Does anyone know the formulas for determining power output? - Page 2

Don't the strain guage meters (like the PT) integrate the electrical current passed through the strain guage circuit over the entire sampling period, and then report that total at the end of the sampling period? The electrical resistance of the strain guages varies with the torque being produced at any given instant. If what you're saying is true, then it should be possible synch up the PT so that the reporting is alway done at the deadspot. I'm pretty sure that'd be a noticeable problem.

I thought the 'magic cadence' phenomenon was due to the number of downstrokes that were being included in the sampling period (ie, some samples only include 2 downstrokes, but every e.g, 3rd sampling period will include 3 instead).

This explanation certainly fits with PT's manual that says that cadences over 140 rpm's may not be measured correctly unless they're measured with the crank arm magnet and cadence sensor. Without the cadence sensor the PT is looking at peaks in the strain to determine when you're pushing _down_ on the pedals. Two peaks per sample would be confusing...

It typically sees at least 2 peaks per sample (one from each leg), but at very high cadences the peaks and valleys would be very closely timed, and pretty non-distinct (ie, more circular force application).

But, this certainly supports the idea that the PT is looking at torque over the entire sampling period, and not just picking the instantaneous value at report time. Otherwise, it would never know when the peaks and valleys occurred.

Yes, and might miss some peaks and valley.

I do some standing starts at high torque and if the sample was based on one instant every 1.2x seconds it could miss some downstrokes and I'd be seeing jumpy torque readings in the file, but I don't.

And why would it take 72 samples every 1.2 seconds but only use one sample instead of averaging many (all 72?) samples before sending that average to be saved in memory? -Simple for a microprocessor to do.

i'm fairly confident it's 60 x's per second.

ric

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I can never find that number when I want it.

always comes just after 59

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I stand corrected!

And while I agree with the last statement in general, I do think for very short durations, the PT has some issues. Anecdotally, a friend I work with is a very good local masters road sprinter. I leant him my SRM for a month while he was deciding whether to take the plunge and get a PM for himself. He did end up getting a PT and at my urging, rode with both for a month. During that month he did 7-8 dedicated sprint workouts and 6-7 group rides with both PMs on his bike. He also raced 3 times with just the SRM. During that period his PT recorded 5s powers above 20w/kg 24 times. The SRM recorded one 5s power of 19.1, but more typically in the low 18 range. Does that matter in regards to his training? Not a bit. But it does make me dubious of PTs ability to measure short durations accurately. Of note, the peak 10s values were much closer, though the PT still had 10 readings 1w/kg higher than any SRM reading.

Scott

Did your friend know about the best way of zeroing the torque before each ride? Why/how would the PT 10s power numbers be closer to the SRM's 10s numbers than the 5s power numbers?

I agree, Scott; even over 10 seconds, the average on the PT may be skewed, though it should smooth out fairly well.

1 W/kg sounds like too much, though....maybe SRM slope issue?

Shrug, that's within 8% or so, and it was a much smaller number of discrepant values. And as one would expect, those 10 also coincided with the highest 5s values recorded by the PT for the month (including one value of 26w/kg, which had 5 identical, succesive watt readings, a clear glitch).

I calibrated the SRM just before lending it to him (and it's had essentially a stable slope for the last 2 years).

Being a sprinter, my friend likes the PT much better than the SRM =).

Scott

Correction: the factor of 2 arises from the fact that there are 2 * pi radians in 360 degrees (e.g., a complete pedal revolution). Neither the SRM nor the PowerTap make any assumptions about how force is applied to the pedals.

Sure. The *equation* assumes that the force is being applied continuously and tangentially for the entire circle. If the direction or magnitude of the force is changing over the course of the circle, then you can't just plug a simple value into the equation. You'll need a meter to perform the integration, in that case.

I was referring to your statement that "...each leg applies force for one-half the circle", and interpreted that to mean that's why there was a factor of 2 in the equation. In fact, no assumptions are made, and none are required, re. the pattern of force application; the power actually delivered to the crank (or the hub) is the same regardless of how you pedal. (The exception to this statement is the Ergomo, which only measures the torque applied to the left crank, and calculates the total power by assuming that the left and right legs are balanced.)

These authors:

http://www.ncbi.nlm.nih.gov/entrez/q...=pubmed_docsum

had subjects perform a maximal 8 s effort from a dead stop on an Axiom trainer in different gear ratios (i.e., different inertial loads) while measuring power using both an SRM Scientific model and a PowerTap. Quoting from the results:

"The PowerTap POmax during the sprint test with low gear ratio
(39/23) was significantly (p = 0.016) lower (8%) compared with
the SRM POmax (Fig. 2). However, there were no significant differences
in POmax when cycling with high (39/14) and middle gear
ratios (39/17) (p = 0.173 and p = 0.153, respectively). The pedalling
cadences at POmax with the low, middle, and high gear ratios
were 149.2 ± 5.3 rpm, 113.5 ± 5.4 rpm, and 93.6 ± 4.6 rpm, respectively."

Anecdotally, I have been unable to detect a difference in my own maximal power (after correcting for chain friction, assuming a drivetrain efficiency of 97.5%) as measured using a PowerTap and an SRM, regardless of the sampling duration (i.e., 1/1.26 vs. 2/2.54 vs. 5/5.12 s). I have not, however, performed any sprints while using both devices simultaneously, plus my maximal power is <1000 W, so this doesn't rule the possibility of subtle differences and/or at higher powers.