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Everything You Need To Know About Crank Length

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Everything You Need To Know About Crank Length

Old 02-24-16, 03:46 PM
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Everything You Need To Know About Crank Length

Eat Sleep Train Smart - Personal Training & Coaching Official Blog: Crank Arm Length: Everything You Need To Know

Hi everyone, I own a small Personal Training and Coaching business and I work heavily in crank arm length testing and evaluation. I recently wrote a post that explains the differences between various crank lengths and why some cranks are more appropriate for certain types of athletes/ cycling disciplines. I would be happy to answer questions related to crank length here.
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Old 02-24-16, 05:13 PM
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Hi, just read your post. I find it interesting but I am a bit confused by a few details (please correct me if/where I am wrong):


Power (N * m / s) = Torque (N * m) * RPM (1 / s)


Force (N) =/= Torque (N m) = Force (N) * d (m)


so


Power = Force * Crank length * Cadence


For the same power, if force goes up, cadence goes down, and if force goes down, cadence has to go up.

I believe that should read 'if torque goes up cadence goes down...' I think that distinction is especially important in the next part


The optimal crank length is one that doesn't require so much force that fatigue occurs too early (too long), but also doesn't make it possible to put any pressure into the pedals (too short)

If we look at the torque definition above, since d is larger for a longer crank, less force is needed to keep the same power using longer arms (if RPM are kept constant) and more force is needed for shorter cranks.


Now, if force is kept constant, then longer arms do necessitate a decrease in cadence to keep constant power and vice versa.


As crank length increases, cadence decreases.
As crank length decreases, cadence increases.

Doesn't seem to me like that is necessarily true. Lets say we're on a single gear bike and we want to travel at a certain speed. A certain cadence will be necessary regardless of crank length, what will change is the force you have to exert on the pedals. Now, if the force required is above what is physically possible, then changing gear to increase RPMs and decrease force would be necessary, which is where I guess the statement for shorter cranks comes from. Seems to me that, since gearing allows us to vary the force-cadence relationship regardless of crank length, the idea that shorter cranks lead to higher cadence would only apply to maximal efforts where the force required to turn the pedals is extreme. For sub-maximal efforts, it seems to me that the same cadence could be kept regardless of crank length. I guess the question is, is force the limiting factor when it comes to power production?


LONG CRANKS pump a large volume of blood per pedal stroke at a slower rate (slower cadence). This occurs because the hip, knee and ankle travel a large distance or range of motion. More muscles are involved, especially in standing, and they contract fully to create a more complete muscle pump. This compensates for the slower cadence, but heart rates still tend to be slightly higher on longer cranks.
SHORT CRANKS pump a small volume of blood per pedal stroke at a faster rate (high cadence). This occurs because the hip, knee and ankle travel a smaller distance or range of motion. Less muscles are involved, and they contract partially which contributes to a less efficient muscle pump. This is offset by the faster cadence which is why short cranks tend to lead to lower heart rates.

Lactic accumulation is lower on long cranks.
Long cranks allow slow twitch (ST) muscle fibers to do more of the work because as the name implies, ST fibers contract slowly. When cadences are lower, muscle contractions are slower which means FT fibers get less of an opportunity to work and leave behind hydrogen ions.
Lactic accumulation or muscle burn is a frequent battle on short cranks.
Short cranks cause fast twitch (FT) muscle fibers to contribute more often. When cadences are high, muscle contractions are faster which means more FT fibers will be more likely to activate.

I will admit to not knowing much about pedaling physiology, but aren't the statements above in contradiction with the 'common knowledge' (which might very well be wrong) that higher cadence = higher heart rate while lower cadence = painful legs? Quick Google search throws a lot of results saying higher cadence = higher hear rate but I haven't seen any data to confirm (I haven't looked for it, don't know if it is out there or not) or deny. I also found this blog post by Hunter Allen:

Since fast-twitch fibers are more powerful than slow-twitch cells, the fast-twitch fibers swing into action at slow cadences, when high muscular forces are needed to move the bicycle along rapidly.


On the other hand, "fast" pedaling rates of 80 to 100 rpm are not too hot for the slow-twitch cells to handle. Slow-twitch cells can contract 80 to 100 times per minute and can easily cope with the forces required to pedal in low gear.
That seems to disagree with your view regarding slow twitch/fast twitch use.

I would certainly love to see some data on this, it is an interesting topic. I hope I didn't come off as negative, that certainly is not the intention, I just want to learn some more and your contribution is definitely appreciated.

As for me, I switched from 172.5mm to 175mm arms by accident (didn't pay attention) and I haven't noticed any difference, really. My cadence is still up there in the 100s usually and my power numbers are similar. However, I do have some issues with my legs touching my torso when I get low (I am not particularly tall) so I was considering maybe trying shorter arms, but I am still not sure I want to bother with.
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Old 02-24-16, 05:40 PM
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Meh. Along with what has been said above you haven't made any effort to address the issue of matching crank length to leg length.

Human limbs are levers that increase limb SPEED at the ends of the limbs while reducing FORCE at the ends of the limbs. If 2 riders had exactly the same muscle strength but different leg lengths then the rider with the SHORTER legs would have more force at the knee with less speed at the knee than the rider with longer legs. The longer legged rider would have more leg speed at the knee with less force at the knee. Therefore the rider with shorter legs could use shorter cranks and have the same pedal force as a longer legged rider using longer cranks.

Why do shorter riders want to use shorter cranks?

One reason is aerodynamics. If the cranks are too long for you leg length then you can't get into an aero position without you legs rising into your chest, and two, the greater range of movement is making your legs go through positions of far less than optimal power delivery. Basically extending your weakness at top dead centre.

I think that a crank arm on a bicycle should be considered something like crank size in a motor. You don't put 6 litre V8 cranks in a 1.5 litre 4 cylinder engine.

Anthony

Last edited by AnthonyG; 02-25-16 at 12:56 AM.
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Old 02-24-16, 09:09 PM
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^ Also my experience: crank length should not allow your thighs to hit your chest when your back is flat. However IME neuromuscular coordination does seem to be developable for a wide range of crank lengths. Testing someone with a variety of crank lengths doesn't speak to that issue.

For sure, higher cadence will give a higher HR at the same power output as a lower cadence, say the difference between 100 and 80. However higher HR for the same power is not necessarily a bad thing. Long distance riders commonly ride at higher cadences than most recreational riders. One might think that doesn't make sense, but there it is. And of course LD riders use slow twitch fibers almost exclusively.
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Old 02-25-16, 11:07 AM
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I appreciate your feedback regarding my post! The questions you have will help improve the clarity of my post. The difficulty in writing about this topic is that I'm trying to explain the Physiological side of crank length. It gets confusing because while the Physics can show advantages, there are disadvantages Physiologically. Let me try to explain.

Originally Posted by PepeM View Post
Hi, just read your post. I find it interesting but I am a bit confused by a few details (please correct me if/where I am wrong):

Power (N * m / s) = Torque (N * m) * RPM (1 / s)

Force (N) =/= Torque (N m) = Force (N) * d (m)

so

Power = Force * Crank length * Cadence

I believe that should read 'if torque goes up cadence goes down...' I think that distinction is especially important in the next part

If we look at the torque definition above, since d is larger for a longer crank, less force is needed to keep the same power using longer arms (if RPM are kept constant) and more force is needed for shorter cranks.

Now, if force is kept constant, then longer arms do necessitate a decrease in cadence to keep constant power and vice versa.
You have all of the equations absolutely correct. I made a mistake in reusing a term for a different meaning. I tried to use a word that better represented perceived effort. Whenever I'd use the term torque, a lot of my clients wouldn't seem to understand what torque is. I think replacing "force" with "pedal pressure" will make it clear that I'm speaking in terms of physiology versus physics.

So if cadences are low, pedal pressures must increase to compensate for the low cadence. This is habitual and only true when the rider is only trying to maintain a certain power output. From a standstill, pedal pressures are higher on shorter cranks vs long cranks.

I'll need to clarify this!

Originally Posted by PepeM View Post
Doesn't seem to me like that is necessarily true. Lets say we're on a single gear bike and we want to travel at a certain speed. A certain cadence will be necessary regardless of crank length, what will change is the force you have to exert on the pedals. Now, if the force required is above what is physically possible, then changing gear to increase RPMs and decrease force would be necessary, which is where I guess the statement for shorter cranks comes from. Seems to me that, since gearing allows us to vary the force-cadence relationship regardless of crank length, the idea that shorter cranks lead to higher cadence would only apply to maximal efforts where the force required to turn the pedals is extreme. For sub-maximal efforts, it seems to me that the same cadence could be kept regardless of crank length. I guess the question is, is force the limiting factor when it comes to power production?
Thanks for this question!

The more appropriate answer is that motor control is the limiting factor to power production. As crank length increases, range of motion increases which means more muscle groups get recruited. The upper limit of cadence decreases because coordinating all of these muscles at a fast rate is beyond the neuromuscular system's capabilities.

In short cranks, the range of motion is smaller and fewer muscles get involved, so it's much easier to coordinate all of the muscle groups at a fast cadence.

Riders on long cranks tend to be limited by pedal pressure, and riders on short cranks tend to be limited by cadence or leg speed. If you run long cranks, the body tries to adjust cadence/gearing to optimize for increasing pedal pressure (low cadence). If you run short cranks, the body favors a cadence/gearing that allows the legs to move as fast as possible. This is what happens when the cranks are too long or too short.

Originally Posted by PepeM View Post
I will admit to not knowing much about pedaling physiology, but aren't the statements above in contradiction with the 'common knowledge' (which might very well be wrong) that higher cadence = higher heart rate while lower cadence = painful legs? Quick Google search throws a lot of results saying higher cadence = higher hear rate but I haven't seen any data to confirm (I haven't looked for it, don't know if it is out there or not) or deny. I also found this blog post by Hunter Allen:

That seems to disagree with your view regarding slow twitch/fast twitch use.
Unfortunately, that's an incorrect interpretation of fast twitch fiber recruitment. Maybe it's a typo? When the two fiber types (there are actually three) were discovered, they were named in a way that was supposed to be easy to remember. Slow twitch = slow. Fast twitch = fast.

There are two ways to recruit fast twitch muscle fibers. As long as the movements are slow, the power is produced primarily by ST fibers. If the movements are fast, the power is generated primarily by FT fibers. If you try to increase pressure in a slow movement, you'll activate FT fibers in a more progressive way (smaller percentage of FT fibers). If you try to apply pressure quickly at an already fast cadence, you'll be more likely to recruit all of your FT fibers. Faster cadences burn out the muscles because FT fibers produce acidic metabolic waste when activated (lactate, hydrogen ions) whereas ST fibers only produce carbon dioxide. The reason why higher heart rates occur at longer cranks is because heart rate and respiration increases as a response to high carbon dioxide levels. This is a way to prove that long cranks favor ST more than FT.

Here's something interesting, it is true that slow twitch fibers can handle 80-100 rpm. The optimal crank tends to be one that naturally creates an upper cadence ceiling of 100rpm. This ensures you're mostly recruiting the fatigue-resistant slow twitch fibers.

Originally Posted by PepeM View Post
I would certainly love to see some data on this, it is an interesting topic. I hope I didn't come off as negative, that certainly is not the intention, I just want to learn some more and your contribution is definitely appreciated.

As for me, I switched from 172.5mm to 175mm arms by accident (didn't pay attention) and I haven't noticed any difference, really. My cadence is still up there in the 100s usually and my power numbers are similar. However, I do have some issues with my legs touching my torso when I get low (I am not particularly tall) so I was considering maybe trying shorter arms, but I am still not sure I want to bother with.
Next time you ride, try a little experiment where you turn off the cadence display, but still let it collect the data. A lot of times people will try to keep their cadence higher than what their body wants to naturally select. When the thighs hit the torso, a lot of times it's because the ankles don't have enough ankle dorsiflexion.

Also try doing a standing 5, 10 and 20 second sprint on your 175mm and compare it to your 172.5mm. You'll see a big difference in power output.

Here's a fun video on crank length testing. Their results fall in line with my description.

https://www.youtube.com/watch?v=eMAxH_Ud8YE

In the meantime, I have a lot of editing to do! Thanks a lot for your input!
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Old 02-25-16, 05:39 PM
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Originally Posted by AnthonyG View Post
Meh. Along with what has been said above you haven't made any effort to address the issue of matching crank length to leg length.

Human limbs are levers that increase limb SPEED at the ends of the limbs while reducing FORCE at the ends of the limbs. If 2 riders had exactly the same muscle strength but different leg lengths then the rider with the SHORTER legs would have more force at the knee with less speed at the knee than the rider with longer legs. The longer legged rider would have more leg speed at the knee with less force at the knee. Therefore the rider with shorter legs could use shorter cranks and have the same pedal force as a longer legged rider using longer cranks.

Why do shorter riders want to use shorter cranks?

One reason is aerodynamics. If the cranks are too long for you leg length then you can't get into an aero position without you legs rising into your chest, and two, the greater range of movement is making your legs go through positions of far less than optimal power delivery. Basically extending your weakness at top dead centre.

I think that a crank arm on a bicycle should be considered something like crank size in a motor. You don't put 6 litre V8 cranks in a 1.5 litre 4 cylinder engine.

Anthony
The reason why I will never recommend a crank length based on leg length is because there's no correlation between leg length, femur to tibia ratio, inseam length, foot length and optimal crank arm length. Those methods assume no ROM restrictions from tissue, fascia, muscle, tendon or ligament which are bigger limitations versus bone length. That's a huge flaw that seems to always be overlooked.

The gold standard to determining optimal crank length is through lab testing where expired gasses, lactic accumulation and even muscle recruitment can provide a complete picture. I'm trying to create a guide to determining optimal crank length through motor control or the body's ability to smoothly coordinate the muscles for a powerful and efficient pedal stroke. I advocate using corrective exercise versus simply giving up and buying a shorter crank to solve the issue- settling for a shorter crank when the longer crank is more optimal places the rider at a serious disadvantage.

Regarding the problem of the thighs hitting the trunk- improving ankle mobility and flexibility can completely eliminate the issue and restore motor control to the muscles of the hip, knee and ankle. It can also allow you to hold a neutral back comfortably again. Just because the thigh hits the chest doesn't mean the crank isn't optimal. The only way to find out is to correct the limitations of the body and re-evaluate. Being able to push the longest crank that the body can efficiently manage poses some serious advantages. In my experience, I have a 200 watt difference in 20s sprint power between my 165mm and 172.5mm.
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Old 02-25-16, 05:59 PM
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One of my favorite equations is:

Circumference = πD = 2πR


It is pretty simple. The Circumference increases linearly with the crank arm length.

Increase the cranks by 5% and the circumference increases by 5%.

So, if you look at cadence not as RPM, but rather feet per second around the circle that your foot is travelling through, then when you increase the crank length, you naturally increase the distance around the circle. Maintain the feet per second for the rotation around that circle, and cadence drops, but with the increased leverage, the power remains the same.

One other thing to mention is that seat height is dictated by the pedal position at the bottom of the crank circle. I still go by the old heel on pedal measurement for seat height, then adjust it from there. This means that the longer the cranks, the lower the seat. The shorter the cranks, the higher the seat.

Personally, I prefer longer cranks. I am flexible enough. I prefer slower cadence. I do max out at about 180mm. I haven't tried longer, but I don't think I could adapt to anything longer as I'm not real tall.
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Old 02-25-16, 06:58 PM
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Originally Posted by CliffordK View Post
One of my favorite equations is:

Circumference = πD = 2πR


It is pretty simple. The Circumference increases linearly with the crank arm length.

Increase the cranks by 5% and the circumference increases by 5%.

So, if you look at cadence not as RPM, but rather feet per second around the circle that your foot is travelling through, then when you increase the crank length, you naturally increase the distance around the circle. Maintain the feet per second for the rotation around that circle, and cadence drops, but with the increased leverage, the power remains the same.

One other thing to mention is that seat height is dictated by the pedal position at the bottom of the crank circle. I still go by the old heel on pedal measurement for seat height, then adjust it from there. This means that the longer the cranks, the lower the seat. The shorter the cranks, the higher the seat.

Personally, I prefer longer cranks. I am flexible enough. I prefer slower cadence. I do max out at about 180mm. I haven't tried longer, but I don't think I could adapt to anything longer as I'm not real tall.
That's good information! If you prefer long cranks, you probably have a larger percentage or dominance of slow twitch muscle fibers. Do you post your rides online? I'm interested in seeing what cadences you tend to naturally select.

I'll be sure to include some bike fitting recommendations as well. Longer cranks also require the saddle to be positioned more forward to prevent the braking effect phenomenon at the bottom of the pedal stroke. I completely forgot to mention the braking effect in my post. I'll need to mention this as well! Thanks!
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Old 02-25-16, 07:34 PM
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A reasonable conclusion based on the limited study that has been done in this area shows that while a rider may prefer 170-175 mm cranks, for whatever the reason (and, thank goodness for that because that is what is most commonly available from bicycle makers) 145 mm cranks would be at least as efficient and likely would enable many cyclist to get even more wattage out those legs.

From my personal experience -- at least on training equipment -- cranks as low as 100 may be even better. And, there is some common sense backing for the conclusion that it has been successfully ergonomically tested by everyone. It is about 8 inches from top to the bottom of a stroke for 100s and that's already more than the ubiquitous standard of acceptance as measured by height of a stair.

For similar reasons I believe that positioning the pedal more to the mid-foot than modern cycling shoes allow for is a lot more natural --e.g., like placing a foot squarely on the tread of a stair. Some might think it is faster to pedal on toes (e.g., envisioning a sprinter coming out of the blocks versus speed walker with swinging hips) but, that analogy fails the reality test. Everyone is different but people have a foot speed that is natural for them and at any given foot speed, going to shorter cranks will result in an increased cadence.

If you couple an increase in cadence with a higher gear (which is what the studies indicate may come naturally from an optimal foot-speed), the only result possible is an increase in power.
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Old 02-25-16, 08:54 PM
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Originally Posted by AnthonyG View Post
Meh. Along with what has been said above you haven't made any effort to address the issue of matching crank length to leg length.

Anthony
Originally Posted by ESTrainSmart View Post
The reason why I will never recommend a crank length based on leg length is because there's no correlation between leg length, femur to tibia ratio, inseam length, foot length and optimal crank arm length. Those methods assume no ROM restrictions from tissue, fascia, muscle, tendon or ligament which are bigger limitations versus bone length. That's a huge flaw that seems to always be overlooked.
I never claimed that leg length was the be all and end all of deciding optimum crank length. I'm well aware of the issues of differently proportioned legs.

I just said that you made no effort to take it into account.

No amount of pontificating on the difference between 170mm and 172.5mm is going to make up for the fact that cranks for short people are about 25mm or more too long in the first place. You need to be in the ball park before worrying about fine tuning.

Anthony

Last edited by AnthonyG; 02-25-16 at 11:20 PM.
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Old 02-26-16, 12:44 PM
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Originally Posted by AnthonyG View Post
I never claimed that leg length was the be all and end all of deciding optimum crank length. I'm well aware of the issues of differently proportioned legs.

I just said that you made no effort to take it into account.

No amount of pontificating on the difference between 170mm and 172.5mm is going to make up for the fact that cranks for short people are about 25mm or more too long in the first place. You need to be in the ball park before worrying about fine tuning.

Anthony
Sorry for misinterpreting what you wrote and I really appreciate your feedback. I updated the post with a section discussing the limitations of using anthropometric methods to determine optimal crank length. I'll also make some revisions to demonstrate how a tall rider and a short rider can share the same optimal crank length. In writing the article, I made the assumption that the people reading it already own a bike and have experience on their current crank length. The purpose of the information was to evaluate how their current crank affects their technique and muscle activation, then determine whether going longer or shorter is more appropriate.

Your statement about short people also reminded me to include information on improving thoracic mobility to reduce compression at the top of the pedal stroke. Just out of curiosity, what approximate height would you consider to be short? Thanks again for your input because it's going to help solidify the content in the article. If you have any other comments, I'd like to hear it.

@PepeM - When I read the "80-100 rpm" statement about slow twitch contractile rate, it sounded strikingly similar to one of the articles in my files which cited studies on fiber recruitment. Fortunately, the study is still available online and I placed the link below for your convenience. Please search for the phrase "muscular efficiency of ST fibers." I mistakenly agreed about the statement that ST fibers can operate efficiently up to 100rpm. ST muscular efficiency drops from 80 to 100rpm and instead, improves the efficiency of FT fibers.

The Relationship Between Optimal Pedaling Cadence and the Isokinetic Contractile Properties of the Quadriceps
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Old 02-26-16, 04:15 PM
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Originally Posted by ESTrainSmart View Post
Sorry for misinterpreting what you wrote and I really appreciate your feedback. I updated the post with a section discussing the limitations of using anthropometric methods to determine optimal crank length. I'll also make some revisions to demonstrate how a tall rider and a short rider can share the same optimal crank length.

Short people and tall people cannot share the same optimal crank length. The whole notion is preposterous.

Your fundamental problem is that you want to set yourself up as an "expert", while also not rocking the boat of current practice. As a way to run a business its probably smart, but Expert? No.

Your possibly labouring under the delusion that bike manufacturers know or even CARE to make bikes to fit people. Maybe someone knows but they certainly don't care enough to do anything about it.

In bike manufacturing these days Manufacturing economics rules the world to the near exclusion to everything else. Sometimes token steps are taken to change handlebar width but thats pretty paltry. The only variation that is generally made is to steepen seat tube angles on smaller frames. This is a relatively cheap variation to make but also WRONG.

Its ALL about making the least variations to a basic design as possible. The more things are done exactly the same the cheaper they become. Its called Economies of Scale. Variation is the enemy of Economies of Scale.

Giving a short person the same crank length as a taller person but simply moving them further forward over the cranks, kind of works for shorter distances, but its not what makes short people really comfortable on a bike. To be comfortable short people need short cranks and RELAXED seat tube angles.

The trouble with this is that what happens next is that given a relaxed seat tube angle with the corresponding SHORT front centre distance, a 700c wheel will no longer fit in the frame so smaller wheels are required.

Bike manufacturers don't want to know. They have market domination and force of precedence on their side so they're not changing for anyone.

Even the small guys get it wrong too. The fundamental problem is lack of easy choice of crank lengths. Its Garbage in, Garbage out (GIGO).

I'M an expert on crank length. The difference is that I'm not running a business and I don't care if my answer is something difficult to achieve. The expert answer is what the expert answer is.

I'm not really here to change what you do. There will be people in the industry who are happy with what you are doing as you are not a challenge to them.

Business is business. That's the cycling Industry for you.

Anthony

Last edited by AnthonyG; 02-26-16 at 06:33 PM.
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Old 02-26-16, 05:45 PM
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More folks could see for themselves if canks were still made like these:

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Old 03-03-16, 03:51 PM
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Originally Posted by AnthonyG View Post
Short people and tall people cannot share the same optimal crank length. The whole notion is preposterous.

Your fundamental problem is that you want to set yourself up as an "expert", while also not rocking the boat of current practice. As a way to run a business its probably smart, but Expert? No.

Your possibly labouring under the delusion that bike manufacturers know or even CARE to make bikes to fit people. Maybe someone knows but they certainly don't care enough to do anything about it.

In bike manufacturing these days Manufacturing economics rules the world to the near exclusion to everything else. Sometimes token steps are taken to change handlebar width but thats pretty paltry. The only variation that is generally made is to steepen seat tube angles on smaller frames. This is a relatively cheap variation to make but also WRONG.

Its ALL about making the least variations to a basic design as possible. The more things are done exactly the same the cheaper they become. Its called Economies of Scale. Variation is the enemy of Economies of Scale.

Giving a short person the same crank length as a taller person but simply moving them further forward over the cranks, kind of works for shorter distances, but its not what makes short people really comfortable on a bike. To be comfortable short people need short cranks and RELAXED seat tube angles.

The trouble with this is that what happens next is that given a relaxed seat tube angle with the corresponding SHORT front centre distance, a 700c wheel will no longer fit in the frame so smaller wheels are required.

Bike manufacturers don't want to know. They have market domination and force of precedence on their side so they're not changing for anyone.

Even the small guys get it wrong too. The fundamental problem is lack of easy choice of crank lengths. Its Garbage in, Garbage out (GIGO).

I'M an expert on crank length. The difference is that I'm not running a business and I don't care if my answer is something difficult to achieve. The expert answer is what the expert answer is.

I'm not really here to change what you do. There will be people in the industry who are happy with what you are doing as you are not a challenge to them.

Business is business. That's the cycling Industry for you.

Anthony
I'm sorry I didn't know you were an expert on crank length. What's your background? If you have novel information that hasn't already been investigated/ discovered, I can get you in contact with researchers who are studying the relationship between optimal crank length and aerobic efficiency, maximal power output, lactic accumulation, motor control and fiber type dominance. Maybe you can scientifically demonstrate how absurd it is that two riders of distinctly different heights can share the same optimal crank length. I'm sure they would love to hear your feedback.

With regards to bike manufacturers, I completely disagree with you. I think many have good intentions, but it's not fair to expect them to account for individual physiological differences. That's the responsibility of the customer to determine, and the reason why I created this how-to post. Manufacturers tend to see the body through an engineer's point of view, which is why they rely on anthropometrics or "sizing chart methods" to determine the geometry and crank length of the bikes they sell. The problem is that a lot of people aren't informed about the other variables that also affect optimal crank length, and I want to fix that. Not all business owners are crooked- some actually care about helping people and are passionate about the sport. The manufacturers and business owners you seem to have a personal vendetta against are the same ones that pressured the UCI into alleviating the saddle tilt rules so that a more anterior pelvic tilt can be achieved.

I'm in the business of Coaching and Personal Training and using exercise to improve performance and injury resistance through improved motor control. I'm not in the crank length optimization business which would be an extremely niche market and a poor business decision by the way. I'm passionate about using my skills and knowledge to help people which is why I'm freely sharing this information with the public.
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Old 03-03-16, 04:59 PM
  #15  
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Much of the information you need on ideal crank length has already been given in this thread. You just dismissed it out of hand and I expect that you will continue to dismiss it out of hand.

I'll try and simplify it. Aerodynamics are far MORE important than absolute peak power at a certain part of the cycle. Having a more even power delivery through the entire cycle is more important than the peak power delivery available at one part of the cycle. Cycling is an endurance sport so comfort for many hours on a bike is of paramount importance.

Now having said this I don't think that you do lose power with short cranks but I'm not claiming that you gain any either.

I've ridden MANY miles/kilometres on bikes that have, 170mm cranks, 165mm cranks, 152mm (6") cranks, 140mm cranks, 135mm cranks, 125mm cranks and 110mm cranks. Only the 110mm cranks were a little too short for me but even then, I'm FAR more comfortable riding long distances with 110mm cranks than 170mm cranks.

I'm observant. I'm advocating NOTHING, that bike manufacturers don't already know how to do. I've owned and seen numerous small vintage bikes that are EXACTLY what I recommend. Short cranks, 110mm on child's bikes, 140mm on adolescents bikes. Relaxed seat tube angles of around 70 degrees. Really short front centre distances, small wheels and low bottom brackets.

I'm not teaching anyone ANYTHING new. The industry knows these things but have deliberately forgotten them. This is in part why I'm a little cranky. Why have they deliberately forgotten them? Because of manufacturing economics and economies of scale. I was a student of Industrial Design. I was taught these things.

Bike manufacturers also do things differently in different markets. When I was out yesterday I noticed a small bike locked up at the shops. 24" wheels and a RELAXED seat tube angle on a fairly newish bike but it wasn't an Australian market bike, It had to be an Asian market bike. Now the cranks were too long still at 165mm I guess but still better than most we get these days.

Bike and component manufacturers don't want to deal with a wide range of different size parts on the same production line. Variety is the enemy of economies of scale.

I have a custom road bike with 650c wheels, 125mm cranks, 70 degree seat tube angle and a low bottom bracket. Most comfortable and fastest bike I have ever owned. I can comfortably get into an aerodynamic tuck without my thighs rising into my chest and hold the position as the relaxed seat tube angle has taken a LOT of weight off my hands and shoulders. When I rode more standard bikes with 165mm cranks, 700c wheels and 75 degree set tube angles which is the industry standard for small bikes I couldn't get into an aerodynamic position to save myself let alone hold it.

The Industry is perfectly happy with guys like you. Your someone who is validating their current approach which makes them feel better about themselves. Cranky guys like me are a complete pain.

Anthony
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Old 03-15-16, 04:18 PM
  #16  
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i think sagan recently switched from 175 to 172.5 cranks. correct me if I'm wrong. why would a sprinter do that if 2.5mm crank length reduction would lead to a 70-100W drop in max power?
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Old 03-15-16, 04:37 PM
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correction, sagan seems to have been using 172.5 for a long time. I feel like at his height (6'), he should be using a bigger crank (like 175) to accentuate his specialty. what do you think?
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Old 03-15-16, 09:53 PM
  #18  
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Originally Posted by spectastic View Post
correction, sagan seems to have been using 172.5 for a long time. I feel like at his height (6'), he should be using a bigger crank (like 175) to accentuate his specialty. what do you think?
https://www.youtube.com/watch?v=Td9kXwI9_fc

Based on his cadence on the road bike in the World Championships, he appears to have maxed out the longest crank he can possibly push. His preferred cadence was around 83-93rpm at high steady state seated power, so he's optimizing Slow Twitch and Fast Oxidative fibers. During the downhill seated attack, he peaked at around 103rpm which corresponds to the optimal cadence for fast glycolytic fiber activation. Since he only attacked for 10 seconds, he timed his attack very well and was only partially fatigued (it takes about 15-20 seconds to completely fatigue FG fibers).

If his cadence ceiling falls any lower, he'll likely experience reduced venous return efficiency, increased lactic accumulation and increased cardiac-related stress/ fatigue. Since optimal cadence decreases with increasing crank length, he'll likely fall below that critical cadence ceiling with 175's and have to stand in order to recruit the FG fibers; whereas, he has the option to conveniently recruit FG fibers seated or standing on the 172.5mm.

From a short search, he apparently used a 175mm in 2013, but used a 172.5mm to win the World Championship in 2015. Through lab testing, I'm sure they had the challenge of deciding whether the higher peak sprint power output on the 175 would have offset the slightly reduced venous return and higher heart rate. Since he's good at drafting, he can avoid these problems with clever positioning, but he'll always have to rely on using riders as stepping stones for the sprint. The 172.5mm likely sacrificed some maximal sprint power output for overall increased power output, so he'd be a lot more competitive without as much help.

In TT's, he's probably also using 172.5mm- A steeper seat tube angle tends to allow a rider to achieve a slightly faster optimal cadence, but still have the torque availability to stand for any climbs (steeper STA's reduce maximal sprint power output, but that's not a priority in TT's).

I'm putting together a list of Pro riders and the crank lengths they use. I think everyone will find the list pretty surprising because there's no real trend. Optimal crank length depends on a lot of factors. "Optimizing" can mean optimizing certain physiological variables at the expense of others or it can mean optimizing based on preferred race strategy.

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Old 03-16-16, 12:08 AM
  #19  
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Originally Posted by ESTrainSmart View Post
https://www.youtube.com/watch?v=Td9kXwI9_fc

Based on his cadence on the road bike in the World Championships, he appears to have maxed out the longest crank he can possibly push. His preferred cadence was around 83-93rpm at high steady state seated power, so he's optimizing Slow Twitch and Fast Oxidative fibers. During the downhill seated attack, he peaked at around 103rpm which corresponds to the optimal cadence for fast glycolytic fiber activation. Since he only attacked for 10 seconds, he timed his attack very well and was only partially fatigued (it takes about 15-20 seconds to completely fatigue FG fibers).

If his cadence ceiling falls any lower, he'll likely experience reduced venous return efficiency, increased lactic accumulation and increased cardiac-related stress/ fatigue. Since optimal cadence decreases with increasing crank length, he'll likely fall below that critical cadence ceiling with 175's and have to stand in order to recruit the FG fibers; whereas, he has the option to conveniently recruit FG fibers seated or standing on the 172.5mm.

From a short search, he apparently used a 175mm in 2013, but used a 172.5mm to win the World Championship in 2015. Through lab testing, I'm sure they had the challenge of deciding whether the higher peak sprint power output on the 175 would have offset the slightly reduced venous return and higher heart rate. Since he's good at drafting, he can avoid these problems with clever positioning, but he'll always have to rely on using riders as stepping stones for the sprint. The 172.5mm likely sacrificed some maximal sprint power output for overall increased power output, so he'd be a lot more competitive without as much help.

In TT's, he's probably also using 172.5mm- A steeper seat tube angle tends to allow a rider to achieve a slightly faster optimal cadence, but still have the torque availability to stand for any climbs (steeper STA's reduce maximal sprint power output, but that's not a priority in TT's).

I'm putting together a list of Pro riders and the crank lengths they use. I think everyone will find the list pretty surprising because there's no real trend. Optimal crank length depends on a lot of factors. "Optimizing" can mean optimizing certain physiological variables at the expense of others or it can mean optimizing based on preferred race strategy.
I would appreciate you putting together a list like that. it would definitely be interesting to see.

I'm surprised that you think a 2.5mm change in crank length would have a big effect on recruitment of fast twitch muscles. I suppose for breakaway efforts, it would be pretty useful to be able to hammer down while seated to maintain aerodynamics, and catch people off guard. however, I'm suspicious on whether it will really matter between 175 and 172.5, especially for someone who's 6'

I've never played around with crank lengths. I went for the longest time thinking I had 175's, and bought 175s for my new builds, only to find out later that they were actually 172.5... I think this is common, for people to not even realize what length cranks they're riding. on the other hand, when I ride my vintage beater from college that has 170 cranks, I can definitely notice a difference in the decreased range of motion. there are probably many other fit factors at play, especially considering the steel beater has platform pedals, but I much prefer the greater range of motion of the 175's, as opposed to the 170s. At peak fitness, I spin my 175s at 90+ rpm no problem throughout a race, and my best sprints have been in the 110-120 range. I don't know for sure, but I think this should be indication that my existing setup is suitable for me. I think at 6 ft, I shouldn't try to max out at 160 rpm or something crazy...

what about stack height of the cleats? I believe the difference between the stack height of a recessed spd (2 bolt) cleat is maybe 2-3mm different from that of a spd-sl (3 bolt) system.

Last edited by spectastic; 03-16-16 at 12:14 AM.
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Old 03-16-16, 07:10 AM
  #20  
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Originally Posted by spectastic View Post
I would appreciate you putting together a list like that. it would definitely be interesting to see.

I'm surprised that you think a 2.5mm change in crank length would have a big effect on recruitment of fast twitch muscles. I suppose for breakaway efforts, it would be pretty useful to be able to hammer down while seated to maintain aerodynamics, and catch people off guard. however, I'm suspicious on whether it will really matter between 175 and 172.5, especially for someone who's 6'

I've never played around with crank lengths. I went for the longest time thinking I had 175's, and bought 175s for my new builds, only to find out later that they were actually 172.5... I think this is common, for people to not even realize what length cranks they're riding. on the other hand, when I ride my vintage beater from college that has 170 cranks, I can definitely notice a difference in the decreased range of motion. there are probably many other fit factors at play, especially considering the steel beater has platform pedals, but I much prefer the greater range of motion of the 175's, as opposed to the 170s. At peak fitness, I spin my 175s at 90+ rpm no problem throughout a race, and my best sprints have been in the 110-120 range. I don't know for sure, but I think this should be indication that my existing setup is suitable for me. I think at 6 ft, I shouldn't try to max out at 160 rpm or something crazy...

what about stack height of the cleats? I believe the difference between the stack height of a recessed spd (2 bolt) cleat is maybe 2-3mm different from that of a spd-sl (3 bolt) system.
It's definitely common for people not to be able to perceive how crank length affects their muscle activation. It depends on the rider's level of proprioception.

The stack height won't affect the length of the lever, so your muscle activation and range of motion won't change as long as you adjust the height of the saddle to account for the stack height.

Although I was mentioning only fiber type (ST, FO, FG fibers) and contractile speed, that only covers the optimal speed for force production. The length of the muscle can also influence your force production massively. If the muscle is excessively stretched or shortened, the muscle won't produce optimal force. Please see the chart below.



When the crank is too long, the primary movers (glutes, quads and soleus) will be excessively lengthened at the top of the pedal stroke and limit force production (right side of the graph). The only way to reach a more optimal muscle length for the pedal stroke would be to stand. When the crank is too short, the muscle will be excessively shortened (left side of the graph). Although the contraction speed may be optimal for activating the right type of fiber, if the muscle isn't at the right length, force production won't be maximized.

The reason why I keep restating the importance of flexibility training is because if the rider is inflexible, focusing on flexibility training can reposition the force-length curve to optimize force production at longer lengths. There's a lot more to flexibility training than just aerodynamics. The primary goal of flexibility training should be to optimize the force-length curve, and the secondary goal should be aerodynamics. I always like to use Robert Forstemann as an example of someone who has excellent flexibility and mobility, and has strength across the entire range of motion. Check out the video below.

https://youtu.be/xddnlX1o1GU
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Old 03-28-16, 08:37 AM
  #21  
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Originally Posted by AnthonyG View Post
...
I have a custom road bike with 650c wheels, 125mm cranks, 70 degree seat tube angle and a low bottom bracket. Most comfortable and fastest bike I have ever owned. ...

Anthony
Curious, how tall are you and what inseam?
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Old 03-29-16, 01:49 AM
  #22  
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Originally Posted by bikebreak View Post
Curious, how tall are you and what inseam?
I'm 5' 1 1/2" with a 27" inseam so I'm long torso/short legged. I ride another bike with 135mm cranks which are fine but I still found them a little long so I went for 125mm. Truth be known 130mm is probably right for me but I'm still fast with 125mm cranks.

Even though I am an odd size when a bike is built to fit short people properly I can fit just fine with some minor adjustments and I'm talking about small vintage bikes here. I've seen any number of perfectly average short people suffering with VERY poor fit because small bike don't fit AVERAGE small people either.

Anthony
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Old 03-30-16, 07:52 PM
  #23  
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Late to this party, but I'll add my two cents worth. I'm 5'4 / 125 lb.

I started experimenting with short cranks about a decade ago, for two reasons: 1. For folding bikes where a shorter crank makes it easier to pack the entire bike in a suitcase for travel. 2. I'm a spinner and not a masher, so it seemed ideal for spinning.

I had Mark Stonich of bikesmithdesign.com shorten a couple Shimano cranks for me, a 105 and an Ultegra, down to 146mm and 148mm respectively. Here's what I found.

--Great for spinning. Your feet travel a smaller circle with a smaller crank. Spinning seems lighter, easier.

--Not so great for anything where you really need to push hard. Headwinds and hills were a major bummer.

In the end, though I still have both short cranks, I've removed them from my bikes and now they're just sitting in my parts bin. I've settled back to 170mm on most of my bikes and 165 on a couple folders. I'm in my happy place now.
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Old 03-31-16, 12:16 AM
  #24  
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The super short crank folks seem to feel that very high cadences are a good thing. Instead of 100 rpm at 172.5 mm, why not 160 rpm at 100 mm? Because very high cadences are inefficient, as you expend considerable energy simply accelerating and decelerating your legs, which have considerable mass. Get on a stationary spin bike, set resistance to zero, and start pedaling at 160 rpm, you'll tire quickly despite there being no load other than your own leg mass. But reduce cadence to 80 rpm, and you can probably pedal until boredom takes over.

So, combine super short (100 mm) cranks, reasonable cadence (100 rpm), and reasonable peak pedal force. Yes, that is nice and comfortable to pedal. But you're not putting out much power.

Last edited by jyl; 03-31-16 at 12:22 AM.
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Old 03-31-16, 04:45 AM
  #25  
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If your cadence is 100rpm, which is a FAST cruising cadence, then your not going to up your cadence to 160rpm just from short cranks. Look, 160rpm is only for experts. I'm happy to sprint at 140rpm even with short cranks.

Yes you increase your rpm with short cranks but nothing silly like 160rpm and to be honest you don't NEED to.

Short cranks is only one part of the story. The story is SYNERGY.

People with short legs find SYNERGY with SHORT cranks. Another VERY important part of the synergy story is placing riders with SHORT legs AND cranks at KOPS or thereabouts. A rider with short legs and short cranks will need a very relaxed seat tube angle of somewhere between 69-71 degrees. I haven't worked out the geometry to figure exactly why this happens but I suspect its because the saddle setback is fairly constant but with a shorter seat tube length the angle needs to be slacker to get the right setback over a shorter distance.

When I rode with short cranks but steep seat tube angles I just didn't get the power I have with short cranks and a relaxed set tube angle.

See some of my posts above. Short legged riders have MORE force at the knee than longer legged riders. SHORT legged riders can push SHORT cranks with MORE force which counteracts the reduced leverage.

Its SYNERGY.

ANYONE can ride with long cranks and get more leverage when the cranks are at 9-3 yet you will struggle greatly to get past 12-6.

Its SYNERGY.

I don't care if you tall guys like short cranks or not. I'm not recommending short cranks for tall riders. I'm recommending short cranks for SHORT riders.

Anthony
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