Test request : Fortius against SRM/Powertap

Anything about Power (watts)

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Exciton
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Test request : Fortius against SRM/Powertap

Post by Exciton » Sat Mar 07, 2009 11:48 pm

An often discussed subject among Fortius users is how Watts measured in Fortius compare to those measured with a powermeter like SRM or PowerTap and there doesn't seem to be any consensus on the accuracy of the Fortius.

So wouldn't it be a great idea for Tacx to do the actual comparison and post the results in this Forum?
A test could be to run the Fortius in Catalyst Watt mode and compare values from 100W to 600W or so.

carlocau
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Post by carlocau » Sat Apr 04, 2009 2:08 pm

Yesterday I rent a powertap wheel in order to calibrate my Fortius.
As many in the forum knows I have a Polar power sensor (Cs 600 wind wireless) but I was not sure of results due to variability of data recorde by polar on roller in function of rpm, cog etc.
So yesterday I had the final answer: my fortius is really precise while Polar on roller is a waste of time.

I run for 10 minutes in order to warm up the whole system after that I calibrate the roller with tyre pressure at 8 bars. I turn the knob in order to reach a calibrate value around 0 (0,1 for the sake of precision).

I made some test in ergo mode, but I have seen that it is slighty depending from speed. If you set f.i. ergo at 200 watt and you pedal with 39x23 at around 16kph (in this gear polar is pretty accurate and coupling quite well powertap) the real power is arpound 193 watt, while when you do the same exercise (200 watt costant with a 53x19 at 30kph the real watt become 203 205.
Anyway it is possible to admit that at a costant power of 200 watt and scale factor to 0 the real power is in a range from 195 to 205 watt .

At this point I made a test in order to find the correct scale factor for my Fortius.

I work in this way: I made three test of 2min at 5% slope mantaining a costant speed. 1 time at scale factor 80, the other at sf=100 and the last at sf 120.

well the results are sumarized as follow:

sf pt polar tacx
120 163 160 218
100 207 206 216
80 249 262 215

I put data in a graph and I note that : first of all the powertap record is completely linear (so it means that scale factor works lienar and it is good) besides polar is not linear, due the fact that it records high values in high power range.
I don't know how to insert pictures in the forum, i f somebody explain to me I'll post all I have made.

As final result I test a ramp test starting from 100 watt to 375 (was the maxiumum I was able to reach! with scale factor 100

The fortius slope and powertap slope are identical with a shifting along x axis due to scale factor difference.

I definitively say that in my case a scale factor of 96 give a perfectly correspondance of tacx watt in respect of powertap.

It is note that powertap doesn't keep into account for drive losses (chain etc) so this 4% could be reasonably match with that amount.

It is also clear that the test was done in relatively short durations and that long rides could influence the tyre calibration. this could imply a reduction of actual watt in respect of reported by Fortius unit.

Carlo

mcorn
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Post by mcorn » Sat Apr 04, 2009 7:30 pm

I have had a PowerTap on my trainer for over a year and regularly compare it with Fortius VR and TTS. I have my TTS Scale Factor to 98 (equivalent to setting Fortius VR Scale Factor to 102) and it generally is within a couple percent of average wattage for the entire ride. We also use a CS600 with power on our outside bike. I don't think it is very accurate, but more of a rough guide.

mlang46
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power tap vs. fortius.

Post by mlang46 » Sun Oct 25, 2009 9:58 pm

I have not gotten anywhere near as good a results as others on this forum comparing the power tap to Fortius nor given the mechanics of Fortius would I expect a better than 10 percent repeatability. Some days it will match well and other days it can be off by 30 percent on average and going downhill it does not match very well at all

According to Phil, the Schleck brothers are insistent that Tacx incorporate power into the Bushido and TTS software through the Antares interface so power can be read directly from a power meter and when they do that the problem will be solved. Meantime I observe the power on the screen and use the heart rate as a measure of intensity because in my opinion, the Fortius is not repeatable enough to train with power.
tacx Imagic fortius owner

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ms6073
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Post by ms6073 » Mon Jan 25, 2010 8:39 pm

I realize this is an older topic but I respectfully am going to have to disagree. While performing some impromptu tests this past weekend to resolve questions about the weight value in a riders profile in TTS 2.0, I used an SRM K-Force Light (wireless) with PCVI which by most is considered to be the 'gold standard' for power meters to adjust the scale factor to within 1-2 watts of the SRM.

To address Mcorn's statement about the accuracy of the Polar CS-600 WIND power sensor, I will have to agree that it is not as accurate as the SRM or Powertap but it is close (+/- 2.5%) based on my personal experiences using it on the wide and my TT bikes. When properly setup, the results are consistent and repeatable but from my perspective, since the Polar unit uses the vibrations of the chain, this method of power measurements tends to make it unsuitable for use while the bike is installed on a TACX (or any stationary trainer) since there is significantly less vibration from road shock and such while the bike is on the trainer.
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mcorn
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Post by mcorn » Mon Jan 25, 2010 9:11 pm

I recently hooked our outside tandem with the Polar CS600 on it to the Fortius trainer. I also have a PowerTap hub, so I am able to compare all three systems in terms of wattage.

I actually think the Polar works a lot better on the trainer compared to outside because there is not a lot of road and other vibration affecting the chain. Things like road shock would degrade, not improve CS600 accuracy.

There is still a fair amount of variation between all three systems in reported wattage. I am assuming that the PowerTap is the most accurate. One thing I have noticed is that during the first 10 minutes of use, the Fortius is reporting considerably reduced wattage.

I have not done systematic comparisons of all three systems (don't really have the time or energy). For me, approximate (plus or minue 5%) is good enough, although I appreciate others may want a lot more. In addition, rather than try to compare instantaneous wattage, which is almost impossible due to different sampling and smoothing functions in the system, I compare average wattage at the end of the ride.

In order to maximize Fortius accuracy, you would need to recalibrate before each ride after a good warmup and be sure that all starting variables (tire pressure, tension, etc.) are the same each time. All of that is too much trouble for me as well. I usually leave everything the same in stand and check tire pressure (150 psi) about once a week.

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ms6073
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Post by ms6073 » Wed Jan 27, 2010 8:05 pm

mcorn wrote:Things like road shock would degrade, not improve CS600 accuracy.
That is correct but can be somewhat corrected by a CS 600 setting that sets update time to more conservative values. My assertion about higher power output is from the wifes observations riding our analog trainer with the CS 600 in which she routinely caps her 60-minute average power achived on the road by 10-15%. Of course without variables such as wind and cold to deal with, it may be that she is simply that much more efficient in the static trainer environment versus the real world.
I usually leave everything the same in stand and check tire pressure (150 psi) about once a week.
Nice, the tube in my trainer tire (Vittoria Ultralight) losses as much as 10-15 psi overnight thus the tire gets inflated before each trainer ride while the PC is powering on.
TTS 4.19.1 | T2800 NEO Smart
MSI GS70 Laptop | Windows 10 x64 | Intel Core i7-4710HQ 2.40 GHz | 16 Gb DDR3 1600 MHz | GeForce GTX 870M
Samsung 32" LED TV

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