The Science of Crank Length: Why Cycling's Leverage Myth is Collapsing

For decades, the cycling world worshipped at the altar of leverage. The logic was simple, seductive, and seemingly undeniable: longer crank arms equal a longer lever, which equals more torque, which equals more power. It was a mechanical truth that dictated bike builds from the local club ride all the way to the WorldTour.

But the gospel was wrong. We are currently witnessing a biomechanical revolution in the sport. A trend of 'Crank Length Madness' is sweeping through the professional peloton, with riders of all heights ditching their traditional 175mm levers for 165mm—or even shorter. And they aren't doing it on a whim. They are doing it because the science of human performance has finally caught up with the physics of the bicycle.[7]

To understand why the shift is happening, we have to look at the fundamental formula of cycling performance: Power equals Force multiplied by Velocity. When you shorten the crank arms, you do indeed reduce the applied force because the lever is shorter. But you simultaneously increase the pedal velocity due to the shorter distance the pedal travels in each rotation.[4]

The power loss from reduced leverage is entirely offset by the power gained from increased pedal velocity. In fact, moving from a 175mm to a 165mm crank results in a negligible 0.35 percent increase in power output—essentially no change at all. The fear of losing watts by going shorter is a myth that has been thoroughly debunked by modern sports science.[4]

The most definitive data on this comes from biomechanics labs testing trained cyclists across a massive spectrum of crank lengths, from 120mm all the way up to 220mm. Researchers measured maximum power output at each length and found that across a 50mm range—from 145mm to 195mm—there was no significant difference in maximum power.[6]

When zooming in on the practical comparison that most cyclists actually care about—145mm versus 170mm—the difference in peak power was less than one percent. The human body is remarkably adaptable, and it turns out that our muscles care far more about the angles they operate within than the pure length of the lever they are pushing.[6]

This brings us to the true advantage of shorter cranks: biomechanics. Every time you push a pedal, your foot traces a circle. Longer crank arms create a large circle, which requires a wider range of motion from your joints. A smaller circle reduces that range of motion, which is where the massive physiological benefits begin to compound.[3]

The most critical point in the pedal stroke is the very top, known as top dead center. With a long crank, your knee is pushed high into your chest, forcing your hip and knee joints into extreme flexion. This closed hip angle compresses the joint, restricts blood flow, and limits your ability to generate force smoothly over the top of the stroke.[7]

Shorter cranks fundamentally change the geometry of the rider. For every 10mm reduction in crank length, the hip angle opens by approximately four to six degrees, depending on saddle height and torso position. This more open hip angle reduces compression forces on the hip joint and relieves strain on the iliopsoas muscle, allowing for a smoother, less restricted pedal stroke.[7]

The impact on the knee is even more dramatic. Reducing crank length from 175mm to 165mm decreases knee flexion by up to 15 degrees at the top of the pedal stroke. That decreased flexion reduces patellar stress by up to 18 percent, making shorter cranks a highly effective intervention for cyclists suffering from recurring knee pain.[7]

Think about the difference between a partial squat and a deep squat in the gym. You can lift significantly more weight from a partial squat because your joints are in a more mechanically advantageous position. The exact same principle applies on the bike. Shorter cranks allow your muscles to produce force in a stronger, more efficient part of their range.[6]

Beyond joint health, there is a massive aerodynamic advantage to be unlocked. Because shorter cranks prevent your knees from pushing as high into your chest, you can lower your handlebars and achieve a more aggressive, aerodynamic position without suffering from hip impingement. You get the speed benefits of a low profile without the biomechanical penalty.[4]

This opened-up torso position also has a profound effect on respiration. When your knees aren't constantly compressing your diaphragm at the top of every pedal stroke, you can breathe deeper and more efficiently. For time trialists and triathletes who spend hours locked into an aerodynamic tuck, this improved respiratory capacity is a game-changer.[8]

There are, of course, specific scenarios where longer cranks still hold an advantage. Studies have shown that longer cranks, typically 170mm or more, can be beneficial in scenarios demanding maximum instantaneous power output at very low cadences, such as standing sprints or grinding up incredibly steep, loose climbs on a mountain bike.[2]

During standing cycling, longer crank lengths require increased propulsion power by the lower limb muscles, and the peak knee extension moment increases with decreasing crank length. For a BMX racer or a track sprinter relying on a massive initial torque spike out of the starting gate, that extra leverage still matters.[1]

But for the vast majority of road, gravel, and endurance cyclists, the benefits of a shorter crank far outweigh the niche advantages of a longer one. The ability to maintain a higher cadence with less fatigue, combined with the reduction in joint strain, makes the shorter setup the clear winner for sustained efforts.[2]

It is crucial to understand that changing your crank length requires a corresponding change to your bike fit. When you install a crank that is 10mm shorter, your foot will be 10mm higher at the bottom of the pedal stroke. To maintain your optimal leg extension, you must raise your saddle by exactly 10mm.[5]

Raising the saddle by 10mm also means your entire body moves up, which requires raising your handlebars by 10mm to maintain the same back angle—or leaving them where they are to achieve that more aerodynamic position we discussed earlier. The crank length is the anchor point that dictates the rest of the bicycle's geometry.[5]

The cycling industry is slowly catching up to the science. While 172.5mm was the default standard on medium-sized road bikes for decades, manufacturers are increasingly speccing 170mm or 165mm cranks straight from the factory. The realization that leverage does not equal power has freed designers to prioritize biomechanical efficiency.[8]

Ultimately, the right crank length is a deeply personal choice that depends on your height, riding discipline, and injury history. But the collapse of the leverage myth means that riders no longer need to fear going shorter. By prioritizing joint health and pedal velocity over a mechanical illusion, cyclists are unlocking a smoother, faster, and pain-free ride.[8]