I-magic power measurement

flyingiguana

Hi, all.

I have an I-magic and am trying to use it to measure power. From what I have read so far, it isn't likely to be accurate but I have not been able to track down a thread that deals with this specifically.

Can anyone tell me whether it's just randomly inaccurate, or whether there is an error range that is reasonably predictable? I know better than to try to find any info on the tacx website - which is unhelpful at best!

As in, does its measurement have any relation to real watts so that I can reasonably accurately adjust the numbers to get real numbers, or is it at least internally consistent so that I can at least measure if I'm getting any ..um..more powerful?

Thanks.

AndROOb

I believe that the I-magic can be calibrated. I recently made enquiries at Tacx about the Tacx Flow, and although this turbo's calibration is being investigated, the initial response was that the i-magic was capable of accurate calibration.

jsal

you will be able to find all kinds of opinions on the net. Here is a link where they claim it is accurate

http://www.scientific-coaching.co.u...rs&sub=catalyst

if you go to the Tacx forum and look at the wattage section you will find a lot of info about what it can and cannot do, also how to calibrate etc.

Here's the link

http://www.tacx.nl/phpbb2/

I think

mac_220

The new version of the software i.e. the fortius version has a calibration function which performs a roll to zero test from 19Mph. If you buy one of the real life videos the version of the catalyst included with the video has the new version with calibration.

I read a web site a couple of weeks ago which had done a comparison test with SRM cranks and the Tacx flow and the tacx excell. they found the calibration and accuracey of the tacx excell was much higher than that of the flow. However, this didn't include the I-magic and at the time of they performed the tests the versions of the fortius software was not available and as a result was not included in the tests.

Regards,

Mac

CycleFast

Thanks for the links.

If you look at the graph (there's a link to a graph) where SRM power readings are compared to Tacx power readings, does anybody have an idea why it says "Tacx data (slope=3)" on the top og the graph page.

Is the Tacx power reading more accurate when using slope=3?

_________
CycleFast

flyingiguana

Thanks all. I will do some research. I have also emailed Tacx (the auto-reply message came back in Dutch - I hope the real response comes back in some form of English....)

On the other hand, in temporarily snowy Toronto today, it was wonderful to get out and do some mountain biking in the snow and not give a hoot about wattage....

Spunout

Even if your trainer has some drift, keep it calibrated and keep your tire pressure the same. Design your power workouts around the same ranges derived from your test.

No matter what you call it, as long as you test/train on the same system of measurement. I think I'll re-calibrate my PT to read 450 Watts at threshold

rob of the og

That's right. The power reading is relatively accurate at a slope of 2-3%. If you want to do power testing or power based sessions in Catalyst then you're best off setting the slope to 3% and controlling the resistance with your gears. I never get quite the near-perfect correlation that that graph suggests, but it's pretty close to my PT at 3%. However I find that at lower gradients the iMagic over reads power quite dramatically.

Phil Stone

The week befor Xmas I set up the i-magic again to run some tests for Tacx & I was able to get spot on the same watts as a SRMPro/Powertap between 150 - 450watts (I did not have the legs to go much harder for long enough). Changes have been made to the drivers & software over the months & now I feel they have it perfect now. For my trainer I needed to set the calibration slope to 95 with a Armadillo tire pumped up to 100psi.

This was with a Real Life Video (Col du Galibier) which uses a modified version of Catalyst.

__________________
Phil Stone

www.tacxvr.com
www.tacx.com

Phil Stone

Just found some data on this laptop from a earlier test. Its only 11minutes but it has very variable power (descent of the virtual Col du Peyresourde & the start of the Col de Azet). You can clearly see the power slope is extremely close to the powertap.

http://www.tacx-video.com/Downloads...erTest-PTap.png

http://www.tacx-video.com/Downloads...werTestTacx.png

Next time I train inside I will do a longer ride on a RLV or if not I will rake up the ride I did on the Alpine Classic. Im 100% happy with the new i-magic slope. I need to test different tires & roller settings to see if there is a repeatable way to get it calibrated with out actually owning a shed full of expensive power meters

__________________
Phil Stone

www.tacxvr.com
www.tacx.com

rob of the og

Phil, those plots looks pretty good and are very encouraging. Hopefully this is repeatable, I'm just downloading all the updates now (I thought they only affected the virtual world) and once I get over my cold I'll compare the output to my PT. Presumably the Fortius software roll-down test does away with worrying about the make of tyre and pressure?

The real test will be setting a constant power programme in Catalyst and seeing if the PT keeps reading a more or less constant reading for a wide range of gears and cadences.