Optimize your cycling power and prevent knee strain. This crank arm length calculator uses biomechanical formulas to determine the ideal crank size based on your inseam and riding style.
Crank Arm Length Calculator
Crank Arm Length Calculator Formula
Where $L$ is Crank Length (mm), $I$ is Inseam (mm), and $K$ is the Discipline Constant (typically 0.216).
Source Reference: Sheldon Brown’s Bicycle Technical Info & CyclingNews Biomechanics
Variables Explained:
- Inseam Length: The vertical distance from your crotch to the floor (measured without shoes).
- Rider Height: Your total standing height, used to validate proportions.
- Discipline Constant: Road riders usually prefer a 21.6% ratio, while TT specialists may opt for more leverage.
What is a Crank Arm Length Calculator?
A crank arm length calculator is a tool designed to find the optimal lever arm distance between your pedal spindle and bottom bracket axis. Choosing the correct length is critical for maintaining an efficient “spinning” cadence and ensuring your hip and knee angles stay within safe biomechanical ranges.
While many bikes come with standard 170mm or 172.5mm cranks, smaller or larger riders often benefit significantly from switching to a length that matches their specific femur and leg proportions.
How to Calculate Crank Arm Length (Example)
- Measure your inseam accurately (e.g., 80 cm).
- Convert the measurement to millimeters (80 cm = 800 mm).
- Multiply by the standard road constant: $800 \times 0.216 = 172.8\text{ mm}$.
- Select the nearest commercial size (e.g., 172.5mm).
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Frequently Asked Questions (FAQ)
Does crank length affect power output? Generally, no. Studies show power output is similar across lengths, but shorter cranks can improve aerodynamics and reduce knee compression.
Should I use shorter cranks for triathlon? Yes, many triathletes prefer shorter cranks (160-165mm) to open up the hip angle, making the transition to the run easier.
What is the most common crank size? 170mm and 172.5mm are the industry standards for medium-sized frames.
How do I measure my current crank length? It is usually stamped on the inside of the crank arm near the pedal threads.