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19-02.-2004, 08:48 PM
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#16 | |
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Community Team
Join Date: Nov 2002
Location: Newport, South Wales
Posts: 3,830
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Quote:
see my first reply (2nd message) in this thread ric
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19-02.-2004, 09:01 PM
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#17 | |
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Registered User
Join Date: Oct 2003
Location: UK
Posts: 211
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Quote:
sound to me like you are confirming such. that's also like ric is agreeing with me. wow! |
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19-02.-2004, 09:09 PM
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#18 | |
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Community Team
Join Date: Nov 2002
Location: Newport, South Wales
Posts: 3,830
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Quote:
Maybe i'm not understanding what you've wrote... however, i'm saying that the Tacx units i've tried over estimate power at high power such that your real power is lower than the Tacx says (whereas you're saying it's higher 410 to 480 W). In other words where the Tacx says you're doing 410 W, a PT or SRM would say (e.g.) 380 W. Ric
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19-02.-2004, 09:40 PM
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#19 |
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Registered User
Join Date: Oct 2003
Location: UK
Posts: 211
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ah - that's what i expected you to say! Sometimes you just expect a negative answer from one particular person. C'est la vie.
I guess we will all know once the PT arrives. Probably then the figure it gives will be wrong! |
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19-02.-2004, 10:10 PM
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#20 | |
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Community Team
Join Date: Nov 2002
Location: Newport, South Wales
Posts: 3,830
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Quote:
The data i presented (based on extensive use of several Tacx units, not a Flow) was prior to your involvement with this thread - if you're trying to infer that i make negative comments in relation to you (which i'm not). Out of curiousity how did you arive at the conclusion that the Tacx is underestimating by such a large amount - 18% (410 to 480 W) I've never seen a PT give incorrect data with these exceptions 1) make sure the torque/zero offset is correct 2) the torque tube isn't damaged One of the great points about the PT is that the data is always correct. i've never seen any drift in the strain gauges. however, after some period of time (usually more than a couple of years) the torque tube needs replacing (but it'll be obvious as you'll suddenly be getting really silly data -- e.g. riding steady at 2KW!) ric
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13-03.-2004, 06:37 AM
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#21 |
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Registered User
Join Date: Jan 2004
Posts: 158
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How is it possible there is a relation between the rpm and power difference between PowerTap and the Tacx????
I don't know if the Power Tap uses rpm to calculate the power? If it is, the PowerTap measuremnt is the one with the greatest fault. if it isn't, I think your measurements weren't long enough. |
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13-03.-2004, 07:57 AM
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#22 |
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Registered User
Join Date: Jul 2003
Posts: 2,592
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The PowerTap uses neither cadence nor speed in its calculation of power. However, if the reed switch that counts hub rpm fails (whcih happens occasionally, or at least used to), then neither power nor speed will be read. (Torque, OTOH, will still be, so that is one way of diagnosing the cause of the problem.)
The reason that the OP observed an effect of cadence on the relationship between power as measured by the PowerTap and estimated by the Tacx trainer is likely due to differences in wheel speed between comparisons. The accuracy of devices like the Tacx (and the Computrainer, and the Velodyne) is dependent in part on how well rolling resistance is accounted for...if that correction isn't exact, then the error in estimated power will vary with speed. |
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