The importance of crank length to the cyclist.

[CoachFergie]

I thought the question was pretty straight forward but I guess not so, I guess, you could start with this simple question: How will the drag change as the crank length (and associated seat height), and nothing else - including handlebar height, changes. To put it another way, how will the drag of the rider change as the height of the head changes compared to the height of the rear end. Now, I intend to ask some other questions also but I think that is enough for this thread right now.

So keeping handlebar height constant you confound any potential result from changing crank length. Keep up the good work Frank

 

[FrankDay]

A change in drag. But because each change in crank length requires a change in seat and handlebar position the study will not show data will not show anything conclusive.

Have you actually read or understood anything I have written in this thread? When one shortens the cranks and raises the seat it is not necessary nor desirable to also raise the handlebars also. In fact, most who try this are able to lower their handlebars, but I will not be testing that. Using your scenario, if one were to actually raise the handlebars as one raise the seat I would predict no improvement in aerodynamics and would not be wasting my time. Why you think this would result in a change in drag is a mystery to me, but I hardly ever understand anything you say. Anyhow, under my conditions (not yours) I am predicting there will be a change, and I want something more than my thought experiment upon which to give recommendations to those who ask.

 

[FrankDay]

So keeping handlebar height constant you confound any potential result from changing crank length. Keep up the good work Frank

It may be confounding to you but it is not confounding in the scientific sense if that is what you are actually trying to measure. LOL

 

[CoachFergie]

Have you actually read or understood anything I have written in this thread? When one shortens the cranks and raises the seat it is not necessary nor desirable to also raise the handlebars also. In fact, most who try this are able to lower their handlebars, but I will not be testing that. Using your scenario, if one were to actually raise the handlebars as one raise the seat I would predict no improvement in aerodynamics and would not be wasting my time. Why you think this would result in a change in drag is a mystery to me, but I hardly ever understand anything you say. Anyhow, under my conditions (not yours) I am predicting there will be a change, and I want something more than my thought experiment upon which to give recommendations to those who ask.

All you are doing is wasting your money to prove something we have known and observed and had the data for for years. Will show you nothing about crank length.

 

[Boeing]

You are wrong. I am right.

And Fargo, Dude Pot Kettle Black

Or to paraphrase your entire contribution to this thread

"My Personal consensus of opinion is that the basic fundamentals of pedaling in a circle and the push of the lever from my actual experience on a regular regular basis and repetitive routine as I commute back and forth, never at any time does a short crank aid in the empty space in the round cadence estimated roughly forward in a path that is straight as pi$$ with the (end) result each and every trial in cold and hot temperature at close proximity we are presented with a difficult dilemma of which Frank Day is selling cranks at a cheap price for his own self interest where as I teach you how I coach over achievers and did I tell you I coach."

please see my sig line

 

[FrankDay]

All you are doing is wasting your money to prove something we have known and observed and had the data for for years. Will show you nothing about crank length.

Well, if you have known about and had this data for years it would seem like a simple thing for you to do to put forth your predictions and demonstrate to the multitudes you really are as smart as you think you are. What will be the shape of the curve we see and what will be the magnitude of the change seen? In fact, it would have seemed even more reasonable for you to have presented that data and knowledge you have had for years 1100 posts ago, but you didn't, why not?

 

[elapid]

All you are doing is wasting your money to prove something we have known and observed and had the data for for years. Will show you nothing about crank length.

How about being constructive? To give Frank Day his due, at least he is willing to do the research and present the data ... data that you keep on asking for. You'd rather shoot him down than suggest an experimental model which would provide the data to your satisfaction.

 

[FrankDay]

Can't remember if this was posted before or not but here is some actual data that someone else has done looking at the effects of crank length on aerodynamics. It does appear they kept handlebar height constant, as I intend to do.

Here is the chart from this data done with 4 pro riders by Cervelo.

At least now we have something to compare my data to. Of course, they only looked at 5mm changes in 3 of those riders and 7.5 mm in the other. I suspect these changes are so small as to make "noise" a real problem in interpretation. I, of course, am going to be looking at much larger changes such that if there is a real change I should be able to pick it up. Along these lines, I just picked up what I am sure are the worlds smallest cranks so I can test the extremes. these cranks are so short I am quite certain I won't be able to generate any power on them. But, this is not a power test but an aerodynamics test so it doesn't matter. I will put them on this weekend and see what I can do.

 

[CoachFergie]

How about being constructive? To give Frank Day his due, at least he is willing to do the research and present the data ... data that you keep on asking for. You'd rather shoot him down than suggest an experimental model which would provide the data to your satisfaction.

If you think this is real research then you are not the academic you think you are.

 

[elapid]

If you think this is real research then you are not the academic you think you are.

There you go again. This is not about me. This is about you and Frank Day perpetuating this thread like two silly kids. As per usual, you want data. Frank Day offers data and you squash his design and methods. One way to stop this stupid bickering merry-go-round is for you to come up with an experimental model that would provide the data to your satisfaction. That's it.

 

[CoachFergie]

There you go again. This is not about me. This is about you and Frank Day perpetuating this thread like two silly kids. As per usual, you want data. Frank Day offers data and you squash his design and methods. One way to stop this stupid bickering merry-go-round is for you to come up with an experimental model that would provide the data to your satisfaction. That's it.

Well, no data yet.

But someone manipulating the data to suit his purposes is not good science. I have already outlined the process many professional teams use testing aerodynamics in the tunnel, testing on the track and then on the road to ensure the aerodynamic positions attained are ones the riders can actually use in competition.

 

[elapid]

Well, no data yet.

But someone manipulating the data to suit his purposes is not good science. I have already outlined the process many professional teams use testing aerodynamics in the tunnel, testing on the track and then on the road to ensure the aerodynamic positions attained are ones the riders can actually use in competition.

I doubt Frank has the finances of a professional or national team. So can you come up with an experimental design that will fit his budget, say for argument's sake wind tunnel testing only, and satisfy your need for data?

I am not being ornary or confrontational or snide or anything else. I would just like to see this thread put to rest one way or the other. If you do not come up with an experimental design and/or Frank does not go through with the experiment or provide the raw data, then this thread will continue chasing its tail ad nauseum.

 

[coapman]

Well, no data yet.

But someone manipulating the data to suit his purposes is not good science. I have already outlined the process many professional teams use testing aerodynamics in the tunnel, testing on the track and then on the road to ensure the aerodynamic positions attained are ones the riders can actually use in competition.

I read an explanation on wind tunnel testing, it works by using the wind to force the bike/rider backward , does it take into consideration the suction created by the reduced pressure behind a rider when travelling at speed.

 

[CoachFergie]

I doubt Frank has the finances of a professional or national team. So can you come up with an experimental design that will fit his budget, say for argument's sake wind tunnel testing only, and satisfy your need for data?

I am not being ornary or confrontational or snide or anything else. I would just like to see this thread put to rest one way or the other. If you do not come up with an experimental design and/or Frank does not go through with the experiment or provide the raw data, then this thread will continue chasing its tail ad nauseum.

Frank is going to prove what he wants to prove but this result will have no meaningful influence on performance.

The research on the crank length in relation to performance has been done and the research on aerodynamics has been done. The only person disputing the unimportance of crank length on cycling performance is the person marketing short cranks.

 

[CoachFergie]

I read an explanation on wind tunnel testing, it works by using the wind to force the bike/rider backward , does it take into consideration the suction created by the reduced pressure behind a rider when travelling at speed.

I'm on the road so away from my journal articles but I suspect this is why the positioning and equipment selection process should be based on more than just changes in drag from the wind tunnel.

 

[CoachFergie]

I read an explanation on wind tunnel testing, it works by using the wind to force the bike/rider backward , does it take into consideration the suction created by the reduced pressure behind a rider when travelling at speed.

I'm on the road so away from my journal articles but suspect this is why we make our decisions on position, equipment selection and aerodynamics from more than just wind tunnel data.

 

[Night Rider]

Hey Fergie, In a few weeks I have some wind tunnel time in which I am going to measure the effects of crank length alone on aerodynamic drag. Why don't you tell us what you predict we will find? Assuming there is an aerodynamic benefit to going to shorter cranks, does it ever stop? What will the shape of the curve look like as we go from 170 shorter, keeping everything else (except seat height) the same? If there is an affect, how big will the maximum effect be? Those are the main questions I am going to try to answer. Give us your predictions now.

Others, of course, are welcome to place their guesses now. If there is enough interest maybe I should come up with a prize. If we get enough guesses, I will come up with something.

Are you measuring power at the same time?
Are you using road bikes (that is what 90% of us come to this forum for).
What is the testing protocol?
When taking photos or video please capture full shots, not cropped from knees or thighs up (like your other article).

Once you report on all that I will place my prediction.

 

[sciguy]

Hey Fergie, In a few weeks I have some wind tunnel time in which I am going to measure the effects of crank length alone on aerodynamic drag. Why don't you tell us what you predict we will find? Assuming there is an aerodynamic benefit to going to shorter cranks, does it ever stop? What will the shape of the curve look like as we go from 170 shorter, keeping everything else (except seat height) the same? If there is an affect, how big will the maximum effect be? Those are the main questions I am going to try to answer. Give us your predictions now.

Others, of course, are welcome to place their guesses now. If there is enough interest maybe I should come up with a prize. If we get enough guesses, I will come up with something.

Frank,

It would seem to me that the results of your wind tunnel investigation will in large part be a function of the rider's initial position while seated with the 170mm cranks. If he/she is positioned resembling a barn door, any reduction of handlebar to seat height will show significant aero improvement. If on the other hand, he/she is initially positioned with a nice flat back, further lowering of relative handlebar to seat height will likely show no decrease and perhaps an increase in drag.

One aero negative created by the use of shorter cranks is an increase in the exposed average leg length at any given moment. With each crank length the fully extended leg would always expose a similar amount of area but the less retracted upper leg will expose more area when using shorter cranks. This may well represent a non-trivial addition to drag.

So my expectation is that if you start with a 170mm crank rider with a nice horizontal back, then drag will actually increase sightly with shorter cranks.

YMMV,

Hugh

 

[blutto]

If you think this is real research then you are not the academic you think you are.

...well, to be completely honest about it your performance here shows that you are not much of an academic either...

...and no, just getting thru school, as you purportedly have done, does not atuomatically confer the academic label...

...fact is, by wrapping yourself in the flag of academia, you are an embarrassment to true academics...

...so sorry to intrude..carry on with your witch hunt ( and btw that low rumble you hear in the background is the late great Arthur Lydiard rolling in his grave...which he does just about everytime you unleash your petty rantling on this thread )...

Cheers

blutto

 

[FrankDay]

Frank,

It would seem to me that the results of your wind tunnel investigation will in large part be a function of the rider's initial position while seated with the 170mm cranks. If he/she is positioned resembling a barn door, any reduction of handlebar to seat height will show significant aero improvement. If on the other hand, he/she is initially positioned with a nice flat back, further lowering of relative handlebar to seat height will likely show no decrease and perhaps an increase in drag.

One aero negative created by the use of shorter cranks is an increase in the exposed average leg length at any given moment. With each crank length the fully extended leg would always expose a similar amount of area but the less retracted upper leg will expose more area when using shorter cranks. This may well represent a non-trivial addition to drag.

So my expectation is that if you start with a 170mm crank rider with a nice horizontal back, then drag will actually increase sightly with shorter cranks.

YMMV,

Hugh

My goal is to run the gamut from what a typical age-grouper's position looks like to going beyond what the best pro's position looks like. My goal is to get some numbers to tell people what might be gained if they take the effort to get lower regardless of where they are starting whether they be a beginner or a pro. There has to be a point where going lower actually will increase wind resistance (for instance, I suspect if we could get so low that the head is between the knees frontal area would be higher than when it is at a minimum and, of course, shape makes a difference also) but I don't know that. If we find out that dropping the head 2 inches further beyond a flat back results in a 5% drop in resistance it might be worth it for someone to make that effort. If we find that such a change only results in a 0.25% drop in resistance (or no change) then it probably wouldn't be worth the effort. The problem that I see is that no one knows what might be expected but it is important to know now that I think it would be "easy" to get lower without compromising much power by going to shorter cranks.

Also, along these lines, I was totally impressed with the position of Tom Boonen in today's P-R bike race which he dominated. His position was the equal of the best TT riders even though he was not on a TT bike. Those chasing him, without much effect, tended to be much higher. To those who think that aerodynamics are not important for ordinary bicycle races I think Tom just put the lie to that argument today.

 

[FrankDay]

Are you measuring power at the same time?

No, I don't care about power for this test. The rider will be pedaling but we will not be testing power, which is typical for wind tunnel testing. I am only looking at rider aerodynamics. Once we establish what the aerodynamics of various positions are any rider can test their sustainable power in those positions outside of the wind tunnel which should allow them to assess what is best for them. It is not perfect but it is more information than they have now.

Are you using road bikes (that is what 90% of us come to this forum for).
What is the testing protocol?

I only have 2 hours. We are using a bike that will allow us to assess the full range of positions and rapid changing of components. The bike will not have any cables or brakes and I expect to be able to change crank length in less than 2 minutes. I am expecting to be able to test about a 9-10 inch difference in seat to handlebar drop. I also hope to get both road bike positions and aerobar positions. Again, we have a limited amount of time but I hope to get one run with just the bike alone so those of you concerned about this can assess the effects of the bike as it modifies the results. Again, I am primarily interested in assessing the effects of rider position on drag, generally agreed upon as the major component of drag in bicycle riders. This means the bike being used is not important.

When taking photos or video please capture full shots, not cropped from knees or thighs up (like your other article).

I will do my best. Not sure what the arrangement is.

Once you report on all that I will place my prediction.

Looking forward to it.

 

[FrankDay]

One aero negative created by the use of shorter cranks is an increase in the exposed average leg length at any given moment. With each crank length the fully extended leg would always expose a similar amount of area but the less retracted upper leg will expose more area when using shorter cranks. This may well represent a non-trivial addition to drag.

So my expectation is that if you start with a 170mm crank rider with a nice horizontal back, then drag will actually increase sightly with shorter cranks.

YMMV,

Hugh

You are wrong about increasing the exposed average leg length. The reason is that moving the saddle up moves more of the upper part of the thighs into the wind shadow of the shoulders. So, even though one foot is lower at TDC, an supposedly increasing exposed area, the total frontal area exposed actually decreases. Only if the rear gets above the highest part of the back does raising the seat increase frontal area, as I see it.

The other issue, where I think you are wrong, is the "nice horizontal back" being the best position. That might be true if the rider didn't have to look where they were going. If looking forward makes the head the highers part of the body (the usual case) then the head is increasing frontal area. I see that there is still something to be gained by going lower until the top of the head is below the top of the back. At this point I think further improvements from going lower will be small, if any - at least that is what I predict. One question I have here though is does an aero helmet do much good when the head is not sticking up? I hope to test this also. If an aero helmet does little good in this position it might be more beneficial for the rider to use a standard helmet to emphasize cooling rather than an "aero" helmet that adds little or nothing to speed but hurts cooling.

 

[sciguy]

You are wrong about increasing the exposed average leg length. The reason is that moving the saddle up moves more of the upper part of the thighs into the wind shadow of the shoulders. So, even though one foot is lower at TDC, an supposedly increasing exposed area, the total frontal area exposed actually decreases. Only if the rear gets above the highest part of the back does raising the seat increase frontal area, as I see it.

The other issue, where I think you are wrong, is the "nice horizontal back" being the best position. That might be true if the rider didn't have to look where they were going. If looking forward makes the head the highers part of the body (the usual case) then the head is increasing frontal area. I see that there is still something to be gained by going lower until the top of the head is below the top of the back. At this point I think further improvements from going lower will be small, if any - at least that is what I predict. One question I have here though is does an aero helmet do much good when the head is not sticking up? I hope to test this also. If an aero helmet does little good in this position it might be more beneficial for the rider to use a standard helmet to emphasize cooling rather than an "aero" helmet that adds little or nothing to speed but hurts cooling.[/QUOTE

Frank are you really positive that I'm wrong on the first one? Would you please post a side view of the upper thigh of a short crank user being in the wind shadow created by the shoulders with the rider in a sustainable position where they can see up the road for at least on hour if not several.
You seem to have forgotten that one of your reasons for advocating the shorter cranks is so that the rider isn't so scrunched up in a tight ball IE the raised thigh is more extended. Can't have it both ways here....

Interesting to be pronounced wrong before the test has been run. No bias at work here

Hugh

 

[FrankDay]

Frank are you really positive that I'm wrong on the first one? Would you please post a side view of the upper thigh of a short crank user being in the wind shadow created by the shoulders with the rider in a sustainable position where they can see up the road for at least on hour if not several.
You seem to have forgotten that one of your reasons for advocating the shorter cranks is so that the rider isn't so scrunched up in a tight ball IE the raised thigh is more extended. Can't have it both ways here....

Interesting to be pronounced wrong before the test has been run. No bias at work here

Hugh

Well, in the video I did and linked to in the first post of this thread I used an animation program I have to demonstrate how the legs move into the wind shadow of the shoulders. And, I could also do so with the pictures that I took for my "aerodynamic musings" essay but didn't use (if I have some time I will try to do that). The issue is the shape of the legs. If the seat is moved up, something back there is moving into the wind shadow of the shoulders which do not move. If one is really high, it is the pelvis, as one gets lower in front it is the "exposed" part of the thighs and the part that moves into the wind shadow is the widest part of the leg. So, even though the upper leg is somewhat "more exposed" because the foot moves down the total area exposed is less because the part moving down is the lower leg and ankle, the narrowest part of the leg, and the part moving up into the wind shadow, is the thigh, the widest part of the leg. Since the amount of movement up and down is the same, more frontal area moves into the wind shadow than moves out so the total area exposed to the wind has to be reduced.

I am not declaring your overall prediction to be wrong, I simply declared your statement that the total exposed area of the leg remained the same to be wrong. I was simply giving you my reasons as to why my prediction is going to be somewhat different from yours. If your prediction is more correct then I will have to revise my thoughts as to what is going on because shape will at some point become more important than frontal area.

 

[sciguy]

Well, in the video I did and linked to in the first post of this thread I used an animation program I have to demonstrate how the legs move into the wind shadow of the shoulders. And, I could also do so with the pictures that I took for my "aerodynamic musings" essay but didn't use (if I have some time I will try to do that). The issue is the shape of the legs. If the seat is moved up, something back there is moving into the wind shadow of the shoulders which do not move. If one is really high, it is the pelvis, as one gets lower in front it is the "exposed" part of the thighs and the part that moves into the wind shadow is the widest part of the leg. So, even though the upper leg is somewhat "more exposed" because the foot moves down the total area exposed is less because the part moving down is the lower leg and ankle, the narrowest part of the leg, and the part moving up into the wind shadow, is the thigh, the widest part of the leg. Since the amount of movement up and down is the same, more frontal area moves into the wind shadow than moves out so the total area exposed to the wind has to be reduced.

I am not declaring your overall prediction to be wrong, I simply declared your statement that the total exposed area of the leg remained the same to be wrong. I was simply giving you my reasons as to why my prediction is going to be somewhat different from yours. If your prediction is more correct then I will have to revise my thoughts as to what is going on because shape will at some point become more important than frontal area.

OK, I went back and looked at the stick figure again. When you say wind shadow of shoulders it would appear you really mean to say upper arms at least in regards to that figure. Interestingly you very distinctly say and label "Lowering the knee here" to show the top of the stroke.
'
So let's look at this a sightly different way. Say we start with someone riding 170mm cranks with a nice horizontal back and compare their frontal area to someone riding 105mm cranks also with a nice horizontal back. Who has a larger frontal area? I think the one with the more extended raised leg will so long as each ride has a horizontal back. Now at this point you're going to pipe in that the rider with the 105mm cranks can lower their bars even farther while I would contend that unless they can see out the top of their skull this is an unridable position of any length of time. Perhaps the real question here should be what is the more important limiter in time trial position, how compacted the rider's hip angle is or their ability to use their neck to see down the road safely.

I see merit in doing wind tunnel testing on this but it will be very important to access the results in regards to whether the the positions tested can truly be ridden for significant lengths of time.

YMMV,

Hugh