Tacx Basic- Trainers Accuracy In Measuring Power?

[marlon1]

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.

 

[acoggan]

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.