lookatmy595 said:
Thanks for the responses folks. Spunout: My PT only seems to have 1 or 2 sec average recording settings. Alex: My average cadence for the hour was 80.
OK, I guessed as much. There is an artifact of the methodology the PT uses in determining power for each data point, known as
aliasing, which would explain the choppy nature of the power reading. If you repeated the exercise at ~ 95rpm, I bet you could make the display read much more smoothly.
Aliasing with a PT:
The PT records a power data point every 1.26 seconds and uses the average of the hub torque readings (sampled at 61Hz) to determine the power for that 1.26 seconds.
When we pedal, we provide torque to the hub which pulses in a sinusoidal manner, with two torque peaks per pedal revolution (one from each leg).
So at a cadence of 43, 95 or 143 rpm the number of torque pulses at the hub will be equal for each 1.26 second time period at 2, 4 and 6 pulses per 1.26 seconds respectively.
It is also a even number of pulses per 1.26 secs at cadences of 71, 119 and 167 rpm but with an odd number of pulses per 1.26 secs (3,5, & 7) - so as long as there isn't much of a leg imbalance, then the power reading from one 1.26 sec interval to another will be consistent.
The further you deviate from those cadences the likelyhood that consecutive 1.26 sample periods will contain different number or fraction of torque pulses (which will raise or lower the average torque recorded for that interval). Since power = torque x cadence, if cadence is fixed then your power reading will vary in step with the torque measurement.
So at 80 rpm (approx mid way between 71 and 95 rpm) the number torque pulses will vary for each consecutive 1.26 second time period and hence the instantaneous power reading will naturally move up and down with each time period.
As you say, it makes no difference when averaged over a longer duration and the solution is to lengthen the power display rolling average interval. You would need the Pro CPU to choose longer display averaging intervals. 3 or 5 secs works well.