https://www.hscycle.com/ I'm 6'8" with a 39 inch inseam, I'm using 190mm cranks, not quite long enough, but the 230mm cranks were just to long for corners, etc. however on a straightaway the longer cranks allowed me to average 23mph, when using the 185mm cranks the average was 18mph. The above link is for custom cranks.

My Revive has 170MM cranks and that seems to be right for my 5’9” body. Now that I also have a dirt bike it has 175 MM cranks and with the seating position required with a flat bar that seems about right. I don’t seem to have any trouble finding a gear to spin in and climbing seems easier with the longer crank. The Revive hates climbing just about as much as I do. The trail bike likes climbing better than I do. But I am not as strong as I once was and I need the extra leverage.

DnvrFox

It should be stamped on the inside of the crank arm close to the pedal. Otherwise just measure center to center.

Just changed cranks on my Wahoo from 170 to 175. I think it makes a big difference in climbing power. I like them.

Here is a great device for experimenting with multiple crank lengths, for example from 150mm to 190mm. Unfortunately, for some reason, the price is $650! It sure doesn't look like it should cost anywhere near that amount.

https://www.hscycle.com/Pages/adjustablecrankset.html

Check this link for another example of crank length adjusters.

https://www.tandemseast.com/parts/cranks.html

Check this out:

175mm= 6.8 inches
172mm= 6.7 inches
170mm= 6.6 inches

We are only talking a little less than an 1/8 of inch difference between
the lengths. I'm really not sure if one will notice a big difference performance
wise unless they go into the 180 mm something or 160mm something if the
were using something like 172 mm crank. I could be wrong in all this since
I have never ventured from my 175 mm cranks, but on a positive note there
must be some benefit between the lengths even though it is only an 1/8' of
an inch otherwise the manufacturers would not bother to make them.

But I will say this, everyone I ride with that has shorter cranks than my 175mm
will get dropped by me on the hills and the rollers. This is why I never considered
changing them to something else.

The formula of: crank length= (1.25 x inseam in cm) + 65 = 170 mm for me.
But I have no intentions on changing to it.

Is this because it is impossible for any of them to maintain a higher cadence than you?

Apparently, you didn't read the very first post of this thread?
Some ONE did!

Are you 50+?

Don't you contradict yourself within your own post? Apparently there IS a difference in performance...

NO, I didn't contradict myself, I said in my opinion I do not feel going from 172mm to 175mm or 170mm to 172mm, even from 170 to 175mm is going to make a big difference in performance. It may help with a "Fit" issue, but it's not going to make a person faster. I also said that the guys I ride with have shorter cranks and I drop them on the hills and rollers. My point in that was, I drop them because I spin faster with my 175 mm cranks than they do with their so called "shorter cranks". Whereas I have read in replies here that going from a 175 mm crank to a 172 mm or 170 mm crank would allow you to spin faster. Well you will not spin any faster just by reducing your crank length 1/8" or 3/16”. You want to spin faster work on the engine….

If you think less than a 1/8" or 3/16" of crank length will make you a lance Armstrong or enhance your performance and add 2-3 mph on your average speed, you are mistaken.

The primary reason to move to a smaller crank arm is comfort. If you use too long of a crank arm and adjust your seat height so as for the angle of your knee to be optimal at the bottom of the stroke, then the angle at the top of the stroke will be more severe. For many people this can result in more pain in their knees over extended usage.

It can also affect performance by making it difficult for the cyclist to maintain their optimal spinning motion. Obviously this would vary from person to person.

This seems to be even more of an issue for recumbent riders, where one's leg position is fixed relative to the pedals.

I use 170mm cranks on my upright bikes and those have worked well. However on my recumbent, 170 was too long. I was either losing some contact with the pedal on the downstroke, or it was coming up too high for comfort on the upstroke. I switched to 155's and my cadence went up by 10 and my speed by 1 mph immediately ... because I was then able to apply power throughout the stroke. And just cruising along is now so much easier than it was with the 170's.

Hmmmm.... aren't facts dependent on your particular view of reality? Remember the story of the man who wanted facts about what the inside of an orange looked like. He had two oranges and he cut one top to bottom and the other horizontal. He was quite confused, because they looked nothing alike. So, that fact was that it depends.

Glad you found a crank length that works for you and your knees.

I may have missed it, but I didn't see any discussion of physical build beyond leg length and height. Yet, my reality is that my thighs and calf muscles are much larger than most folks (even seasoned cyclists) for someone my height. Hence, the longer cranks allow me to use some of that muscle mass in ways that a shorter crank won't.

I'm not certain this is a significant factor for selecting your optimal crank arm length. Having more muscle might not really help if one is using too long of a crank arm, as it would prevent you from using that muscle as efficiently as you might on a shorter crank.

Granted, it might enable to you to shove a longer crank where someone else might struggle. But that doesn't mean you would be able to hit your maximum power output on the longer one. We are looking for maximum mechanical efficiency here.

FACTS are what actually has HAPPENED! I can't explain it much simpler to you than that!

Why would you ASSume, I remember some story about an ORANGE, when I NEVER heard it?

I guess that's your version of "facts".

Until you have experimented with different length cranks AND the subtle gear changes appropriate to those different length cranks, you are simply guessing about what the best length is!
I've made up a custom cassette from 2 different cassettes. Thus I have the ability to fine tune my gear ratios VS crank length. I've probably made 4-5 minor changes to get the cassette dialed in for MY riding. That will probably change in the Winter, when it's colder & windier.
So far, I've used 175, 170 & 160MM cranks.
The 175's are definitely too long for my bad knee, as explain in the original post.
I'm slightly faster on the 160's than the 170's, but they seem to feel a bit too short. I hoping some 165's get left on my doorstep by the crank fairy. Then I can wonder if 162.5 or 167.5 is better
Budget constraints prevent me from buying some. (My 160's were $7.50 +S&H)

Bill, you really ought to watch the name calling and read my first post in this thread. I have experimented with different length crank arms. I was simply trying to point out that the same crank length will not be experienced the same way by people with different perspectives. Of course, there are those who think their perspective is the only one.

As another has pointed out the actual differnece in length between the crank arms we're discussing aren't that significant either. Not having the ability to actually measure my power output, I don't know which would allow me the most power. I think it would be an interesting experiment to factor muscle mass into crank arm length and power output..... along with inseam size and perhaps even different lengths between the upper leg and lower leg. I'm also trying to get my mind around the idea that a longer lever allows one to do more work with less expenditure of energy and figure out how this relates to the other simple tool of wheel and axle. We've got both of these in play here, but to what extent I sure don't know. But, I would suspect that for all but the most astute if they were on a trainer and blindfolded, they wouldn't be able to tell the difference.

When I was test riding a lot of bikes, I found out early in that process that I was more comfortable with a 170mm crank than a 175mm. However when checking out bikes, I sometimes forgot to check on it, paying more attention to other aspects of the bike.

I remember taking a couple of longer test rides (20-30 minutes) on the bike I eventually chose, the Fuji Absolute 1.0 . On nearly every other bike, the size that fit me best was a 17"-18" frame. On my test ride on the 17", I liked much about the bike, but couldn't get super comfortable on it. Upon getting back to the LBS, they told me that the Fuji 17" was a bit small for its size and that perhaps I should try the 19". So out I went on the 19".

It took about 5 seconds to recognize that it was a better fit. My riding position felt much more natural, my arms & shoulders were more relaxed. It was great except for one thing, on every stroke it felt like I was trying to step over something that was a little bit too high. Instead of a smooth pedaling motion, I felt like I had to stretch to get over the hump at the top of the stroke. This was on each and every stroke.

Initially I thought that perhaps it was the same on the 17" but the other issues had disguised it. So I got back on the 17" and there was no such sensation. So finally I got around to checking the crank length. The 17" had 170mm cranks, the 19" had 175mm.

At the time of purchase the LBS swapped the cranksets for no extra charge, so I took home the 19" w/170 cranks.

There is no way I'd want to frequently ride a bike that had 175 cranks.

Now, all of that said. It is more likely that one would notice the effects over time if they are using too long of a crank arm. Because if the knee angle at the top of the stroke is too severe, and you ride 3000 miles with every stoke subjecting your knee to that angle, then you are more likely to have knee problems.

Either this or they are able to spin a bit faster right away. That's the way it worked for me on my bent. But that was a change of 15mm and fixing a very noticeable problem.

Interesting thread and it helped to answer some things for me.
When I first tried out my Giant, I noticed right off how much more easily it seemed to glide than my LeMond.
Now, granted, the LeMond has a 42T middle vs the 39T on the Giant.
Still, it was just easier all the way around.

Where I really started to notice it was on the climbs when both bikes are down in the 30T front ring.
After riding the OCR for a few months, the Reno felt sluggish, like it was riding on sticky roads or in the mud. I knew it has 175mm cranks so I finally looked at the Giant's. The verdict? It has 172.5mm

It has been mentioned how small of a difference it is between some of the crank arm sizes. One should note that as it relates to one's knee angle at the top of the stroke, this difference is doubled.

You adjust your seat to the proper height in relationship to your maximum leg extension on the downstroke. When using a 175 crank, your seat height will be set 5mm lower than if you use a 170 crank, because the pedal is 5mm lower at max extension.

Thus on the upstroke, a minimum extension, the pedal on the 175 crank will be 10mm closer to your seat, the 5mm from it being lower plus the 5mm from the crank being longer on the upstroke.

This is all fine and dandy if one's legs are long enough to accommodate a 175mm crank (or 180 or 185). However if they aren't then this is more than enough difference to negatively affect the knee angle and mechanical efficiency.

In the case where someone is better fit to a 165, then using a 175 puts the top of the stroke 20mm out of optimal position, that's 4/5 of an inch. We all know that adjusting pretty much anything on your bike, be it the seat height, the stem, the height of your handlebars, etc., by 20mm makes a noticeable difference.

In the case of my recumbent, going from the stock 170 to 155 cranks, changed this parameter by 30mm, or 1.2".

So who might consider using a crank of less than 175mm? The formulas vary and there is no universally agreed upon algorithm. Lennard Zinn, who builds custom cranks, uses:

Inseam in mm x 0.216 = optimal crank arm length.

This would be your total leg inseam, not your pants inseam. My leg inseam is 760mm, which works out to 164mm.

An 80cm (31.5") inseam yields a 172.8, or a 172.5 crank.

Anything above 80cm (let's say 32") would need a 175 or larger crank. At this point Zinn recommends dropping the multiplier from .216 to .21, so as to not overestimate the length.

However Zinn & others will take it a bit further on a custom measurement, wherein they will take a femur measurement and adjust the crank length to that.

The more common algorithm of (leg inseam * 1.25) + 65, will yield a shorter crank length than above. Using my measurements, this one recommends a crank length of 160 instead of 164. And would not recommend the use of a 175 unless your leg inseam was over 34".

I wonder what formula my brother would use?
About 40 years ago, he was in a car accident when he was 15 and one leg didn't grow.
He later had the long leg shortened nearly 2" below the knee. Not enough to even them up, still being about an inch different.

As far as crank lengh goes, it's trial and error. Any mathematical formula would be like a mathematical formula giving you the proper weight of a bike or gearing for you. If it feels good and right, then do it!
Bill, last year I met a fellow who had a very badly damaged knww which was at least partially fused and his one leg shorter than the other. He had a really short crank on the one side and a longer one on the good side. He had derived the length purely by trial and error, but the one on the injured leg was vry short.
Also depends on the weight and gearing of the bike. I have 155's on one bike of mine which is appropriately geared down and it feels right. I spin a lot with it.
I have a lighter bike which has the original 170's on it and because of the light weight that one "feels" right to me.
Of course, a lot of it is what you get used to. But at least with recumbents, they seem to spin better with shorter cranks.
I have no opinion about uprights bikes, although to me it makes sense if bike 'fitting' is as important as the web sites seem to indicate, that fitting would also have to include riding with the correct length crank for YOU, rather than the usual stock 170's or 175's.
Like Denver, I wish someone wojuld make adjustable cranks so anyone could go out and experiment. It's tough to try different lengths otherwise.

When I went from 175's on the mountaing bike to 170's on the road bike I had an immediate increase in comfort. I have a 34" inseam. We are all different and get different results. Shorter cranks work for me.