Saturday, January 21, 2017

Sunday, January 15, 2017

Outdoor velodrome with an anemometer


On an outdoor velodrome (250m length), the rider selected the "Defined lap length" mode in the app and did a first run of 28 laps, then a second run of 22 laps. The Weather Meter anemometer, which is mounted on the front of his TT bike, records axial apparent speed:


We clearly see that the wind, during the session, is not constant at all, and has a range of [1-5] km/h. The mean axial wind, on the whole session, is -0.19 km/h. It indicates that the calibration factor used in the app to take in account the stagnation effects seems slightly underestimated. It is confirmed by the computation of real CdA vs apparent CdA (air vs bike speed in the drag force): 

First run:
0.303 +/-0.006 (CV: 1.8%)
0.301 +/-0.005 (CV: 1.8%)
Second run:
0.308 +/-0.003 (CV: 0.9%)
0.304 +/-0.005 (CV: 1.7%)

The apparent CdA is lower than the real CdA during the two runs, which is unexpected. Nevertheless, even if there is a biais on the real CdA, we can notice two things:

-Variability is low for the real CdA, which is a very good point for the validation of the anemometer
-During out and back ride or velodrome testing, apparent CdA is accurate enough when wind is low (less than 5 km/h) as the effect is nearly cancelled in the drag force