Out-back / road testing

Estimated reading: 2 minutes

Overview

  • For more accurate aerodynamic testing outdoors, when a track is not available, you should ride in both directions along an ideally quiet section of road.  The wind generated by passing traffic will degrade the measurement.
  • The road section does not need to be straight, but tight corners should be avoided.  You should aim to be able to ride at a roughly constant speed and power during the test.
  • The more out and backs you perform, the more accurate your CdA will be.

Protocol

  1. Press the “start/stop activity” button on the Garmin to start recording data to fit file.
  2. You should ride in one direction along the road stretch for between 500m and 1km. The longer the better.  At the end of your segment, brake sharply, turn around and then ride back the other way back to your starting point. For the turnaround ride slowly (below your preset minimum speed) for at least ten seconds to ensure the lapping process works properly.
  3. During each out or back segment, aim to maintain a constant position on the bike, and ideally a constant speed.
  4. Don’t forget to press the “start/stop activity” button again to end the recording.

The more out and backs you perform, the more accurate your CdA will be.

Calibration

For each out and back segment, Aerosensor calculates a “Cal factor”, shown on page 6 of the display as “Prev cal factor” for the most recent segment.  Average these values for all of your out and back segments to get your calibration factor.  This should then be multiplied by the aero device calibration factor set in the CIQ app to get the new calibration factor.  You can then set the aero cal parameter to this new value.

For example, lets say you perform two out and backs.  Your aero cal parameter set in the CIQ app is 1.18.  The values you get are as follows.  Note that typically the odd laps will be acceleration and turnaround segments.  Because of ambient wind the value in one direction will generally by higher than in the other.

Lap 2   1.05

Lap 4   1.15

Lap 6   1.03

Lap 8   1.14

The average of these values is 1.09.  1.09 x 1.18 = 1.29.  You should go into Aerosensor settings -> parameters and enter this new value.

This value varies a little with your body position so for accurate CdA data periodically repeat this process.

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