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Mountain Biking: Should You Consider Shorter Crank Arms?

November 22, 2023

A recent study by a research institution investigated the correlation between a cyclist's height and the ideal crank arm length, revealing surprising findings that challenge common beliefs.


For most mountain bike riders, crank arm lengths typically range from 165mm to 175mm, with 170mm being the most common. However, what if I told you these standard sizes might be too long for almost all mountain bikers? Let's delve into the scientific principles behind crank arm length and why shorter cranks might offer advantages for most riders.


Seven scientific studies examining power output, pedaling efficiency, and various crank arm lengths presented unexpected conclusions. Surprisingly, these studies indicated that crank arm length seems to have negligible effects on power output or efficiency unless riders use extreme lengths like 120mm or 220mm. Even comparing lengths differing by 145mm and 190mm showed minimal impact. It turns out that with shorter cranks, riders naturally pedal at a higher cadence, offsetting the decrease in power output caused by the lever effect.


Moreover, shorter crank arms reduce the time required to increase power during sprints, help riders reduce fatigue when pedaling out of the saddle, and potentially decrease hip and knee joint stress.


Taking all these findings into account, along with the reduced risk of pedal strikes on rough terrains with shorter crank arms, it's compelling to consider shorter cranks for most riders.


What about taller riders and shorter crank arms?


Recent research explored the relationship between a rider's power output, crank arm length, and leg proportions. The study suggested that the optimal crank arm length is around 20% of the rider's leg length. Crucially, any crank length between 15% and 25% of leg length didn't significantly affect a rider's power output.


This implies that for an average 175cm tall male, the ideal crank arm length is around 161mm, while for an average 162cm tall female, it's about 150mm. Hence, the standard 170mm cranks might actually be too long for most riders. Previous suggestions that taller riders should use longer cranks for maximum power output have been challenged by the latest research. Even riders towering at 2 meters wouldn't notably decrease their power output using 145mm cranks.


In summary,


If you're concerned about pedal strikes on rugged terrain, shorter stature, or experience knee or hip pain while cycling, shorter crank arms might be more suitable. (Consulting a professional bike fitting service is advisable if experiencing joint discomfort on the saddle.) Any of these reasons warrant considering shorter crank arms. I'm taller than average and never encountered issues with standard cranks, but I still felt improvements in ground clearance and reduced leg discomfort with shorter ones.


However, it's important to note that complementary adjustments such as changing chainring sizes, adjusting saddle height, or even bike compatibility might be necessary. While standard 170mm cranks might suffice for many, they might not be optimal, especially for riders frequently tackling challenging terrains. In reality, riders of different heights require varying crank arm lengths for optimal pedaling efficiency and stability. Thus, if you're particularly short or tall, adjusting your crank arm length could enhance your cycling experience."


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