There has always been an ongoing debate on what size crank arm is best for you,
Some people have mathematical formulas so you can calculate crank length, some have charts,
I won't list them all here, since depending on which source you use you come up with a different size.
Not much help is it !!
Then you have the people that say that crank length should depend on what type of pedalling style you have.
If you are a faster cadence SPINNER, you should have smaller size cranks to help you spin faster using less energy and if you are a lower cadence MASHER you should have longer cranks to generate more leverage and therefore power.
In all honesty I have tried crank sizes from 170 mm to 175 mm and have a hard time distinguishing any difference in power generation or my ability to turn the pedals faster.
It is just too much of a subtle differance for me to feel it.
Track sprinters of all sizes and heights with Quads like Godzilla sometimes use crank lengths in the 165mm range and are generating 1500 watts over short distances with a cadence in excess of 200 rpm's
Just as an aside, Sir Chris Hoy, one of Great Britain's best track sprinters of all time can leg press 631 kilograms 5 x in a row. Thats over 1388 lbs. Now you know what I mean by Godzilla quads.
Ever ask yourself... Why they aren't using longer cranks?
Most crank length formulas use a function of Inseam length in cm's as part of the calculation
For example :
crank length (mm) = Inseam (cm) X 1.25 + 65 ( Not my formula by the way )
For a person with an inseam of 81cm (typical of someone who is 5' 8" tall) the suggested crank length would be...
81 X 1.25 = 101.25 + 65 = 166.25 mm
Typically most people this height are using 172.5 mm cranks though
WHY?
Most people this tall, are usually riding bikes somewhere in the 52 - 54 cm frame size range and 172.5 mm cranks is the standard size cranks that come on these size bikes from every Manufacturer.
Why? Who knows
One important question rarely addressed however is....
How does crank length relate to an individuals leg anatomy ?
For arguments sake, lets take 2 people that both have an inseam length of 81 cm.
These people would typically be around 5' 8" - 5' 9"
One has a femur length (upper leg) of 42 cm and a tibia length (lower leg) of 39 cm
The other has a femur length of 40 cm and a tibia length of 41 cm
Should both these people be using the same size crank??
The answer is NO THEY SHOULDN'T
WHY?
The answer lies in the knee and hip angle at the top of the pedal stroke for both cyclists
Both these people having the same total leg length would have the same saddle heights to achieve equal extension at the bottom of the pedal stroke as long as they have approx. the same flexibility
Good extension at the bottom of the pedal stroke would result in a knee angle between 145 -148 degrees for most average cyclists. Can be up to 155 degrees for pros and elite cyclists.
Now lets come around to the top of the pedal stroke
Would both these cyclists have the same knee angle and hip angle at the top of the pedal stroke?
The answer is NO,
The person with the longer tibia will have a more acute (tighter) knee angle and hip angle than the person with the longer femur.
For good efficiency through the top of the pedal stroke, we typically need a minimum of 70 degrees of knee angle or more, preferably 74 - 75 degrees if possible for smooth transition over the top.
The person with the longer femur would typically have that.
The person with the longer tibia however may only have 65 degrees of knee angle at the top of the pedal stroke making transition through the top quite difficult resulting in having to raise the hip to bring the leg through,
IE Rocking in the saddle
We can't raise their saddle any more or they will be overextending, so what do we do
The answer lies in crank arm length
The person with the longer tibia would benefit greatly by having a shorter crank
WHY ?
A shorter crank would open up the riders knee angle and hip angle at the top of the pedal stroke making for a smoother transition.
Here is how it works
For every 2.5 mm shorter your crank length is you would have to raise your saddle height by the equivalent 2.5 mm to maintain the same extension at the bottom of the pedal stroke.
When you come around to the top of the pedal stroke, your foot is also 2.5 mm lower in relation to your hip than the longer crank would be.
To make a long storey short
For every 2.5 mm shorter your crank is you gain 5 mm of knee and hip room at the top of the pedal stroke..
This means that if the cyclist with the long tibia's uses cranks 5 mm shorter than the cyclist with the longer femurs he can achieve a 1 cm (approx. 4 degrees) less acute knee and hip angle at the top of the pedal stroke allowing for much better efficiency than with the longer cranks
This change would give the cyclist with the longer tibias (lower leg) a knee angle of 69 degrees instead of 65 degrees with the longer cranks.
Combine this with moving the cyclist with the longer tibia's' saddle forward a little more in relation to the cyclist with the longer femurs ( this further opens hip angle) and we will be able to match the knee and hip angle at the top of the pedal stroke for both riders
YOU SEE... SHORTER CAN BE BETTER,
Even if you have longer femurs than tibias, you may want to cosider slightly shorter cranks if you have trouble coming through the top of your pedal stroke efficiently.
For the cyclist with the long tibia's it is IMPERATIVE, for the cyclist with the long femurs it's a BONUS
These are some of the important subtleties of a professional bike fit.
Any questions or to book a fit.... Give me a call
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