Calibrate PP with a Wahoo Kickr Core

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darthmonkey
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Calibrate PP with a Wahoo Kickr Core

Post by darthmonkey »

As I've had a multitude of issues with calibration on the road, (calibration completes but seems about 25% over-reading) I'm now wondering if it's at all possible to calibrate my PP to match my Kickr Core static trainer power.
If so, what would I need to do?
Velocomp
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Re: Calibrate PP with a Wahoo Kickr Core

Post by Velocomp »

Where is it "high"? On the flats or on hills?

Post a ride file and I will adjust it.
John Hamann
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darthmonkey
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Re: Calibrate PP with a Wahoo Kickr Core

Post by darthmonkey »

I'll go one better than posting a single ride, I'll post all of them, as they all overestimate by roughly the same amount.
https://1drv.ms/f/s!AkvX-bEguFc_i5UkXUZOMPYJ-9HnYg

One in particular is probably the best, I was riding home, found a flat spot, did an out and back calibration ride, then pushed it (on the flat) and achieved a 1117w peak at 50kph which by my other bike would have been around 1kw.

The ride up to it was 'iBike_01_30_2019_1804_10_km', then I stopped and restarted the PP to ensure it's reading correctly, and did a calibration ride:
'iBike_01_30_2019_1831_6_km' after which I tried a hard effort, and then continued my ride home.

If required, I can provide power numbers for my Kickr FTP, and Step tests.
Velocomp
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Re: Calibrate PP with a Wahoo Kickr Core

Post by Velocomp »

I opened your 'iBike_01_30_2019_1831_6_km' ride and picked a section where speed, wind and slope conditions were constant. You'll see this in the attachment below.

On that section of road, I input the measured parameters to "aerocoach australia", which is a nice website for checking things.

http://www.aerocoach.com.au/power-from-speed/

Basically, on the website you input rider/bike weight, bike speed, wind speed, Crr, CdA, and slope. It returns power.

The website was within 3 W of your PP (186 vs 189). So, it appears that PP is running its calculations properly.
Screen Shot 2019-03-17 at 11.49.22 AM.png
Screen Shot 2019-03-17 at 11.49.22 AM.png (168.59 KiB) Viewed 3182 times
Your view is that PP is always about 25% high compared to your Wahoo Kickr. If you are correct then your power in this section should have been 139W.

For your wattage to be "wrong", one or more of the following would have to be true for the section selected:

measured wind speed (25.3 km/h) is wrong
measured bike speed (30.4KM/H) is wrong
CdA (0.390) is wrong
bike/rider weight (106KG) is wrong
slope (0.40%) is wrong

I'm going to assume you entered your bike and rider weight correctly.

Looking at the elevation graph, it appears that you are riding on a nearly flat section of road. Note that slope reading and elevation readings come from two different sensors; if slope were wrong, then the elevation graph would reflect that. You don't have that problem.

You have wheel circumference set to 2136 mm (700 x 28c wheel). I have no way to determine if that is correct. However, if your wheel circumference were 25% smaller that would reduce your power by 25%. I don't know of any road bike wheel that is that small.

Your CdA is 0.390. If you were riding on the hoods this seems correct for a person of your weight. If you were riding in the TT position, then your CdA would be too high; this would cause high watts. The CdA that adjusts down your watts to 139W is 0.209. This CdA in the range of an elite time trialist, with a small body, riding in the TT position, with skin suit and aero helmet.

If your CdA is correct, then a high Crr would cause high watts. However, even if your Crr was "0" (frictionless tires and pavement), that would not reduce watts down to 139W.

The final factor is wind. To reduce watts to 139, you'd have to be in a -12km/h tail wind. You MIGHT achieve that level of tailwind if you were riding in a group, but the profile of your wind doesn't suggest you were riding in a pack.

So, looking at your data, if you really want to reduce your power by 25%, you can adjust your CdA to 0.209. I don't recommend making this change unless you think it matches your ride position and equipment.
Attachments
iBike_01_30_2019_1831_6_km.ibr
(60.55 KiB) Downloaded 209 times
John Hamann
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darthmonkey
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Re: Calibrate PP with a Wahoo Kickr Core

Post by darthmonkey »

Thanks very much John, for the in depth look at my ride.

First, Isaac software doesn't appear to provide an average wind speed over a distance, so after some research, and some maths, I wanted to closely replicate your findings. I used the steady cadence part of the stop to stop section, as I know it's a very slight gradient, and I did have a tailwind, but was unsure what strength it was. I came out with tailwind speeds of;
Mean: 4.34kph
Median: 3.75kph( I know there's no such thing as an 'average' wind speed, so I used the median).
This is from 10 points over the specified section, and it's slightly different than your finding at 195.7w vs 185w(pp) - This shows the PP was actually under reading by 10w, but still fairly close.

Although I appear to have a higly predictive heart rate, and I'm accutely aware that the PP is not accurate when wind is involved. I can't say with any conviction that it's over or under reading (or both) in any situation.
I'd love to go on feel, and although my heart rate seems to fairly accruately predict my power (150bpm for 210-220w), I'll have to be a bit more scientific with my perceived exertion to get anywhere near the stage where I can tweak the settings correctly.

What I can see I need to do now, is attempt exactly the same pace of 30.4kph on the smart trainer on a 0.4% gradient over 2km. I should get a power and heart rate reading. The power should match the aerocoach readings very closely. If not, I'll tweak the settings until they do.
Once they do, I'll be able to get an accurate difference between the aerocoach settings, and the PP.
With that difference, I can do that section again, once at the same power, and once at the same heart rate. At that point I'll come back to you to see where I should tweak the isaac settings.
Once that is complete, I'll do the same speed (and another with the same hr) over that section, with as low wind as possible, and see if it translates to anywhere near the same power/hr.
Velocomp
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Re: Calibrate PP with a Wahoo Kickr Core

Post by Velocomp »

To use Aerocoach properly is tricky. The software assumes that the variables you enter are unchanged. In real riding, that is not easy to achieve!

In ride files I look for sections where things are reasonably constant: bike speed, wind speed, and slope. I also look at the power; hopefully it is not varying too much (power can be significantly affected by small changes in slope, wind, and cadence).

If too large a section is selected, then it's likely that the variables are changing. If too small a section is picked, then its likely that transient changes in power won't yield a good average value.

For the above reasons, agreement within 10W is close enough.

One other thing: though HR does correlate to power, HR can be tricky, too! At the five minute mark you did a 30 second sprint of 1000W (nice!). Your HR spiked too, but it took about 2 minutes afterwards for it to recover to its prior value. So, if you're going to try to match HR and power, make sure you pedal at an even pace for several minutes.
John Hamann
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