So why is that?
Well, there are probably several reasons why, but unfortunately none of the reasons have much to do with logic, science, or what's right for the human body. Perhaps the best way to describe why bike cranks are all nearly the same length is, 'manufacturing expediency' and 'tradition'. Occasionally, one even reads individuals in cycling and in the bike industry asserting that either crank length has no effect at all, or that cranks sized over a very small size range in tiny increments (such as 2.5 mm) are sufficient to fit nearly all riders. Others assert that the mechanical advantage differences provided by different crank arm lengths are irrelevant because of bicycle gearing.
Well, I submit that these conclusions are, if not ridiculous, patently false. Below, I provide some logic-based evidence to support a seeming revolutionary idea, the theory that bicycle crank arms should be sized in proportion to the rider's legs (i.e., short legs will generally work best pedaling shorter cranks, and long legs will generally work best pedaling longer cranks). Further, I believe that there's evidence to support the idea that, for a given sized cyclist riding a specified cycling event, such as, say, a 40 km (25-mile) cycling time trial, there exists an optimum (and proportional) crank length for a rider of that particular size in that type of event.
But even for those that accept the general idea of proportional crank length, the question often comes up: aren't cranks already proportionally sized if the you can easily get them in 170 mm, 172.5 mm, and 175 mm lengths?
Well, not exactly. Say you assume that someone with a 31-inch (78.7 cm) inseam should ride 170 mm cranks. Well, if you accept a proportional crank-to-leg relationship, then 175 mm will fit a rider with just a 31.9-inch (81.1 cm) inseam.
So with most bike cranks, you actually end up with less than one inch of leg size range. And cranks longer or shorter than those like 165 mm and 180 mm lengths (which are often difficult to find and very few riders use) will only increase the proportional inseam range a little more (from 30.1-inches to 32.8 inches). So, if you consider the real human family and were to go out and measure the random inseams of a few taller male riders and a few smaller female riders, you'll very quickly discover how ridiculously small that range actually is.
And just as bicycle seat height has been shown (in laboratory research) to influence rider power output and efficiency, crank length also very likely influences power output. But like seat height, if crank length changes by only 1 mm, it's unlikely that the performance difference will be large, it may actually be so small as to be basically undetectable. However, if crank length changes by larger increments such as 5 mm, 10 mm, or 50 mm, then the story changes entirely.
To support the idea that bicycle cranks in a size proportional to the rider make a significant difference in cycling power output, I will use a bit of logic, a little common sense, and some basic knowledge of human biomechanics and physiology. I will also make just a few assumptions along the way, but they'll be assumptions that even skeptical readers should be able to accept fairly easily. So, read, look at the figures, follow the reasoning, and then decide for yourself.
While there have been a few research studies published that have tried to examine some aspects of human performance and bike crank length, little of the research to date has shed much light on the relationship between submaximal cycling performance and crank length. Also, many of the crank length studies have had some fairly significant limitations regarding the variables examined, the methodology used, the sample size, etc. Doing a truly thorough real-world study to adequately examine the relationship between bicycle crank length and rider leg length in endurance cycling would not be an easy feat, as it would take quite a bit of time and might cost a substantial sum, neither of which I have at my disposal.
But there is an alternative.
Using just a few basic assumptions, we can explore this idea by setting up a dinner napkin "thought experiment." Using just known information and basic logic, we can actually explore whether or not bicycle crank length could have an effect on sustained cycling power output and rider efficiency, all without actually doing a real study. While the results of this virtual study will clearly not be the final word on this topic, they will point the way to where the answers lie.
To begin this thought experiment, we'll start with just a single virtual individual as a test subject, one, say, about 5'8" (~ 1.7 meters) in height. I think we can all safely assume that if he or she is very fit, experience tells us that he will likely be able to generate high levels of power with cranks 170 mm long.
So for our simple thought experiment, let's assume our imaginary 5'8" tall test subject is a very fit endurance athlete (a runner would probably do fine), but one who has never trained on a bike (I think this last requirement might be important, because if our subject has trained a lot on a bike, his body has already begun adapting to the crank length he was using, most likely to cranks 170 mm long, which are the most commonly available).
Let's start our case study by generating a baseline. We'll begin by testing our human subject and determining his maximum oxygen uptake (max VO2) on an ergometer (an indoor bike trainer) with the standard 170 mm cranks just so we can get a rough idea of his maximum ability to generate work.
Once we have the rider's max VO2 measured, then we'll start testing the rider with a few different length cranks and see how many average watts (as measured at the rear wheel) he can generate for a somewhat sustained effort at, say, 85% of his max VO2. The idea behind this is, if one crank length was indeed 'better' or more efficient than another length, then our subject would be able to generate more sustained average watts with the 'better' length crank.
Ok, so let's start this experiment by testing our subject with just three different length cranks. On this and all the following protocols, we'll of course do each and every test when the rider is completely rested so we'll get accurate and repeatable results. We will test the subject at 85% of his VO2 max with cranks that are 170 mm long and then two totally 'crazy' crank lengths: a test with ultra-short 50 mm cranks, and a test with ultra-long 300 mm cranks. And in each test, we will adjust the ergometer's seat height so that it will remain constant relative to the pedal position at bottom dead center (measured parallel to the seat tube, or 'seat angle'). We will also allow the test subject to self-select his own pedaling cadence, but we'll run him through a range of normal cadences, identify ones that result in the highest sustained wattages at 85% of his VO2 max), and encourage our rider to use the higher wattage cadences.
So what kind of data will we collect? Well, seeing as this experiment is imaginary, I obviously cannot supply absolute numbers, but I would venture that (and most experienced riders and exercise physiologists would agree) if we took a physically fit 5'8" tall human subject through the test protocol generally described above, our first 3 data points would very likely look a lot like this:
(click on any of the figures below to enlarge them)
Fig.1
If we can agree that this is indeed the kind of data that we'd collect in the first part of our experiment, the next interesting question would be what would happen if we could take the same test subject and test him with the same basic protocol as before, but this time test him on every single crank length (in, say, 5 mm increments) in the entire range between 50 mm long cranks and 300 mm long cranks (e.g., crank lengths of 50 mm, 55 mm, 60 mm, 65 mm, 70 mm, and so on, all the way to crank lengths of 280 mm, 285 mm, 290 mm, 295 mm, and 300 mm) ?
So if we went ahead and did that as the next part of our experiment, what kind of data would we collect as we tested through this quite substantial range of cranks lengths? And if we then graphed all those data points and drew a curve through them, what would this curve look like? Again, since this experiment is imaginary, I obviously cannot supply absolute hard numbers, nor an absolute curve. But based on even fairly basic knowledge of human physiology and biomechanics and on the understanding that the human organism is an analog, and not a digital, machine, I think we can agree that the curve though the data points collected in this part of the experiment would most definitely not look like this:
Fig. 2
But, of course, this begs a more interesting question, if the data curve would not look like fig. 2, then what in the world would it look like? And what is that curve going to tell us?
Well, since we used the same basic test procedure in every test so far, basic logic would say that the curve would have to at least intersect the first three data points that we collected in the first part of our experiment (data shown in fig. 1). This means that the curve is going to tell us that somewhere between 50 mm cranks and 300 mm cranks, for our test subject there is some optimal crank length range where our rider is able to generate more sustained watts (at 85% of his max VO2) than he could with the extremely short or extremely long crank lengths. So, if we can agree on this general line of reasoning, I suggest that the curve through all these data points might very well look like this:
Fig. 3
Of course, the data curve would look like fig. 3 only if it just so happened that a 170 mm crank length was the optimal length for our test subject.
But what would the curve look like if this was not the case, what would we see if perhaps there was some crank length longer or shorter than 170 mm where our subject was able to generate more watts? Could we have a viable curve that would still intersect our original three data points and not follow a highly unrealistic "digital" shape (like shown in fig. 2) ?
Absolutely, yes. This is what the data curve might look like if cranks somewhat longer than 170 mm were better for our particular test subject:
Fig. 4
And this is what the data curve might look like if cranks somewhat shorter than 170 mm were better for our test subject:
Fig. 5
You could (and should) do this same series of tests with other human subjects, people much shorter or much taller than our 5'8" test subject. To do the same basic tests on different sized subjects, it would probably be smart to proportionally adjust the baseline test crank length and the sizes of the "crazy length" test cranks (i.e., make the range of all the test cranks lengths longer for tall test riders, and shorter for small test riders), but I think it's obvious that you would get essentially the nearly same data curves and could come to basically the same conclusions.
As an aside, in this test, the "crazy length" super long and super short cranks were not chosen because there is something magical about the 50 mm or 300 mm crank lengths. No, those dimensions were chosen arbitrarily because, for an average-sized rider, those two lengths are incredibly short (or incredibly long) and, based on experience and common sense, those crank lengths are very likely to be less efficient for a 5'8" tall rider than the standard 170 mm long cranks. (But if you were to do this test on a rider that is 7' tall, a 170 mm long crank would probably be an almost 'crazy short' length crank for him, and a 300 mm long crank might not be nearly so 'crazy long.')
This experiment does not prove that any particular rider should be riding with "long" cranks or "short" cranks. What it does try to show is that there is a physiological, biomechanical, and logical basis for the conclusion that there is an optimum crank length out there for riders of every single size, and that it may be close in length to 170 mm, or it may not. (Actually, given the very wide range of leg lengths out there in the cycling world, I suspect that for most riders 170 mm cranks will not be optimal.)
Essentially, all other things being equal, my theory (based on the reasoning above) is the revolutionary idea that one's optimal crank length is likely to be somewhat proportional to your leg length. In reality, this theory is far from revolutionary but, simple as it may be, it seems to run headlong against accepted cycling "tradition."
If any of this makes sense to you, take a look at the link at the very top right of this page (the link on the page to Kirby Palm's crank length formula). On his page, Mr. Palm proposes a remarkably well thought out formula for determining crank length based, not on what's convenient for bike manufacturers, but on the rider's leg length. Read his supporting info, measure your own legs, put your measurement into the formula, and prepare to be surprised. My guess is that easily 9 out of 10 riders who do that will discover that the cranks they are using right now are nowhere near the right size.
I welcome ideas and comments. And if you're in the academic world, and have lots of funds and lots of time, this might be an interesting thesis project ...
