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Re: Crank arm length from fit perspective [FindinFreestyle] [ In reply to ]
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Excellent post! Thank you.
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Re: Crank arm length from fit perspective [FindinFreestyle] [ In reply to ]
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Perhaps everyone can just agree to ignore the water troll on this thread? Be nice to keep the signal to noise ratio in check on one of his favored topics for a change.....

Tech writer/support on this here site. FIST school instructor and certified bike fitter. Formerly at Diamondback Bikes, LeMond Fitness, FSA, TiCycles, etc.
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Re: Crank arm length from fit perspective [FindinFreestyle] [ In reply to ]
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FindinFreestyle wrote:
zinny wrote:
Thanks for the thorough answer findinfreestyle. From LAI's response, I figure he had a negative or at least not beneficial experience from going "short". Which I haven't really considered.

So with that thought, while the majority of the examples seem to be people "over cranking". Are there any of people "under cranking" and its affects? Could that negatively affect fit?


I think you can go too short, and I think you can largely "feel" your way to that as well. For starters, many (most?) riders can get to such a low position with reasonably short cranks, that going lower via shorter cranks would be of questionable benefit to aerodynamics anyway.

For me, having the luxury of an adjustable crank fit bike at my disposal for many years, I hovered between 160 and 155 as my lower limit. At a long legged 6 feet tall with a 78cm seat height at my old 172.5s, going down through 170, 165, 160 all felt great. All shorter lengths felt better than the longer. Swapping back and forth between 160 and 155 (many, many times) has been interesting. The shorter the crank, the higher the natural RPMs, and I have tentatively reached the conclusion that 160s is ideal for me. I can comfortably ride 17cm of drop at that length with zero power loss from a well developed road position.

On my recently ordered PC fit bike, I am going to utilize both the standard 155-185mm cranks as well as the shorter 140-170mm set. Shorter riders are certainly not under cranked at 155mm, and those shorter adjustable cranks are probably the one piece of hardware I have wished for over the last few years. I am a little curious how the 140 - 150 length treats me, but I am fairly confident that the 155s are getting a bit under my range. It's subtle, but once noticed, it's consistent with every trial.

For the record, I also think all the 'testing' Dave C. is doing with HR and power and speed over courses is NOT a path to finding a proper range of crank lengths. I think it is a bottomless rabbit hole, and I hope he can keep it off this thread.

I am 6' long legged as well and tried from 170 in 5mm steps but going from 165 to 150 and found too short. I went back to 160 and felt great. I settled on them for the season but have 155 I will trial now. At 150 when I was on I felt powerful and fast but at times I was chasing the gear engaged with power through part of the pedal stroke but then chasing the load. You certainly cant have a lazy/poor pedal stroke or takes some time to adapt to such a small pedal circle. I have found over time though that my cadence has dropped with a shorter crank now in the low 80's.
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Re: Crank arm length from fit perspective [h2ofun] [ In reply to ]
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Tell me again, how do I block seeing someone’s post? I am so tired of seeing posts about “my adjustable cranks” that I’m going crazy. 3 million posts later, and still no conclusion? STFU!

I'm closer to the feathered end of the spear than the point.
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Re: Crank arm length from fit perspective [David_Tris] [ In reply to ]
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David_Tris wrote:
Tell me again, how do I block seeing someone’s post? I am so tired of seeing posts about “my adjustable cranks” that I’m going crazy. 3 million posts later, and still no conclusion? STFU!

Easy, click on my name and click on hide posts, I am gone

Dave Campbell | Facebook | @DaveECampbell | h2ofun@h2ofun.net

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Re: Crank arm length from fit perspective [David_Tris] [ In reply to ]
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David_Tris wrote:
Tell me again, how do I block seeing someone’s post? I am so tired of seeing posts about “my adjustable cranks” that I’m going crazy. 3 million posts later, and still no conclusion? STFU!

Look at the how to unsee water boy thread. There is a built in hide posts option but it only hides threads someone started, not all of their posts
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Re: Crank arm length from fit perspective [stevej] [ In reply to ]
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stevej wrote:
Sheldon Brown would disagree with you.

https://www.sheldonbrown.com/gain.html

I actually think this is wrong or at least misleading. The chain doesn't travel the diameter of the crank length. on a 53-tooth ring, the chain will travel the same distance regardless of crank length from a single rotation of the crank arm. You can test this by marking a spot of the big ring, and then rotating this chainring all the way around, advancing the bike while you do this until it reaches that mark again. Then measure that distance. It will be the same regardless of crank length.

I think you can get more leverage with longer crank arms and your foot velocity would be different, but that is completely separate from how fast a gear will rotate the back wheel and therefore the speed the bike will be propelled.
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Re: Crank arm length from fit perspective [nightfend] [ In reply to ]
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nightfend wrote:
stevej wrote:
Sheldon Brown would disagree with you.

https://www.sheldonbrown.com/gain.html

I actually think this is wrong or at least misleading. The chain doesn't travel the diameter of the crank length. on a 53-tooth ring, the chain will travel the same distance regardless of crank length from a single rotation of the crank arm. You can test this by marking a spot of the big ring, and then rotating this chainring all the way around, advancing the bike while you do this until it reaches that mark again. Then measure that distance. It will be the same regardless of crank length.

I think you can get more leverage with longer crank arms and your foot velocity would be different, but that is completely separate from how fast a gear will rotate the back wheel and therefore the speed the bike will be propelled.

An important part of gearing is the force you have to apply to the pedal at a certain rpm to generate a specific amount of power. As crank length goes down, pedal force must go up to maintain the same power at the same rpm.

-------------
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Re: Crank arm length from fit perspective [RowToTri] [ In reply to ]
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That's true but its not the whole picture. There are two more things to consider.
1. For the same rpm, shorter cranks mean lower pedal speed. That means lower muscle shortening velocity. Muscles produce more force as they shorten more slowly. So yes, you need more force, but the muscle can produce more. For more on this google muscle force-velocity relationship.
2. When people switch crank lengths, they generally change seat height to maintain the same distance to the pedal in its most extended position (maximum leg extension). With shorter cranks your leg is more extended at every point in the cycle except that one most extended point. When your leg is more extended you are stronger. For example, you can lift more for a partial squat than for a deep squat. This is why short cranks don't compromise maximum torque as much as the simple length ratio. Only about half as much as the ratio would predict.
So your muscle is stronger at lower velocity and your leg is stronger due to what we call "biomechanical gear ratio" (the ratio of the movement of the distal point of the limb to the length change of the muscles involved in the action).
Cheers,
Jim


RowToTri wrote:
An important part of gearing is the force you have to apply to the pedal at a certain rpm to generate a specific amount of power. As crank length goes down, pedal force must go up to maintain the same power at the same rpm.
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Re: Crank arm length from fit perspective [Bio_McGeek] [ In reply to ]
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Bio_McGeek wrote:
That's true but its not the whole picture. There are two more things to consider.
1. For the same rpm, shorter cranks mean lower pedal speed. That means lower muscle shortening velocity. Muscles produce more force as they shorten more slowly. So yes, you need more force, but the muscle can produce more. For more on this google muscle force-velocity relationship.
2. When people switch crank lengths, they generally change seat height to maintain the same distance to the pedal in its most extended position (maximum leg extension). With shorter cranks your leg is more extended at every point in the cycle except that one most extended point. When your leg is more extended you are stronger. For example, you can lift more for a partial squat than for a deep squat. This is why short cranks don't compromise maximum torque as much as the simple length ratio. Only about half as much as the ratio would predict.
So your muscle is stronger at lower velocity and your leg is stronger due to what we call "biomechanical gear ratio" (the ratio of the movement of the distal point of the limb to the length change of the muscles involved in the action).
Cheers,
Jim


RowToTri wrote:
An important part of gearing is the force you have to apply to the pedal at a certain rpm to generate a specific amount of power. As crank length goes down, pedal force must go up to mainta

in the same power at the same rpm.

So why then am i hearing that me trying to target a lower rpm is bad? Or super short cranks are bad?

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Re: Crank arm length from fit perspective [h2ofun] [ In reply to ]
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h2ofun wrote:
So why then am i hearing that me trying to target a lower rpm is bad? Or super short cranks are bad?

Because as with everything, there are limits to what your body can do indefinitely. You yourself have noted that on the short cranks you can't 'hold' 300 watts at 70 RPM. If that is because your legs can't produce the requisite force for a high number of repetitions, it may be that you can still achieve that power level by bringing the pedal speed up, with slightly reduced force. That requires either longer cranks, higher cadence, or a moderate combination of both.

Less is more.
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Re: Crank arm length from fit perspective [FindinFreestyle] [ In reply to ]
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FindinFreestyle wrote:
I think you can go too short, and I think you can largely "feel" your way to that as well. For starters, many (most?) riders can get to such a low position with reasonably short cranks, that going lower via shorter cranks would be of questionable benefit to aerodynamics anyway.

On top of that, going substantially shorter than what's needed can also have a negative effect on the bike's handling. I found no performance difference when using 145s instead of 165s, but I did not like the effect that raising everything 20mm had on my bike's handling...

"I'm thinking of a number between 1 and 10, and I don't know why!"
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Re: Crank arm length from fit perspective [Bio_McGeek] [ In reply to ]
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Bio_McGeek wrote:
That's true but its not the whole picture. There are two more things to consider.
1. For the same rpm, shorter cranks mean lower pedal speed. That means lower muscle shortening velocity. Muscles produce more force as they shorten more slowly. So yes, you need more force, but the muscle can produce more. For more on this google muscle force-velocity relationship.
2. When people switch crank lengths, they generally change seat height to maintain the same distance to the pedal in its most extended position (maximum leg extension). With shorter cranks your leg is more extended at every point in the cycle except that one most extended point. When your leg is more extended you are stronger. For example, you can lift more for a partial squat than for a deep squat. This is why short cranks don't compromise maximum torque as much as the simple length ratio. Only about half as much as the ratio would predict.
So your muscle is stronger at lower velocity and your leg is stronger due to what we call "biomechanical gear ratio" (the ratio of the movement of the distal point of the limb to the length change of the muscles involved in the action).
Cheers,
Jim


RowToTri wrote:
An important part of gearing is the force you have to apply to the pedal at a certain rpm to generate a specific amount of power. As crank length goes down, pedal force must go up to maintain the same power at the same rpm.

I don't know much about how shortening velocity affects muscle strength. But is it not true that most people self-select a cadence that is at least close to the same angular velocity of the cadence they used on the longer cranks?

-------------
Ed O'Malley
www.VeloVetta.com
Founder of VeloVetta Cycling Shoes
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Re: Crank arm length from fit perspective [RowToTri] [ In reply to ]
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Are you aware of some data on self selected pedaling rates across crank length? I'd be keen to see it.
The only data I'm aware of are from Tom Korff's lab. He has left academia so we may never see them published. In our conversation he indicated that pedaling rate did change with length but not as much as the ratio of the length change.

RowToTri wrote:
I don't know much about how shortening velocity affects muscle strength. But is it not true that most people self-select a cadence that is at least close to the same angular velocity of the cadence they used on the longer cranks?
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Re: Crank arm length from fit perspective [Bio_McGeek] [ In reply to ]
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I don't know of a study. But it is "conventional wisdom" that cadence increases when cranks are shortened and I guess I just assumed that this was the body's way of maintaining percieved effort which in turn made me assume that pedaling velocity is important to percieved exertion, perhaps more important than rpm.

-------------
Ed O'Malley
www.VeloVetta.com
Founder of VeloVetta Cycling Shoes
Instagram • Facebook
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Re: Crank arm length from fit perspective [RowToTri] [ In reply to ]
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RowToTri wrote:
I don't know of a study. But it is "conventional wisdom" that cadence increases when cranks are shortened and I guess I just assumed that this was the body's way of maintaining percieved effort which in turn made me assume that pedaling velocity is important to percieved exertion, perhaps more important than rpm.

I have dropped my cadence a lot from what I used to spin on 200's to what I am spinning now on 150's.

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Re: Crank arm length from fit perspective [zinny] [ In reply to ]
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My wife is 5'-1" (156cm) and just recently went in for a fit. Early this year she had upgraded from an older P2 with 650 wheels to a newer P2 with 700's. (reason for 700's was flatting in a race where SAG had absolutely no 650 tires. She had two tubes, but the tire blew, and she had to DNF.) Anyway, she got switched down from 165 to 155 cranks after telling the fitter she felt like her perceived power had dropped off on the new bike versus the old. He said he has seen that before, switched cranks, and she appears to be back faster. Lack of power meter in use means none of this is quantifiable, but for her it helped.

I'm closer to the feathered end of the spear than the point.
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Re: Crank arm length from fit perspective [zinny] [ In reply to ]
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zinny wrote:
Since it seems like everything to do with fit has a method to it, does anyone know if there is a way to measure this, say since my "x" is this, a good guess at a optimal crank length would be "y".


You know, you're not that short ... ;^)

But to answer your question, yes, there is a formula that you can use to get a ballpark fit number. It is based on 170mm cranks, but I think that it is close enough. While this info was drawn up with the intention of showing a possible relationship between crank length and performance (and of which there may not be lots of evidence), it can easily be used for crank length vs fit. It might provide a good target number with which to start the fit process.
Link:

http://www.nettally.com/palmk/crankset.html

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Last edited by: DarkSpeedWorks: Dec 6, 17 15:58
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Re: Crank arm length from fit perspective [DarkSpeedWorks] [ In reply to ]
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DarkSpeedWorks wrote:
zinny wrote:
Since it seems like everything to do with fit has a method to it, does anyone know if there is a way to measure this, say since my "x" is this, a good guess at a optimal crank length would be "y".


You know, you're not that short ... ;^)

But to answer your question, yes, there is a formula that you can use to get a ballpark fit number. It is based on 170mm cranks, but I think that it is close enough. While this info was drawn up with the intention of showing a possible relationship between crank length and performance (and of which there may not be lots of evidence), it can easily be used for crank length vs fit. I might provide a good target number with which to start the fit process.
Link:

http://www.nettally.com/palmk/crankset.html

I used that as my basis years ago to buy and ride 200 cranks. It may have been way way off for me at 6'5

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Re: Crank arm length from fit perspective [h2ofun] [ In reply to ]
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I am not saying that it is some kind gospel.

Just that, for fit purposes, the number is often a good starting point.

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Re: Crank arm length from fit perspective [DarkSpeedWorks] [ In reply to ]
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DarkSpeedWorks wrote:
I am not saying that it is some kind gospel.

Just that, for fit purposes, the number is often a good starting point.

Just offering that is what I thought also, and it was WAY off, just for FYI. Wish I had known years ago. :(

Dave Campbell | Facebook | @DaveECampbell | h2ofun@h2ofun.net

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Re: Crank arm length from fit perspective [zinny] [ In reply to ]
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I posted this in a similar thread:
Here's my simplistic approach to fitting for aerodynamics and crank length together.
1. Decide how close to horizontal you want your torso. Closer to horizontal is generally more aero but requires really holding your head up to look ahead. You could do this part of the fit with cranks stationary and horizontal, its just to find out how low you're really willing to go.
2. Once you have dialed in the seat-relative-to-elbow pad position for how low you want to go, you can determine crank length. Proper crank length would be the one that gives you a reasonable hip angle as you come over the top. This is also influenced by fore-aft saddle position which serves to rotate your body about the bottom bracket. Of course you must adjust seat and elbow pad height to accommodate each crank length. If you're tall and thin, regular sized cranks will probably work just fine. If you're under about 5'10" or thickly built, you'll likely need shorter cranks (unless you want to sit up pretty high).
3. Done.
I went through this process to set up a UCI legal position (nose of saddle 5cm behind bb) for myself for track cycling. I wanted absolutely horizontal torso. At 5'8" 160lb, I needed 145mm cranks to make that rideable. For triathlon I wouldn't need to go that short because the rules allow a much more forward saddle position. Probably a good place to start any fitting procedure is with the saddle as far forward as the rules allow.
Cheers,
Jim
PS I am not a bike fitter except for myself. I have, however, published several papers on cycling crank length and cycling aerodynamics, and spent untold hours in wind tunnels.
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Re: Crank arm length from fit perspective [zinny] [ In reply to ]
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the one important thing for me with regard to using short cranks is that I find it easier to go up a hill while on the tri bars. When I have been on 'normal' cranks say 170 I have to sit up. This may be due to my inherent inflexibility but the ability to stay aero while putting out a reasonable TT power up a hill is a huge improvement.

I read that a smaller crank doesnt change drag but intuitively this does not make sense .. surely by turning a smaller circle we are creating less turbulence hence saving drag. The size of the hole we creating is now slightly smaller as well so doesnt this have an effect?? surely it must???
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