Does anyone know the formulas for determining power output?

[RapDaddyo]

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

ric

I can never find that number when I want it.

 

[ric_stern/RST]

I can never find that number when I want it.

always comes just after 59

 

[scotmart]

RapD is right that torque is averaged over 1.26 seconds and with the recording interval set at 1s (really 1.26s), there's no data discarded. Only longer recording intervals will discard data, but there's scarcely need for other recording intervals.

The reason PT has aliasing, or "magic" cadences as you put it, has nothing to do with the recording interval but with sampling interval. Regardless of what recording interval you set, the PT averages torque over 1.26s; that is, it is time-based instead of based on one crank revolution. Therefore, the sampling interval may contain, for example, two peaks in torque because of the variation over the pedal circle. The next sample will probably have two troughs and so on.

That's why PT data is a bit noisy, but over the long term the numbers are good.

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

 

[WarrenG]

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?

 

[jws]

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.

Scott

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?

 

[scotmart]

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

 

[acoggan]

For rotating machinery, Power = Torque x angular velocity

As measured at the cranks:
Power (watts) = Pedal Force (Newtons) x crank length (meters) x Cadence (rpm) x 2*pi/60

This assumes that the pedal force is the net force applied tangentially to the cranks, and that each leg applies force for one-half the circle.

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.

 

[frenchyge]

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.

 

[acoggan]

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.)

 

[acoggan]

I do think for very short durations, the PT has some issues.

These authors:

http://www.ncbi.nlm.nih.gov/entrez/..._uids=16320172&query_hl=6&itool=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.