CRANK ARM LENGTH

[Titanium Rocket]

Apologies of this has been beaten to death before:

 

What are the implications of shortening your crank arm length, say from 175 mm down to 172.5 or even 170 mm? If you calculate the % difference in the circumference described by one revolution, it's negligible.

 

Surely you can achieve the same thing by staying with a 175 mm crank arm, and changing chainrings: going from a 40 tooth, down to a 38 tooth in the case of MTB?

 

Or is there a whole bunch of physics and physiology I'm missing here - power transmission, torque, effect on endurance, etc?

[Captain Fastbastard Mayhem]

it basically comes down to the amount of power you'd have to put down to make the crank perform a single revolution. 

 

As the longer crank has a longer lever arm, you'd technically expend less effort in order to get it to rotate once than if you had a shorter crank.

 

And no, changing the chainring does not do the same thing, as that effects the gearing - not the effort you expend to get the crank to go one rotation. Well, it does, but you'd have to go through more than one rotation in order to go the same distance, so your overall work done to go 1m remains the same.

[NotSoBigBen]

it basically comes down to the amount of power you'd have to put down to make the crank perform a single revolution. 

 

As the longer crank has a longer lever arm, you'd technically expend less effort in order to get it to rotate once than if you had a shorter crank.

 

And no, changing the chainring does not do the same thing, as that effects the gearing - not the effort you expend to get the crank to go one rotation. Well, it does, but you'd have to go through more than one rotation in order to go the same distance, so your overall work done to go 1m remains the same.

Mmmmm Captain ...... When I was doing some juggling with cranks for my SS MTB I was told that despite a 32*20 having the same 'ratio' as a 34*22 the 32 would feel easier to pedal due to some leverage effect - could you perhaps comment if they were talking poofie or not?

[jcza]

No difference in power unless you go shorter than 145mm or longer than 195mm. Will try find the link to the research.

[openmind]

Mmmmm Captain ...... When I was doing some juggling with cranks for my SS MTB I was told that despite a 32*20 having the same 'ratio' as a 34*22 the 32 would feel easier to pedal due to some leverage effect - could you perhaps comment if they were talking poofie or not?

Poofie. Changes in gearing are just that - they have no "leverage" effect at the pedal.

 

With longer cranks you're applying less force over a longer distance (arc) so it feels easier (due to the longer lever) but the physics will show that your expending the same amount of energy. That said, longer cranks may suit your physiology better, e.g. if you have long legs, so you may be more efficient with longer cranks. I'm 1.88m and happily ride 180mm cranks on all my bikes. 

[jcza]

Here is the link - http://link.springer.com/article/10.1007%2Fs004210100400

 

Summary: For many athletes, the idea “longer is better” has changed in part because of Dr. Jim Martin’s 2001 study titled “Determinants of maximal cycling power: crank length, pedaling rate and pedal speed” (Eur J Appl Physiol (2001) 84: 413-418).

 

Jim’s study involved 16 bike racers of various heights doing maximal sprint power tests, typically less than four seconds duration. During the study, they repeated the efforts while systematically testing the following crank lengths: 120, 145, 170, 195, and 220mm. Believe it or not, the test results showed no statistical difference in maximum power among the three middle crank lengths (145, 170 and 195mm).

 

The saddle height (measured to the pedal) was maintained throughout and the researchers did not adjust fore-aft saddle position or handlebar height despite changes in pedal-to-knee relationship and handlebar drop with the various crank lengths. For years crank length tests had been inconclusive and the general working knowledge came more from experience and intuition than science.

 

Now athletes can choose the crank length they like without worrying they’re affecting power.

[Nayr]

Would it not make sense, from a pure maths point of view, that the only effect a crank arm length will have is when it is under max strain i.e. starting on an extreme incline, it should be easier to turn over a longer crank arm on the same gear than a shorter one, but i suppose leverage will come in to effect as well... The effect while peddling theoretically should be nil because power remains constant, and therefore comes down to comfort of rider

[JA-Q001]

Reading the bit in the link, I find one problem with his test. His test was done with short sprints and high cadence (110 t0 132 rpm), this will always be more optimal in a shorter crank range. If you take a longer ride, where it is better to spin at maybe 90rpm, you will get greater efficiency with a longer crank, because the effort needed would be less for the same leg speed.

 

I did do a calculation on this a few week back, I will search for it and link it again.

[popcorn_skollie]

I honestly thought that changing arm lengths from 175 to 170 would make little to no difference in this regard. I was wrong. I could feel it in the climbs. Having said that I must add...

 

I'm not quite sure how to explain this but bare with me. With the shorter cranks when I'm on longer climbs and I feel like I'm starting to get tired. I find it easier and more efficient to slow down a notch before I feel the need to drop gear. Kind of like resting but still pedaling. With longer arms I would change gears more often

[JA-Q001]

Would it not make sense, from a pure maths point of view, that the only effect a crank arm length will have is when it is under max strain i.e. starting on an extreme incline, it should be easier to turn over a longer crank arm on the same gear than a shorter one, but i suppose leverage will come in to effect as well... The effect while peddling theoretically should be nil because power remains constant, and therefore comes down to comfort of rider

 

 

No, because you do not get to a even point, you are always decelerating, so you would always need to put power down to maintain a constant speed. This would mean that leverage will always play a role.

[JA-Q001]

I honestly thought that changing arm lengths from 175 to 170 would make little to no difference in this regard. I was wrong. I could feel it in the climbs. Having said that I must add...

 

I'm not quite sure how to explain this but bare with me. With the shorter cranks when I'm on longer climbs and I feel like I'm starting to get tired. I find it easier and more efficient to slow down a notch before I feel the need to drop gear. Kind of like resting but still pedaling. With longer arms I would change gears more often

 

But, what cadence to you climb at. If its higher (90, 100, +), the shorter crank will enable you to do this, as your legs are moving slower.

 

If you ride a lower cadence (below 80), the longer cranks should enable you to keep a lower force needed to maintain speed, basically grinding a gear easier.

[jcza]

Also have a look at the Sheldon Brown website for more

[JA-Q001]

https://community.bikehub.co.za/topic/142305-170mm-vs-175mm-mtb-crank/page-2

 

Here is the link to the previous discussion.

[linnega]

http://sheldonbrown.com/gain.html

 

Pretty much where you guys got to in the last discussion - crank length effectively changes gearing (although it is marginal when talking 5mm).

[carbon29er]

There has been so much written about this topic over the years yet there is no scientific proof that a longer or shorter crank changes your power output.

 

Only reason for a normal sized adult to go with a different with a length crank is comfort.  Longer crank lowers saddle height but increases chances of pedal strikes.

[jcza]

There has been so much written about this topic over the years yet there is no scientific proof that a longer or shorter crank changes your power output.

 

Only reason for a normal sized adult to go with a different with a length crank is comfort.  Longer crank lowers saddle height but increases chances of pedal strikes.

 

I thought European Journal of Applied Physiology is scientific proof?