Cda Analysis

[janholleman]

Can sombody explain me this screen dump.........What it means with Time adv and Dist and.....
How do i need to read this graph. Also i add the Ridefile......

[KenS]

Your profile says your CdA is 0.359 m^2 but by comparng against your DFPM iBike calculates that most of the time your actual CdA is higher (top graph) so in this ride you were generally not as aerodynamic as your profile indicates.
The result is you are going slower than you would if you had the CdA in your profile. So you lost 40 seconds over the course (middle graph)
And you finished 300 metres behind your more aerodynamic self (bottom graph)

[janholleman]

Thanks a lot.....But if you check this profile then you see that the wattage average is very close.
But the Max power of Power2Max is 442 Watt and the Max Power of Ibike is 631 Watt allthough you dont see it in the curve the 631Watt........This is a lot of difference.
Is there any explenation for it?

is still dont understand the seconds lost and the meters lost.........over the complete ride.....
Because what do i need to take.......the end of the graph......or....... the middle....i dont see totall

This is a new profile........

[Velocomp]

We haven't talked much about CdA analysis for a while, and this thread could be instructive for everyone, so I thought I would chime-in.

1) I took Jan's 1901_9KM ride, and did Analyze/Return to Original Download, to get back to the data actually recorded in the ride file. After doing this, I saw that Newton watts were 172.4, and P2M watts were 169.8. So, overall calibration is very close and I used the original data, not the modified data.

2) I then did Tools/CdA analysis. Screen shot is attached. I selected CdA in the top window, time advantage in the middle, and elevation in the bottom. Why? Because climbs/descents can cause the rider to shift position, altering CdA. Also, I used one minute filtering, so that trends could be seen

3) Here's how I interpret the screen shot

For the first 30 seconds minute, the rider is settling-in. At 30 seconds his ride position yields the default CdA value of 0.359

At about 2 minutes, his ride position becomes a bit more vertical, causing CdA to rise above his default value. Overall, his value exceeds 0.41 until about minute 10. Now, this change could be one of a few things:

1) The P2M drifted a bit. The Newton records temperature for each second, and in this ride outdoor temperature remained constant. However, heat is generated in the crank (due to cyclist power) and it's possible the P2M crank is drifting a bit, until it gets warmed up. REMEMBER: any minute changes in readings, from either PM, will cause shifts in CdA.

2) The Newton has drifted a bit. This doesn't seem likely, because the Newton takes only a few seconds to acclimate, and the drift doesn't begin until minute 2.

3) The cyclist has changed ride position, perhaps sitting up a bit. There are some hills in this section; perhaps he sits up a bit, to get more power...

Notice that, when his ride position is less aero (CdA is higher), he begins too lose time advantage. What this means is that, due to higher measured CdA than his default value of 0.359, he is less aero, and therefore traveling more slowly for his level of power output. At minute 11 his overall time loss is about 15 seconds.

4) Around minute 11, his CdA drops, and he begins to gain-back the time lost. What's interesting here is that there is a slight downhill; perhaps the cyclist tucks a bit, which lowers CdA, and causes time to be gained...

5) At minute 16, there's another hill, and a bit of time is lost.

Overall, the time advantage is -10 seconds, suggesting that the rider's overall CdA was higher than the default value of 0.359.

To put it in context, though: this is 10 seconds out of a 1,000 second (18 minute) ride. So, this is a 1% difference, beyond the measurement accuracy limitations of both devices.

[Velocomp]

Thanks a lot.....
But the Max power of Power2Max is 442 Watt and the Max Power of Ibike is 631 Watt allthough you dont see it in the curve the 631Watt........This is a lot of difference.
Is there any explenation for it?

Max watts is are tricky thing to interpret, and it's easy to get hung-up on them.

This particular situation is caused by a power spike at 5:54, where the cyclist starts pedaling. The Newton records 631W for one second; the P2M, 442W. Which is correct? We can't be sure; the actual operation of the internal clock timer in each PM will determine when its sensors are sampled. Also, filtering algorithms will affect second-to-second output. The Newton uses very little filtering; we don't know about the P2M.

But the more important answer is: it doesn't matter. This is a single, 1-second-long power reading, out of the 1,000 readings made. Look for 5 second averages, 30 second, and 1 minute wattage averages. These are representative of what the human body is actually producing and are the things to assess.

[janholleman]

When you know that the The P2M drifted a bit..............
You read that in Wattage bar or.........and what do you see.....so that i can explain it to the people and used it when i am indoor or outdoor with people who have a DFPM....Is there a time that the DFPM is on good temperature.....or something else!!

Sorry again but need to learn. Dont want to tell fairy tales.

[Velocomp]

When you know that the The P2M drifted a bit..............
You read that in Wattage bar or.........and what do you see.....so that i can explain it to the people and used it when i am indoor or outdoor with people who have a DFPM....Is there a time that the DFPM is on good temperature.....or something else!!

Sorry again but need to learn. Dont want to tell fairy tales.

We don't know that the P2M drifted. What we do know is that outdoor temperature readings were very stable during the ride (the Newton records this), between 66F and 67F. There's no reason to believe that the Newton drifted. We also know that DFPMs experience the full-forces of the rider, and that they are electro-mechanical devices. Inevitably, they will heat somewhat internally. Finally, we know that power readings diverged (very slightly) after the first couple minutes of riding.

It's possible that the P2M drifted, causing the watts divergence. It's also possible that the rider changed position slightly on the bike; that, too, would cause the change. We can't tell...

CdA is a fun measurement, but it requires very careful setup. Remember, quantities are being measured that vary by only a few percent; that is within the error tolerances of the power meters...

[janholleman]

Today i went with my son for little testing......And we make a round of 5Km. When we came back i shutdown ibike and start again. When i came home i download all details and he make 4 seperate rides. but i start isaac again i have 1 ride off 22Km when i open the file again from isaac then i become again 4 rides. I find in the last part of the last 2Km a difference of 10 Watt
....Try to find where it is coming from but.......so Ibike give 143 Watt and Power2Max 133 Watt.
Why one ridefile when i start Isaac?

There are 4 rides in the file and 3 are in my opinion close enough with wattage.

I had look to.....all but can not find why.

Jan.

Thanks

[janholleman]

Calibration Goed DT SWiss.ibr The file calibration Goed DT Swiss.ibr is done with my broken cervel RS Size 54.
Headtube 16 Cm plus 2 cm spacers effectief toptube 548 mm and stem 9 cm.
CdA Value 359......This is a comfort frame.
Ibike position on Stem

I was thinking that i sit deeper on the Cube bike so the CdA value will be lower but my conclusion is that the top tube and the stem lengt are to short what make me more up with my back.......

Allthough the cervelo is a more comfort frame then the Cube with the more agressive race geometry but the CdA value is higher on the cube then on the Cervelo.
Post below is Cube Calride

[janholleman]

Look to the second file Calride 2.ibr.
Is done with a Cube size 53. Headtube 13 cm plus 2 cm spacers effectief toptube 531 mm stem 9 cm.
CdA Value 695......
Ibike position on steer

[Rolando]

...
We don't know that the P2M drifted. What we do know is that outdoor temperature readings were very stable during the ride (the Newton records this), between 66F and 67F. There's no reason to believe that the Newton drifted. ...

Where can I see the range of temperature readings? I only see in the Advanced Stats the avg. temp. value. A graph may be valuable.

[Velocomp]

...
We don't know that the P2M drifted. What we do know is that outdoor temperature readings were very stable during the ride (the Newton records this), between 66F and 67F. There's no reason to believe that the Newton drifted. ...

Where can I see the range of temperature readings? I only see in the Advanced Stats the avg. temp. value. A graph may be valuable.

Export your ride file as a csv file. In the excel file, you'll see the temperature values vs time.