Agree with the last two, but my experiences don’t match the shorter cranks (im 6’1ish). With 170mm cranks, midfoot cleat position, and steep STA (75), I get a lot of pressure under my patella. It goes away instantly with 175mm cranks. On the road and track bike, I can go between 165 and 172.5 without any pain.
I think the steep STA pulls me too far over the pedals. I feel like I am pushing backward on the 3’oclock pedal position.
But that goes back to your second point: if we had longer chainstays, we could have slacker STA’s, which IMO work fine with shorter cranks.
I find shorter cranks better, I’m 181cm/5’11"ish proportionally speaking I have short legs and long torso. Centre of BB to seat rail for me is 690mm, though I may be a bit conservative on height as I found having the seat too high causes me to get tendonitis in the front of my ankle, measurement might change by a few mm from saddle to saddle as well. I used bb centre to seat rail because it makes it easy to figure out what size dropper/length of seatpost/dropper insertion depth is but no one lists the stack height of their saddles that I know of so it takes that variable out.
For me 175mm cranks suck, 170s are ok and 165s are good. Haven’t tried anything shorter, I think I will get some aliexpress 155s and put them on a DJ bike and see how they are for that purpose.
On the crank question, there’s actually a decent amount of research that exists. I’ll dig up some links at some point if someone is interested but long story short there’s not a meaningful difference in power output between something like 140-170. Most riders start doing worse as cranks get longer than that.
In the era of rigid seatposts, short cranks (say, 165 and under) were mostly a nonstarter because you have to raise the saddle so high to get enough leg extension that it gets hard to descend. Think anti-dropper. So almost all mountain bikes ended up at 175 plus or minus 5mm.
With dropper posts, that’s not as much of an issue (or a non issue entirely, depending on the terrain) so you can lower the BB height considerably, run shorter cranks, and have a bike that handles much better with a lower COG. But we’re stuck on 175.
Now, before you jump down my throat, yes, you can have a preference for longer cranks, you can have bad knees that require you to have a certain length, etc. There’s nothing wrong with 175 cranks. But in many areas (enduro/trail bikes that aren’t aimed at max climbing speed) it’s silly to run long cranks when you could be an inch lower with 150s (or even shorter).
I’ve ridden very short cranks my whole life by default, until last summer when I got some 190s on my mtb. They are still “short”, proportionately. Zinn would have put me on 210-220s.
I’ve learned that I will happily trade a bit of ground clearance or bb height for a wider stance with the cranks in a horizontal position. I can hop better and I feel like I have more space to move my weight around.
I will agree w/r/t bikes that keep the wheels on the ground. On road and track bikes, I have never had a problem with 175s, and I’m 6’9”
Ya, that is super interesting, I am only 50mm taller, but my saddle height is 95mm taller.
I think all those studies were done on ergo trainers. And I found those to be true. On the road and track, I notice no difference in power and speed with 165 vs 172 cranks. Shorter cranks also have the benefit of opening up my hip angle so I could get a more aero position. On the MTB I find it harder to spin because the terrain is more dynamic and steeper. I end up mashing more, which is why I think I was more sensitive to the knee pain.
Saddle height is another can of worms. I think 25 or more percent of older road cyclists have their saddle too high and some have developed inactive glutes as a result. A pro racer friend pointed it out, and now I see it everywhere.
My saddle height on my trainer is 860 saddle top to bb with 175s…
The study I am most familiar with was for max power/sprint efforts. There was also one done by Mike McCalla (xc wc pro) for steady efforts that showed slightly lower o2 consumption with shorter cranks.
All of them have some flaws but the general message that crank length doesn’t matter much for most people is probably true.
I have not desire to jump down your throat Walt! I think those studies don’t have people’s long term fitness in mind. I’m just a big range of motion guy and what @anon68659156 said about having a better stance when in the horizontal position makes sense as well. I could probably produce more power only squatting to partial depth, but that might have consequences down the line if that’s all I ever did. Had the opportunity to ask Jeff Jones why he spec’ed 170mm cranks on all sizes and he pointed back to these same studies you are referring to.
Oh, you can pick apart all those studies, for sure. There’s not a lot of money in bikes, and there’s not a lot of money in academia (don’t ask how I know…) and when you combine the two…
That said, from a meta-analysis/30,000 foot level, it’s pretty clear that there’s no evidence for crank length (setting aside really really long/short ones) having any effect on how fast you go when pedaling. That has a lot of interesting implications for bike (especially mountain bike) design, but so far nobody has really followed up on it (unless you count me building myself low BB bikes with short cranks on them, I guess).
I don’t have any strong opinion about the long term fitness thing. I’d expect regularly riding a bike is going to be just fine for your long term fitness regardless of the crank length, though.
Ok, playing devil’s advocate here - if there’s no evidence for crank length having any effect on speed, then it comes down to what is your primary goal aside from speed. If the goal is ultimate handling then short cranks make some sense. If the goal best pedaling biomechanics, then I posit that cranks should be proportional. Yes any pedaling is going to be a net benefit for your long term fitness, but I know a fair few cyclists that have zero ability to touch their toes, squat while keeping their feet flat on the floor, or do various other humanoid activities.
Indeed, whether your goal is speed or handling (or both) short cranks probably make sense.
Cycling is a crap sport for flexibility, core strength, balance, etc, etc, etc, so I’m not particularly concerned about 25mm of leg range of motion. Longer cranks aren’t going to help with any of the flexibility/balance/core strength deficiencies that lots of serious cyclists have, so why worry about it? It seems like a bizarre thing to be concerned about, but maybe I’m not understanding you correctly.
I agree it is a crap sport for your mobility; cyclists definitely need to be doing other exercises to counter-balance the hamstring shortening, hip flexor tightening, etc. Longer cranks aren’t going to make that go away of course. Still, I don’t think it’s bizarre to be wanting cranks that feel like you’re utilizing at least a decent amount of range. Aside from the change in knee angle at the top of the stroke, your feet are also traveling along a much shorter path with the 25mm of crank length change you mentioned.
For example, with 170mm cranks my cadence had to be higher than I’d like to not feel like I was mashing the pedals. There’s got to be an efficiency loss from rapidly cycling through a small range of motion vs slower cycles through a longer range of motion. Think of walking up tiny shallow stairs vs taller deep stairs. Everybody will have an ideal stair height AND depth. With 180s my cadence is naturally slower because my feet are traveling a longer path each revolution and my target leg force output matches better with my target cadence. Essentially, I can pedal at my favored cadence without mashing. Somebody more clever than me needs to model this up…
I’ll have to measure my knee angle on the 180s, but using the tibia length formula, 41% of 18.5-19 inches gives me a 192-198mm crank length. It’s odd that their slider tool maxes out at recommending a 175mm crank…
I did it once about a year ago. It put me in a good range and I tweaked it to make it feel as good as possible. Surprisingly, the AI bike fit improved my comfort and ever so slightly my output compared to a professional fit I had done a couple years prior.
The tibia slider produced a strange result for me too. Inseam slider seemed more accurate. Everything with a grain of salt I guess. Plenty of interesting info though.