AeroPod, PowerPod and Newton

The average power measured by my PowerPod for my rides seems about 25% too high. I have been analyzing my rides in Isaac and also comparing the results using https://www.gribble.org/cycling/power_v_speed.html.

Here are the numbers that I have been using in both Isaac and Gribble:

Weight of rider + bike + gear = 86.5 kg
CdA: 0.32 (riding in drops)
Crr: 0.004
Cm: 1.0

Here are two examples of recent rides and what I am seeing:

On May 25, I rode 47 km. Isaac is saying that my average power was 229 watts. That should have given me an average speed of 35.8 km/h. However, my average speed was only 32.9 km/h. If this average speed of 32.9 km/h is correct, then my average power was really only 183 watts (and not 229 watts).

On May 20, I rode 85 km. Isaac is saying that my average power was 243 watts. That should have given me an average speed of 36.6 km/h. However, my average speed was only 33.6 km/h. If this average speed of 33.6 km/h is correct, then my average power was really only 194 watts (and not 243 watts).

Could you please help?

Please post the .ibr ride file.

Attached are the two examples. Note both rides ended where they began and there was no drafting (I rode alone).

Hi -- Could you please get back to me or should I contact technical support? Thanks!

You can't use bike calculator websites in this manner.

Your ride was over a wide variety of wind and elevation conditions. These bike calculators assume that the riding conditions are CONSTANT at all times.

From the Gribble website:

"Remember that the speeds involved are steady-state, i.e., the speed you will be able to sustain under the given power, assuming all other parameters stay constant."

To use bike calculators properly, you need to select short ride segments, where conditions don't vary much.

I selected a short segment, around km 24, where bike speed was nearly constant, terrain was nearly constant. Isaac gives 224W for this segment; Gribble gives essentially the same number.

Your PP is working perfectly