Myth concerning Crank Arm Lengths

[David Henderson]

Dr. James C. Martin, PhD research shows that there is little difference in efficiencies between your current crank arm length and any other commercially available crank arm length.

There are several other interesting finds from his research.

In summary: Common crank sizes are nearly all equal in efficiencies. Cranks size can be chosen for reasons such as ground clearance for cornering/obstacles (shorter), aerodynamics (shorter), or rehabilitation/flexibility (longer). Sprinting 120 rpm is best. 60 rpm is better than 100 rpm aerobically (generally lower cadences are more efficient than higher). Natural pedal stroke is best (do not pull up), crank length has no effect on fatigue, no effect on metabolic efficiencies and very small effect on maximum power. A big gear sprint is better than a small gear sprint (for 30seconds) .

For some illustrations and further comment see: http://myworldfromabicycle.blogspot.com/2010/08/dude-your-crank-lengths-fine-you-just.html

I hope this helpful.

Regards,
David Henderson

 

[artemidorus]

Time to exhaustion is longer at the typically-preferred higher cadences of experienced cyclists (85-100/min) rather than 60/min. It is true, however, that VO2 is lower and gross efficiency is higher at lower cadences.

 

[tafi]

There is a problem with trying to draw conclusions from altering any one particular parameter of bike fit:
No-one can ever really just change one parameter of bike fit in isolation from all the other parameters.

Adapting to different length cranks often means other changes in fit geometry are required. That being the case, you can't tell if the changes are complimentary or contradictory (most people are looking for only subtle effects). A different geometry will lead to different changes required for different crank lengths and if you add the fact that no-one person conforms to an "average standard", you have a complete mess of different bodies, fits, cranks lengths and adjustments, all of which cloud the results. I doubt there is a statistically reliable way to draw any conclusion about this.

In short I agree with you: There is no need to worry about your crank length. NOT because there is no difference between lengths, but because it is impossible to tell what the differences are with any reliability.

 

[vspa]

I read once that Indurain shifted to a longer crank arm in april or may every season once he started getting into shape.
Hinault also said that people doing higher cadences had a longer life as a cyclist, low cadences being harder on the body.
FWIW

 

[swampy1970]

Originally Posted by tafi .

There is a problem with trying to draw conclusions from altering any one particular parameter of bike fit:
No-one can ever really just change one parameter of bike fit in isolation from all the other parameters.

Adapting to different length cranks often means other changes in fit geometry are required. That being the case, you can't tell if the changes are complimentary or contradictory (most people are looking for only subtle effects). A different geometry will lead to different changes required for different crank lengths and if you add the fact that no-one person conforms to an "average standard", you have a complete mess of different bodies, fits, cranks lengths and adjustments, all of which cloud the results. I doubt there is a statistically reliable way to draw any conclusion about this.

In short I agree with you: There is no need to worry about your crank length. NOT because there is no difference between lengths, but because it is impossible to tell what the differences are with any reliability.

There's this never heard of device called a power meter. Let me summon the Time Lords from Dr Who and invent a stop watch. Maybe the Dark Lord of the Sith can integrate the two. Power, weight, time - give me a big hill and make changes one at a time and we'll find, with reliability, what the effect of those changes are. You can do that on a flat course to and use the "Chung method" - but again you'll need a powermeter and conditions that remain similar throughout the day.

 

[dhk2]

swampy, your PM method seems simple, but can it really be repeated to reliably detect minor changes in power output due to small changes in the bike or the fit (eg, a 1-2% difference in crank length or seat height)? Do you use PE, heart rate, or some other measure to control the intensity, or just tell the subject go "all-out" for a given period and measure the output? Seems to me that normal variations in our physiological state would account for at least a few percentage point of output difference throughout a given day or week without changing anything on the bike.

Further, as tafi stated, you can't change a dimension like crank length without affecting other parameters. In addition to changing the angles of leg/hip flexion, changing crank length is effectively changing the gearing of the bike. Consider that if you lengthen the crank arms, the legs have to move faster to maintain a given cadence, like using a slightly-lower gear. So, even if the longer crank arms seem to allow the subject to produce more power, the results could be due to the small change in effective gearing rather than the change in the crank length itself.

OTOH, you're not looking to publish a scientific paper here. If doing some repeats with your PM on a couple of different crank lengths helps you to select the ones that you like best, and give you confidence in your choice, that's not a bad thing.

 

[David Henderson]

Originally Posted by vspa .

I read once that Indurain shifted to a longer crank arm in april or may every season once he started getting into shape.
Hinault also said that people doing higher cadences had a longer life as a cyclist, low cadences being harder on the body.
FWIW

The lower cadences can definitely stress the knee, especially if not properly adapted. I think it's ideal to have a range of cadences when riding, sprinting high, burst of speed high, hill climbing and time-trialing lower. Generally we do this naturally without much thought. It's nice to have science to quantify the effects.

 

[tafi]

Originally Posted by swampy1970 .

There's this never heard of device called a power meter. Let me summon the Time Lords from Dr Who and invent a stop watch. Maybe the Dark Lord of the Sith can integrate the two. Power, weight, time - give me a big hill and make changes one at a time and we'll find, with reliability, what the effect of those changes are. You can do that on a flat course to and use the "Chung method" - but again you'll need a powermeter and conditions that remain similar throughout the day.

Come on Swampy, even the gold standard SRM power meter has a 2% uncertainty. That means you can't measure differences in power output of less than 4%.
The uncertainty of the method is increased if PE enters the equation (how accurately can you judge a subjective quantity?) or if conditions are assumed to be consistent (which never happens outdoors).
We all know that a few percent difference can be important but we can't reliably measure any differences that small. It is that simple.