Does crank length matter?

Interesting video and conclusions. I wonder if it holds true for mountain bikes? More opportunity to be fully geared out, sometimes for significant stretches, on a mountain bike. But also significant opportunities here give the way bottom bracket height has crept ever down. I'd love an extra 5 mm of pedal clearance.

I've ridden 170mm cranks for as long as I've been serious about cycling (LONG time). Tried 172.5 cranks when doing TT's or hilly races, but never felt right on them. The crazy thing is, for those of us who are metric-challenged, 2.5mm is about 1/10", and represents a 1.47% increase in crank length! Minor change, but man could I feel it. at 5'9" with an inseam of ~30.5", 170mm have worked well, but studies like this make me curious to try 167.5mm cranks. I have always been a spinner, and getting beyond the concern of a mechanical disadvantage of shorter cranks has me intrigued. Time to see if my LBS has a pair I can try.

I'll throw in another data point. I am 5'11" but with very short legs (30 inch inseam). Nonetheless, having two bikes, I observed that I was much more comfortable and felt more powerful (even in high gears) on the 175 cranks that came with my mountain bike. Since then my road bikes have been fitted with 175. And I am happy.

Very entertaining. One is forced to question however, how many uncontrolled variables were involved. I noted that saddle height was apparently adjusted to compensate for crankarm change, but was saddle fore/aft also changed with crankarm length? There are several other considerations, but as in all cases where the human variable is involved, crafting true apples-to-apples comparisons is nearly impossible.

For me, since we know that mechanical leverage is increased with longer crankarms and leverage is desirable (especially when climbing and accelerating), performance cyclists should use the longest crankarms they can. The limit on that isn't rider height but rider flexibility. To maintain optimal leg extension and fore/aft knee position, saddles can adjusted to compensate for crankarm length changes (e.g. down and forward for longer crankarms) but saddles cannot be adjusted to compensate for the knees being raised higher at the top of the pedal stroke. So, longer crankarms send knees higher which requires greater rider flexibility because the torso/upper leg angle is closed by the longer crankarm, plus the torso/upper leg angle is closed even further by the lower saddle position.

If there is a significant slowing of foot velocity through the top of the pedal stroke, then most likely the cyclist is using a crankarm length that is too long for their flexibility (primarily hamstring). This is especially evident among aggressively positioned high level triathletes, many of who are going to shorter crankarms since they've started to realize that a more consistent foot velocity throughout the entire pedal stroke leads to higher average road speeds.

That means for example, a super flexible short cyclist might in fact be better suited for a longer crankarm set than their very inflexible tall friend. That's a key human variable I never see accounted for in testing such as in this and similar articles/studies. Consider a test protocol where foot velocity is constant for various crankarm lengths (part A), then road velocity is constant for power output (part B), then wattage is constant for various crankarm lengths (part C), then VO2 is constant for various crankarm lengths (part D); then you're onto some useful information.

I viewed this "scientific" approach with open ears and eyes BUT....
in using a human guinea pig there in lies the problem, the rider for instance is used to the use and feel of crank length A, is then adapting to crank length B and C, physiological changes that then incur differing resultants in the test data.
And I believe there are very obvious other over sights as follows

A crank arm is a simple lever with the leg an extra 4 pivot points, a force applied at a give distance from the fulcrum gives a give torque etc to the resistance,

1: The riders foot position, pointing the toes down, flat foot changes the point of force applied changing the leverage through the ankle
2: The riders foot size, refer to above
3: Has the seat position bean modified forward or aft with the changing crank length which alters his/her relative position to the pedals and there by altering the rest of the legs movements AND handle bars changing the chest position and there by the breathing.

To do this scientifically, take the rider from the bike and give them a rest and do the maths on paper, the numbers can't be altered as can a rider.

I'm sure an engineer with more time on their hands than I can do the sums and even throw in a logarithm taking into consideration bone length variants etc

Interesting but a bit old hat. I have known all this for 30 plus years. There are personal considerations: style and comfort. Also standard bike frames are basically designed to use 170 cranks. I have used 167.5 on Track bike and a little road use with track bike.. But used 170 mostly always, but tried up to 175, hate anything over 170., I am near 6'2''. adjusting to what you have makes a difference It is very obvious shorter cranks lower heart rate. Max power in this test means nothing because gears were not changed. Having used very heavy 1400 pound weight in leg press machine I learned a less bent leg at high reps is possible. This points to using shorter cranks. The greatest advantage of shorter cranks is a very scrunched over super aero position on standared road bars. I am as or more aero on my road bars 40 cm wide,than tri bars amazingly enough. But just for a short period of time as position is hard to hold. Getting top of head near top of back, bent over with a very bowed back. It,s hard to look down the road and is good for a few seconds. With shorter cranks the knee and thigh don't come up to hit the body when in the most extreme areo position, and opens up the hip so it does not come up so high. A lot of interesting concepts that are hard to discuss easily here. The bottom line, it is worth looking at 162.5-165 on a normal frame design, even if you are tall. You will need lower gears for up hills and spin more -as pitch changes that slow RPM are far more extreme when climbing than flats. Just like Lance use to do.