Longer term drift?

[mds]

Has anybody noticed a longer term drift in their calibration? Up until May I was using an Aero of 0.439 and getting nearly zero watts on the coast screen at high speed - 35 to 40 mph. By the end of May, coast watts had gotten larger so I reduced Aero to 0.428. By the first of July coast watts had increased again so I reduced Aero to 0.418. Today on the same section of road it is to high again. Same bike, same equipment, same clothing, same postion as nearly as I can make it and my weight has not changed much. I am going to try an Aero of 0.406 now. Any idea why coast watts might creep upwards over a period of months?

[lungbuster]

Are the watts increasing as the weather warms up? You may find that the different seasons/times of the year affect aero.
Warmer weather reduces friction through the air. Hence the reason why some people warm the air in velodromes when trying to ride fast times to get selected for National teams.
Wayne

[mds]

Yes. Watts are increasing with warmer temps. I looked up standard air density: at +5C it is 1.269 kg/m^3, at +30C it is 1.164. Ratio of 1.164/1.269 is about 92%. Interestingly, my summer/winter Aero ratio 0.406/0.439 is also about 92%. I wonder if this is just a coincidence? Maybe my unit's temperature compensation system is not working properly?

[rruff]

I'd advise checking your wind scaling periodically. If it drifts slowly this is not a big deal since you can adjust it. Mine varies randomly more than the amount of drift you've mentioned.

[mds]

Thanks rruff. Is it possible to check wind scaling on an iSport?