Erratic power readings, but AVG okish

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warbs
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Joined: Sun Apr 15, 2018 5:40 pm

Erratic power readings, but AVG okish

Post by warbs »

Hello,

Been a happy powerpod user for a few years now.

Starting last summer I've lost confidence in my power readings, and after a winter on smart trainer and zwift, I'm less convinced my PP readings are accurate.

The average according to Strava is probably not far off, but according to the PP I'm regularly pushing over 400 watts for far longer than I can on a trainer.

I have 3 bikes I use my PP with and want to avoid the hassle of recalibrating if possible

Can you take a look at today's ride (attached) and let me know the best course of action?

Cheers

Chris
iBike_04_02_2020_1319_13_Miles_HiDef.ibr
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Velocomp
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Re: Erratic power readings, but AVG okish

Post by Velocomp »

I looked at your ride file and I checked your PP calibration. Calibration looks fine.

In general your ride file looks fine, too, though I do notice one place, in a descent near mile 5.5, where watts seem high. You are descending a hill while facing a pretty high headwind.

If I were descending into a headwind I would likely go into a tuck. However, when you go into any kind of a tuck on a descent, you will go faster compared to riding in your usual position, because your CdA goes down. However, since PP doesn't know you're in a tuck, it interprets your higher speed as a result of more pedal-power. Watts go up...

Exacerbating this potential problem is also the fact that you descended into a headwind. Once again, any error in assumed CdA would be amplified.
John Hamann
warbs
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Joined: Sun Apr 15, 2018 5:40 pm

Re: Erratic power readings, but AVG okish

Post by warbs »

Thanks for checking...

Perhaps the winter training has properly paid off this year 😁

Cheers

Chris
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Re: Erratic power readings, but AVG okish

Post by Velocomp »

warbs wrote: Sat Apr 04, 2020 2:36 am Thanks for checking...

Perhaps the winter training has properly paid off this year 😁

Cheers

Chris
You are a strong cyclist, your ride file most definitely shows that!
John Hamann
warbs
Posts: 12
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Re: Erratic power readings, but AVG okish

Post by warbs »

Hello Again,

I'm still not convinced my powerpod is working properly.

I've recallibrated a number of times, and get the same high readings.

On sunday I rode 40 miles over around 2:15 hours. Last time I measured my FTP a couple of months ago it was 288W, it's probably not very different now. On Sunday's ride, I apparently averaged 273W, and peaked over 1000! I know from Wattbikes what it takes to get above 600 and to 1000, it doesn't last very many seconds!

And just looking at the wheel/wind graph in Isaac, it was a light wind day (less than 10mph) but from the graph it looks like a headwind for around 75% of a loop, the wind didn't change direction during the ride.

during the ride there were plenty of periods where I was cruising, but the power readings were above 300, I wasn't pedalling that hard!

the weight in my profile is maybe a couple of lb's high, but it's not far off.

Ride file attached, any ideas?

cheers

Chris
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Velocomp
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Re: Erratic power readings, but AVG okish

Post by Velocomp »

Your recalibration was incorrect. Wind speed readings were way too high.

I applied the profile from your earlier ride (which produced reasonable wind speeds) to this ride and I got much lower watts--around 211.

Your latest ride file, with your original profile, looks good.

I would respectfully suggest you stop tweaking the calibration of your device all the time, and instead simply use it! On my bike I have 3 Velocomp devices, and I have not calibrated them in MONTHS! :D

You are a strong cyclist and it is very easy to get short bursts of power that are very high, particularly when accelerating. That is what your ride file shows.
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John Hamann
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lorduintah
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Re: Erratic power readings, but AVG okish

Post by lorduintah »

As personal experience - when I first got my PP3 I was checking the calibration at the end of almost every ride. Every so often I would save the suggested changes - even when Isaac was only estimating small changes - the result for the most part was a wandering wind scale and friction values or slope.

I now have one calibration, which I repeated -just a few days later using the calibration as described by the manual - a few weeks later - those coefficients barely moved from the original. I don't mess anymore - stability and reproducibility reign. Check calibration leaves you on a walk away from what are likely the best factors.

I am with John (who has a whole lot more experience with these little devices than I) - ride - don't tweak. Do one careful calibration the way the directions state - then leave it alone. You can peek at check calibration if you must - just don't hit ACCEPT.
spangelsaregreat
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Re: Erratic power readings, but AVG okish

Post by spangelsaregreat »

I would second the point of doing a good calibration and sticking with it.

Overall PowerPod does its job well but it has some quirks that are annoying that lead to this search for a better calibration.

My personal experience is PowerPod will give odd readings when:-

- riding downhill with a tailwind. I don't agree with the "you must have been in a tuck" argument. It just seems that in certain circumstances, regardless of riding position PowerPod will give high watts.

- riding in crosswinds very occasionally gives high watts.

- off road you get some low power readings in sticky surfaces, mud, grass etc.

- riding in the rain just stops it working due to the wind port getting blocked.

All of the above are annoying but lets balance it against the positives. PowerPod is considerably cheaper than its rivals. It is easy to swap between bikes. The level of support via this forum is pretty unique for a product (I doubt the creator of SRM answers queries). I have seen other friends have lists of issues with DFPMs so we are not alone!

Regards
Velocomp
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Re: Erratic power readings, but AVG okish

Post by Velocomp »

spangelsaregreat wrote: Thu Apr 30, 2020 10:09 am I would second the point of doing a good calibration and sticking with it.

Overall PowerPod does its job well but it has some quirks that are annoying that lead to this search for a better calibration.

My personal experience is PowerPod will give odd readings when:-

- riding downhill with a tailwind. I don't agree with the "you must have been in a tuck" argument. It just seems that in certain circumstances, regardless of riding position PowerPod will give high watts.

- riding in crosswinds very occasionally gives high watts.

- off road you get some low power readings in sticky surfaces, mud, grass etc.

- riding in the rain just stops it working due to the wind port getting blocked.

All of the above are annoying but lets balance it against the positives. PowerPod is considerably cheaper than its rivals. It is easy to swap between bikes. The level of support via this forum is pretty unique for a product (I doubt the creator of SRM answers queries). I have seen other friends have lists of issues with DFPMs so we are not alone!

Regards
1) Downhills can be a challenge for opposing force measurement, for a variety of reasons, one of which is riding position.

2) Crosswind performance has been improved in V3 units

3) Yes, going off road on "sticky" surfaces will cause low watts if your PP is set to a smooth road Crr.

4) Plugged wind ports during rain can cause problems. You can "repair" the blocked section after the ride in Isaac.

5) We use the forum not only to help you, but to learn from you. It is great fun to see how people actually use our devices, and from that to learn about things we can change to make our products better.
John Hamann
spangelsaregreat
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Re: Erratic power readings, but AVG okish

Post by spangelsaregreat »

John

1. Out of interest what are the other challenges? Riding position I understand but what other factors cause this?

2. Yes I have noticed it less of an issue with the V3.

3. Slightly confused by this one, I thought the option for road surface is not as relevant after you changed the software to allow for very bumpy (cobbles etc) surfaces. What initial setting should you be using for a Gravel set up?

4. My experience of the blocked port is (other than one occasion) once the port is blocked it stays blocked for the entire ride. Unless you have the blocked part where you are riding in one direction it is difficult to chose a value to replace in the blocked section. Have you ever considered using an L shaped pitot tube like some planes have? Would that reduce blocked tubes?

Regards
Velocomp
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Re: Erratic power readings, but AVG okish

Post by Velocomp »

spangelsaregreat wrote: Thu Apr 30, 2020 2:55 pm John

1. Out of interest what are the other challenges? Riding position I understand but what other factors cause this?

2. Yes I have noticed it less of an issue with the V3.

3. Slightly confused by this one, I thought the option for road surface is not as relevant after you changed the software to allow for very bumpy (cobbles etc) surfaces. What initial setting should you be using for a Gravel set up?

4. My experience of the blocked port is (other than one occasion) once the port is blocked it stays blocked for the entire ride. Unless you have the blocked part where you are riding in one direction it is difficult to chose a value to replace in the blocked section. Have you ever considered using an L shaped pitot tube like some planes have? Would that reduce blocked tubes?

Regards
1. If default CdA is "incorrect" then downhill watts can be either too high or too low

3. We do our best to compensate for changes in road roughness, but it is imperfect. For gravel, I would use a default Crr of 0.008

4. If the port is blocked, you can clear it by stopping and blowing it out. There are other ways of battling the problem, (Newton used some) but it is expensive and, for most riders, not worth the added cost.
John Hamann
spangelsaregreat
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Re: Erratic power readings, but AVG okish

Post by spangelsaregreat »

John

Default Cda of 0.008? Don't you mean CRR?

Regards
Velocomp
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Re: Erratic power readings, but AVG okish

Post by Velocomp »

spangelsaregreat wrote: Fri May 01, 2020 9:58 am John

Default Cda of 0.008? Don't you mean CRR?

Regards
OOPS...fixed :D
John Hamann
warbs
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Re: Erratic power readings, but AVG okish

Post by warbs »

Thanks all for the feedback, and now an interesting thread.

Just for the record, I don't fiddle with my PP or Isaac for fun, life's to short :) My PP had a fault a couple of months ago where my laptop would not recognise it. Tech support advised a reset, which fixed it, but it then demanded a calibration - the first one for a year or so. That calibration went from 1-50 normally, then back to 0 and up to 100...and I've had little confidence in it since.

Now I've submitted to a battle of wits with Isaac, I can see the adjustment John's made to my profile...strangely those settings are identical to before the reset (ie. all of 2019)...so who knows what happened to first calibration after the fault. Nor why the most recent was so far off on a very low wind day, with a textbook straight flat out and back ride.

I think my PP is under a curse, but the good news is after the most recent round with Isaac I have my own set of spells....

(Went out today on bike 3, and that reported exactly what I'd expect).

Thanks again for the help and advice.

cheers

Chris
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