Comparison Fortius & Powertap

22. Februar 2010
2 Comments

There are lot’s of discussions around the accuracy of Tacx Fortius & i-magic wattage outputs and their calibration. Last week i did my own comparison, by doing an LDR VCF Ride and recording wattage in addition to my Fortius’s own recording with a Powertap powermeter. The ride was paced (until 5km to go) and all calibration & scale settings have been untouched, which is an obligation when riding in virtual leagues. For the sake of easier comparison the Powertap file has been flattened by using 5 sec power averages (which is the format of the Fortius HRM file too).

The results:

Fortius shows always more watts than Powertap does. Overall averages have been: Fortius 274W, Powertap 210W, 65W average difference.

Difference in watts Powertap to Fortius:

Difference is low in the beginning (warm-up phase of Fortius break) and increases constantly until first climb is over. Biggest differences of over 100W in steep downhill sections, where i – basically – didn’t pedal at all.

Zoom into a hilly section with lot’s of power spikes:

Difference small on power spikes, because Fortius is too slow to follow those spikes so that Powertap catches up to generally too high Fortius power outputs. Difference is high, when going downhill.

Comparison of flat sections:

Little difference in 1st flat part during warm-up phase of the Fortius break. Difference seems to depend on speed, because it’s less, on the 4th, slower section. Interesting is comparison of Powertap output on 1st & 3rd section, which both have been at same avg. speed: Looks like not just displayed wattage of Fortius is going up, but also resistance of the Fortius break is becoming less over time, thus requiring less power to ride at the same speed, when the break is warmed-up.

Merge Powertap wattage into Tacx HRM file

22. Februar 2010
Add Comment

If you’re using a Tacx VR trainer for your indoor trainings, you might find the following how-to useful, which describes on how to merge a Tacx HRM export file with a Garmin Edge 705 TCX file (which records power while workouts via ANT+ from a Powertap Elite+ powermeter).

Why you might need this: Accuracy of displayed wattages on Tacx VR trainers isn’t very good. Even if you calibrate your trainer frequently, there are several factors which cause wrong wattage outputs during workouts. e.g. motor break heating up. The best way to get comparable results is to use an SRM or Powertap powermeter. Unfortunately the Tacx software can not directly read data from those powermeters. Only way to get this done is to record the same workout/ride in parallel with the Tacx software and the powermeters computer (in my case a Garmin Edge 705) and merge the files afterwards manually before you can analyze your ride on your favorite analysis software.

Prerequisites:

  • HRM file, exported from Tacx Analyser (in this example named „tacx.hrm“)
  • TCX file from your Garmin device (in this example named „edge.tcx“).
  • Now comes the tricky part: You need to have Perl installed and the CPAN module DateTime (and i think one more CPAN module of which i can’t remember the name – You’ll get notified during installation anyway).
  • Download and install the latest version of the perl Workout library, a great set of tools to manipulate HRM, TCX, GPX, … files.
  • If you don’t know how to install Perl, a CPAN module or a perl library you can try to search on Google for howto’s and instructions. But most likely it’s better if you stop at this point and go find a friend who’s familiar with Perl and helps you out.

If you have the prerequisites done, let’s merge the files:

  1. Open the tacx.hrm file with a Texteditor and write down the StartTime (line 6).
  2. Open the edge.tcx file with a Texteditor and write down the StartTime which is within the first lines of the file.
  3. Close both files.
  4. Calculate the difference between the StartTimes in seconds. Note: Edge saves in GMT and Tacx in local time-zone. You need to respect this. It’s just about finding the difference between the 2 clocks which is typically in the range of a few seconds. e.g. in my example files: Edge: 7h26m04s, Tacx: 8h27m50s. Difference = 106s.
    Note: If you take care that both of your devices have set the same time in their settings, you can skip this step.
  5. Start the convert tool:
    wkmerge –otype=hrm –fields=ele,dur,cad,dist,time,hr –recint=5 –delta=106 –debug edge.tcx tacx.hrm merged.hrm
    This takes the edge.tcx file and merges Elevation, Duration, Cadence, Distance, Time and Hearrate from the tacx.hrm file into it. The output file is called merged.hrm, the format is hrm using a 5 sec interval and a 106s delta between the two files.

That’s it. It’s quite a lot of manual work, but at least for me this method works and produces nicely merged files, which is great help if you want to deeper analyse your performance on a specific ride.

The „perl Workout libraries“ are able to read Powertap CSV files as well as SRM files. Therefore the method described above should also work for SRM & Powertap computers as well as for Polars WIND system.

Sport

20. Oktober 2009
2 Comments

Hier klicken um die Blogbeiträge aus der Kategorie Sport anzuzeigen

Habe mein Leben lang Sport betrieben und unterschiedlichste Sportarten wettkampfmässig ausgeübt. Am intensivsten Volleyball – wo ich einige Jahre in 1. & 2. Bundesliga gespielt habe. Nachdem ich meine Volleyballkarriere 2007 beendet habe (Na gut, im Winter/Frühjahr 2009 habe ich ein kleines Comeback gegeben …) bin ich auf der Suche nach neuen Herausforderungen. Neben Familie und Job ist das Zeitbudget beschränkt, trotzdem zieht es mich zu – leider sehr zeitintensiven – Ausdauersportarten hin: Radfahren, Langlaufen, Marathon … Liegt wohl daran, dass ich immer älter werde.

Das beste an den Ausdauersportarten ist aber, dass man sie  super messen und technisch erfassen kann. Stichwort GPS. Verwende eine Reihe von Software & Hardware Tools, die das Trainingseinerlei abwechslungsreicher gestalten:

Jahresleistung Ausdauertraining

Germknödelindex* 2012:

gki2012

h2012

km2012

* Der Germknödelindex gibt an wieviele Germknödel man pro Tag zusätzlich essen müsste um den durch das Training entstandenen Energieverlust wieder auszugleichen. Auf Basis 500kcal/Germknödel und einer 1:1 Umrechnung von geleisteten kJ auf kcal (= ca. 24% Effizienz) laut Powermeter (nur Rad) sowie einem geschätzten Kalorienverbrauch von 800kcal/h (Lang)laufen.

Über mich

16. Oktober 2009

Beruflicher Werdegang

Sportliche Karriere

Habe mein Leben lang Sport betrieben und unterschiedlichste Sportarten wettkampfmässig ausgeübt. Am intensivsten Volleyball – wo ich einige Jahre in 1. & 2. Bundesliga sowie Beachvolleyball gespielt habe. Nachdem ich meine Volleyballkarriere 2007 beendet habe, beschäftige ich mich seit 2010 mit Ausdauersport – im speziellen dem Rennradfahren. Seit 2011 fahre ich auch bei Lizenzrennen.

Germknödelindex* 2021:

*) Der Germknödelindex gibt an wieviele Germknödel man pro Tag zusätzlich essen müsste um den durch das Training entstandenen Energieverlust wieder auszugleichen. Auf Basis 500kcal/Germknödel und einer 1:1 Umrechnung von geleisteten kJ auf kcal (= ca. 24% Effizienz) laut Powermeter (nur Rad) sowie einem geschätzten Kalorienverbrauch von 800kcal/h (Lang)laufen. Ab 2015 Kalorien laut strava.com/veloviewer.com.

P.S.: Habe bewusst nichts von meiner Familie erwähnt. Das ist Privatsphäre und soll auch so bleiben.