Comparing setups

Absolute CdA values depend on your mass, Crr and air density being right. Differences between setups tested the same day, on the same course, with the same parameters, are much more reliable. That's what MyCdA is best at.

Baselines

Mark one saved analysis per ride as the baseline (the switch next to Save), usually your normal setup. The ride's saved analyses on the activity page are then compared with it:

As a rule of thumb, 0.005 m² of CdA is worth about 5 W, or 20 seconds over 40 km, at that speed. At lower speeds the savings in watts are smaller, but the percentage stays the same.

Telling real differences from noise

Every test has some scatter: gusts, a slightly different line, a change in temperature. Before believing a difference, compare it with the scatter of repeated tests of the same setup:

• The standard deviation (SD) under the saved analyses and the Tests table measures that scatter. The coefficient of variation (CV) is the SD as a percentage of the mean: 1–3 % is good field testing.
• A difference smaller than the SD of repeated runs is probably noise. Test again before changing equipment for it.
• Average several runs per setup: the average of 4 runs is about twice as precise as a single run.
• Alternate setups (A, B, A, B). A steady trend through the session (for example as the wind picks up) then affects both alike instead of looking like a difference.
• One run far from the others is usually a disturbed run (a gust, a car, a lost position). Open it and look at its virtual elevation before including it.

Comparing between days

Comparisons between rides are harder: the air density, wind, tyre pressure and even your power meter's zero differ. Give each ride its own conditions in its ride settings, especially air density, and include your baseline setup in every session, so each day has its own reference.

Something missing or unclear? Ask the user community or contact us.