Position notes and aero values (CdA, Crr)

What lives in this section

The BikeSetup record carries three position and aero fields beyond the basic fit measurements:

• Position notes (free text): extension type, bar angle, cockpit setup, anything that does not fit a numeric field.
• Last CdA (m squared): the most recent measured CdA for this bike and position.
• Last Crr (rolling resistance coefficient): the most recent measured Crr.

CdA

CdA (coefficient of drag times area) is the bike position's aero efficiency. Lower is faster.

• Value range on the BikeSetup record: 0 to 1 m squared, three decimal places (e.g. 0.245).
• Where it comes from: the CdA Field Test or the CdA calculator directly.
• When it updates: after each CdA measurement. The latest becomes last_cda.

CdA is specific to the position, not just the bike. If the athlete changes saddle, bar position, or even kit, the CdA changes. Retest after meaningful changes.

Crr

Crr is the tire rolling resistance coefficient. Lower is faster.

• Value range on the BikeSetup record: four decimal places (e.g. 0.0040).
• Where it comes from: tire manufacturer data, third-party rolling resistance labs, or a back-calc from a known CdA test.
• Reference values (mounted, race surfaces):
• Race tubeless on smooth tarmac: 0.0035 to 0.0050.
• Training clinchers: 0.0050 to 0.0080.
• 28-32 mm gravel tires: 0.0080 to 0.0150 (surface dependent).

Why both matter for race planning

Both CdA and Crr feed the Bike Power Plan calculator at step 4 of the race planning wizard. Together they determine the watts-to-speed mapping for the race course.

A 0.010 CdA improvement at 40 km/h saves about 8 to 12 W. A 0.0010 Crr improvement at 40 km/h saves about 5 to 8 W. The two together compound into meaningful split changes (1 to 4 minutes over a 70.3 bike leg).

Position notes: what to write

Examples of useful position notes:

• "Profile Design Aeria extensions, 12 cm S-bend, 5 deg drop".
• "Bar angle 7 deg down from horizontal".
• "Cockpit drop 130 mm from saddle to pads".
• "Pad arm length: 280 mm centre to elbow".

The goal is enough detail to recreate the position from parts on a bench.

See also: CdA calculator, CdA Field Test, BikeChangeLog.