The optimal crank length is a controversial topic. The current trend is toward shorter cranks. Crank length is primarily based on body height, although leg length, flexibility, and riding style also play a role. The topic recently made headlines when world champion Tadej Pogacar, who is 1.76 m tall, switched from a 172.5 mm crank to a 165 mm crank. Tadej Pogacar was not the first to make such a radical change. A shorter crank length has advantages, but also disadvantages. The lengths of cranks on road bikes usually range between 165 and 175 millimeters. Occasionally, you will also find 160 millimeters on small road bikes (sometimes even 155 mm in the professional/time trial sector).
Using shorter cranks changes the geometry of the pedaling motion. This usually means that you have to adjust the saddle setting. Shorter cranks mean that your foot moves less up and down. At the bottom dead center, the foot is higher, and at the top dead center, it is lower. Since shorter cranks mean you don’t go as far down at the bottom dead center, the saddle needs to be raised slightly. However, it is not enough to consider the change in one dimension. Shorter cranks change the hip angle, especially at the top dead center. Since other angles, such as those at the foot and knee, also change, further adjustments must be made. Many riders therefore move the saddle slightly further forward in order to maintain the usual knee position above the pedal axis (KOPS). Sometimes the angle of the saddle is also changed if pressure points change or slipping occurs. This results in changes in the load on joints and muscles. Changing the crank length is therefore not a trivial matter.
The physics behind cycling performance:
Power is measured in watts [W] and is the work done or energy used [joules J] per unit of time [seconds s].
Power = work (energy) / time
Work is defined as force acting along a path. The energy required for this or generated in the process is the corresponding energy.
Work (mechanical) = force x distance (distance: distance along which the force acts!)
When cycling, the distance along which the force acts is not a straight line, but a circle or a section of a circle. Only force that acts in the direction of movement contributes to the power on the bike. Forces acting in other directions (shear forces) do not result in additional power or even reduce it (keyword: smooth pedaling). To calculate the distance (circumference) along which the force acts on the pedal, the crank length and the angle traveled must be known. For a full revolution, the angle is always 360 degrees. The distance traveled is part of a circular path or, in the case of a full revolution, a circle. However, the distance traveled by the pedal varies with the crank length. With shorter cranks, the distance is correspondingly shorter. The shorter the crank, the smaller the circle, the shorter the distance, and the less work or energy generated at constant force.
The force acting on the crank via the crank arm is known as torque. The longer a crank is, the higher the torque at the same/constant force.
Torque = force x lever arm (crank length).
To calculate the power, the time it takes to move the crank arm must also be taken into account. The faster the crank is moved with the same force, the less time is required and the higher the power output. In a circular motion, we refer to the angular velocity ω at which the system moves. The angular velocity is directly related to the cadence. The angular velocity ω is calculated from the cadence f as follows: ω=2π x 60f
The power output on the bike is:
Power = torque x angular velocity.
The faster you pedal, the higher the rotational speed and angular velocity, and the higher the power output for the same amount of force. In other words, the higher the cadence at constant force, the higher the power output. At the same cadence and force, shorter cranks generate less power than longer cranks. A higher cadence increases power linearly without requiring more force. An increase of 5 mm in crank length results in approximately 3% more power with the same force and cadence. However, only a few more revolutions per minute are required with shorter cranks to achieve the same power.
The table below shows the resulting power at different cadences for different crank lengths with constant force.
| Cadence (rpm) | 165 mm (W) | 170 mm (W) | 175 mm (W) |
|---|---|---|---|
| 50 | 129,6 | 133,5 | 137,4 |
| 55 | 142,6 | 146,9 | 151,2 |
| 60 | 155,6 | 160,2 | 164,9 |
| 65 | 168,5 | 173,6 | 178,6 |
| 70 | 181,5 | 187,0 | 192,4 |
| 75 | 194,5 | 200,3 | 206,1 |
| 80 | 207,4 | 213,7 | 219,9 |
| 85 | 220,4 | 227,0 | 233,6 |
| 90 | 233,4 | 240,3 | 247,3 |
| 95 | 246,3 | 253,7 | 261,1 |
| 100 | 259,3 | 267,0 | 274,8 |
| 105 | 272,3 | 280,4 | 288,6 |
| 110 | 285,2 | 293,7 | 302,3 |
| 115 | 298,2 | 307,1 | 316,1 |
| 120 | 311,2 | 320,4 | 329,8 |
Research and considerations:
The current state of research on crank length, which is still very limited, suggests that differences of ten millimeters or more have no effect on performance, as the lower leverage is “almost automatically” compensated for by a higher cadence, but there are details to consider. For riding under “normal conditions” – such as riding on predominantly flat or undulating terrain with cadences in the mid-range – cranks can be selected on the basis of criteria other than performance. These are, for example, purely fit-related considerations. These could include: pain-free, more comfortable, or more aerodynamic sitting and riding.
Advantages of shorter cranks:
A shorter crank definitely makes it easier for the rider to get into a lower position, as shorter cranks mean more favorable joint angles. The knees are not raised as high toward the chest, breathing is easier, hip mobility is less strained, and the movement on a small track is overall “smoother” than with long cranks. All of this leads to greater stability in the saddle and makes it easier to achieve an aero position. The wind resistance decreases with shorter cranks. Higher cadences are easier to achieve and are easier on the joints and sometimes also on the muscles. This results in a lower moment of inertia, which makes it easier to “rev up” faster during sprints and on the track. The shorter crank length has clear advantages for short and intense rides.
Disadvantages of shorter cranks:
However, the higher cardiovascular strain at higher cadences should not be underestimated, which can be a problem in long-distance races and generally during high-intensity exercise and in hot weather. The adjustment period can be a temporary disadvantage, which is why changing the crank length should not be done in the middle of the season. Since the optimal position on the bike is very individual, changing the crank length can lead to reduced efficiency for some riders. This must be checked on a case-by-case basis. When riding uphill and/or riding at a low cadence, the shorter crank length can have a negative effect on performance due to lower torque (with the same force).
Conclusion:
Shorter crank lengths are recommended for smaller riders, those with short legs, and those with a very athletic, aerodynamic riding position. In general, shortening the crank length could be a possible solution to problems such as back or knee pain, and is worth considering. Higher cadences are easier to achieve. For long-distance rides/races, especially in the ultra range, and when tackling large elevation gains, where lower cadences are predominantly used – also due to higher fatigue – the use of shorter cranks should be critically examined. In general, individual differences in optimal positioning on the bike and performance should be taken into account.
Some literature:
Park S, Roh J, Hyeong J, Kim S. Effect of crank length on biomechanical parameters and muscle activity during standing cycling. J Sports Sci. 2022 Jan;40(2):185-194. doi: 10.1080/02640414.2021.1982516. Epub 2021 Sep 28. PMID: 34581253.
Barratt PR, Korff T, Elmer SJ, Martin JC. Effect of crank length on joint-specific power during maximal cycling. Med Sci Sports Exerc. 2011 Sep;43(9):1689-97. doi: 10.1249/MSS.0b013e3182125e96. PMID: 21311357.
Ferrer-Roca, Ventura et al. “Acute effects of small changes in crank length on gross efficiency and pedalling technique during submaximal cycling.” Journal of sports sciences vol. 35,14 (2017): 1328-1335.
Barratt PR, Martin JC, Elmer SJ, Korff T. Effects of Pedal Speed and Crank Length on Pedaling Mechanics during Submaximal Cycling. Med Sci Sports Exerc. 2016 Apr;48(4):705-13. doi: 10.1249/MSS.0000000000000817. PMID: 26559455; PMCID: PMC5638423.
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