Forum home› Road cycling forum› Training, fitness and health

Tacx flow, power calibration with SRM or Powertap

chas.readmanchas.readman Posts: 19
edited January 2010 in Training, fitness and health
Hi
has anyone calibrated a tacx flows power against a SRM or Powertap system?
I have been told these are the only believable power meters and wondered how my tacx flow compared.
If the flow is set up at say 0 offset, does anyone have SRM readings for say 100, 200, 300 and 400 watts?
Cheers Chas

Posts

  • amaferangaamaferanga Posts: 6,789
    I think you'd have to calibrate your Tacx Flow as it'll probably be different to someone elses. Another Tacx Flow could easily read +/- 5-10% compared to yours.

    The important thing is that you make it repeatable.
    More problems but still living....
  • chas.readmanchas.readman Posts: 19
    The Tacx flow can be calibrated each time it is used, I have found it needs to be warmed up for repeatability. I just wanted to know if anyone had put a SRM or powertap system on a flow and how the values compared.
    Thanks
  • AnonymousAnonymous Posts: 79,667
    chas.readman wrote:
    The Tacx flow can be calibrated each time it is used, I have found it needs to be warmed up for repeatability. I just wanted to know if anyone had put a SRM or powertap system on a flow and how the values compared.
    Thanks

    I did, I was 12 watts more powerful than what both were reading.
  • amaferangaamaferanga Posts: 6,789
    chas.readman wrote:
    The Tacx flow can be calibrated each time it is used, I have found it needs to be warmed up for repeatability. I just wanted to know if anyone had put a SRM or powertap system on a flow and how the values compared.
    Thanks

    Have a look at the post by liversedge.

    I have an Elite Crono digital turbo trainer with a power display and (ignoring the fact that it takes about 30 - 40 minutes to stabilize :cry: ) I'd estimate that it over-reads by around 10% (based on my sustainable power from a research study I took part in).
    More problems but still living....
  • lloydy75lloydy75 Posts: 78
    I had a blast on my Tacx Flow earlier today and recorded a 5 mile effort after calibrating both my turbo and Powertap.
    During the warm up when the Flow was reading 200w the PT was around 170w with the Garmin recording the 30 second average. During the effort, the average on the PT was 292w and the turbo was reading around 335 to 350 with the turbo set at +1.
    Hope this helps.
  • chas.readmanchas.readman Posts: 19
    Thanks lloydy thanks exactly the information I was after. I can now compare my power output off the turbo with power data and see where I am at.
    Cheers Chas
  • lloydy75lloydy75 Posts: 78
    With the icy weather I did another turbo session yesterday, intervals this time, and it was worth noting that with the deviation on the Flow and PT that come the race season if you're NOT using a PT and you are doing that interval session and feeling good remember that those numbers you are looking at are below PT users so push harder and gain more! ( as you should do any way!)
  • amaferangaamaferanga Posts: 6,789
    Another point to note would be that its unlikely that any two Tacx Flows will have an identical power curve.
    More problems but still living....
  • chas.readmanchas.readman Posts: 19
    amaferanga wrote:
    Another point to note would be that its unlikely that any two Tacx Flows will have an identical power curve.

    Why.
    The resistance is electromagnetic and you can calibrate on startup using tyre pressure / resistace. I am aware they over read. I don't have a power meter for the road bike but I find the power output useful for longer intervals, I know it doesn't measure power instantly and is no good for very short intervals.
  • AnonymousAnonymous Posts: 79,667
    chas.readman wrote:
    amaferanga wrote:
    Another point to note would be that its unlikely that any two Tacx Flows will have an identical power curve.

    Why.
    The resistance is electromagnetic and you can calibrate on startup using tyre pressure / resistace. I am aware they over read. I don't have a power meter for the road bike but I find the power output useful for longer intervals, I know it doesn't measure power instantly and is no good for very short intervals.

    As long as it is consistent then nowt else matters really!
  • amaferangaamaferanga Posts: 6,789
    chas.readman wrote:
    amaferanga wrote:
    Another point to note would be that its unlikely that any two Tacx Flows will have an identical power curve.

    Why.
    The resistance is electromagnetic and you can calibrate on startup using tyre pressure / resistace. I am aware they over read. I don't have a power meter for the road bike but I find the power output useful for longer intervals, I know it doesn't measure power instantly and is no good for very short intervals.

    Unless I am missing something, the calibration is ensuring your turbo is repeatable. I can't see how it would make it identical to another Tacx Flow.

    But as NapD says, its the consistency that matters.
    More problems but still living....
  • andrewgturnbullandrewgturnbull Posts: 3,861
    amaferanga wrote:
    Another point to note would be that its unlikely that any two Tacx Flows will have an identical power curve.

    Hi there.

    'Curve' is the most important word in this sentance. It's not like the Tacx will always over-read by x% or y watts - the relationship is not linear. If this was the case, then Tacx would have sorted their calibration to be accurate all the time!

    In tests that my boss did with his power tap on my Tacx flow, the flow consistently over-read up to about 300w, where it seemed to be pretty much spot on, then under-read above that.

    Hope that helps!

    Cheers, Andy
    http://www.andyturnbull.co.uk
    http://www.stirlingtri.co.uk
  • andrewgturnbullandrewgturnbull Posts: 3,861
    ps Computrainer consistently under-reads compared to both power tap and SRM. I found this out to my cost in the uni lab last week...
    http://www.andyturnbull.co.uk
    http://www.stirlingtri.co.uk
  • dnrdnrdnrdnr Posts: 27
    Flow overreads by 40-50 W compared to PT
  • RChungRChung Posts: 163
    chas.readman wrote:
    has anyone calibrated a tacx flows power against a SRM or Powertap system?
    Yes, but I'm not sure my experience is typical of all Flows since mine is an early model and perhaps they've improved since then. We discussed this on the Google Wattage list back in 2004 when the Flows first came out. Basically, it was wildly optimistic (but consistently wildly optimistic) in trainer mode; however, in ergo mode the load generator was just overmatched and the Flow was horrendously, astoundingly, laughably, unusable. Here's a summary plot:
    tacx_correction.png
    The plot shows the relationship in ergo mode between the "set" power, the wheel speed, and the error when compared to a verified PT. Just focus on the black dots for now. The y-axis is the %error, the x-axis tells you the ratio between set power and wheel speed.

    If ergo mode worked well, set power and wheel speed would be unrelated. That's not what we see--in fact, the relationship between set power and wheel speed is very strong (and negative). That means as wheel speed increases, the Flow becomes less and less able to increase load in order to hold set power constant.

    You can adjust the "scale" factor on Tacx ergos, and that's what the red and green dots are. As you can see, they just move the curve up and down but don't change the slope of the black dots.
  • guv001guv001 Posts: 688
    amaferanga wrote:
    Another point to note would be that its unlikely that any two Tacx Flows will have an identical power curve.

    Totally agree

    On my own flow calibrated to 0 my PT is about 25 - 40 lower.

    I borrowed a Flow over Xmas same bike etc calibrated to 0 this time the PT was 70 - 80 lower.
  • lloydy75lloydy75 Posts: 78
    I did the Fight club Sufferfest session the other day, an hour workout after calibrating and the flow was reading 380 when the PT was 310 for the interval parts.
    My Flow is about 5 years old with nearly 5000 miles on it, not sure if newer ones are more accurate.
Sign In or Register to comment.