Ironman Power compared to Stages Power

Anything about Power (watts)

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andyarceye
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Ironman Power compared to Stages Power

Post by andyarceye » Tue Nov 03, 2015 5:26 pm

I am sure this has been discussed to death and there will not be a complete and correct answer.

For 11 months I have trusted my Stages power meter completely to tell me I am putting out a certain amount of watts. However I recently bought a Tacx Ironman smart trainer which is telling me I am doing mu more power than the stages.

I calibrate the tacx on each use but it constantly a considerable amount higher than the stages, an example would be stages showing 200 and Tacx showing 250. ( not an exact comparison, but now too far wrong )

Now of course I am in a dilemma where I don't know which is correct. Is the Tacx correct and is the stages wrong because it is only measuring 1 leg then doubling ( implying my right leg has way more power but remains un-measured ).

Personally I feel it is the tacx which is wrong for the following reasons
1. I do a real life simulation ride of my commute home, and the tacx has me out of the saddle and 2 gears lower for the same hills as I do in proper real life.
2. While in real life simulation on a 7% hill I am out of the saddle at 6kph. In proper real life I can do in the same hill, in the saddle around 16kph.
3. My 20 minute ftp has jumped from an unimpressive 231 watts ( stages power meter ), to 274 watts ( tacx power )

I do real life simulated rides all the time but I dare not upload them to Training Peaks as it will give me what I believe to be a hugely exaggerated ftp.

From this I can only assume I have missed out on setting something up correctly.

Calibration is done before each use, and I adjust tension until it is right in the middle of the green. I have correctly given my weight of 76Kg ( rather heavy ) beyond this I don't know what else I can set up.

I could, if it is any help upload a ride from the Tacx, which was also recorded on my garmin using the stages power meter, it will show how much difference there is, but I would only keep it there for a short time as I belive it is artificially making me appear to be something I am not, and it would double up on miles.

Andy

thebigoneinfront
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Re: Ironman Power compared to Stages Power

Post by thebigoneinfront » Tue Nov 03, 2015 6:29 pm

Do you calibrate the brake after warming it up? Makes a significant difference. It's best to calibrate after training and then just maintain the same tyre air pressure. There is no need to calibrate every time. If you keep the tyre air pressure constant, calibration every few weeks to compensate tyre wear is sufficient.

risslerp
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Re: Ironman Power compared to Stages Power

Post by risslerp » Tue Nov 03, 2015 7:04 pm

Stages is correct.

andyarceye
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Re: Ironman Power compared to Stages Power

Post by andyarceye » Thu Nov 05, 2015 7:22 pm

thebigoneinfront wrote:Do you calibrate the brake after warming it up? Makes a significant difference. It's best to calibrate after training and then just maintain the same tyre air pressure. There is no need to calibrate every time. If you keep the tyre air pressure constant, calibration every few weeks to compensate tyre wear is sufficient.
I would say "yes" I calibrate when its warm, I normally warm myself up before taking on a real life simulated ride, but I calibrate every time just the same as I do with the stages and I would do with any other power meter.
To be honest, if I was to go through calibration 10 times in a row without touching anything, I would probably get 10 different readings from the low side of the green area to the high side.

I will do as you suggested as of now and see how the power seems, if it appears to be very wrong still, I will just ignore whatever it says.
risslerp wrote: Stages is correct.
Until I used the tacx I would agree with you 100%, I trusted the stages to be correct, but now I have two measuring devices giving very different answers so I have no clue which to trust, if any at all. My doubts in the stages, are due to it measuring one leg then doubling the result, I may be right leg dominant, which means it is giving a low reading based off a weak leg.


I think the best thing for me to do now, is to not trust the tacx power at all as it is calculated, and if Strava's calculated power is anything to go by, then it will not even be ball park, I will only trust the stages as a guide. My answer now if someone asks me my ftp with simply be "I don't know because I can't trust the measuring devices".

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ms6073
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Re: Ironman Power compared to Stages Power

Post by ms6073 » Thu Nov 05, 2015 10:07 pm

andyarceye wrote:now I have two measuring devices giving very different answers so I have no clue which to trust
The one mounted to the crankset on your bike will see far more use than your Ironman, so best to depend on that one. There are so many posts about power discrepancies between Tacx trainers and either TTS or the apps for tablets and smartphones, and Tacx is about the only one of the major application providers that does Not provide a setting in the applications to use power data from an external powermeter to set the brake resistance!
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Re: Ironman Power compared to Stages Power

Post by thebigoneinfront » Fri Nov 06, 2015 12:09 pm

andyarceye wrote:My answer now if someone asks me my ftp with simply be "I don't know because I can't trust the measuring devices".
I think that's the correct mind-set. Few of us humble amateurs outside of labs will have any chance to really calibrate or test powermeters or trainers for accuracy. People typically will more trust what they are used to, which overall will be the PM used outside, but that is psychology, not physics. Overall, I think the best one can do is take these numbers only as relative measurements to follow one's own (hopefully) progress.

zacha
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Re: Ironman Power compared to Stages Power

Post by zacha » Tue Nov 10, 2015 1:46 am

Hi!

have the same problem with the genius smart. (which is the roughly the same as the ironman.) the stages is correct. I have a stages, a 4iiii, a vortex smart and a genius smart. stages, 4iiii and vortex more or less agree on power values. The genius overreads power. I have a support case open with tacx. They sent me a newer firmware for the genius which seems to have lowered the power readings of a bit but still is too high. According to my measurements between 10 and 15%, seems it gets closer the higher your power output is. That is really annoying, I currently run trainerroad with all my workouts set to 112% and record power data from my stages to get accurate numbers. For Zwift I pair my stages too just b/c using the genius would be like cheating, I set personal bests on each and every segment using the genius for the first time, just to see that zwift suggested in the end that my ftp would have improved by 20%.

an option to use external power meter would not be very helpful... as this is related to the software used and you would always have to have a power meter mounted. there should be an option instead for calibrating the power meter JUST IN the firmware of the trainer, so you could do a calibration once with a power meter and afterwards get accurate readings in every software and device. at least letting the user calibrate a dumb +- x percent should be possible. and why this is important? because if you want to use your power data from your trainings to pace a race or time trial you need accurate data or at least CONSISTENT data- not so much of interest if your ftp is really X but very important that each device you are using agrees that it is X.

take a look at the figure attached.
Attachments
genius_smart1.png

zacha
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Re: Ironman Power compared to Stages Power

Post by zacha » Tue Nov 10, 2015 5:20 pm

okay seems I found a way to get the calibration done correctly. Ignore what the Tacx app says about calibration, it is simply not reliable.

- put 8 bar on your trainer tyre
- put your wheel in the trainer and tighten it nearly as much as possible (just so that the screw is not falling out yet)
- warm up the trainer (start the tacx training app set slope to -5 and ride 5 mins at 50kph)
- loose the screw on the turbo for about 3 full revelations (1080 degrees)
- do the calibration in the app. the app should now show "too loose" if not again loosen the screw and repeat (mine just was a bit below the acceptable area)
- leave the calibration and tighten the trainer by about 1,5 revelations (540 degrees)
- set the slope to +-0 and ride at 220-250W for about a minute until you have a steady power reading, compare data to your external power meter data
- if the app shows a too high power value tighten the screw of the trainer by a quarter revelation
- if the app shows a lower value loosen the screw of the trainer by a quarter revelation
- compare data again and repeat until you have reasonable power reading.

DO NOT RECALIBRATE in the TACX app!

Seems I got my genius now spot-on. At least I am able to shift the power reading from the genius upwards and downwards with this method. Will try to compare readings in the evening again and upload a figure. At least the principle should have become clear. Calibrate and afterwards shift pressure on the tyre until reading is correct.

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jpalcover
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Re: Ironman Power compared to Stages Power

Post by jpalcover » Tue Nov 10, 2015 5:28 pm

Make sure you are using the latest firmware version for the Tacx Genius (3.17)

http://www.tacx.com/en/service/software ... s/i-genius#

And the latest version of Tacx Trainer Software (TTS4.16.4)

http://patches.tacxdata.com/

http://patches.tacxdata.com/data/files/ ... 1604_3.exe

Then compare again.
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zacha
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Re: Ironman Power compared to Stages Power

Post by zacha » Tue Nov 10, 2015 7:07 pm

I don't use any TTS software. I just compare ant+ sensor data that should be correct without the need of any specific software. And I have firmware 3.25 which tacx gave to me after I had catastrophic power readings from the genius (with 3.17 installed). All I can say to you after having calibrated the trainer according to tacx' instructions I got the results shown above. And they are miserable. I have much better results now. diagrams will follow. I don't say this is the way to go but it is definitely a posibility to get comparable readings from the trainer which is essential for ftp based training especially in erg mode.

edit. here is a diagram after my latest tests fiddling around with the trainer tension as I decribed above.

While the stages is much more reactive (and possbily always will be as the data from the genius seems smoothened just as the data from my vortex) now AVG power is within 0,5 percent! Remember: the deviation I had after a standard calibration with the tacx app was 12%.

So believe it or not. Tacx say they have tested the genius against an SRM and got proper results but obviously it is just ocassion to get a proper calibration. That is just my 2 cents.

stages genius deviation
avg 193,7682927 194,2987805 0,002737743

that is the avg data to th diagram shown
Attachments
genius_smart2.png

sUpgannto
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Re: Ironman Power compared to Stages Power

Post by sUpgannto » Tue Nov 17, 2015 10:39 am

Easy to read, it is updated throughout most of the site

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ms6073
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Re: Ironman Power compared to Stages Power

Post by ms6073 » Tue Nov 17, 2015 5:31 pm

zacha wrote:an option to use external power meter would not be very helpful... as this is related to the software used and you would always have to have a power meter mounted.
That is confusing. You comment about using TrainerRoad but the power output of the trainer in is not well aligned with that of your powermeter, yet discourage use of the feature intended to eliminate the discrepancy by setting the trainers resistance based on your powermeter. If you have enabled the option in TrainerRoad and still see a power discrepancy, then that would be an issue with TrainerRoad who have admitted on other forums that the ANT+ FE-C implementation for Tacx trainers needs work. Unfortunately they have not been able to address the issues because they do not yet have any Tacx SMART trainers available for testing.

Image

While it is natural to assume that a center-of-the-green zone calibration would be the expected benchmark, realistically this varies from rider to rider and anywhere in the green zone should be considered a valid calibration. What I have derived from posts on the forums is that more powerful and/or heavier riders tend to prefer setting the roller tension on the brake to prevent slipping, which typically means a calibration pointer located more to the right of center. Riders with lower power-to-weight ratios will typically opt for calibration values that are to the left of center. As you have already observed, those with external powermeters wishing to ensure parity with the trainer, may have to spend a bit more time experimenting with roller tension on the brake to find out what produces the best results.
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Re: Ironman Power compared to Stages Power

Post by zacha » Wed Nov 18, 2015 12:24 am

ms6073 wrote:That is confusing. You comment about using TrainerRoad but the power output of the trainer in is not well aligned with that of your powermeter, yet discourage use of the feature intended to eliminate the discrepancy by setting the trainers resistance based on your powermeter. If you have enabled the option in TrainerRoad
I have not. This option does not exist in Trainerroad's current beta. I think they might have removed it I read it was not working well because of the lag it produces. Anyway you are able to pair a power meter so you see the actual power output. Anyway that has no impact on trainer control. So if trainerroad suggests riding at 100% ftp it will just try to set the trainer to 100% of your ftp and actually this will be lower (in the case I described above). So to get a workout correctly adjusted to your ftp you either have to have a trainer that is calibrated correctly or try fiddling around with the intensity setting in trainerroad. But if you choose the latter you will record wrong power data later on, unless you have paired an additional power meter.

But I didn't want to discuss special settings in a particular software. That were just examples of the impact this problem causes. Really no need of advice which workaround I could choose in which kind of software. I won't. I have a smart trainer with ant+ power meter and I want that trainer to give accurate power readings that can't be to much of requirement. And I won't have a power meter on my indoor bike all the time that is just ocassion, that there is one in place at the moment. I have more bikes than power meters and I won't always swap them around just to correct the power readings of my smart trainer.
ms6073 wrote: While it is natural to assume that a center-of-the-green zone calibration would be the expected benchmark, realistically this varies from rider to rider and anywhere in the green zone should be considered a valid calibration.
A valid calibration for me is is calibration where the trainer sends accurate power readings. I cannot reach this with the calibration tool as I described. The calibration is different every time and although I got it always within the "acceptable" area the power data is never correct. Anyway I was able to get it to agree with my stages as I described and I won't touch it until it drifts again.

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Re: Ironman Power compared to Stages Power

Post by ms6073 » Wed Nov 18, 2015 2:14 am

zacha wrote:This option does not exist in Trainerroad's current beta.
Sorry, you have me confused. If the feature has been deprecated, then what are these items?
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Re: Ironman Power compared to Stages Power

Post by zacha » Wed Nov 18, 2015 12:17 pm

didn't I talk about current beta version? I have no access to an I-* device whatsoever, so no idea how it looks in ios. and as I said. it is absolutely not of interest if there may or may not be such option in Trainerroad. this is simply off topic.

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ms6073
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Re: Ironman Power compared to Stages Power

Post by ms6073 » Wed Nov 18, 2015 8:33 pm

zacha wrote:didn't I talk about current beta version?
Apologies. Just traded some texts with Nate from TrainerRoad and PowerMatch is not yet implemented in the current Windows BETA but should be implemented by weeks end.
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Re: Ironman Power compared to Stages Power

Post by zacha » Thu Nov 19, 2015 1:39 am

Okay so might be I misunderstood this but I thought I read somewhere on the web that they would not continue this feature because of the lag you have with it. possibly a just a misinformation. but in general wouldn't you agree that a trainer that has a power meter (or at least gives power readings) should not need an extra power meter on the bike? So I think there should definitely be an option that makes it easier to get accurate power readings from the trainer and furthermore I think there should definitely be more information about this topic because people usually trust that they get correct data from the trainer if they set it up according to the manual and just jump on it and take a ride. but this isn't necessarily true. it isn't even sure that you will always get consistent results from the trainer, e.g. if you have to fit another rear wheel or have to make adjustments to the trainer tightness. I personally did not imagine that the range of the power readings would be that dramatically large. I thought it might be within 5 % or so even if the calibration is not perfectly adjusted, but I learned from testing that you can modify power readings by at least 15% just by making adjustments like described above. And 15% is A LOT. Training 15% harder according to my ftp I could not withstand. Training 15% easier would mostly feel like easy pedalling. And finally I am reviewing power data in golden cheetah for CP estimations. E.g. CP20 estimation 15% too high - means miserably failing in my next TT.

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Re: Ironman Power compared to Stages Power

Post by Arked » Tue Nov 24, 2015 1:04 pm

@andyarceye

Stages is very susceptible to giving wrong total power numbers due to left/right leg imbalance.
2. While in real life simulation on a 7% hill I am out of the saddle at 6kph. In proper real life I can do in the same hill, in the saddle around 16kph.
So both tries are made at the same wattage? If that so this indicates your Stages reads low not the opposite. Take your historical ride data from Strava etc., find couple steep climbs and calculate your estimated power using online power calculator (f.e. bikecalculator.com). Then compare the results to Stages power readings. This might point you in the right direction.

I use Stages myself and after months of checking it with dozens of climbs I'm 100% sure that it reads over 10% low for me. The difference between my Stages and calibrated Vortex Smart is 13%. So in my case trainer readings are closer to real power (in terms of physics) than power meter.

That's why I use Vortex in Zwift;) Not for cheating but because with Stages I would be in serious disadvantage.

Yet I agree that it should be possible to set trainer calibration to agree with power meter readings. In my case it would be simple -13% and I'd have consistent power data. Now I have to record my trainer and power meter data concurrently to be able to use erg mode and keep power data consistency (trainer power data goes to Strava, Stages power data goes to Golden Cheetah database).

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ms6073
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Re: Ironman Power compared to Stages Power

Post by ms6073 » Tue Nov 24, 2015 5:47 pm

Arked wrote:I use Stages myself and after months of checking it with dozens of climbs I'm 100% sure that it reads over 10% low for me
Do you set the zero-offset prior to each training ride? What about powermeter calibration? If my SRM were that far off the mark, I would be hounding the manufacturer for a replacement.
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Arked
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Re: Ironman Power compared to Stages Power

Post by Arked » Sun Dec 06, 2015 10:50 pm

Yes. I zero-offset it prior to each ride. As I mentioned Stages measures only one leg power and then doubles it. So if your left leg is weaker or stronger than the other it will report total power too low or too high in comparison to power meter that measures your total power output. I don't think there is something wrong with my Stages. My power readings are still consistent and comparable between rides.

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