Race recap: CdA estimate from your bike FIT

What this is

When a race has an actual bike-leg FIT file linked, the post-race recap shows an automatic CdA estimate in the bike leg card. Same physics as the standalone CdA calculator, just running on the FIT we already have — no calculator visit needed.

How it works

For each second of the ride that looks like real bike riding (speed > 5.5 m/s ≈ 20 km/h, power > 60 W, |grade| < 4%), the estimator solves the standard cycling power equation for CdA:

P × drivetrain_eff = ½·ρ·CdA·v³ + Crr·m·g·v + m·g·v·grade

then averages across all valid samples (with a trimmed mean to suppress gust / draft outliers).

Air density (ρ) — from your FIT when possible

Air density matters a lot for aero calcs (a 4% drop in density shifts CdA by 4% the other way). The recap uses per-sample density computed from FIT temperature + altitude:

• Pressure from altitude: barometric formula, P = 101325 × (1 - 0.0065·h/288.15)^5.255.
• Density from pressure + temp: ideal gas, ρ = P / (R_d × (T_c + 273.15)), R_d = 287.058 J/(kg·K).

The bike-leg card shows what density was used + where it came from. Example readouts:

• "Air density: 1.179 kg/m³ (derived from FIT — 26°C, 4 m elev)." — Kristof's Shanghai race, hot day at sea level.
• "Air density: 1.225 kg/m³ (default — no temp/alt in FIT)." — older file or indoor recording, falls back to standard sea-level value.

Multisport-file handling

If your bike file is actually a single multisport FIT (Garmin records swim → T1 → bike → T2 → run as one file), the recap's CdA estimator handles it the same way: the minimum-speed filter (5.5 m/s) cleanly drops swim and run portions from the solution, since neither can sustain bike-speed velocities. You get a clean CdA for the bike portion only.

Assumptions surfaced for transparency

Underneath the CdA value, the card lists exactly what the estimator used:

• Rider weight (from your athlete profile)
• Bike weight (default 8.5 kg unless overridden)
• Crr (default 0.0042 — typical road tubeless at race pressure)
• Drivetrain efficiency (default 97.5%)
• Density + source (see above)
• Sample count: e.g. "5856 of 9471 samples kept after speed/grade/power filters"

If the numbers look surprising, the assumptions block tells you what to question. If you want a more rigorous estimate, open the CdA calculator with the activity picker — same physics, but the calculator's FIT-mode gives you the tuning knobs (smoothing, filter bounds, etc.).

When the estimate is hidden

The bike-leg card omits the CdA estimate when:

• Athlete weight isn't on file — can't compute the gravity / rolling terms.
• Fewer than 60 valid samples remain after filtering — too few clean bike-riding seconds for a reliable fit.
• The activity isn't a bike file — only the bike leg gets this section.

See also

• CdA calculator deep-dive
• CdA field test assessment
• CdA glossary