Power numbers don't add up in Isaac

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lph
Posts: 2
Joined: Tue May 08, 2018 9:28 am

Power numbers don't add up in Isaac

Post by lph »

I observe the following stats for the inbound portion of a ride:

Code: Select all

Distance:   11.10 mi (1:02:11)
Energy:     333.4 kJ
Cals Burn:  318.7 kcal
Climbing:      30 ft
Braking:     -0.0 kJ (-0.0%)
          Min   Avg    Max
Power       0   89.4   486  W
Aero        0   34.7   132  W
Rolling     0   69.8   100  W
Gravity  -314    1.5   301  W
Speed     0.0   10.7  15.4  mi/h
Wind      0.0   10.8  19.7  mi/h
Elev      992   1054  1101  ft
Slope    -6.0   0.04   7.0  %
Cadence     0   79.7   110  rpm
Why don't the average power numbers add up? Shouldn't the average power(89.4) = average Aero power (34.7) + average Rolling power (69.8) + average Gravity power (1.5)? The sum of the average power components (34.7 + 69.8 + 1.5 = 106) does not equal the average power (89.4). The speed at the beginning and end of the ride were both 0, so no power was converted to/from kinetic energy. The discrepancy is 16.6 watts.
Velocomp
Velocomp CEO
Posts: 7793
Joined: Fri Jan 11, 2008 8:43 am

Re: Power numbers don't add up in Isaac

Post by Velocomp »

Please post your entire ride file.

In general the component numbers don't add up to the total number. There's lots of averaging that goes on when computing the components, whereas the total power number doesn't average the constituent parts; it adds the instantaneous components, "live".

The other thing to comment on is that your total power is less than 100W, and it appears that your rolling watts are the largest component. This seems odd to me, but when I see your entire ride file I will be able to better understand what is going on.
John Hamann
lph
Posts: 2
Joined: Tue May 08, 2018 9:28 am

Re: Power numbers don't add up in Isaac

Post by lph »

This was meant to be a slow out and back recovery ride on my mountain bike on a dirt trail. I'm only going 11 mph. The trail was rough. The average power is correct for miles 22 to 44, verified by a DFPM. The segment that I posted above was from mile 22 to mile 33. The first portion of the ride was messed up when the Power Pod was jostled about by bumps and rotated in the mount. No matter how hard I tighten the thumb screw, the Power Pod still moves in the mount. There has to be a more stable mount for riding on rough trails. The first portion of the ride was messed up due to this problem. When you compare the DFPM measurements with the Power Pod power measurements, from mile 22 to 44, the average power measurements agree to within a watt or two. However, the two disagree by about 8-10 watts for miles 0 to 22. I'm trying to determine the cause for this mismatch.

Isn't the average power computed as the total energy expended divided by time? And isn't the total energy expended equal to the total energy expended due to aerodynamic resistance plus rolling resistance plus change in potential energy plus change in kinetic energy. Conservation of energy then dictates that the powers must add up.
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