165 mm cranks, and I vastly prefer my new, shorter cranks. I have long limbs, so with the longer cranks, my thighs were massaging my breakfast when I was in an aero position. Not great.
While I don’t think your reasoning is quite right, your plan is sound: you should opt for slightly easier gear if you want to keep the torque (which determines the force on your joints) approximately the same. When I went from 172.5 mm cranks to 165 mm cranks, I subtracted 2 teeth from my chain ring (I’m rocking a 1x setup on my road bike), and I have been super happy.
But I don’t think you should expect higher speed at the top end: at the end of the day, power is power. Getting shorter cranks doesn’t magically give you more power (unless you were biomechanically limited in some way). Moreover, if you look at the speeds mere mortals reach while pedaling hard, I don’t think that’ll be much faster than 50–55 km/h. And for that, 50:12 or 50:13 is more than enough, assuming you spin at roughly 90–100 rpm. You only need harder gears on the downhill, and even here, the difference isn’t as massive as most people think it is.
Thanks, I’ll keep all this in mind. I think what these images are suggesting not that I will have more power, but that the gearing for 160MM cranks at 50/34 is going to feel like more 53 and 36 at the top and low ends of 170MM. That’s if I understand these charts right.
I’m not sure about that. In my experience with the shorter cranks, my natural cadence range has extended upwards, from 100-103 rpm to perhaps 103-107 rpm. That’s why au opted for easier gears. Apart from that I feel much better on the bike. 
That helps a lot!
At 193cm tall, I went from 175mm to 155mm. I’ve noticed my cadence has increased a little bit and that means I’ve put more force into the pedals for the same power. I didn’t change my gearing at all and it still feels the same as with the longer cranks.
My suggestion would be to get the cranks, get familiar with them, then make any gearing decisions based on how you’re climbing or sprinting as you ride around.
I also changed to 165mm cranks, coming from 172.5mm cranks. I am 185cm tall.
Feel much more comfortable, cannot imagine going back to longer cranks. My cadence increased 5-10 rpm. Kept the same gearing and feel fine.
The answer @fasterthanever is with something called gain ratio (the distance the pedal moves as a ratio of how far the bike moves). Probably best if you google it I might mess you up by explaining further. Edit: Thought the title was gear not gain. Thought I was being helpful but, you are way ahead with gain ratio: Gain ratio=radius ratio x chainring teeth/rear cassette teeth. Longer cranks=less radius ratio which=less % gear than a shorter crank.
Having been on 165’s from 172.5’s for many years (geared bike not fixed!!) nothing changes with power or cadence. You’ll find you just self select by feel and shift as required. What you might notice is how a shorter crank feels over the top. That is where these shine. If they can reduce even slight impingement then they will improve pedaling for sure.
This. Yes, if you ride the exact same gears, your cadence will increase slightly with shorter cranks, but most people automatically self-adjust by shifting gears to stay within their preferred cadence range.
Yes, exactly, you’ll get used to them in no time. For some people the cadence might stay the same, for others like me, the cadence will increase slightly. But your body will do this automatically.
Oh, and from everything I have read, there is no downside to going to a shorter crank (provided it doesn’t introduce any fit issues).
That’s my understanding as well. I started cycling 6 years ago with a 170MM but then switched to 165 and didn’t really notice it. When I switched back (via another bike) to 1`70 I didn’t notice it much either. Going from 170 to 160 after a few years will likely be at least slightly noticeable this time.
I have a mix of 165, 170, and 175 across all my bikes and don’t really notice any difference in pedaling.
My cadence definitely didn’t change. Not sure why crank length would automatically affect that. I think that would have to be a conscious effort. Power didn’t change either.
I like short cranks because of potentially enhanced aero on a road bike (higher saddle for a flatter back, all else equal) and lower chance of a pedal strike when pedaling through a corner in a crit. On an MTB, it’s mostly reduced pedal strikes over rocks, roots, and ruts.
Changing crank length is very much unlike changing Q-factor or cleat position IME. Those are immediately noticeable. You could make me blindfold test 170mm vs 165mm cranks on various bikes and I’d probably get at least a few of them wrong.
With a shorter lever you need to apply more force to output the same power at the same cadence. So if your body wants to keep your force and power constant, it needs to raise your cadence instead. Also, because the circles your feet have to inscribe are smaller, you require less kinetic energy to make them go around in circles.
I heard that some people do the opposite change, they go for longer crankarms on e. g. time trial or tri bikes to slow their cadence a little.
I’m not saying smaller cranks are more efficient for that reason, just explain why in some athletes you might see a change in cadence.
Same here. I went for shorter cranks, because with 172.5 mm cranks, my thighs were often massaging my stomach when I was in an aero position. When doing the last fit with my old road bike, the owner of my LBS was kind enough to take off a pair of cranks from a bike in the show room that were 165 mm long, and heureka, it instantly felt so much better.
