Troubleshooting CdA

  1. Your CdA is measuring too high or low

    This is a common one and usually simply resolved or explained. These are the possibilities

    • You are using ‘Loop’ mode.
      ‘Loop’ mode requires minimal braking to be a true calculation of your CdA. For general riding ‘Dirty’ mode is more suitable as this attempts to filter out braking and anomalies.
    • Your Performance Profile has errors in it. As part of the CdA calculation weight and rolling resistance are important. If these parameters are far from their true values your CdA will be incorrection.  Check that you have entered in your combined weight with bike, clothing etc. CdA calculations are especially sensitivie to incorrect weight inputs for hilly rides.
    • You dragged your brakes down hills. This is very difficult to separate from non braking data as there isn’t a large enough difference to create outliers to filter. Especially when there are long descents.
    • Your brakes were dragging on the rim.
    • You didn’t calibrate your power meter or it received a bad auto calibration during your ride.
    • You were wearing unusually baggy clothing such as a rain coat.

  2. No CdA displayed for my ride

    • If you don’t upload power data to Strava we can not calculate your CdA. If you want to protect your power data. Try uploading the TCX file directly to myWindsock.
    • Ensure your are still in Analytics mode.  Go to > Mode

  3. Some of my test Loops are missing

    • When doing field testing ensure you have the highest position recording setting on your bike computer. For Garmin devices this is ‘GPS + GLONASS’.
    • Experiment with the detection sensitivity. Lowering the Loop Tolerance will detect more loops but be increasingly less accurate.
      > Aero > Loop Tolerance

  4. My CdA is very different from my previous rides

    • If weight, rolling resistance or drive train loss parameters are changed your CdA will also be different. Keep an Aero test profile that keeps this values the same to help compare CdA from ride to ride.
    • Incorrectly calibrated power meter.

  5. Only a small section of my out and back Time Trial has been measured.

    Expand the minimum loop distance to capture a greater proportion of your time trial. > Aero > Minimum Loop Distance

 

Author: Ben Norbury

Ben Norbury is four times Manchester & District Middle Distance and Best All Rounder Time Trial Champion from 2016-2019. During this time he developed software to eliminate uncertainty of the weather conditions out of his pre-race preparation.