Bytager the publication "The correct length of cranks"
All equipment manufacturers offer cranksets with different crank lengths, but at the same time most bikes are sold complete with components determined by the brands. However, whatever your size, Crank length really matters in how you pedal. Should you get bigger ones, smaller ones, or stick with the basic offering?
By Guillaume Peephole – Photos: Scott-Sports, Pixabay.com
Shimano, Sram or Campagnolo offer their crankset models with different crank lengths and different chainring combinations. The same goes for specialist crankset equipment manufacturers, such as Rotor, Stronglight, Praxis, etc., or all power meter manufacturers. A few years ago, Look even offered an interesting solution with a Zed 2 crankset that could be adapted to three lengths (170, 172,5 and 175 mm) thanks to the positioning of an insert. These three lengths are the most common, but depending on the crankset models, you can also find cranks from 165 to 180 mm.
However, on most production bikes you don't really have a choice in crank length. Determined by internal brand statistics, crank lengths depend on the size of the bike, just like the length of the stem, the width of the handlebars or saddle, or even the developments. All this without taking into account the morphological particularities of each of you, your more or less pronounced flexibility, your practice, your experience or your habits.
We also know that good positioning is essential for efficiency, comfort and performance on the bike. And often, it comes down to one or two millimeters so that the body can give its best, with a bike that behaves like a true extension of the body.
On a production bike, the cyclist who knows what he wants or who goes through a thorough postural study often has to negotiate with the bike shop to have the position components exchanged, in order to ride the ideal bike. This is the least he can expect with a bike costing €5000, €7000 or even more than €10. But it is obviously much more complicated with online sales, where bikes are standardized as much as possible. And when modifications are possible, you often have to pay more. Then there are the solutions of opting for a montage a la carte, or purchase an additional pedal board in the correct size.
=> SEE AS WELL : Finding a Crankset with the Right Crank Length
How is crank length determined?
For bicycle brands, crank length is determined by the size of the bike. On XS and S bikes we often find 170 mm cranks, on M and L 172,5 mm, and on XL 175 mm.
But to be a little more specific, we should rather look at the length of the crotch., or rather the ratio between the leg length and the total height. Because for two people of 1,75 m for example, there can be big differences in morphology, with small legs and large trunk on one side, and large legs and small trunk on the other. A more precise formula to find the right crank length could be the length of the crotch to which we apply a coefficient of 0,216. However, we can see that this does not work for the extremes, since we would need 150 mm cranks for someone with a 70 cm crotch (which can be the case for small women), and 190 mm cranks for someone with a 90 cm crotch. In both cases, these crank sizes do not exist.
Other aspects must also be taken into consideration, such as the cyclist's joint and muscle flexibility, the position he naturally adopts in the saddle, and of course his habits.
The lever arm
Even if we stick to just the three most common crank lengths (170, 172,5 and 175 mm), these differences of a few millimeters are not negligible in terms of position and sensations. Between 170 and 175 mm, the difference in length is not noticeable to the eye once the crankset is mounted on the bike. However, on each crankset revolution, the length of the circle (difference between two extreme points) described by the crankset movement varies by 1 cm between these two crank lengths. At the circumference of the circle, the difference is 3,14 cm (109,9 cm – 106,76 cm). At the muscular, joint and tendon levels, it is easy to understand that this difference influences the amplitude of the gesture, especially since it is multiplied thousands of times per outing.
In theory then, larger cranks increase the circumference of the circle, and therefore the lever arm. With the same muscular force, a larger lever arm gives more force to the transmission, and therefore makes it easier to take a large gear. But this must still be weighted in terms of power, which is the product of force and rotation speed. If the enlargement of the circle is accompanied by a slower rotation speed due to the amplitude required to accomplish the gesture, there is no advantagetagand to mount cranks that are too long.
Some studies have even shown that for the same athlete, crank lengths of 160 to 190 mm do not change power production, this being primarily determined by the general physical capacities of the runner. However, we must not neglect once again the muscular, articular and tendon constraints of a movement that is too wide in relation to the size of the crotch. Constraints that can sometimes cause deep injuries, in addition to the usual aches and pains when changing a movement that has become almost automatic.
Regarding the position, a change of cranks in one direction or the other must be accompanied by a change in the saddle height., in the same proportions as the modification of the crank length, so as to maintain the same extension of the leg when it is "stretched". If you go for example from 172,5 to 170, you must increase the saddle height by 2,5 mm.
=> SEE AS WELL : Finding a Crankset with the Right Crank Length
The advantagestagTheoretical smaller cranks
Let's first talk about an aspect that is sometimes overlooked but can be important when your practice is truly engaged., whether on route or off-road: shorter cranks are less likely to touch the ground, whether in turns (while pedaling), or when crossing obstacles (rocks, roots, branches) and going over slopes in mountain biking, cyclocross or gravel. On time trial or triathlon bikes, choosing shorter cranks can allow manufacturers to lower the position of the bottom bracket, thus lowering the center of gravity of the bike and the rider in order to gain in aerodynamics and stability.
The other benefit of opting for shorter cranks is the forward and low aerodynamic position at the front. The choice of crank length changes the opening of the angle between the hip and the trunk. With large cranks, the angle is more closed, which can limit the ability to lower forward by creating a lot of constraints on the hip joint, with possible associated pain.
This is also a trend observed among very high-level runners, who adopt very aggressive positions. Alejandro Valverde, for example, went back to 170 mm cranks some time ago, as he approached his forties, despite his height of 1,76 m.
Finally, the advancetage often considered a determining factor in shorter cranks is the supposed ease of "turning the legs" more easily., and therefore to increase the pedaling cadence. With the consequences of less fatigue over long distances, less accumulation of muscular waste and better reactivity during accelerations.
Not so simple
Beyond theory, what is actually happening on the ground? Is it really ahead?tagthem to choose shorter cranks for use on route ?
I was able to experience this very recently when receiving a bike from test. I have been riding regularly for 36 years, covering thousands of km each year. I started at the age of 14 with 165 mm cranks (for 1,50 m at the time). When I joined the Juniors, having grown a little (no laughing!), I went to 170 for a year, then to 172,5 for the year I turned 18. This was the era of Armel André, an ergonomist from the automotive world who was one of the first to work on positioning software. A crank size a little high for my 1,67 m, but made possible by my flexibility and speed at the time. I never changed again, with the exception of a few attempts at time trials or mountain biking with 175s (which gave me the feeling of climbing stairs two steps at a time) or with a few bikes test delivered with 170s. This latter length often bothered me, giving me the impression of not being able to extend my legs and limiting me during sustained efforts.
Recently I received a bike ordered with my specifications for a test over several weeks. Despite my request, a mistake by the mechanic meant that it was sent with 170 cranks. In absolute terms, this is not a big deal, because I have another crankset, in 172,5, that I can mount on this bike, while the test. However, I needed an adapter for this, which took more than three weeks to arrive.
So I did over 1000km with this bike in 170. I could already perceive its qualities, but without taking great pleasure in riding it. I felt like I was "stuck". I had trouble getting going again while dancing, I felt myself crashing on the false flats, and on the longer bumps I couldn't get myself to the right intensity. And the succession of outings didn't change much. Strange muscular sensations that were not corroborated on Zwift, with another bike, equipped with the right cranks.
Then the part finally arrived, and we were able to mount the correct crankset. From the first few hundred meters, I discovered another machine. I was finally able to find the right coordination as a dancer, between the movements of the legs and those of the upper body. On the flat, I had the impression of being able to increase my cadence by "sending" my legs forward each time I wanted to accelerate. And even more astonishing, I found a perfect balance (50/50) between the power of the two legs, which was not the case with the 170.
By going straight from 170 to 172,5 cranks on a bike that I had already had time to tame, I also realized that I was pedaling with a very slightly less rounded gesture., with perhaps slightly more pronounced dead times at the high and low points of the crank rotation, but I nevertheless had the impression that these very small releases of muscular pressure at each pedal rotation promoted a “micro recovery” that I did not feel with the 170.
In early 2017, during the test of the Bikefitting postural study system, the operator had noticed that my pedaling efficiency was better with the 172,5 cranks, whereas with my height, he had naturally directed me towards 170. An observation also made a few months later during the test Specialized's Retül positioning system. After hundreds of thousands of miles with 172,5s, my body is so used to them that it feels a little lost and less efficient with another length.
=> SEE AS WELL : Finding a Crankset with the Right Crank Length
Crank length is therefore a much more complicated subject than it seems., beyond theories that are as interesting as each other, but which can only be verified in the field or after a good dynamic postural study, which takes into account not only your morphological particularities, but also your muscular characteristics and the way you ride.
=> SEE AS WELL : All our Gear articles
Bytager the publication "The correct length of cranks"
Is there an increase in power with longer cranks that would allow for tackling steeper slopes with the same brackets?
With longer cranks, you have more lever arm, therefore more "force" especially with a low pedal frequency. But we must never forget that power itself is a combination of force and cadence.
Is there an increase in power with longer cranks that would allow for tackling steeper slopes with the same brackets?
With longer cranks, you have more lever arm, therefore more "force" especially with a low pedal frequency. But we must never forget that power itself is a combination of force and cadence.
I have been pedaling for 20 years on a mountain bike with 175 cranks for my 1.68m. I find it an advantagetagand on the hill, in terms of reactivity. On route (with thin tire wheels) I can spin with a longer gear ratio, which puts me way ahead of the bikes of route. But maybe there is a connection with my difficulty finding a comfortable saddle (pelvic movements?). My new gravel bike comes in 170, I feel like I'm running out of juice, but I'm sitting well.
I have been pedaling for 20 years on a mountain bike with 175 cranks for my 1.68m. I find it an advantagetagand on the hill, in terms of reactivity. On route (with thin tire wheels) I can spin with a longer gear ratio, which puts me way ahead of the bikes of route. But maybe there is a connection with my difficulty finding a comfortable saddle (pelvic movements?). My new gravel bike comes in 170, I feel like I'm running out of juice, but I'm sitting well.