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Cycling Software

Optimum Bicycle Crank Length

JavaScript Crank Length Calculator | Windows Crank Length Calculator | CrankCalc

It stands to reason that bigger riders need bigger bikes and longer cranks. But what is the optimum? The calculator below is designed to answer this question.

I'm a tall rider with an inside leg measurement of 33 inches (about 84 cm). I currently use 170 mm cranks, and they 'feel' just fine. Enter my data and the calculator informs me that I require cranks that are 169.7mm long which seems pretty close to me. I think that my cranks would need to be at least 5mm longer or shorter before I could 'feel' any appreciable difference ;-)

To use the calculator enter your inside leg measurement (in some contries this is called inseam) in either centimetres or inches. The optimum crank length is calculated for you when you click the button marked 'Calculate Crank Length'. Bicycle cranks are normally measured in millimetres, but you can also optionally calculate this in inches.

Javascript Bicycle Crank Length Calculator

Riders inside leg measurement =
Calculated optimum crank length =
Calculator by Machinehead Software

How Bicycle Cranks Are Measured

Bicycle Crank length diagram The crank length is the distance between the centre of the pedal and bottom bracket spindles.

How It Works

The Racing Bike Book, By Dave Smith, Ben Searle & Steve Thomas (foreword by Sean Kelly), ISBN 1-85960-319-X, gives a table of recommended crank sizes in millimetres for inside leg measurements between 72 and 96 centimetres. From this table of values the following linear relationship can be derived:

Recommended crank length in millimetres = (1.25 * Inside Leg in cm ) + 65

The program in based on the above relationship.

Windows Crank Length Calculator

A crank length calculator based on the same guidelines is also incorporated into my bicycle power calculator.

Windows crank length calculator


Once you have decided on the optimum crank length for your inside leg measurement, my CrankCalc program can be used to determine if the new cranks will also require a new frame. Shorter cranks require the seat to be positioned further back and higher up (relative to the bottom bracket) than long ones.

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