Glossary: CdA (aero drag coefficient times area)

What CdA is

CdA is the product of the drag coefficient of the body plus bike system and the frontal area it presents to the wind. Measured in m squared. Lower is faster.

The aerodynamic drag force at speed v is approximately 0.5 * rho * CdA * v^2, where rho is air density. At 40 km/h, drag is the dominant resistance for cyclists; rolling resistance is secondary.

Typical CdA values

• Sitting up on a road bike (no aero): 0.40 to 0.60 m squared.
• Hands on hoods, road position: 0.30 to 0.40.
• Aero road, hands on drops: 0.28 to 0.35.
• Aggressive aero road position: 0.25 to 0.30.
• TT or tri bike, decent fit: 0.22 to 0.28.
• Well-fitted TT position: 0.20 to 0.24.
• Pro time-trial position: 0.18 to 0.21.

How big is the win

At 40 km/h, a CdA reduction of 0.01 m squared saves roughly 8 to 12 W of drag power. Over a 70.3 bike leg that compounds to 1 to 3 minutes of split time at the same power.

How the Hub captures CdA

• Per-bike: last_cda field on the BikeSetup.
• Per-test: from a CdA Field Test or the CdA calculator directly.
• Per-change: BikeChangeLog entries can record the CdA-before and CdA-after of an equipment or position change.

The Bike Power Plan calculator uses the bike's last_cda to project the race bike split.

What changes CdA

In rough order of magnitude:

• Body position: huge. Aero bars, hands in drops, shoulders narrow.
• Helmet: typical 5 to 15 W savings going from open road helmet to TT helmet at 40 km/h.
• Skinsuit / race kit: smooth fabric can save 5 to 15 W.
• Wheel choice: deep section wheels save 5 to 15 W.
• Hand and arm position small changes: 2 to 8 W.

CdA is position-specific

If you change your position (extensions raised, saddle moved, bar angle), the CdA changes. The recorded value is for the position you held in the test. Retest after meaningful changes.

See also: CdA Field Test, CdA calculator, Position notes and aero values.