Tacx genius - fantasy watts

Anything about Power (watts)

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kikko
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Tacx genius - fantasy watts

Post by kikko » Sat Dec 16, 2017 12:32 am

Hello.

Some weeks ago I bought a tacx genius, reading that the power readings will be withing 5% accuracy.

Having done quite some hours on it my conclusin is clear: the wattage readings are way off. I started cycling in june this year and relying on genius wattage reading im producing 350 watts FTP at 88kg.

Reading online these results are very unlikely.

I have tested multiple softwares, using both ant+ and BLE with the same results. I have performed freequent calibrations and even checking the calibration after longer sessions to rule out "cold" wheel or motor.

What can I do to verify wether it is wrong or not? what can I do to correct this issue? and just for information, im not going to put down a 1000 dollars in a powermeter to verify wether a new product is faulty or not.

Thanks

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Re: Tacx genius - fantasy watts

Post by mcorn » Sat Dec 16, 2017 1:16 am

I think 5-10% is about the best you can expect. Without a separate power meter, it is almost impossible verify. I think if you already have a semi-accurate idea of your true FTP, that gives you some notion of the accuracy of the Genius.

Sometimes you can get better prices on power meters. I have an older ANT+ PowerTap wheel/hub set that I got for less than $400 on closeout from Performance Bikes. With an ANT+ power meter like this, all power reporting is based on the power meter and not the brake, so it will be a lot of more accurate.

I assume your tests are not on courses with downhills as the Genius motor function may contribute to inaccuracy in power readings, but I can't say for sure.

Perhaps others can give you feedback on how reliable they find the Tacx FTP test without a power meter.

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kostyap
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Re: Tacx genius - fantasy watts

Post by kostyap » Sat Dec 16, 2017 1:17 am

In order to measure power you need power meter. It is either part of the trainer/software combo as in your case and may be inaccurate or the real one. If you do not want to buy one invite friend with the power meter for visit. Otherwise are no miracles here.

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Alex Masalovich
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Re: Tacx genius - fantasy watts

Post by Alex Masalovich » Sat Dec 16, 2017 12:16 pm

For a simple evaluation of the correctness of the power measurement, a power meter is not required. Use the bike calculator.

A simple solution: set the slope to 0% and maintain a speed of 30 km/h. What power is transmitted from Genius?
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kostyap
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Re: Tacx genius - fantasy watts

Post by kostyap » Sat Dec 16, 2017 1:05 pm

Alex Masalovich wrote:
Sat Dec 16, 2017 12:16 pm
For a simple evaluation of the correctness of the power measurement, a power meter is not required. Use the bike calculator.

A simple solution: set the slope to 0% and maintain a speed of 30 km/h. What power is transmitted from Genius?
LOL. Trainer can transmit absolutely legit power while the real power (the one you put into bikes drivetrain) can be drastically different

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Re: Tacx genius - fantasy watts

Post by Alex Masalovich » Sat Dec 16, 2017 6:51 pm

kostyap wrote:
Sat Dec 16, 2017 1:05 pm
LOL
I'm talking about a simple primary evaluation that does not require expensive equipment. And I do not see anything super-funny here.
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Re: Tacx genius - fantasy watts

Post by kostyap » Sat Dec 16, 2017 7:07 pm

Alex Masalovich wrote:
Sat Dec 16, 2017 6:51 pm
I'm talking about a simple primary evaluation that does not require expensive equipment. And I do not see anything super-funny here.
Trainer/firmware/software combo are using the same/similar formulas as those online calculators for their power/resistance calculation. If you drill down those formulas you would find that somewhere along the way there is a Torque/Pedaling Force/Whatever equivalent is included and that is the key. In case of Genius they do not measure torque directly using strain gauge or any other similar tech. What they do instead - they derive torque based on measurements of some electric values taken from motor. In the best case the accuracy of such measurements could be acceptable but sometimes could be be showing water level in my kettle instead.

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Alex Masalovich
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Re: Tacx genius - fantasy watts

Post by Alex Masalovich » Sat Dec 16, 2017 7:23 pm

kostyap wrote:
Sat Dec 16, 2017 7:07 pm
Trainer/firmware/software combo are using the same/similar formulas as those online calculators for their power/resistance calculation.
Must use. I suggest checking it out first.
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Re: Tacx genius - fantasy watts

Post by kostyap » Sat Dec 16, 2017 7:38 pm

Alex Masalovich wrote:
Sat Dec 16, 2017 7:23 pm
kostyap wrote:
Sat Dec 16, 2017 7:07 pm
Trainer/firmware/software combo are using the same/similar formulas as those online calculators for their power/resistance calculation.
Must use. I suggest checking it out first.
And I suggest better understanding of a subject. They can convert formulas to internal tables instead for better efficiency or do any other transform they wish for the same sake. It does not matter. There are frigging laws of physics involved and once you use volts and current on the motor instead of for example values coming out from strain gauge you are risking loosing great deal of accuracy.

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Re: Tacx genius - fantasy watts

Post by mcorn » Sat Dec 16, 2017 8:23 pm

The other thing that might make the Genius less accurate (like the Fortius before it) is it is operating simultaneously as a motor and brake. So with no direct, independent measurement of torque on the spindle, it seems like there are a very large number of variables and assumptions in this type of application. As I recall, when I had my Genius on the same bike as the PowerTap, there were much larger variances in reported wattage in courses that had significant downhills compared to those that did not.

In addition, it always seemed that the run-down/coastdown calibration procedure was somewhat suspect. By comparison, the calibration for direct torque sensors seems a little more straightforward. But even with those, I think the bicycle torque sensors are typically calibrated at zero torque rather than over a range of inputs, which I would think would be more accurate.

simondk
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Re: Tacx genius - fantasy watts

Post by simondk » Fri Jan 26, 2018 9:45 am

I have the same situation. Tacx smart genius, and not trusting the consistency of the power data at all.
This week I finally bought a crank power meter (4iii precision - left only). And I have done some comparison using Zwift and BigChainVR:
- in ERG mode, the power from Tacx seems much too high. And the larger the gear I use, the bigger the power difference. Up to ~50 Watts!
- in normal mode the power from the Tacx and 4iii are closer: 4% average more on Tacx. The Tacx seems generally more generous going downhill.

With Zwift and the Tacx in ERG mode I first noticed the issue with FTP tests. Done in the small chainring I get an FTP of 285W. In the Big ring 301W.
And when I am climbing the Zwift mountain in normal mode I am at about 275W for 22 minutes (same perceived effort: FTP = 261W).
I also climbed the Alp d'Huez using BigRingVR. Effort was maximum for me. It took me 1h and my average power from Tacx was 245W.
So a lot of different numbers for my FTP...I'm pretty sure my FTP is no where near 300W (I wish it was...).

I know one can say that a left only crank power meter can also be off due to imbalance. But at least the power, I think, will be more consistent whether going up or down hill, and whatever gear is used for ERG mode.
Yesterday I setup Zwift to use the 4iii as power and cadence source, and the Tacx genius as smart trainer for the resistance. Worked great!
And the power figures reported versus effort and heart-rate felt much more realistic.

So my feeling of the Tacx being way off the 5% accuracy claimed seems confirmed. Be it not the absolute power (as my 4iii reference can also be off), but the consistency of power versus gear and terrain. My feeling, as early posted by someone, is also that it is due in some way to the electronic spin motor. The motor is kicking in more in downhills and, I think also, in ERG mode with big gears. This seems to be adding to the measured power. Some kind of mechanical doping I suppose ;-)

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