Does changing crank length probably have a greater impact than we previously thought?

Lately, there has been more and more discussion about crank length. The Tadej Pogačar phenomenon, for example, rode the 2024 Giro d'Italia with a 165 mm crank and won the race with an impressive lead of almost 10 minutes! Nowadays, Wout van Aert and Jonas Vingegaard also cycle with shorter cranks, which only further fuels the discussion about using these lengths.

For some riders, the length of the crank is crucial for a good cycling position, while others do not appear to be affected by it. Shorter cranks can make pain-free riding possible for one rider, while it causes pain for another. This makes choosing the right crank length very confusing. Through this blog, I'm going to try to provide a little more clarity about this.

The length of the crank has been largely ignored by the bicycle industry for decades. Everyone agrees that bikes should come in different sizes to fit cyclists of different heights. That goes without saying. So why not include the length of the crank?

Let's take a look at some of the common sizes of various components:

• Frame sizes: 48 cm to 62 cm, a range of 25%
• Stems: 40 mm to 140 mm, a range of 111%
• Steering width (road): 38 cm to 46 cm, a range of 19%
• Cyclist height: 1.52 m to 1.96 m, a range of 24%
• Inseam length: 66 cm to 91 cm, a range of 32%
• Crank lengths: 165 mm to 175 mm, a range of 6%

Here, the minimum range of 6% in crank length immediately catches the eye. Maybe it's the production methods of the past, cost savings or unwillingness to change. Probably a bit of everything.

Currently, the industry standard is organized so that short people qualify for 165 mm and tall people for 175 mm. However, this scale is not very strongly substantiated. Perhaps a wider range of crank lengths should be available and the average of these lengths should shift. This is an interesting discussion.

Another approach could be:

• 155 mm and 165 mm cranks for most cyclists (average)
• 145 mm and 175 mm cranks for cyclists at the ends of the spectrum
• 135 mm and >175 mm for a few exceptions

The current shifts in crank lengths in 2.5 mm increments may be too small to actually see significant differences. On the other hand, 10 mm changes would show this.

Changes in crank length could play a major role in comfort, stability and speed. This is especially true for cyclists who agree with the following points:

• Physical complaints (knee, hip, back)
• You move a lot on the bike
• Your knee moves outwards at the top of the pedal stroke
• Have a history of discomfort on the bike
• Want more speed
• Want a more fitting bike
• Feeling like something isn't quite right

This illustration (Figure 1), where only the crank length and saddle height change, shows the following: Shortening the crank will stretch the leg more, reducing knee and hip angles.

Reduced knee and hip flexion: Shorter cranks can reduce knee or hip pain. Excessive bending of the knee or hip can cause increased stress in these joints with the risk of possible pain or injuries. With a shorter crank, the legs bend less at the top of the pedal stroke, which increases comfort. The reduced range of motion of the hip also helps people with hip constriction (impingement).

Breathing easier: At the top of the pedal stroke, a shorter crank moves the leg away from the abdomen and chest. As a result, there is less compression in the abdomen/chest cavities, which offers more breathing space.

More speed: Get a more aerodynamic position and more speed by taking a lower stance with extra torso to leg space due to shorter cranks. You see this a lot on today's time trial bikes, especially in triathlon.

More stability on the saddle: Low back pain, saddle pain, and loss of strength can be caused by rocking the hips back and forth. This often occurs when the cranks are too long, the cyclist is not flexible enough, or the saddle is too high. The wobble occurs with each pedal stroke when the cyclist lifts the hip due to a crank length that is too long to get his leg over the top of the pedal stroke. Shorter cranks can position the legs within a comfortable range of motion for the cyclist, increasing the stability of the hips while riding.

Pedal punches: Shorter cranks move the pedal farther from the ground, resulting in less chance of hitting rocks or asphalt while pedaling around corners.

Toe overlap: Does your toe ever touch your front wheel? Then you have to deal with toe overlap. Shorter cranks also help reduce that.

Determining the correct crank length using knee and hip angles The best way to predict the correct crank length without actually trying different cranks is determined by knee and hip angles. When these joints are bent beyond a certain limit, the excessive stress of pedaling can cause pain, discomfort, or pelvic instability on the saddle.

The crank length should be determined by which angle (knee or hip) is most restrictive for the cyclist.

Knee angle: No method is perfect, but looking at the knee angle (how bent the knee is) at the top of the pedal stroke can be a method of predicting crank length.

Schultz's article (published in 2009) explains that there is a strong link between chronic knee pain and how bent the knee is at the top of the pedal stroke. The thigh muscle (m. rectus femoris) runs from the hip over the knee joint. At the knee joint, the kneecap is encapsulated in this muscle. When our leg is straight, the kneecap lies relaxed over the knee joint. But when we start to bend the knee, the kneecap is just pressed into the knee joint. The greater the bend, the more compression is created. This mechanism is illustrated in a simplistic way in Figure 2.

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In this article, clear numerical guidelines for knee angles are provided as follows:

• Knee angles smaller than 66° are most likely to cause knee pain.
• Knee angles between 66° and 69° can cause knee pain.
• Knee angles greater than 69° are not likely to cause knee pain.

The illustration below (figure 3) shows that the knee angle increases as the crank length decreases.

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‍Hip angle: The hip angle between the thigh and torso is also essential for comfort, speed, and performance. The measurement should be taken at the point where the hip angle is smallest (usually just before the dead center at the top of the pedal stroke). See this in action: Figure 1 above. Measuring the hip angle exactly is difficult because of the complex cooperation of the hips with the flexible spine and the legs, and it is this flexible spine that makes it difficult for a layman to measure the exact orientation of the hips. Cyclists arch and arch their backs to varying degrees, which rotates the hips and blurs the angle between the upper leg and hip-to-shoulder.

Numerically speaking: Schultz's article recommends a 45° lower limit. You often see 45°-55° in other bike fit literature. The guidelines for ideal hip angles are not sharp and clear, and measuring them is difficult, so there is a lot of room for interpretation. The goal here is to minimize contact between the torso and legs. If you touch your chest or stomach with your thighs with each pedal stroke, a change may be necessary. If the leg goes too high (hip angle too small) at the top of the pedal stroke for your given upper body position, you can either shorten the crank length or raise your torso.

Testing during a professional bike fit This is the most comprehensive option for determining the correct crank length... and all other aspects of cycling position. With the help of an adjustable pass bike, different crank lengths can be tried out one after the other, so that you can objectively measure between the different lengths. A side note here, however, is that our body and brain are used to the current crank length. As a result, a change of this length will initially feel strange and may give the feeling that it is not efficient. The feeling of efficiency will only occur after several kilometers.

What does crank length do to your power? Research suggests that power is not strongly linked to crank length. No discussion about crank length is complete without reference to the study by Martin, James & Spirduso ( https://debiomechanieker.nl/wp-content/uploads/2022/11/Determinants_of_maximal_cycling_power_Crank_length.pdf ) over the maximum power of cranks with lengths between 120-220 mm. Here, a length of 145 mm came out on top, followed by 170 mm, which was only a few percent away.

Brief conclusion The length of the crank is a very personal component that can be strongly related to certain complaints during or after cycling. It can sometimes be difficult to attribute the complaints solely to the crank length, but if many solutions have already been sought and none of them have led to a solution, it could just be related to your current crank length.

Current cranks have a limited range of lengths and are not in line with the wide range of riders out there. 155 mm and 165 mm cranks are now presented by many bike fitters as the “new average” crank lengths.

There are many benefits to shortening the crank length and fewer benefits of extending it.

A change in crank length of 2.5 mm is not worth it. 10 mm changes in crank length result in real, significant changes in bike position. This change will also not immediately feel efficient for the person, due to the brain's habituation to the previous position. It always takes a period of time to get used to a new attitude.

The length of the crank can be determined in various ways. Crank length decisions are best made based on current complaints, instability, knee/hip angles, driving style and a bit of trial and error.

Changing crank lengths can be expensive, so test this thoroughly before making this switch! If you are pain-free and riding optimally, feel free to skip a crank length change. Save money and keep cycling.