Originally Posted by
sjanzeir
I've been wanting to experiment with this for a while: Running shorter crank arms than the usual 170-175-millimeter OEM ones that come on most bikes in my frame sizes (15-17.5/S-M.) Nothing wrong with some experimenting; I'm 5'3" inches/160 centimeters tall; and the Wise Oracle that is the internet says that ideally, I should be running 150-155mm cranks - which is totally ridiculous.
What was perhaps a little foolhardy on my part, though, is that I went ahead and up(down?)graded not one, but both of my folding bikes to 165mm cranks. I did not expect to be having this creeping desire to want to go back to the stock 170mm cranks.
All I expected was for there to be a subtle difference in cadence and power at worst. It turns out, the differences really weren't all that subtle. Now my legs tend to spin up faster - and spin up to a significantly higher cadence they do - than I'm accustomed to, in lower gears than I'm used to, with a little less torque than I had come to expect. Simple physics; no surprise there.
It's been a week or so and I've only ridden the bikes a few times, and just around the neighborhood at that. So I should probably give them more time so as to fully figure out exactly how I feel about this and what I need to do.
In the mean time, I'd really appreciate some of your experiences with running shorter cranks!
Choose your cranks based on biomechanics (how much your knees and hips like to flex) and maybe your preferred cadence - the gears will even out the power output. Since power is essentially torque x RPM (with some constants thrown into the equation), if you use shorter cranks, you might lose some torque, but you can compensate with higher cadence (which the shorter cranks will favor) and a lower gear - you should manage similar power output. The only time the added torque of longer cranks become an advantage is when you're run out of gears and long cranks might be the difference between grinding up a steep climb and walking.
Years ago, when I started paying attention to bike sizing (when size selection became more involved than "can you stand over it? Good - it's the right size"), I used an online bike fitting guide which suggested that 172.5mm cranks were appropriate for my height (5'8"). Thinking back, a crank calculation based on inseam might have been a better starting place, but I've used 172.5s for the last ~30 years with no issues, so I guess the height-based method is fine for people of average proportions.