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The importance of crank length to the cyclist.

The importance of crank length to the cyclist.

02 Aug 2011 23:20

I would like to open a discussion regarding the importance of crank length to bicycle racing or cycling in general.

I have posted an "essay" of my thoughts as part of my web site here. I would be interested in constructive criticism to help me fine-tune my arguments or correct any obvious errors.

In summary I feel that shorter cranks do several things for the cyclist.

1. Shorter cranks will improve power output for most.
2. Although this goes completely against the conventional wisdom, shorter cranks can reduce knee stress
3. Shorter cranks allow better aerodynamic positioning without sacrificing power.

And, in general, we are talking substantially shorter than what most would consider to be a short crank. Our data suggests that around 100 mm crank length would be near optimum for most. I am currently riding 105 mm cranks with good feeling.

I believe there are good reasons that explain the above benefits and we can discuss them if any desire.

If possible I would like to open an actual discussion of this issue based on facts and data rather than bias and opinion.
FrankDay
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03 Aug 2011 00:50

Is the data you have applicable for all disciplines of cycling? Any variation for cycling modes that do not require sustained power ie: some forms of xc mtb, downhill or bmx?
Tapeworm
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03 Aug 2011 01:22

If you're looking for constructive criticism, a couple of quick thoughts:

1."There is a clear trend in the data and there is a big difference between racing significance and "statistical significance". It also seemed obvious that if there had been more participants that the 145 mm cranks superiority would have eventually reached statistical significance."

I don't think you get what statistical significance means. The fact that is wasn't statistically significant due to (among other things) small sample size means that if you added more people, statistically there is a reasonable chance that rather than eventually confirm the results, it would have actually changed them. If something isn't statistically significant, it means the study could not adequately conclude it. You can't just look at the results and assume they'd stay the same with more riders. If you could - then it would already be statistically significant!

2.You have plenty of very interesting and perhaps accurate observations about why shorter crank lengths could be more effective. I don't feel qualified to evaluate those. However, you have no science behind your theories, just ideas - as I suggested, your conclusions from the Martin study are pretty much the opposite of the scientific approach. What you have here is the starting point for more study. Which I would certainly encourage you to do.
User avatar egtalbot
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03 Aug 2011 01:24

Would you change chainring size also or what? Although I don't know the physics of why the current crank arm length it does not really seem to make total sense.
User avatar veganrob
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03 Aug 2011 02:40

FrankDay wrote:I would like to open a discussion regarding the importance of crank length to bicycle racing or cycling in general.

I have posted an "essay" of my thoughts as part of my web site here. I would be interested in constructive criticism to help me fine-tune my arguments or correct any obvious errors.

In summary I feel that shorter cranks do several things for the cyclist.

1. Shorter cranks will improve power output for most.
2. Although this goes completely against the conventional wisdom, shorter cranks can reduce knee stress
3. Shorter cranks allow better aerodynamic positioning without sacrificing power.

And, in general, we are talking substantially shorter than what most would consider to be a short crank. Our data suggests that around 100 mm crank length would be near optimum for most. I am currently riding 105 mm cranks with good feeling.

I believe there are good reasons that explain the above benefits and we can discuss them if any desire.

If possible I would like to open an actual discussion of this issue based on facts and data rather than bias and opinion.


I've studied a book that spends a few chapters on crank arm length by Ed Burke called The Science of Cycling and your understanding of crank arm length is completely flawed.

The reason why crank arms are longer than you think they should be is because longer crank arms allow for LESS power (i.e. less effort) to result in the same speed. The goal is to use less power to generate the same cadence.

The study you cited merely indicates that 145mm crank arm length is the ideal length for generating maximum power for ONE rotation. That has nothing to do with your goal as a road cyclist. Longer crank arms are better because you can generate more torque and more power based on less effort than you could with a shorter crank arm length.

Sure a shorter crank arm length might allow for a rider to generate more power in 1 rotation (which is why track riders generally use crank arms that are 10mm shorter than they would on the road), but that won't help you on hills. Shorter crank arms are a severe hindrance when climbing since longer crank arms will allow you to generate more torque with less effort.

You don't seem to understand that longer crank arms are always better and the only reason riders can't go bigger and bigger is because at some critical point, the circle they spin becomes too large.

You're looking at maximum power for 1 rotation instead of what you should be looking at: highest speed based on least amount of effort (i.e. power efficiency) for a 2 hour ride. Longer crank arms are far more efficient than shorter crank arms for that task.
User avatar TERMINATOR
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03 Aug 2011 02:51

Well said 'Terminator'. I 2nd that.

One other thing to consider is the potential for injury with shorter cranks, ie. tendonitis.
User avatar Indurain
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03 Aug 2011 03:20

FrankDay wrote:I would like to open a discussion regarding the importance of crank length to bicycle racing or cycling in general.

I have posted an "essay" of my thoughts as part of my web site here. I would be interested in constructive criticism to help me fine-tune my arguments or correct any obvious errors.

In summary I feel that shorter cranks do several things for the cyclist.

1. Shorter cranks will improve power output for most.
2. Although this goes completely against the conventional wisdom, shorter cranks can reduce knee stress
3. Shorter cranks allow better aerodynamic positioning without sacrificing power.

And, in general, we are talking substantially shorter than what most would consider to be a short crank. Our data suggests that around 100 mm crank length would be near optimum for most. I am currently riding 105 mm cranks with good feeling.

I believe there are good reasons that explain the above benefits and we can discuss them if any desire.

If possible I would like to open an actual discussion of this issue based on facts and data rather than bias and opinion.


What the huh??? 100 - 105 mm cranks? I'm 6'0" with 170 mm road cranks. Who, or what, would use 100 mm?
User avatar on3m@n@rmy
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03 Aug 2011 05:55

measure from top of femure to ground and devide it by 18.5% and see what you get I am just 6'2" and just get into 175 cranks. if you climb with cadence use the next size down otherwise like me hump big gears up hills use the closest. fit.

Its not the power you turn up getting out of a corner its how much power that you can get away with.
So if you get out of every corner with less power and still stay on the bunch it is better than humping big gears and long cranks useing more power than needed.
in theory you should end up with more energy at the end of a race.
brianf7
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03 Aug 2011 05:56

Tapeworm wrote:Is the data you have applicable for all disciplines of cycling? Any variation for cycling modes that do not require sustained power ie: some forms of xc mtb, downhill or bmx?

Every kind of racing requires some kind of sustained power, the only difference is how long is it sustained. Anyhow, I think this "rule" holds for all kinds of cycling, the only issue is how big a deal is it. Riders who ride primarily upright, such as mountain bikers, will probably see a smaller variation than those who are spending most of their time in the aero position (triathletes). But, I think Martin's data was accumulated in riders riding mostly upright and they maximized at 145mm. Even if there was no power advantage to going shorter to a mountain biker, if there was no power disadvantage the mountain biker would see greater ground clearance, another advantage of shorter cranks.
FrankDay
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03 Aug 2011 05:57

TERMINATOR wrote:I've studied a book that spends a few chapters on crank arm length by Ed Burke called The Science of Cycling and your understanding of crank arm length is completely flawed.

The reason why crank arms are longer than you think they should be is because longer crank arms allow for LESS power (i.e. less effort) to result in the same speed. The goal is to use less power to generate the same cadence.

The study you cited merely indicates that 145mm crank arm length is the ideal length for generating maximum power for ONE rotation. That has nothing to do with your goal as a road cyclist. Longer crank arms are better because you can generate more torque and more power based on less effort than you could with a shorter crank arm length.

Sure a shorter crank arm length might allow for a rider to generate more power in 1 rotation (which is why track riders generally use crank arms that are 10mm shorter than they would on the road), but that won't help you on hills. Shorter crank arms are a severe hindrance when climbing since longer crank arms will allow you to generate more torque with less effort.

You don't seem to understand that longer crank arms are always better and the only reason riders can't go bigger and bigger is because at some critical point, the circle they spin becomes too large.

You're looking at maximum power for 1 rotation instead of what you should be looking at: highest speed based on least amount of effort (i.e. power efficiency) for a 2 hour ride. Longer crank arms are far more efficient than shorter crank arms for that task.


While it is tough to credit Mr. Burke for acknowledgement (based on other "sporting contributions") the point is exactly that: apply less power via fitness and cadence and fatigue goes down. The aero advantages are always defeated by the ergonomic factors unless the effort is so short as to make the duration of power insignificant. It is also directly related to the individual rider's physiological advantages and challenges, ie; some tall and thin riders can spin well and apply adequate big-gear power for short durations. They like long cranks for both situations. Shorter riders that have much low rpm power may like the same length for the same reason. Gotta match the length to the rider and the task.
Oldman
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03 Aug 2011 06:09

egtalbot wrote:If you're looking for constructive criticism, a couple of quick thoughts:

1."There is a clear trend in the data and there is a big difference between racing significance and "statistical significance". It also seemed obvious that if there had been more participants that the 145 mm cranks superiority would have eventually reached statistical significance."

I don't think you get what statistical significance means. The fact that is wasn't statistically significant due to (among other things) small sample size means that if you added more people, statistically there is a reasonable chance that rather than eventually confirm the results, it would have actually changed them. If something isn't statistically significant, it means the study could not adequately conclude it. You can't just look at the results and assume they'd stay the same with more riders. If you could - then it would already be statistically significant!

I understand exactly what statistical significance means. I simply looked at the sigma of the data scatter of Martin's data and it is clear that the trend holds whether you are looking at the mean or the entire range. With that kind of presentation it is not unreasonable to infer that additional data would follow the same pattern, although the reason the experiment needs to be repeated with larger numbers is to see if this inference is valid. That was the point. Looking at that data it is reasonable to infer that there is likely a real difference if the number of subjects in the study had been increased.
2.You have plenty of very interesting and perhaps accurate observations about why shorter crank lengths could be more effective. I don't feel qualified to evaluate those. However, you have no science behind your theories, just ideas - as I suggested, your conclusions from the Martin study are pretty much the opposite of the scientific approach. What you have here is the starting point for more study. Which I would certainly encourage you to do.

Thanks. The fact that the science doesn't exist yet to validate my hypothesis is not a particularly good reason to not act on this data. It is easy enough for each athlete to test this for themselves to see if there is anything to it for them. We have yet to have an athlete test this without such a finding, although they all have not gone as short as I have.
FrankDay
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03 Aug 2011 06:28

TERMINATOR wrote:I've studied a book that spends a few chapters on crank arm length by Ed Burke called The Science of Cycling and your understanding of crank arm length is completely flawed.

The reason why crank arms are longer than you think they should be is because longer crank arms allow for LESS power (i.e. less effort) to result in the same speed. The goal is to use less power to generate the same cadence.

I think you are the one who doesn't understand. Longer cranks allow less force on the pedal to generate the same power. The reason this works is because the pedal speed is faster with longer cranks - power is force exerted through a distance per unit time. The reason shorter cranks might allow more force to be applied without causing more energy to be expended is because at the same cadence, the pedals are moving slower so it is easier to apply more force against them. If Burke has some science to support his arguments (as opposed to some theoretical arguments) as to why longer cranks are better I am all up for hearing about it.
The study you cited merely indicates that 145mm crank arm length is the ideal length for generating maximum power for ONE rotation. That has nothing to do with your goal as a road cyclist. Longer crank arms are better because you can generate more torque and more power based on less effort than you could with a shorter crank arm length.

That is a great argument for the penny-farthing type bicycle but it doesn't work for today's bicycle because the torque that matters is the torque delivered to the wheel, not the torque delivered to the bottom bracket axle. there are additional leverage points between the two so this can be easily modified by choice of gearing and wheel size. And, as I point out in the article, the force that can be delivered to the pedal is also modified by the leverage of the knee joint, which decreases as the joint angle increases much beyond 60º. Longer cranks force the knee to bend more at TDC than shorter cranks. Analysis of this issue requires more than simply looking at the leverage of the crank arm.
[/QUOTE]
Sure a shorter crank arm length might allow for a rider to generate more power in 1 rotation (which is why track riders generally use crank arms that are 10mm shorter than they would on the road), but that won't help you on hills. Shorter crank arms are a severe hindrance when climbing since longer crank arms will allow you to generate more torque with less effort.[/QUOTE]
Power is power. climbing rate is totally dependent upon power. You will climb best on the crank length that lets you generate the most power, whatever that is.

You don't seem to understand that longer crank arms are always better and the only reason riders can't go bigger and bigger is because at some critical point, the circle they spin becomes too large.

show me the data that supports this argument.
You're looking at maximum power for 1 rotation instead of what you should be looking at: highest speed based on least amount of effort (i.e. power efficiency) for a 2 hour ride. Longer crank arms are far more efficient than shorter crank arms for that task.

Show me the data. Our preliminary data does not support your statement.
FrankDay
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03 Aug 2011 06:30

on3m@n@rmy wrote:What the huh??? 100 - 105 mm cranks? I'm 6'0" with 170 mm road cranks. Who, or what, would use 100 mm?

Well, I am 6' 2" and my testing suggests that 105 (or thereabouts) is optimum for me. What testing have you done to support your choice?
FrankDay
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03 Aug 2011 06:33

Oldman wrote: Gotta match the length to the rider and the task.

How do you do that matching without testing?
FrankDay
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03 Aug 2011 06:59

Apologies, i am not that technical, so to confirm if I have understood

what you are saying is that a rider can reduce the size of their cranks then this means they can lower their saddle height from the ground (so the max distance from foot to saddle remains the same). However because of the shorter cranks the min distance from foot to saddle is increased. this would allow the rider to reduce the front of the bike even more because they have less of a restriction caused by when their foot is at the top of the stroke.

however people have not done this due to the impression that reducing the cranks would reduce the power. but your data implies this is not the case and shorter cranks can produce the same power.

the thing i do not understand is why no one has tried this before. Without understanding the science it seems with the amount of effort teams are now putting into product development and aerodynamic positioning this seems a real logical point to attack. what is restricting me from lowering my position more? leg at top of stroke. how can we reduce that. shorten cranks. no loss of power - winner. or if there is a loss of power then is the aero gain greater than the power loss. is it possible there is another reason (back strain, balance etc) that is a greater restriction than the leg spacing that would prevent the aero gains made from reducing the cranks.

seems so obvious a thing to experiment with
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03 Aug 2011 08:56

I'm not a scientist so the following is only a thought and not a theory. Does using longer cranks increase the distance over which a rider can exert power on the pedals? Obviously the pedals will travel further with a radius of say 180 mms, than with 170 mms, but I assume the `dead-spot' might be greater with the 180s. My limited reading of biomechanics books (now about 30 years out of date) suggested that in many sports athletes were looking to increase the distance over which they could apply power to an object (think of a golf swing), is this relevant to cycling?

For the record I'm 6' 1" and moved to 180 cranks 25 years ago, I wouldn't change back. I tend to spin, and won a medium gear 25 (72 inches, or about 42x16) on 180 mmm cranks years ago.
Hawkwood
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03 Aug 2011 09:07

daveinzambia wrote:what is restricting me from lowering my position more?

There is a legal limit to how low the bars can be.

Shortening of crank length generally requires a raising of saddle height (and vice versa).
User avatar Alex Simmons/RST
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03 Aug 2011 09:51

It would be interesting to see data for Indurain. I've read that Campagnolo produced special 190 mm cranks for him for time-trials, but that his position wasn't that aerodynamic.
Hawkwood
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03 Aug 2011 10:32

Alex Simmons/RST wrote:There is a legal limit to how low the bars can be.

Shortening of crank length generally requires a raising of saddle height (and vice versa).


You are right.The saddle height will go up as the diameter decreases.To realize an aero benefit,the frame geometry would have to change to lower the BB.
With such a redesigned frame, maybe you could raise your bars and adopt the superman position while staying within the current UCI rules.
simo1733
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03 Aug 2011 14:31

daveinzambia wrote:Apologies, i am not that technical, so to confirm if I have understood

what you are saying is that a rider can reduce the size of their cranks then this means they can lower their saddle height from the ground (so the max distance from foot to saddle remains the same). However because of the shorter cranks the min distance from foot to saddle is increased. this would allow the rider to reduce the front of the bike even more because they have less of a restriction caused by when their foot is at the top of the stroke.
When cranks are shortened the saddle must be raised to keep the "bike fit" at BDC the same as before. But, shorter cranks means the knee is lower at TDC which means it is further away from the chest (because the handlebars do not change). Because there is less "cramping" now at the top the rider can consider lowering the front even more.
however people have not done this due to the impression that reducing the cranks would reduce the power. but your data implies this is not the case and shorter cranks can produce the same power.

the same or more power, at least to a point. There has to be a point where power will start to drop (it is impossible to generate any power with a crank length of zero). Perhaps it is this fact that power has to start dropping at some length that has caused people to think that will always drop at any length if one goes shorter.
the thing i do not understand is why no one has tried this before. Without understanding the science it seems with the amount of effort teams are now putting into product development and aerodynamic positioning this seems a real logical point to attack. what is restricting me from lowering my position more? leg at top of stroke. how can we reduce that. shorten cranks. no loss of power - winner. or if there is a loss of power then is the aero gain greater than the power loss. is it possible there is another reason (back strain, balance etc) that is a greater restriction than the leg spacing that would prevent the aero gains made from reducing the cranks.

seems so obvious a thing to experiment with

Yes it does which is why I tried to explain in my "essay" why people haven't done so. I guess it is just one of those things that everyone "knows" to be true such that it would be a waste of time to do the experiment.
FrankDay
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