View Full Version : Bicycle Crank Length: A Formula

07-29-2010, 09:56 AM

For those simply interested in the formula itself, it is simple enough. First, the rider must measure his inseam. It is not recommended that the pants size be used as this may not be accurate enough, an inch one way or the other makes a pretty big difference. Stand barefoot on a hard floor, straight, feet fairly close together, back against a wall, and have an assistant insert a clipboard between your legs. Lift gently, sliding the clipboard as high as it will comfortably go while keeping it firmly against the wall, thereby assuring it isn't tilted. While holding the clipboard this way, measure the distance from the top edge of the clipboard to the floor, in inches. Don't round to the nearest inch; measure as accurately as possible.

L(mm) = 5.48 x I(in)

Now, multiply the inseam measurement (in inches) by 5.48. This provides a good estimate of proper crank length, in millimeters, for general road cycling or racing.
For those who prefer metric units, simply convert the equation by 2.54 cm/in.

L(mm) = 2.16 x I(cm)

i dont know if this is on the forum already, i didnt see it in the search. i just ordered my cranks set but was unsure if they were right. i wanted short pedal length to go easy on my knees, as been mentioned in other post. according to this formula, my pedal length would be 180 mm, so the standard 170 mm would be short for me. so i am going to try it and see how that goes.

07-29-2010, 10:35 PM
An interesting discussion with some very interesting assumptions for upwrongs.
For recumbents it gives too long a crank length.
The majority of recumbent riders tend to prefer shorter cranks.
The reason being that in the recumbent position you can push more power to the pedals (mashing) leading to more stress on your knees. The other reason is that long cranks mean you may be bending you knees more than 90' which also places more stress on them.
This calculator recommends I use 145mm cranks, I am using 155mm cranks which feel (for me) much better than 170's. I wanted to use 150's for what felt the most effective for me.

07-29-2010, 10:42 PM
I like math, but where the heck am I going to find a 197.63mm cranks? :-p