Ironman Power compared to Stages Power

Anything about Power (watts)

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zacha
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Re: Ironman Power compared to Stages Power

Post by zacha » Sun Feb 14, 2016 12:04 am

For everyone still not believing the Genius/Ironman measures power too high, they guys at bikeradar came to the same conclusion:

http://www.bikeradar.com/road/gear/arti ... All+feeds)

Interestingly the kickr snap seems to suffer a similar problem, even worse, but at least the company responded and adressed it.

Kubik
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Re: Ironman Power compared to Stages Power

Post by Kubik » Sun Oct 30, 2016 10:18 pm

I've got exactly same problem with Ironman + Stages, the watts readings on Tacx are 15-20% higher than those of stages.
At first I suspected left-right leg imbalance may be the reason, but I ruled this out with a simple test:

while doing flat 115wt recovery valley in TrainerRoad in ERG mode, I unclipped my right foot and kept pedaling only with my left. The readings were as follows: Tacx - 115 watts, Stages - 180 watts. Since stages is doubling what it gets form the left foot, it must mean it was getting 180/2=90watts from my left foot at that time, which since my right foot was unclipped was all that the rear wheel was getting too.
So while my power output according to stages was 90watts, Tacx was showing 115w. That's a huge difference, and I must say I am disappointed as all the times I was dying but hanging in there with some Zwift rides, now means I would be toasted and dropped after the first 15 minutes of real watts.

I will play around with calibration (although I do it regularly) and hope Tacx rocognizes and fixes the isse soon.

zacha
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Re: Ironman Power compared to Stages Power

Post by zacha » Sun Nov 06, 2016 7:29 pm

I have given up with the genius' power guesser. I am now always using an on-bike power meter. I don't know why tacx does not fix this as every test agrees that this is a problem with the genius. The vortex did a decent job despite the usual problems (with wrong power readings when the tyre is cold) so generally this should be possible.

andyarceye
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Re: Ironman Power compared to Stages Power

Post by andyarceye » Thu Feb 02, 2017 3:02 pm

Very late I know , but I have an update.
I know its not as scientific as it could be but it has given me some extra thought.

Previously I had always thought the Tacx Ironman was reading high and Stages was correct.
However, I have had a smart meter installed in my house which accurately shows amount of electricity being used and I had a surprise.
Apparently the Tacx gives back electricity to the house, which was was very sceptical of to say the least. Since this was a way of testing if the tacx does in fact give any significent amount of power back the Home smart meter would show it. Test if Tacx was full of sh*t was my thoughts.

So I do a trainer session with some hard intervals with results I could not explain other than to trust the Tacx.
Intervals showing I was feeding back 250 - 300 watts back to the house showed a drop on power usage on the smart meter by very close to the amount the Tacx said I was giving back for the same duration ( some time delay before reacting ).
So as a result the number the tacx said I was giving back to the house was backed up by the smart meter, So if that was a close result then why would the overall power it said I was producing be wrong. ??
Important... I unplugged the fridge / freezer and turned of home heating, so nothing which could automatically turn on or off could influence the smart meter, Prior to starting the test the house was pretty much static in electricity usage.

And now the real surprise.
When I calibrate my Stages it calibrates to 886 ( it is supposed to calibrate at 900 , but stages say +-50 is acceptable ) . The power readings on the Stages were 15% lower than the Tacx, is it a coincidence that the Stages calibrates 14 points lower than it should and shows to be a 15% lower reading ??

To make matters worse here, my previous Stages did have a problem and was sent back, however prior to it being returned it started calibrating higher than 900 and my power readings were going up accordingly. Just before It was returned it was calibrating near 1000 ( I don,t remember the exact number ) and was showing my power to appear almost double ( Yes 100% higher than the Tacx ) , Coincidence ??

All I can say is based on my own experience and what little testing I have done if you have a Stages take note of where it calibrates , if it is below 900 you will be getting a lower than real life and if its over you will be getting a higher than real life reading.

By using the word "calibrating" when referring to the stages, I do of course mean Zero offset.


Something else to change things up.
My pedalling style may well be influencing the stages, I know I get higher power results when I deliberately pull up hard on the left pedal.
My cadence according to my independent cadence counter AND tacx always show exactly the same which is 5-6 higher than the stages.
At this point in time I still don't trust either to be really accurate, but I have more trust in the Tacx than I do the stages.

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kostyap
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Re: Ironman Power compared to Stages Power

Post by kostyap » Thu Feb 02, 2017 4:36 pm

andyarceye wrote:
Thu Feb 02, 2017 3:02 pm
...I was feeding back 250 - 300 watts back to the house showed a drop on power usage on the smart meter by very close to the amount the Tacx said I was giving back for the same duration ( some time delay before reacting ).
So as a result the number the tacx said I was giving back to the house was backed up by the smart meter, So if that was a close result then why would the overall power it said I was producing be wrong...
It goes like this:

Actual mechanical power when pedaling is converted to electrical power being fed into your grid.

In between there are:

1) Mechanical losses: basically what you apply to pedals and what reaches motor's shaft are quite different values. Calibration supposed to account tor this but is prone to drifting, tire slip, etc. etc.
2) Electrical losses. Motor working as a generator has some losses in winding that depends on current RPM and temperature, then you have losses in electrical circuitry that back-feeds your power grid and so on.

So due to (1) and (2) the mechanical power you are producing and electrical power being fed to your grid could be 2 very different values.

andyarceye
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Re: Ironman Power compared to Stages Power

Post by andyarceye » Fri Feb 03, 2017 1:57 pm

Kostyap:

I have no doubt the amount produced by me and the amount given back are different, but that isn't what I said.
I wasn't producing 250 - 300 watts. that is what the software said I was giving back after what I assume is some sort of calculation to account for losses, which is quite a bit lower than what was being produced.

But as I said, The amount the Tacx software said ( calculated ) I was giving back was very close to the amount the smart meter showed as a reduction in usage.

When I have a 100W light bulb on in the house and I turn it off the smart meter shows the drop in usage with very close accuracy, when the tacx said I was giving back to the house, the amount it said I was giving back was also quite accurately represented by the smart meter ( I also mentioned it was delayed a little ).

But that wasn't the point of the post.
The point was, and still is.
I never trusted the Tacx compared to the Stages when I first got the tacx ( I ignorantly assumed the stages was correct ). But since I have another way of checking the tacx results, my doubts have changed towards the stages being inaccurate depending on where it calibrates ( zero offset ), 900 gives pretty accurate results, higher than 900 gives high readings , lower than 900 gives low readings. Example based on my experience, 910 will give approx 10% higher results 890 will give approx 10% low results.

I guess the moral is, If you buy a power meter of any kind never trust its actual numbers they may not may not be complete BS, use them only as relative numbers, if your ftp improves by X% while using the same meter with the same zero offset, it has improved, but the actual numbers are irrelevant.

What I would like to try. is a pedal based power meter ( garmin vectors perhaps ) so I can get a left right balance check and also some results to compare with what I already have, but, as I use speedplay pedals there isn't really an option on the market that I am aware of, Brimbrothers were working on a shoe based power meter based on speedplay, but it appears they have closed, I wonder if they refunded everyone's upfront payments ?

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malfukt
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Re: Ironman Power compared to Stages Power

Post by malfukt » Fri Feb 03, 2017 5:26 pm

Kostyap :D :D good informative reading.. had smart meter for 18mnths and never crossed my mind to see if tacx powerback was correct..

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kostyap
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Re: Ironman Power compared to Stages Power

Post by kostyap » Fri Feb 03, 2017 5:58 pm

malfukt wrote:
Fri Feb 03, 2017 5:26 pm
Kostyap :D :D good informative reading.. had smart meter for 18mnths and never crossed my mind to see if tacx powerback was correct..

Vary use between GENIUS & NEO mainly using TCA (app) which doesn't show powerback (unlike tts which does) so never even checked if NEO does give powerback
Actually what they report electrical power being pumped back to grid could be accurate as electric values and can be measured nicely. The discrepancy is between that number and actual mechanical power.

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