The importance of crank length to the cyclist.

[FrankDay]

Thanks for the excellent data set!!!!!! Wasn't there some guy that predicted the outcome on this thread earlier? Hmmmmmm Didn't he say mean leg length exposed will increase with shorter cranks and increase overall drag?

In addition, a quick view of the 180mm position shows the rider in what many would fell to be a very ridable position with regular cranks where one doesn't need to fight the cranks "to get them over the top" as one does with PCs.

YMMV,

Hugh

Yes, you did predict this outcome of sorts but I disagreed with your analysis that it was due to increased exposure. Looking at the average exposure of the two legs actual direct frontal area exposure has to decrease with shorter cranks but it seems the effective frontal area seems to increase (at least that is the only way I can explain this data).

While the 180 position looks very rideable I can assure you it was not. Mike might come on here and explain more what he felt but we could watch him and after a few minutes he could barely keep his legs synchronized. We would have never known about this difficulty on regular cranks. To say he wouldn't have to fight the cranks to get them over the top ignores the fact that it takes energy to push them over the top if one cannot do it themselves, the point of PowerCranks. If a rider cannot generate reasonable power or have reasonable endurance in any particular position it it pretty much worthless for racing, regardless of what the drag numbers say.

That was the concern of the coach for the rider before us. Len was able to reduce his drag numbers by lowering his front but the coach was concerned as to what this might to to his power. It is the best balance of power and aerodynamics that should result in the best racing.

 

[Martin318is]

Let the comments and criticisms begin!

Just wanted to say that I am really glad you have gone to the effort of doing this testing (and being so clear in stating its known limitations up front) and sharing the info with us.

And quite happy to see that your new attitude to crank length matches my own: "It depends!"

Would you agree that it would be very interesting in the future to add some further constraints to the set up for the testing? I was thinking something like: Pre-ride each of the positions on trainers and track the heartrate and power the test rider is able to ride at for say 20mins. To be included in the wind-tunnel tests, each position has to stay within a set of HR and power ranges. That way you would be testing the aerodynamic effects of the positions while knowing that they were rideable and not sacrificing power or efficiency...

(Note, I am not at all criticising what you have done, just thinking randomly about future test protocols if someone else has the time and money to do it... Especially as to baseline power and HR would take several days so that fatigue is not affecting the testing.)

What do you think?

 

[sciguy]

Here is a link to a video taken by Mikes wife in the wind tunnel with Mike on 85 mm cranks and the flat stem. It is on facebook so I hope he has set access so everyone can see it. Looking at that I would have expected if we could get the head a little lower we would save quite a bit. Perhaps so if we had lowered a little but lowering the head a lot didn't do much. Dropping the stem 4 inches from this position gave us a savings of 11 gm from a total of about 3000.

The video is not viewable for me ........... probably need to be one of Mike's facebook friends to get to the page.

Hugh

 

[sciguy]

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

Just to be clear as to what I had predicted

 

[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.

I hate to nit pick but when one goes to shorter cranks without changing handlebar height it is not correct to say "the fully extended leg would always expose a similar amount of area" because more of the upper part of the leg will move into the wind shadow of the arms/torso, at least if the rider is in any reasonable aero position. The upper part of the other now less retracted leg will also move more into that wind shadow. The overall effect is less exposure to the wind and I predicted a drop in resistance for what should be an increased self drafting effect.

I agree you pretty much predicted the actual outcome but I would say you got the mechanism wrong as I analyze the situation. It seems to me that, because of the close proximity of the two legs, the closing of the gap between the feet causes an increased interaction of the two legs together with the wind resulting in what appears to be an increase in frontal area when there is an actual decrease. Shape also is important when it comes to aerodynamics.

 

[TarmacExpert]

So, here are the raw results (drag in grams, not a force number but what they gave us).

180 road helmet 3095 grams
180 aero helmet 2947 and 2991 grams (two runs)
142 aero helmet 2949 grams
85 road helmet 2984 grams
85 aero helmet 2973 grams
85 aero helmet, bars down additional 4 inches 2962 grams
85 aero helmet, bars down 4 inches and angled 45º 3028 grams
0 aero helmet, bars down 4 inches and angled 45º 3062 grams

Thanks for sharing the data, Frank. What I find really interesting from that, together with the photos, is that there was only 148g of drag difference between best and worst. When I have compared power vs speed for different riders in TTs I've taken part in, I have found up to 100W difference in power required to ride at the same speed, even for riders within a 10% weight range who visually all appear to have pretty good positions. If the magnitude of position variation that you looked at only changed the drag by 148g, it seems the aero differences between riders must largely be down to inherent morphological factors.

If you ever had the chance to do more testing, what I would see as very interesting is quantifying sciguy's point about the change in thigh angle. So I would first of all establish the most aero saddle to pad drop. Then change the crank length, moving the saddle up or down as appropriate, but always also moving the bars up or down by the same amount as the saddle. It would be very interesting to quantify how much more aero longer cranks might be with saddle to pad drop held constant.

 

[FrankDay]

Thanks for sharing the data, Frank. What I find really interesting from that, together with the photos, is that there was only 148g of drag difference between best and worst. When I have compared power vs speed for different riders in TTs I've taken part in, I have found up to 100W difference in power required to ride at the same speed, even for riders within a 10% weight range who visually all appear to have pretty good positions. If the magnitude of position variation that you looked at only changed the drag by 148g, it seems the aero differences between riders must largely be down to inherent morphological factors.

If you ever had the chance to do more testing, what I would see as very interesting is quantifying sciguy's point about the change in thigh angle. So I would first of all establish the most aero saddle to pad drop. Then change the crank length, moving the saddle up or down as appropriate, but always also moving the bars up or down by the same amount as the saddle. It would be very interesting to quantify how much more aero longer cranks might be with saddle to pad drop held constant.

We didn't get any drag numbers until we were halfway through the testing. I can tell you we were blown away by the failure to show any change despite the huge position changes. It really took me awhile to be able to wrap my head around it and come up with a mechanism that could explain our findings. Like I said earlier, I believe this explanation can also explain the Cervelo data in which one rider saw a decrease, one an increase, and two no change in drag with shorter cranks. We might be able to predict this if the starting positions were quite different. And, as you indicate, rider morphology may also be important.

That having been said, we made no effort to optimize the drag at each of these positions and there may be more to this than this data suggests. For instance, we do find that shortening the cranks generally allows riders to lower their front end also without losing power. I think for us to really understand what is the true potential of this change we need to look at how power changes with position, power changes with crank length (at any given position), and how the position can be tweaked with crank length.

Taking an unsophisticated analysis of this data (and ignoring the context that one position was "unridable") one might conclude that position doesn't matter at all. We all know that to be rubbish. I am really upset that we didn't have a more realistic 180 position for this rider as that unrealistically low number will mislead a lot of people. However, this data does suggest that once one has reached a certain position, that further changes may make little difference in aerodynamics and that then the rider should concentrate on power and comfort.

I would like to go back and investigate some of these nuances a little more. They are offering another opportunity in the fall and I will see if I can fit it in. Ideas as to how to design the time spent for the best benefit would be welcomed.

 

[sciguy]

I am really upset that we didn't have a more realistic 180 position for this rider as that unrealistically low number will mislead a lot of people.

Frank,

How in the world can you call this an unrealistic position? Perhaps for someone riding Power Cranks it is but for anyone on conventional cranks it's just not very radical.

 

[FrankDay]

Frank,

How in the world can you call this an unrealistic position? Perhaps for someone riding Power Cranks it is but for anyone on conventional cranks it's just not very radical.

You can't tell anything from a picture as to whether a position is realistic from a power perspective regardless of the cranks they are on. Many people might be able to ride that position on 180 cranks but, also, many can not. Mike could not. He might have been able to have done it on 170 or 160 cranks but then his drag numbers would not have been so good because he would have been on shorter cranks. You can cram almost anyone into any position in a wind tunnel and get numbers you want if you have no regard to power production. Mike simply could not have ridden that position and maintained power be it on PC's or regular cranks. We could have had him even lower as we had him in a lower position on the track but changed because it was too hard. If we had started there we would have seen the highest position being the most aero by a long shot. Boy, would that have been fun to explain. It is why I was somewhat concerned about putting out the raw data since it is so likely to be misinterpreted. If we had started him out with a higher position then we would have seen his numbers improve before leveling out, something that pretty much everyone would expect.

 

[simo1733]

It makes you wonder when you see Velonews claim that a Cervelo S5 will save you 82 secs over 40K when compared to a round tubed bike with the same wheels.

 

[sciguy]

Mike simply could not have ridden that position and maintained power be it on PC's or regular cranks.

So you had him try with regular cranks? Must have missed that part That would be the only way I would be able to come to that conclusion. Below is a picture of a friend riding his high narrow position during an IronMan triathlon. He is over 50 years old, 6'4" tall and rides 175mm cranks. This is only his third year doing triathlon. He has done considerable testing of his position in a velodrome using a power meter. He is able to produce just as much power in this position as sitting more upright as long as he does a good portion of his training in this position.

 

[TarmacExpert]

So you had him try with regular cranks? Must have missed that part That would be the only way I would be able to come to that conclusion. Below is a picture of a friend riding his high narrow position during an IronMan triathlon. He is over 50 years old, 6'4" tall and rides 175mm cranks.

It's hard to compare because of the angle of Frank's picture and I think yours isn't at the max thigh height of the stroke, but from holding a goniometer up against the screen, the hip angle in Frank's picture is about 12 degrees tighter than in yours.

 

[sciguy]

It's hard to compare because of the angle of Frank's picture and I think yours isn't at the max thigh height of the stroke, but from holding a goniometer up against the screen, the hip angle in Frank's picture is about 12 degrees tighter than in yours.

I'd say impossible to compare hip opening as my friend's pedal isn't remotely in the most upward position. We can however take a look at their backs and in that regard I think It's pretty clear my friend is significantly more rotated horizontally. That would lead me to believe his hip opening would be a bit tighter than Mike's. This is my friend's "high narrow" position he also races with his arm rests 8mm lower and pads set a bit wider. Another variable that influences hip opening is seat angle. It would be nice to know what Mike's effective seat post angle was for the testing.

YMMV,

Hugh

 

[FrankDay]

So you had him try with regular cranks? Must have missed that part That would be the only way I would be able to come to that conclusion. Below is a picture of a friend riding his high narrow position during an IronMan triathlon. He is over 50 years old, 6'4" tall and rides 175mm cranks. This is only his third year doing triathlon. He has done considerable testing of his position in a velodrome using a power meter. He is able to produce just as much power in this position as sitting more upright as long as he does a good portion of his training in this position.

Come on. Does the fact your friend, who is at least 6 inches taller than mike, and riding shorter cranks than Mike can attain that position without losing power means anything regarding any other rider? Heck, according to my data he would be more aerodynamic if he could stay in that position and move up to 200 mm cranks (you can do it on regular cranks if you like). Why don't you have him try that and see how he does?

Edit: Here is a link to some pictures showing the range of positions ridden by pro triathletes. Notice some are quite flat with very closed hip angles (#7) and some are not (#2).

And, why hasn't a single person commented on my aero helmet finding?

 

[FrankDay]

I'd say impossible to compare hip opening as my friend's pedal isn't remotely in the most upward position. We can however take a look at their backs and in that regard I think It's pretty clear my friend is significantly more rotated horizontally. That would lead me to believe his hip opening would be a bit tighter than Mike's. This is my friend's "high narrow" position he also races with his arm rests 8mm lower and pads set a bit wider. Another variable that influences hip opening is seat angle. It would be nice to know what Mike's effective seat post angle was for the testing.

YMMV,

Hugh

What happens to your friends hip angle will depend upon when he drops his heal. In that picture his knee is still well below his elbows. And, if if drops his heal as he comes over the top it may not come up much higher. That is not very closed.

 

[sciguy]

Perhaps there might be a slight difference when the left crank is in the 10 o"clock rather than 2 o'clock position???????? How does his back orientation look compared to Mike's? It seems that Mike's picture has gone missing so closer analysis will be difficult. It would be great to have a nice perpendicular shot to really tease some measurements from.

Cheers,

Hugh

 

[sciguy]

There wasn't enough presented to really come to a firm conclusion. I'd love to see what the drag would have been with a plain skid lid for the 142 cranks. I'll have to say that I like the look of that position and would by eye, expect it to be equal or better than the 180 position. But then again, the eye isn't always the best judge.

One thing I never mentioned is that for the past 30+ years I've been sleeping with a chick with her masters in fluid dynamics. Perhaps something useful has rubbed off

Hugh

 

[FrankDay]

Perhaps there might be a slight difference when the left crank is in the 10 o"clock rather than 2 o'clock position???????? How does his back orientation look compared to Mike's? It seems that Mike's picture has gone missing so closer analysis will be difficult. It would be great to have a nice perpendicular shot to really tease some measurements from.

Cheers,

Hugh

I don't know why the picture disappeared. I uploaded again and have fixed it.

Yes, it would be nice to have perpendicular measurements but it wasn't possible using my camera. Perhaps, if we ever get the video off of the tape that was given to us, we can get some screen shots.

Your friend has a very flat back judged by looking how flat his front is. Basically, I see him in the best position we attained during our time in the tunnel. Since he is flexible enough to get into that position already then my data suggests he should be riding the longest cranks he can that don't compromise power. 175 isn't very long for a 6' 4" guy.

But, look at the range of those supposedly "best" positions attained by the top 15 women at Kona last year. It is clear that not everyone, even amongst the pros who have an extra incentive to get as aero as possible, is as flexible as your friend. Many of these women are about 16 inches shorter than your friend but riding essentially the same crank length. Don't you think some of them would benefit from going much shorter to allow them to get their upper body flat or nearly flat?

One thing that was good about that "too low" 180 crank length position is it helped me to understand the potential importance of separating the feet. I expected that to be the worst position and it wasn't - how to explain? So, we can conclude it is about balancing the conflicting interests here to achieve the best outcome. In Mike's case, while that 180 position was very aerodynamic it would have cost him a lot of power. This is emphasized by the fact that as soon as he moved from 175 cranks to 145 he saw his speed increase dramatically. So, if one is particularly high (Mirinda Carfrae, Virginia Berasategui, others) then it seems the biggest improvement for these folks would be to shorten crank length to allow flattening the torso. Whereas, those with already flat torso's (Caroline Steffan, Karin Thuerig, your friend) would benefit most by experimenting to maximize crank length until power is affected. And, those in between may have to experiment with both approaches to see which works best for them.

That is my take away.

 

[sciguy]

Your friend has a very flat back judged by looking how flat his front is. Basically, I see him in the best position we attained during our time in the tunnel. Since he is flexible enough to get into that position already then my data suggests he should be riding the longest cranks he can that don't compromise power. 175 isn't very long for a 6' 4" guy.

Frank, below in blue is how my friend approaches getting aero in his own words. As you can see he is using the longest cranks possible for a nice horizontal position. I do agree that there are probably some folks that would benefit with shorter cranks that would enable them roll into a more aero position. That said, my 5'2" 60 year old wife rides 170s and can ride just about as aero as my friend. BTW IIRC Jordan Rapp who is nearly as tall as my friend rides 170s and has one of the better looking positions in all of Ironman distance racing.

However it's "supposed to be done", in choosing my position, I went with aerodynamic considerations first. Once my back was horizontal, and head in line, I moved my saddle forward until my thigh stopped bashing my ribs. At the height of my stroke my thigh just grazes my suit, but not my rib cage. My ankle at its most closed is about 90* if you consider the bottom of my foot, but I have high arches so from the perspective of the top of my foot, my toes are dropped below my ankle. As you and I worked on last spring, my ankles feel better more on the open side of things. FWIW, my elbows are as vertical as possible without my knees hitting them. My elbow width is determined by the Tula bar. I'm comfortable with elbows between 9cm and 12cm wide, and pick whichever has the least drag. I just work with the body I inherited. Space limitation, something bumping seems to determine my position more than my ability to train power and endurance in a given position does.

 

[sciguy]

Ok, now let's go back to the wind gradient concept that I brought up the other day. Real world stuff that will affect my wife in a couple of days. The course will be pretty much straight into a head wind then straight down wind. The current forecast for the PE area is for winds of 30kts and higher for the whole time that folks will be riding Ironman South Africa. I would contend that it would be a real advantage to be just as low to the ground as possible for the head wind portions of the loop and then actually sit bolt upright for the down wind portions. Since the greatest benefit will come from optimizing the slower upwind portion of the loops ridden where athletes will be spending a huge amount of time it will be imperative to be as low and aero as possible. Here is a time where being perched up high in short cranks would have real potential to slow you down.

Agreed?

Hugh

 

[TarmacExpert]

I would contend that it would be a real advantage to be just as low to the ground as possible for the head wind portions of the loop

Someone else mentioned that exact same idea to me recently, that being lower might be advantageous into a headwind, and if you field test in still air you aren't going to be able to measure the effect, because you won't have the same air speed gradient with height above ground that you have with a headwind.

Back onto the theme of getting the thighs high, a good example of someone who has a position that very noticeably gets the thighs really high up is UK female time triallist Julia Shaw, who rides for the drag2zero team, so we can assume her position has had a fair bit of wind tunnel optimisation:

 

[coapman]

So you had him try with regular cranks? Must have missed that part That would be the only way I would be able to come to that conclusion. Below is a picture of a friend riding his high narrow position during an IronMan triathlon. He is over 50 years old, 6'4" tall and rides 175mm cranks. This is only his third year doing triathlon. He has done considerable testing of his position in a velodrome using a power meter. He is able to produce just as much power in this position as sitting more upright as long as he does a good portion of his training in this position.

This is a highly dangerous position and should not be permitted in TT's. There should be a limit on the forward position of the head.

 

[FrankDay]

Frank, below in blue is how my friend approaches getting aero in his own words. As you can see he is using the longest cranks possible for a nice horizontal position. I do agree that there are probably some folks that would benefit with shorter cranks that would enable them roll into a more aero position. That said, my 5'2" 60 year old wife rides 170s and can ride just about as aero as my friend. BTW IIRC Jordan Rapp who is nearly as tall as my friend rides 170s and has one of the better looking positions in all of Ironman distance racing.

However it's "supposed to be done", in choosing my position, I went with aerodynamic considerations first. Once my back was horizontal, and head in line, I moved my saddle forward until my thigh stopped bashing my ribs. At the height of my stroke my thigh just grazes my suit, but not my rib cage. My ankle at its most closed is about 90* if you consider the bottom of my foot, but I have high arches so from the perspective of the top of my foot, my toes are dropped below my ankle. As you and I worked on last spring, my ankles feel better more on the open side of things. FWIW, my elbows are as vertical as possible without my knees hitting them. My elbow width is determined by the Tula bar. I'm comfortable with elbows between 9cm and 12cm wide, and pick whichever has the least drag. I just work with the body I inherited. Space limitation, something bumping seems to determine my position more than my ability to train power and endurance in a given position does.

The only criticism I would have of your friends approach is he hasn't tested to see if shorter cranks might result in more power. He is particularly cramped and it might be that this doesn't affect his power but unless he tests he cannot know.

 

[FrankDay]

Ok, now let's go back to the wind gradient concept that I brought up the other day. Real world stuff that will affect my wife in a couple of days. The course will be pretty much straight into a head wind then straight down wind. The current forecast for the PE area is for winds of 30kts and higher for the whole time that folks will be riding Ironman South Africa. I would contend that it would be a real advantage to be just as low to the ground as possible for the head wind portions of the loop and then actually sit bolt upright for the down wind portions. Since the greatest benefit will come from optimizing the slower upwind portion of the loops ridden where athletes will be spending a huge amount of time it will be imperative to be as low and aero as possible. Here is a time where being perched up high in short cranks would have real potential to slow you down.

Agreed?

Hugh

No. Just because the **** is up does not mean the entire body is up.

1. Shorter cranks allow you to lower the front end, since the knee moves away from the chest. This would actually lower the highest part of the back.

2. Shorter cranks move the pedal away from the ground which would allow you to design a bike with a lower BB, lowering the entire body all at once.

3. The upper leg has been moved down into lower speed air. As the wind tunnel has shown this presents a problem in still air but is, perhaps, much less a problem, or an advantage, in a strong head wind.

 

[TarmacExpert]

1. Shorter cranks allow you to lower the front end, since the knee moves away from the chest. This would actually lower the highest part of the back.

You're still stuck in this mindset that you change the saddle to pad drop when you change the crank length. You need to separate out the two things. First of all establish your optimal saddle to pad drop that makes you most aero. Then change crank length, moving both saddle and pads by the same amount. This way you isolate the two things you're trying to measure. Once you have your most aero saddle to pad drop there is no reason to increase the drop when you move to shorter cranks because it will only make you less aero. If you can't ride with your optimal saddle to pad drop, then you need to train in that position till you can, it's typically not a particularly extreme position, more people leave the wind tunnel with their drop reduced than leave with it increased, i.e. most people in their quest for lowness actually go lower than is most aero. If you look at the guys in the UK who ride for the drag2zero team, all of whom have wind tunnel optimised positions, they typically do not have extreme positions. And it was one of their guys who mentioned about most people leaving the tunnel having moved up not down.