Sunday, August 12, 2018

Power meter data quality

Yesterday, for the first direct comparison during a turbo session, I got results, which confirmed my suspicions, between my two current powermeters, a Power2max NG-Eco (2 months old) and a PowerTap G3 (recently changed). Unfortunatly, I deleted some of the data. Today, during a ride with intervals of several minutes, I recorded the following averages:


One (or the two?) gave an unexplicated difference during the first lap of the two runs (the relative difference does not depend of the gear ratio and is constant at +/-0.5% during the whole lap). Auto-zero feature was activated on the two power-meters and I stopped pedalling during the laps. I have some guesses about the suspect. More to come.  

Tuesday, January 2, 2018

Cheapest Android phone to use CdaCrr bike computer

After some research (cause people asking me when the app will be available on iOS, my answer is: sorry, never), here is the cheapest way to buy an Android phone to use the app (prices are given for December 31th, 2017) :


So for 83$*, you should have an ANT+ enabled Android phone (follow this) to run the free version of the app (classic lap mode) and aero field test on velodrome or other closed loop. There is also GPS and Bluetooth on this phone, so more CdaCrr modes could be used. Not tested yet, but I plan to buy and give a try soon.

*: You would need also an ANT+ powermeter (~450$), an ANT+ speed sensor (~35$), a phone bike mount (~10$). And a bike of course :-)

Edit 05/01/2018
I bought the Blackview A7 and it seems great for the price, but unhappily, it does not have USB host feature so we can't use an ANT+ stick :-(

Edit 14/01/2018: 
-List of ~130 Android phones with ANT+ built-in on www.thisisant.com
-List of ~460 Android phones with USB Host on www.gsmarena.com

Sunday, December 3, 2017

Virtual elevations

Last week results. The anemometer improves CdA accuracy when drafted a lot by cars. Supposed Crr at 0.004 may be too high.

Monday, August 7, 2017

Track model

According to Google Maps, the 250 meters track has straight of length 74.8 meters and turn radius of 16.0 meters.

The measured incline along the black line in the turns is ~30°, for 6° in the straights.

Friday, March 17, 2017

Velodrome session with high wind

Back to the outdoor velodrome. I wanted to see if the anemometer could improve precision with high wind conditions. The results, after three runs of 10 laps (trying to keep the same relaxed position on the hoods of my road bike) are:


We can notice that the wind, measured near the ground (the anemometer was mounted on the drops), is in the 5-15 km/h range (weather station indicated 22 km/h this day). 

Quick updated conclusions for the moment:
  • with low and constant wind conditions (0-5 km/h), data anemometer is useless as the effect is nearly cancelled (CV~1-2%)
  • with higher wind conditions (>5 km/h), anemometer is mandatory to maintain acceptable precision (CV~1.5-3%) whereas for apparent CdA (wind data is not included into the model) variability is way of.
  • variability from run to run is still an issue in my protocol (0.343, 0.353, 0.342) even if it should be lower with the use of a time trial bike.

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