How to calculate Crank Length

Most cyclist when buying their bicycle will just look for the frames and major parts like the grouppo, serious and competitive cyclist needs to know more than that, like frame size, frame fit, measurements of the frame, weight of the parts and many other details, But I bet this one eludes many cyclist out there, the CRANK Size, one of my colleague asked me, hey how do you determine the size of your crank, bet you didn’t know that crank arm comes in 170 *most common, 172, 175 and various length.

So you ask, why do I need to know the crank length ? I am just going to pedal my bike round the neighborhood, well as I said, to go into depth about knowing things in specific details is for the serious and competitive cycling persona. Knowing the crank length can improve on cadence speed and also minimize knee injury.

So all the mambo Jambo and still no way to calculate, well here is it. Calculating the crank length.

So you take a ruler and measure the length of L and divide by the inseams of your leg from your tight to your ball of your feet, there you , that’s how you calculate the crank length. JUST KIDDING please don’t follow that, just read the below and Garner more knowledge… if you love cycling like me !

What is Common

While cranks are available in lengths from 140 mm to 185 mm, most production bicycles are equipped with 165 or 170 mm cranks. While 165 – 170 mm cranks are appropriate for the average rider of 5′ 8″ to 5′ 10″ in height, they may not be the best choices for riders above or especially below this range.

A Starting Point

Given that the average adult male is 70 inches tall and has a femur length of 43.2 centimeters, a 170 millimeter crank is 39.5% of the femur length. I selected this as a starting point for calculations. 39% and 40% yield 168.5 mm and 172.8 mm crank lengths respectively, which would appear to be an acceptable range. 38% and 41% yield crank lengths of 164 mm and 177 mm respectively and these lengths may still be acceptable for some riders depending on their riding style and cadence.

Why proportion crank length to femur length?

The leg and crank arm forms a lever system. In this system, the crank arm rotates and the femur rotates through a limited angular range. In affect, the knee is moving mostly up and down with a small for and aft displacement. The lower leg and foot form a linkage between the pedal axis and knee and has little bearing on the transmission of power.

In a standing position, the hip joint range of motion is 98 degrees or less. That is to say, that the femur can be raised just slightly above the horizontal position when the knee is lifted. In a normal riding position, the pelvis and lower back are rotated forward 20 – 30 degrees relative to the legs to compensate for the seat tube angle. The remaining range of motion precludes raising the femur to the horizontal position without experiencing binding at the hip joint that forces the pelvis to be lifted from the saddle.

So why do we care about all this?

Longer cranks provide more leverage, so you can push larger gears. These longer cranks cause a greater angular change at the hip joint and it is this angular change that limits the cadence that can be produced. In other words expect to spin lower RPM’s with longer cranks

If the crank length is excessive or outside of the acceptable range, the binding discussed above will take place and limit your riding efficiency.

Shorter cranks while providing less leverage, will allow you to spin higher RPM’s. This will in most cases reduce the stress on the knee joint.

Longer cranks will require that you use a shorter top tube length or a higher handlebar position while shorter cranks will allow longer top tube lengths to be used and a lower handlebar position.  

Lets put this in perspective!

The shortest commonly available cranks produced today are 165 mm. The 50th percentile female is 63.6 inches tall and has a femur length of about 38.9 cm. The 165 mm crank is about 42.5% of the 38.9 cm femur. This is like putting a 70 inch rider on 184 mm cranks. While this crank length might be preferred by some small percentage of riders in the 70 inch height range, it would undoubtedly be primarily for off road use.

Lets take a look at this another way. The 50 percentile male is 68.8 inches tall, and the 97.5 percentile female is 68.5 inches tall. The average femur length for both groups is 42.3 cm, and a median crank length (39.5%) of 167 mm. Given this data, I think it is fair to assume that based on averages, 165 mm cranks are appropriate for men of 68 inches (~49 percentile, and women of about the same height (~97 percentile) or about 27% of the adult (US) population. If we assume that a crank length of as high as 41% of the femur length will provide acceptable performance, those 165 mm cranks can be used by as many as 75% of the males and 25% of the females, or about 50% of the adult (US) population.

 

 

 

 

About these ads

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s

Follow

Get every new post delivered to your Inbox.

Join 4,059 other followers