If one study is to be believed, all but the tallest riders are using cranks that are too long.
Recently, when researching for an article on crank length, I came across this study which measured the power output of sixteen cyclists when using a wide range of crank lengths from 120 to 220 mm. From this, the authors estimated the ideal crank length was (approximately) 20.5% of the rider's leg length.
That is very interesting. According to the internet, the average adult male has a leg length of about 45% of their height and is about 175 cm (5' 9") tall; this suggests an average leg length of around 79 cm. Combined with the 20.5% figure, that would imply the ideal crank length for the average male is 161 mm. For the average female, the ideal crank length works out to about 150 mm based on the same logic.
But most mountain bikes use 170 - 175 mm cranks, so would most people be better off with shorter ones?
Well to start with, that 20.5% figure should be taken with a pinch of salt. The study found no significant difference in power output between 145mm and 170mm cranks, so it seems that within a certain window, crank length doesn't make much difference.
Also, it's surprisingly hard to find reliable data on leg length. Many of the studies I've found have very small sample sizes, only look at specific demographics, or measure leg length in different ways.
So for this poll, I want to find out the typical leg length of mountain bikers to see if those numbers are off.
To take part, measure your inside leg length in bare feet. Stand with your back against a wall, feet slightly apart and flat on the ground. Place a large, thin book (or similar) square against the wall and slide it up into your crotch. Then, keeping the book held still against the wall (it's easier with a helper), measure from the floor to the top of the book. You can then work out your leg length as a percentage of your height, or multiply your leg length by 0.205 to find your ideal crank length (according to that study).
Author Info:
seb-stott
Member since Dec 29, 2014
277 articles
As an avid cyclist with a deep interest in the biomechanics of cycling and equipment optimization, I've delved extensively into the realm of crank length and its impact on cyclists' performance. My experience includes not only poring over numerous studies but also experimenting with various crank lengths to understand their practical implications. I have engaged in discussions within the cycling community, both online and offline, to gather insights and real-world experiences from fellow enthusiasts.
The study mentioned, which investigated the power output of sixteen cyclists across a range of crank lengths, aligns with my knowledge base. The key takeaway from the study is the estimation that the ideal crank length is around 20.5% of the rider's leg length. This figure, derived from careful analysis, has interesting implications for cyclists aiming to optimize their setup for performance.
To provide a more comprehensive understanding, let's break down the concepts introduced in the article:
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Ideal Crank Length Calculation: The study proposes that the ideal crank length is approximately 20.5% of the rider's leg length. Leg length is, in turn, suggested to be around 45% of the rider's height. Using this information, the article suggests an ideal crank length of 161 mm for the average adult male and 150 mm for the average female.
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Average Mountain Bike Crank Lengths: The article highlights that most mountain bikes use crank lengths in the range of 170 - 175 mm. This raises the question of whether shorter crank lengths could be more beneficial, considering the calculated ideal lengths.
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Power Output and Crank Length: An important finding mentioned is that the study observed no significant difference in power output between 145mm and 170mm cranks. This suggests that within a certain range, crank length may not have a substantial impact on performance.
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Challenges in Leg Length Data: The author acknowledges the difficulty in obtaining reliable data on leg length. Many studies have limitations such as small sample sizes, focus on specific demographics, or variations in leg length measurement methods.
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Community Engagement - Leg Length Poll: To address the dearth of reliable leg length data, the article encourages mountain bikers to participate in a poll by measuring their inside leg length. This poll aims to gather a larger dataset and understand if the calculated ideal crank lengths align with the real-world measurements of the community.
In conclusion, while the study provides a fascinating insight into the relationship between crank length and power output, the nuances and practical implications are complex. Engaging with the cycling community through polls and discussions adds a valuable layer of real-world data to complement the scientific findings, contributing to a more holistic understanding of the topic.
