CdA Field Test (bike assessment, ~55 min), Yousuli Help

What this assessment is

The CdA Field Test is the seeded bike assessment that pairs with the CdA calculator to estimate your CdA (coefficient of drag times area, in m squared) from a controlled outdoor ride.

Domain: bike. Estimated duration: 55 minutes. Linked calculator: cda.

The 4-step protocol (as seeded)

Warm-up (15 min): ride 15 minutes easy building to moderate effort.
Choose course (5 min): select a flat, wind-sheltered course of 3 to 5 km. Ideally an out-and-back.
Steady power ride (30 min): ride 3 to 5 laps at steady power (target race watts). Hold position constant throughout.
Record data: download FIT, identify clean laps, read off averages.

Inputs the form asks for

Average Power (W, required).
Average Speed (km/h, required).
Total System Weight (kg, required): rider plus bike plus gear plus bottles.
Wind Speed (km/h, optional): if you have a measurement.

Outputs

CdA in m squared.

Test design notes

Calm day matters most. Even moderate wind (10 km/h) changes effective drag by a meaningful amount. If you have to test on a windy day, do out-and-back laps so the wind averages out.
Hold position constant. CdA is for the position you rode in. If you sat up to drink mid-test, the drag estimate is wrong.
Steady power matters because the math assumes constant inputs. Spiking power up climbs or coasting on descents breaks the model.

What CdA tells you

Typical road CdA: 0.30 to 0.40 m squared.
Aggressive aero road: 0.25 to 0.30.
TT or tri bike, well-fitted: 0.18 to 0.25.
Pro time-trial position: 0.18 or below.

A 0.01 reduction in CdA at 40 km/h saves about 8 to 12 W. That matters more than most equipment changes.

CdA Field Test (bike assessment, ~55 min)