Crank Length

Cranks are getting shorter and we are getting faster

I believe shorter cranks could be the way of the future for making more power, more easily.

Up until now we have had a formula to help us decide what to use, also we can use our best judgment as to what feels good.

The crank length Formula.  A man named Kirby Palm is a cyclist and an engineer and wrote a crank length equation. He feels it is accurate and everyone that has used the equation is happy. The formula is:

INSEAM (cm)  x  2.16 = CRANK LENGTH (mm)

The myth. Kirby, has applied a constant. This implies that you will have the exact same leg length proportions, both functional and physical as the next rider. This is not the case. Steve Hogg points out some issues: limb proportion (femur/tibia, fibula) being very different,  foot size is different, the riders flexibility and specific knee issues are not accounted for.

If there was to be a formula that took into account all the different dimensions of the leg, foot and cleat placement; I feel you need to account for discipline and position as well. And also to a certain extent, rider preference as well. If some one does not like something it probably won’t work.

Why are our cranks lengths the lengths they are. Traditionally it has been what works and what is available.

Most cranks come in 4 common sizes, 175 mm, 172.5 mm, 170 mm and 165 mm, 165 mm being the least common and the other three just depend on the bike size. The smaller the bike, the shorter the crank.

To find out if  crank length had an effect on max power a crank length study was done in May 2001 by Dr. Jim Martin from The University of Utah, Department of Exercise and Sport Science and looked at three determinants of maximal cycling power: crank length, pedaling rate (cadence) and pedal speed.

The study tested crank lengths of 120, 145, 170, 195, and 220 mm. What Martin found is illustrated below.

Maximum power is developed with 145 mm cranks, then 170 mm and you can see the loss of power with the 120 mm and the 220 mm cranks.

Another study from Dr. Martin and John McDaniel; who holds a PhD in Exercise Physiology, recorded athletes’ oxygen consumption while cycling on a stationary bike in the lab. Crank lengths of 145, 170 and 195 mm; pedaling rates of 40, 60, 80 and 100 rpm; at intensity levels of 30, 60 and 90% of lactate threshold.

They found, 95% of the oxygen consumption changes were due to change in intensity levels and the crank length changes and cadence accounted for only a 3% change in oxygen consumption from 145mm to 190 mm. Smaller changes in crank length and cadence would yield smaller changes i.e. less than 3 %.

Another interesting connection they found was, as pedal speed decreased (not cadence, but pedal speed), oxygen consumption by the body went down. Why this is the case is likely due to the muscles being able to move slower, closer to maximum efficiency; and further from the maximum power or sprint.

Pedal speed for any given cadence will decrease as the crank length decreases. And inversely as the cranks get longer the speed of the pedal will increase for the same given cadence. This is due to the time it takes the pedal to move through one revolution with different circumferences.
For example a 145 mm crank has a pedal rotation circumference of 910 mm and for a 175 mm crank it’s 1,099 mm. This means at 90 rpm in one minute the pedal on the 145 mm cranks will move 81,900 mm and the pedal on the 175 mm crank will move 98,910 mm, a total difference of  just over 17 m. That movement is lost in the leg’s range of motion through the top of the pedal stroke when it’s very inefficient, some people call this area of the stoke the “dead zone”.

John Cobb feels the advantage of the short crank lies in the ability to get low in the torso. This is great for anyone on a TT or Triathlon bicycle or for road racers looking to have a lower functional body position.As the cranks get shorter, the saddle can come up a similar amount to maintain a similar leg extension. This opens up the hip and knee angle at the top of the stroke too by twice the amount shortened in crank length by bringing the foot down at the top of the stroke with the shorter crank and the amount you are able to raise the saddle and hip. This keeps the knee out of the rider’s rib cage allowing them to get lower in the front end and helps eliminate the dead spot at the top of the stroke too when the knee is at it’s most flexed point allowing the upper leg muscles to begin acting on the femur much earlier in the pedal stroke.
Steve Hogg also makes the point that knee pressure will decrease with shorter cranks. And I feel it is likely knee tracking will become straighter as abductor and adductor muscles are not pulled so tight during knee flexion.Lean angle is around 10º greater for a 145 mm crank when comparing it to a 175 mm. And this means faster out of the tight corners.There seem to me to be many advantages to shorter cranks, but how short do you go and what are the implications for gear ratio and gain ratio.

I will be trying a pair of 145 mm cranks on my road racing bike for three months, we will see how it goes. Fusion Peak is happily stocking John Cobb’s short cranks and can offer lengths in 145 mm, 155 mm and 160 mm in both compact and regular chain sets.


05 Nov 2014

By Aaron Dunford


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