Bicycle Crank Length: Lies, Misinformation and Old Wives' Tales

One thing there is no shortage of is excuses not to accept this formula for crank length.

Old Wives' Tale #1: Tall riders who haven't tried a proper length crank often claim that they don't want a longer crank because it will hurt their "jump". Of course, tall riders who have tried a proper length crank don't make this argument -- they know better. Not only will having the proper length crank actually help one's jump, but the overall sprint performance is one of the things that will be noticeably improved. A tall rider with a suitably long crank and a big ring a tooth or two larger than normal can be a sprinting monster!

Traditionally, big riders have never been known as sprint specialists. Guess why!

Old Wives' Tale #2: Some people who have never tried longer cranks claim they will hurt your knees. Again, the riders who have tried longer cranks know better. This erroneous conclusion may arise from the obvious fact that the knees will be moving through an increased range of motion. However, the damage cyclists do to their knees is not due to moving the knees through an excessive range of motion; the range of knee motion in cycling is nowhere near the limits of the knee joint. The damage is caused by applying too much force while the knee is bent, too often and too continuously. The angle at which the knee is bent when force is applied -- during the downstroke -- is not significantly different with a long crank than with a short crank; as an example, at the 90 forward position (middle of downstroke), the pedal is a little higher with a longer crank, but it is also the same amount farther forward, and the effects on knee angle are pretty much a wash. The damage done to knees is actually caused by the continuous and repeated application of too much force, and going to a longer crank is a cure for this problem. With a longer crank, a rider can generate the same power at the same cadence with less force. When you are generating a constant amount of power for hours on end, the use of a longer crank results in a lower amount of force being used for those hours on end.

Allow me to point out something here: In Junior and Midget classes, there are gearing limits imposed in order to protect the knees of minors. Notice there are no limits on crank length. The problem being addressed is not the range of motion the rider's knees are subjected to, but the amount of force applied. Of course, these limits should be altered to take crank length into consideration, allowing taller gears when longer cranks are used for taller riders. At present, a smart Junior or Midget could cheat by using very short cranks, endangering his knees but allowing him to outspin his competitors.

Old Wives' Tale #3: Some people claim that too long a crank will ruin their spinning motion. They are correct, of course -- too long a crank will ruin their spinning motion. But following this formula does not result in too long a crank. The fact is, using too short a crank will also ruin one's spinning motion; tall riders using undersize cranks are often guilty of riding with their ankles rigid, and sometimes even with their toes pointed. While they may recognize these actions are bad, they still feel inclined to ride that way -- especially after a few hours in the saddle. The problem isn't with the rider's bad habits, the problem is the crank length -- put the proper length crank on the bike, and a proper pedalling motion becomes much more natural.

Old Wives' Tale #4: Some people worry that a long crank will make a "stomper" out of them. Well, yes and no -- if you're tall, having a longer crank will make stomping possible, even tactically useful under some conditions. But it won't force you to stomp. A properly sized crankset allows the rider to use the full range of pedalling styles whenever the need arises; he doesn't have to always be spinning, or always be stomping, or any other limitations. On the other hand, if he is using a 170mm crank when he really needs a 190mm, he'd better be spinning like a madman or he's gonna get dropped.

Old Wives' Tale #5: Somebody is always concerned that going to a longer crank will cause a drop in cadence, resulting in a loss of power. Fact is, this will probably happen the first time you ride the longer crank; you definitely need to do some stretching exercises and some training to become accustomed to the new length, since your muscles are all knotted up from spinning that little toy crank for so many years. Once fully accustomed to a proper crank length, however, experience has shown that a rider's cadence is more a function of his coordination than crank length. Increasing crank length will cause a drop in cadence, to be sure, but it's usually only a very slight drop; one tall rider reported that his max cadence with a 165mm crankset was 145, and after he became accustomed to his new 190mm crankset his max cadence was 141. As a matter of fact, a large, persistent drop in cadence is usually a good indication that the crank is, in fact, too long.

Since the rider's coordination is the big factor, a normal-sized rider going to a shorter crankset will often gain little benefit in increased cadence -- far less than the corresponding decrease in torque. Interestingly, nobody ever seems to suggest going this direction, even though we are actually promoting the same thing by expecting taller riders to use a 170mm.

Of course, a very short rider going from a 170mm crank to one more properly sized for him probably will experience a significant increase in cadence, because the 170mm was too long. He'll also be a lot more comfortable.

Misinformation #1: Several of the scientific studies done on crank length seem to indicate that the shorter the crank, the better the pedalling efficiency. Of course, this is hogwash, and every cyclist knows it. The reason these studies come to such inane conclusions is bad procedure, usually by doing all testing at a fixed power output and fixed cadence. The fixed power output is low, lower than anyone would actually care about. If you were asked to produce a constant, low power output and compelled to maintain a high cadence while doing it, wouldn't you choose a short crank? Of course you would. But that has nothing to do with reality. If you wanna talk about efficiency at low power output, let's talk about a long crank at a low cadence. But who cares? The question is, how do we get the most power output? And, of course, the answer is not in short cranks, but in the longest crank we can efficiently turn. Which is what this formula is all about.

The following are some more examples of misinformation gleaned from Edward C. Zimmermann's Competition Road Racing Bicycle Size/ Proportions Analysis site.  I really don't intend to pick on Mr. Zimmermann; it's not his fault that there is so much misinformation and ignorance in the cycling community.  He has merely included all this "convention" and other commonly accepted dogmas on his site, and done a remarkably good job of it, too, so it makes a very convenient source for responding.  Note that, if you visit his site, it appears you may have to enter a whole string of measurements and run a calculator before you get to see all the wisdom and analysis therein.

Misinformation #2:  Zimmermann:  "Proportional models have been suggested by some, viewing crank length from the perspective of total angle of movement. The problem with these first order models (magic formulas such as 28.4% of C, 20.5% of E, 10% of K or the Palm factor) is that they don't consider the distribution of force during a stroke, muscle usage, bio-mechanical stride preferences and, especially, the knee stress at the top of the stoke just when power is applied. While some of these models might seem reasonable they all lead to exaggerated crank lengths for the longed limed."  Ironically, the reason these formulas exist is that they do consider these factors!  These factors are considered to be just as important for tall and short people as they are for the average-sized rider, and such formulas based on the belief that the same angles and geometries that work best for the average-sized rider will also work best for the taller and shorter rider.

The convention of using a 170mm crank (or a 172.5 or a 175, all of which are so close to the same that the differences are nearly insignificant) for one and all would seem to require two interesting axioms:  First, that tall and short riders are best served by angles and stroke geometries that are different than those used by average-sized riders; and second, that the angles and stroke geometries that are ideal for tall and short riders just happen to be the angles and stroke geometries that result from using the same length crank that the average-sized riders are using.  I'm sorry, but that's too much of a coincidence to be believed.

Of course, the comment regarding knee stress "at the top of the stroke just when power is applied" is simply incorrect.  Any analysis of stroke dynamics shows that there is nearly zero stress at the top of the stroke; the vast majority of the stress occurs between the 90 forward position (middle of downstroke) and the bottom of downstroke.  Interestingly, the angle of the knee joint during this part of the downstroke is not significantly affected by crank length changes.

And, as you've already read several times if you've made it this far, cranksets that are a half inch longer or shorter than 170mm are not "exaggerated crank lengths".  They are, in fact, quite reasonable variations; the only reason they sound exaggerated is due to the silly ranges of sizes offered by crank manufacturers, not due to actually being exaggerated.

Misinformation #3:  Regarding the need for higher bottom brackets to effectively use longer cranks, Zimmermann says, "Even if the UCI where to allow for a morphological clause and permit higher bottom brackets, the bicycle would provide inferior handling (high bottom bracket, short stem)."  There's no reasoning offered for this, but it is a simple matter to explain just how misguided it is.  It is commonly known that a high bottom bracket results in a poor-handling bike because a high bottom bracket also means a higher seat (the distance from BB to seat being constant because the crank length is not changed in this assumption), which in turn means the entire rider -- arms, torso, legs, 100% of him -- is that much higher, raising the center of gravity and making for an unstable platform.  However, we aren't talking about just raising the BB height because we feel like it; we're talking about raising the BB height to put the pedal at bottom of stroke right back where it was before we changed to a longer crank!  The seat location is unchanged, the handlebar position is unchanged, the entire rider with the exception of the lower half of one leg is exactly where it was before.  In fact, the most significant change to handling is likely to be a slight increase in frame stiffness; the higher BB makes for a shorter down tube and a shorter seat tube, making the frame stiffer.

Just where Zimmermann got that "short stem" idea is a mystery; the BB height doesn't affect stem length, they are totally unrelated.

Misinformation #4:  Zimmermann:  "Cyclists with longer femurs tend to push the saddle back and this too increases the effective crank length--- and given the general correlation between foot and femur lengths, the same crank arm length is effectively longer (at relevant point of force application) for larger cyclists. A longer crank often means pushing the saddle forward, reducing the effective length."  I dunno where that "effective length" stuff comes from, but the reason taller riders push the saddle back is because their crank is too short, so the pedal at the mid-downstroke is not far enough forward for them.  Once the bike is fitted with the correct length crank, the tall rider will put his saddle back where it belongs because the pedal will be in the right place during the downstroke.

Misinformation #5:  Zimmermann:  "The assumption is that if different lengths (from these) made sense then over the history of cycling one would envitably have witnessed them."  There are two reasons not to buy this logic: first is the difficulty involved in making framesets with varying bottom bracket heights in the past (the problem is no longer an issue due to modern framebuilding techniques), explained on the tradition page.  The second is apparent in the cycling community today: the resistance to such a simple concept as varying crank lengths in proportion to rider size is incredible.  Anyone who feels that alternative concepts were fully tested at some time in the past must somehow believe the cyclists of the past were far more innovative and enlightened than those of today.  If you think this must be the case, consider this: the bicycle existed for 50 years before anyone thought of putting pedals on it!  The history of technical innovation in cycling is pathetic at best; it's better now than it has ever been, and we still can't convince manufacturers to offer a reasonable range of crank lengths!

There's a whole lot of other pseudo-science and faulty analysis on Zimmermann's site, but for the most part it has been addressed at other places on this page or this site.

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Of course, if you have questions or comments, you are welcome to send e-mail to me at  palmk@nettally.com.