Artifact or valid data? (Power geeks only)

[Windbreaker]

I've been doing a lot of IDT work lately - the only way that I can get a consistent training done. Apart from starting to get my head around not being able to generate the same power as on the road (that's a different story altogether) I am starting to notice a couple of things.

 

Here is an extract from my warm up this morning. Note the smooth plots for the power & torque graphs shortly after I start then as the cadence increases from 95 to 100 the plots take a sawtooth pattern.

 

I am wondering whether this is an indicator of my pedal "smoothness" decreasing at the higher cadence or whether it is an artifact of the PT system. I am using a mag trainer but I doubt very much that is the cause.

 

I know that the PT samples at 1.26s so I would expect to see any artifacts at a multiple of this i.e. 75 rpm.

 

Thoughts?

 

[bruce]

This is known as "precession" and is effectively a beat frequency that occurs due to the 1.26s sampling rate and cadence combination.

google "powertap precession" and you'll see plenty of discussion.

[bruce]

From the "Training with Power FAQ"

 

Other than the Polar units, power-measuring devices display current power as an average over some short time period, which leads to a problem known as the ?precession effect.?  That is, unless you are pedaling at a rate where one or several revolutions are exactly completed in each averaging interval, an extra quarter-revolution can occur, and that partial turn of the crank may be either a power stroke or a dead-center (and perhaps the opposite for the next sampling period), which will produce a less consistent reading, especially for short intervals; the maximum power value captured in the PowerTap?s display memory, for example, is significantly affected, since it is the highest average value achieved over just 1.26 seconds.  Thus, averaging over one (or just a few) crank revolutions would reduce variability in the current power display, track power more nearly as a rider senses it, and result in more accurate maximum values for instantaneous power.  Recorded power values could, and perhaps should, still be based on time.<?:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" />

 

[Windbreaker]

 

Ok, I remember reading about this somewhere and satisfied myself about the generally jagged nature of the power curve at the time ... but neither 95 nor 100 rpm is an exact multiple of 1.26. hence my question why there is a such a significant difference between 95 & 100 rpm. If you can find a harmonic of the two frequencies 1.2/sec & 95 rpm and/or 1.26/sec & 100 rpm that explains this then ....

 

Not sure if your postings tell the whole story though.

Windbreaker2008-02-15 10:37:12

[Windbreaker]

Here is another extract from a different ride. Again note the

significant range. I specifically didn't post this one because the

cadence is lower & closer to the 1.26 number.

 

 

 

Still think that it is an artifact related to the sampling rate?

 

 

[bruce]

What is the "time" scale on this picture?  Or maybe post the raw data view.

 

The speed and cadence curves definitely "ripple" at the same frequency as the power curve, just the amplitude is less, indicating the damping effect of the flywheel if on an idt, or your body mass if on the road.

[Windbreaker]

Bruce, here is the data for the last image.

This is all IDT (Magnetic) work

 

Temp:

    Duration:      3:48

    Work:          53 kJ

    TSS:           5.7 (intensity factor 0.951)

    Norm Power:    n/a

    VI:            n/a

    Pw:HR:          n/a

    Pa:HR:          n/a

    Distance:      1.832 km

        Min    Max    Avg

    Power:           160    283    232     watts

    Cadence:         53    84    65     rpm

    Speed:           23.6    31    28.9     kph

    Pace             1:56    2:33    2:04     min/km

    Hub Torque:      7    11.4    9.6     N-m

    Crank Torque:    19.3    46.5    34.6     N-m