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Free decision tool
Compare the cranks on your bike with a candidate size, then see the coupled geometry before you buy or adjust anything. The assistant gives you a convention-based starting band, the signed crank-length difference, the saddle move needed to preserve bottom-of-stroke leg extension, and the resulting change in top-of-stroke pedal-to-saddle space.
Free · no account · no upload · exact geometry without promised watts, joint angles or pain outcomes
Use the length stamped on each crank arm. Your measurements stay in this page; nothing is uploaded.
Your comparison
You will get the signed component change, the saddle-height compensation and the exact change in top-of-stroke space. No watt, pain or joint-angle promise is inferred from those dimensions.
Crank length is the centre-to-centre distance from the bottom-bracket axle to the pedal axle. A change alters the radius of the pedal circle. That gives us two useful mechanical consequences without needing a body model. If you replace a 172.5 mm crank with a 165 mm crank, the pedal hangs 7.5 mm closer to the bottom bracket at the bottom of the stroke. Raise the saddle 7.5 mm and the original bottom-of-stroke pedal-to-saddle distance is restored. At the top, both changes work in the same direction: the pedal is 7.5 mm lower relative to the bottom bracket and the saddle is 7.5 mm higher, creating 15 mm more pedal-to-saddle space.
Those dimensions are geometry, not physiology. They do not tell us the exact change in hip or knee angle for a particular rider because limb proportions, foot angle, cleat position, saddle setback, pelvic motion and riding posture all matter. They also do not promise that a symptom will improve. The responsible sequence is to predict the mechanical delta, make one reversible change, then use the on-device camera check to observe how you actually move.
The starting range shown by the tool reuses OpenBikeFit's existing inseam-and-discipline convention. It uses practical 2.5 mm bands rather than pretending that one body measurement identifies a uniquely correct crank. Time-trial and mountain-bike contexts bias that band shorter for different reasons: top-of-stroke room in a closed aero posture for TT, and pedal-circle clearance in technical terrain for MTB. Treat the band as a shopping filter, not a prescription. A candidate outside it is not automatically wrong, and a candidate inside it is not automatically right.
The best-known crank-length power experiment deliberately tested an enormous 120–220 mm span. Martin and Spirduso found maximal power differed by only about four percent across that extreme range when pedalling rate and pedal speed were considered. Barratt and colleagues later found that when cadence was optimized, crank length did not materially rearrange how maximal power was distributed between the hip, knee and ankle. These results do not mean every crank feels identical; they mean a small real-world swap should not be sold as a guaranteed watt gain.
Ferrer-Roca and colleagues studied much smaller changes closer to an actual purchase decision. They did not find an efficiency or heart-rate benefit from the crank change, while the longer option increased peak hip and knee flexion. That supports a directional statement—crank length changes the movement range demanded over the stroke—without giving us a universal ideal or a promised clinical outcome. Acute lab studies also do not establish long-term comfort, adaptation or injury effects for one rider.
Cadence is part of that boundary. A shorter radius can feel more “spinny,” and riders may naturally use a different rpm. Compare in easy gearing and let your cadence settle rather than forcing the old gear or rpm. If you want the complete explanation, read crank length: what actually matters, or inspect the claim, sources and limitations in the public OpenBikeFit methodology.
These studies support the limited claims above. The practical inseam bands remain fitter convention, clearly separated from the peer-reviewed findings.
Peer-reviewed · 2001
Maximum power differed by only ~4% across 120–220 mm cranks — pedal speed and cadence determine power, not crank length.
Open source recordPeer-reviewed · 2011
Crank length per se does not determine maximal power; with cadence optimised, hip/knee/ankle power distribution stays unchanged.
Open source recordPeer-reviewed · 2017
±5 mm of crank length doesn't change efficiency or heart rate, but a longer crank increases peak hip and knee flexion — the biomechanical case for shorter cranks.
Open source record