Pedaling Efficiently
- Thread starter Joseph Tsui
- Start date
"Looking forward to possibly not understanding"? Frank, you've already been shown the analytical approach Alex used to make this claim and you didn't understand it so your ability to understand the evidence based on the analysis is a settled matter. It's not "possibly not understanding" it's "definitively didn't understand."Fday said:
OK, well this will probably take some explaining.... I'll try to be brief and not go into the full shebang....Enriss said:
Below is a chart of my own power meter data from doing pursuit like effort at an indoor velodrome. It was one of many such efforts on the day. It shows the speed (dark blue) and power (yellow) plotted against distance.
For most of my runs, I was alone on the track but there were a couple of others there that day that arrived about halfway through my training session.
Now one can use such speed and power data to create what's known as a "Virtual Elevation" profile. This is described in detail by Robert in his paper on the subject, which I can find the link later if you want to read about it.
In essence it enables one, using the maths which described the relationship between power and resistance forces of cycling, to generate a virtual elevation profile from the speed and power data using inputs for rolling resistance (Crr) and aerodynamic drag (CdA).
This virtual elevation profile can then be compared that with the known elevation profile, which as we know for an indoor velodrome such as the DGV (world class facility used for 2000 Olympics) is flat, provided you stay at the same height on the track*.
So in this case, you grab the relevant power meter data, plot the VE, and adjust the CdA/Crr input until you see the VE line level out, since that represents actual elevation of a flat track.
It is a very quick and handy method to assess aerodynamics (or changes in aerodynamics).
So here it is for this pursuit run. On a track, the VE line oscillates in a pseudo-sinusoidal manner, just as you would expect with the variations in height of COM as well as the variable power and wheel speed from the transitions in/out of the bends.
So you adjust the CdA input to make the VE horizontal.
In the VE plot above - you can see that I have done that, and the VE plot remains level (oscillates about a horizontal line) until about the 1.4km mark, where it starts to head downwards, as if I was beginning to ride down a very shallow gradient.
But of course I can't ride gradually down hill on a track if I'm on the black line riding a pursuit effort. So what can cause the VE plot to show this?
Several things - it could be a tailwind (don't have those indoors), or a lower of aerodynamics or, as in this case, the exact moment another rider entered the opposite side of the track and started to circulate, creating just enough air disturbance to be seen by the VE plot.
I have seen this effect replicated quite a number of times with my own and other rider's data. When doing aero testing at the track, it actually effect the results.
The VE is exceptionally sensitive when applied in such benign conditions. The smallest things can be picked up.
* I have also seen this technique pick up changes in VE when moving up from the cote d'azur onto the track, a rise of ~ 10-15cm.
It looks like at the start of the effort, watts are roughly constant while speed is rising, and yet this isn't showing up as a downhill effort on the VE calculation? At the end when the other rider enters the track, are we to assume that he immediately stirred up the air enough to create the noticeable difference, and that the air didn't get any more stirred up as he accelerated?
Anyway, you've piqued my curiosity.
Anyway, you've piqued my curiosity.
Yup. The power may be constant but it's high so it gets translated into acceleration. Alex accelerates until the total drag forces exactly equal the total propulsive force.Enriss said:
Air has relatively low mass so it doesn't take much to get it stirred up. Basically, increasing the rider's speed "broadens" the volume of air that gets disturbed, it doesn't make a small volume of air go faster.
Thanks for the data. That is all very cool. I have a couple of questions and see a few problems with the interpretation.Alex Simmons said:OK, well this will probably take some explaining.... I'll try to be brief and not go into the full shebang....
Below is a chart of my own power meter data from doing pursuit like effort at an indoor velodrome. It was one of many such efforts on the day. It shows the speed (dark blue) and power (yellow) plotted against distance.
For most of my runs, I was alone on the track but there were a couple of others there that day that arrived about halfway through my training session.
Now one can use such speed and power data to create what's known as a "Virtual Elevation" profile. This is described in detail by Robert in his paper on the subject, which I can find the link later if you want to read about it.
In essence it enables one, using the maths which described the relationship between power and resistance forces of cycling, to generate a virtual elevation profile from the speed and power data using inputs for rolling resistance (Crr) and aerodynamic drag (CdA).
This virtual elevation profile can then be compared that with the known elevation profile, which as we know for an indoor velodrome such as the DGV (world class facility used for 2000 Olympics) is flat, provided you stay at the same height on the track*.
So in this case, you grab the relevant power meter data, plot the VE, and adjust the CdA/Crr input until you see the VE line level out, since that represents actual elevation of a flat track.
It is a very quick and handy method to assess aerodynamics (or changes in aerodynamics).
So here it is for this pursuit run. On a track, the VE line oscillates in a pseudo-sinusoidal manner, just as you would expect with the variations in height of COM as well as the variable power and wheel speed from the transitions in/out of the bends.
So you adjust the CdA input to make the VE horizontal.
In the VE plot above - you can see that I have done that, and the VE plot remains level (oscillates about a horizontal line) until about the 1.4km mark, where it starts to head downwards, as if I was beginning to ride down a very shallow gradient.
But of course I can't ride gradually down hill on a track if I'm on the black line riding a pursuit effort. So what can cause the VE plot to show this?
Several things - it could be a tailwind (don't have those indoors), or a lower of aerodynamics or, as in this case, the exact moment another rider entered the opposite side of the track and started to circulate, creating just enough air disturbance to be seen by the VE plot.
I have seen this effect replicated quite a number of times with my own and other rider's data. When doing aero testing at the track, it actually effect the results.
The VE is exceptionally sensitive when applied in such benign conditions. The smallest things can be picked up.
* I have also seen this technique pick up changes in VE when moving up from the cote d'azur onto the track, a rise of ~ 10-15cm.
Question. It appears that on average the "elevation" is highest when the speed is highest and the watts are the lowest? Wouldn't this occur in the curves and shouldn't the association be exactly the opposite since the speed of the upper body is much less than the speed of the wheels (I assume that is the speed you are using, not speed of the CM) so it would look like the aerodynamics improved wouldn't it (or you had a tail wind). Of course, the Crr is increased in the curves, further complicating this analysis but the analysis assumes a constant Crr. Anyhow, why is the elevation variations the way they are when there is not another rider on the track?
Question. Does anyone know what the status of the buildings ventilation system was during this trial? Did any fans turn on or off or change speed?
Problem. Somehow you are able to measure a virtual elevation gain of less than a meter when another rider enters the track (aren't most tracks banked?) using a power meter with an accuracy of what?
Problem: What wind speed does this change represent? Is it possible to measure this wind speed?
Problem: What change in CdA does this represent? How do we know the rider maintained the same position when the distraction of another rider on the track entered the mix?
Problem: Why doesn't the rider influence himself since he is disturbing/stirring the same air and then riding through it just a few seconds later. If it was a simple stirring of the air wouldn't we see a small dropping in the elevation as a single rider kept going until an equilibrium was established? Instead we see in increase in the elevation at the beginning as I read the data. How is this explained? Why is this phenomenon only seen when another rider is added to the mix?
Problem: Why is the elevation seemingly increasing again the last half lap or two? Did the other rider get off the track then?
Problem: This result requires an extremely accurate power meter and an extremely well-trained rider who is able to maintain the exact same aerodynamic position when the distraction of another rider enters the track. Possible but highly unlikely.
Problem. There are no controls.
Problem. Can the method be used to tell how many other riders are on the track?
Problem. Does it matter where the other rider is on the track. Is the effect the same if the rider is a half lap or a quarter lap behind the measured rider?
Anyhow, what I would like to see is a controlled and double blinded study (the rider can't know if there is another rider on the track or not and the interpreter of the data not knowing either) can predict when another rider enters and/or leaves the track before I accept this as true. Too many variables are not controlled here.
It would also be cool if one could measure the wind speed around the track since this is how you are explaining this and that this speed only depends upon what riders on the track are doing. Anyhow, there are other ways of explaining this data so, I guess it is possible what you say, but your evidence is anecdotal and, even if true, is pretty much useless.
One more thing, isn't this method touted as being useful for the rider to test their aerodynamics under all sorts of conditions. Wouldn't the normal interpretation of this data say that the rider did something to improve his aerodynamics towards the end? Now, with this interpretation you have "assumed" the CdA is constant so any variation comes from air disturbances from other riders. It seems to suggest the method can't be used to determine CdA if there is another rider within 100 meters or so.
Anyhow, I believe I understand what you have done. I liken this to the search for extraterrestrial life. You have managed to find some patterns in the power meter noise and drawn the conclusion that bike riders cause wind currents in the air. While I don't accept that your data is particularly valid since there are no controls I will accept your conclusion. While interesting it has essentially zero practical usefulness as far as I can tell.
Of course. You don't understand the analytical method (and the ways in which the data were collected) so you don't understand the evidence. Isn't that exactly what I said would happen? Though, to be fair, it didn't take Nostradamus to see that coming.Fday said:
I must have missed the methods section of that paper. Is it possible to understand how the data was collected without being told how the data was collected?RChung said:
I simply pointed out some potential issues with what was presented. If you can address those issues I invite you to do so.
What you have done is intriguing even though it is difficult for me to see any practical value to the exercise.
Frank, believe your questions are good, and your explanation is certainly plausible. Although I don't understand Alex's results fully, I join you in being skeptical of his explanation that his speed increase at a steady power output towards the end of the trial is due to another rider entering the track.
Seems to me any measureable air disturbance generated by a rider on the other side of the track would if anything act to increase drag, not reduce it, because of wake turbulance effects. Will freely admit my ignorance of aerodynamics though.....never really understood the concept of the Reynold's Number, which seemed to come up often in calculations
Seems to me any measureable air disturbance generated by a rider on the other side of the track would if anything act to increase drag, not reduce it, because of wake turbulance effects. Will freely admit my ignorance of aerodynamics though.....never really understood the concept of the Reynold's Number, which seemed to come up often in calculations
Yes, it is hard to believe, isn't it? That's why it's so cool. Even I was surprised that something so small turned out to be detectable. And note that Alex wasn't giving the totality of his evidence: he's consistently observed these effects across several rides.dhk2 said:
It seems like what you are saying is Alex is the only person who has actually demonstrated this effect. It suggests a link but unless controls are in effect, the methods are known, and the results shown to be repeatable by others with some statistical certainty the conclusion must be seen as suspect.RChung said:
This is hardly anything more than what we see with PowerCranks. People have reported all sorts of surprising results that they attribute to the device. Many here feel such results are impossible and point to the fact the results have not been scientifically verified. While those who use the product and have seen the results remain convinced, those who have not are not.
It seems what Alex has shown is a possible ability to do this but there is no scientific evidence to support his conclusions.
Well, I see their explanation as being similar to putting a spoon in a cup of water and stirring it up. The problem is the mass of the air in a velodrome is quite large and the stimulus for stirring of a single rider is quite small such that the mass movement of air must be incredibly tiny. The effect is probably bigger on a 250 meter track than a larger track but still it must be incredibly tiny. If a butterfly flapped its wings in Brazil could he pick it up. Theoretically I suppose so but practically? How such an effect could be picked up using real people riding bicycles with commercial grade power meters seems impossible. He has made a case for it but he certainly hasn't proved the case as far as I am concerned.dhk2 said:Frank, believe your questions are good, and your explanation is certainly plausible. Although I don't understand Alex's results fully, I join you in being skeptical of his explanation that his speed increase at a steady power output towards the end of the trial is due to another rider entering the track.
Seems to me any measureable air disturbance generated by a rider on the other side of the track would if anything act to increase drag, not reduce it, because of wake turbulance effects. Will freely admit my ignorance of aerodynamics though.....never really understood the concept of the Reynold's Number, which seemed to come up often in calculations
Perhaps he could put a fan of the horsepower of the rider on the track and blow air in one of 4 directions randomly (with movement, against movement, across the track outwards, towards the infield) and only look at the data when he is on the opposite side of the track and see if he can pick out when the fan is blowing in each direction. At least then the rider and observer are both somewhat blinded (as long as they don't know the fan direction) as to what is happening such that there would be some blinding to this little experiment. As it is, it is simply an interesting observation.
Fday said:
Bit a difference. We are not claiming anything beyond the data.
-Data shows speed goes up and power goes down in the bankings.
-Data shows riding a big gear and lower cadence does not alter this situation
-Data shows more riders crash and burn in a pursuit riding too big a gear than too small
Compared to what you and the other people of "faith" think they see or feel or reading too much into a result without taking all the factors into account.
No, the opposite in fact. Look again, use the vertical lines to help. you will see virtual elevation climbs as speed decreases.Fday said:
There are no ventilation systems or fans at the DGV.Fday said:
I think you are confusing accuracy and precision with this question.Fday said:
Approximately 0.005m^2. Not sure what wind that equates to but not much. I could work it out but CBA since it will make no difference to your understanding.Fday said:
In fact it is seen, when a solo rider rides the track for long enough, and I have data showing that effect as well.Fday said:
They moved up high on the track after gaining speed for a few laps, which reduced the influence a little.Fday said:
I ran this analysis post-hoc Frank.Fday said:
It is my data. I consider myself highly experienced at doing such efforts and have probably done upwards of 15,000 laps on this velodrome.
If you think maintaining the same position is unlikely then there would be no way I could convince you. There was video of my pursuit from the UCI World Masters Championships showing my ride style but I can't recall the link.
I didn't present a scientific study, just the data Frank.Fday said:
Of course not. I prefer just to count the other riders when I want to know that.Fday said:
Yes, it does matter. The effect is more obvious as you approach a rider from behind. In this case the rider eventually moved up track reducing the impact since they knew I was doing efforts and that is good track etiquette.Fday said:
Ummm, let's blind a rider riding their bike.Fday said:
FFS - have you ever been to a track before Frank?
Most wind measuring devices are not sensitive enough to pick up such low (average wind) speeds, they would however detect the immediate rush of air as a rider passes by.Fday said:
Occam's razor Frank.Fday said:
The VE method is very useful and can be applied with varying degrees of precision. When the conditions are exceptionally benign, such as in an indoor velodrome, then the level of precision in detecting changes is quite high. A you move to less benign conditions, then the usefulness doesn't disappear, just the level of precision.
Fday said:
I think you are the one from another planet.
Au contraire.Fday said:
Field testing of aerodynamics at a velodrome is an exceptionally useful exercise. But I don't expect you to understand that.
I've seen exactly the same thing as Alex (on the same velodrome....Dunc Gray) when aero testing.
The effect ,with four other riders entering and leaving the track at various times,
even though they were all conscious of the fact that I was testing and so moved up the track to leave me a free path on the black line, was enough to "waste" a full morning of testing.
Well not exactly waste, since by observing the resultant files, I could clearly see that I needed to caefully choose the times I tested to ensure "clean" and useful data was captured.
I was seeing CdA numbers all over the place when looking at back to back runs in multiple identical configurations. (I had my lap top trackside, and was checking data after every couple of runs).
After the "wasted" day, I returned to the track the next morning, and using the same test protocol, suddenly saw VERY consistent data from run to run.
I'm fortunate that I can often get to the track at times when I'm often the only one there. Real handy when conducting any aero testing.
Mike Lawson
The effect ,with four other riders entering and leaving the track at various times,
even though they were all conscious of the fact that I was testing and so moved up the track to leave me a free path on the black line, was enough to "waste" a full morning of testing.
Well not exactly waste, since by observing the resultant files, I could clearly see that I needed to caefully choose the times I tested to ensure "clean" and useful data was captured.
I was seeing CdA numbers all over the place when looking at back to back runs in multiple identical configurations. (I had my lap top trackside, and was checking data after every couple of runs).
After the "wasted" day, I returned to the track the next morning, and using the same test protocol, suddenly saw VERY consistent data from run to run.
I'm fortunate that I can often get to the track at times when I'm often the only one there. Real handy when conducting any aero testing.
Mike Lawson
You are correct. I don't know what I was looking at before.Alex Simmons said:
How do they keep it cool in the summer or is it open to the outdoors? An indoor velodrome with no ventilation?
Either way. Both are important for this purpose I would think.
Well, sure it does. I have been in the wind tunnel and it would seem that such variations are seen regularly in riders trying to hold position, let alone actually riding. What is becoming clear is it is a tiny effect, less than a second for a 3000 m pursuit.
How much effect/benefit do you think a single rider sees from this effect during an hour attempt?
Really?
That is cool. Your data is what it is and your analysis is what it is.
Correct. Part of my point.
Well, if it is possible to detect one rider on the other side of the track, the effect should be proportionally larger for more riders on the track, don't you think?
Well, we were told the technique was so sensitive it could detect a rider on the other side of the track. What is sounds like is this run didn't detect the other rider until one was in the "draft zone". Is that correct?
blinding doesn't mean can't see. It means the subject cannot know what the conditions are.Ummm, let's blind a rider riding their bike.
FFS - have you ever been to a track before Frank?
Yes, but we are being told the technique can pick up a rider on the other side of the track.
The implication of this discussion has been that just having another rider on the track interfered with the analysis. It seems it makes a difference how far the rider is away, not just that there is one. I can see now that the effect has been perhaps oversold as to how big the effect is.
I was referring to using the technique to detect other riders on the track. Not much pracital usefulness to that.
I think you are the one from another planet.
Au contraire.
Field testing of aerodynamics at a velodrome is an exceptionally useful exercise. But I don't expect you to understand that.
I have believed in the benefits of field testing of aerodynamics. I have recommended "the Chung method" to many. It has been "advertised" as an accurate method as long as you ride a loop and "know" the rolling resistance. Wind and other issues were seemingly accounted for. My concern here was if the outcome can be made less accurate by something as minor as another rider on the other side of the track it makes me wonder if it is as useful as I had thought. In view of the answers that have been given here I don't think the error are so large as to invalidate the technique for aerodynamic testing.
It gets very hot in the velodrome (and quite cold in the winter) - this is not a climate controlled venue. There are vents in the upper reaches for ventilation but no air flow track side.
You can't use this analysis to pick the number of riders as their individual sizes, CdA, pace and formation would alter the nature of the impact seen. Three riders riding abreast would be different to three riders in a line.
I can certainly tell you that as soon as a Derny driver enters the track, the impact is quite substantial.
As for blinding, how can a rider not know there is a another rider on the track?
For an attempt on the hour, in this velodrome for Jayson's hour record. the effect of the lone rider stirring the pot, plus I also presume tyres warming up (as I can't separate the two to know the precise impact of each) and hence the lowering of effective Crr (or if you like the combined CdA/Crr pair), is worth up to 375 metres or 1.5 laps. In the context of an hour record, I would call that substantial.
It takes about 10-15 minutes for it to stabilise on the opening run and a little less time on subsequent runs if there has been a short break. I have seen this effect multiple times. Pursuit runs are too short to detect this effect.
One day I'm going to use the method to test and see if we can detect and/or quantify a reduction in effective CdA of having a rider behind on your wheel.
You can't use this analysis to pick the number of riders as their individual sizes, CdA, pace and formation would alter the nature of the impact seen. Three riders riding abreast would be different to three riders in a line.
I can certainly tell you that as soon as a Derny driver enters the track, the impact is quite substantial.
As for blinding, how can a rider not know there is a another rider on the track?
For an attempt on the hour, in this velodrome for Jayson's hour record. the effect of the lone rider stirring the pot, plus I also presume tyres warming up (as I can't separate the two to know the precise impact of each) and hence the lowering of effective Crr (or if you like the combined CdA/Crr pair), is worth up to 375 metres or 1.5 laps. In the context of an hour record, I would call that substantial.
It takes about 10-15 minutes for it to stabilise on the opening run and a little less time on subsequent runs if there has been a short break. I have seen this effect multiple times. Pursuit runs are too short to detect this effect.
One day I'm going to use the method to test and see if we can detect and/or quantify a reduction in effective CdA of having a rider behind on your wheel.
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