CdA Analyser

Overview

CdA Analyser is a Python-based desktop application for cyclists and aerodynamicists who want to extract CdA (aerodynamic drag coefficient × frontal area) values from .fit file ride data.

It processes power, speed, elevation, and wind data from a structured test ride and outputs CdA calculations alongside visualisations to help understand where drag is coming from.

ℹ Screenshots

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What is CdA?

CdA (drag coefficient × frontal area, in m²) is the primary metric for quantifying how aerodynamic a cyclist is. A lower CdA means less energy wasted overcoming air resistance at a given speed.

CdA can be estimated from field tests by measuring power output and speed under controlled conditions and solving the equation of motion:

P_aero = CdA × ρ/2 × (v + v_wind)² × v
P_total = P_aero + P_rolling + P_gravity + P_drivetrain

Features

.fit file import — reads Garmin/Wahoo .fit files directly
CdA calculation — solves for CdA using the virtual elevation method or direct power-speed method
Lap-based analysis — calculate CdA per test lap for back-to-back position comparisons
Environmental correction — account for air density (altitude, temperature, humidity)
Rolling resistance estimation — Crr estimation from coasting segments
Visualisations — CdA vs. time, virtual vs. GPS elevation overlay, power/speed traces
CSV export — export per-lap CdA results for spreadsheet analysis
Configurable — set rider+bike mass, wheel circumference, drivetrain efficiency

Supported test protocols

Flat constant-speed laps — classic velodrome or flat course protocol
Virtual elevation (Chung method) — any course, using GPS altitude cross-reference
Ramp test sections — isolate specific segments of a ride

Installation

pip install cda-analyser
cda-analyser

Or from source:

git clone https://github.com/domoriks/cda-analyser
cd cda-analyser
pip install -r requirements.txt
python -m cda_analyser

Quick example

from cda_analyser import FitRide, CdACalculator

ride = FitRide.load("my_aerotest.fit")
calc = CdACalculator(ride, mass_kg=72.5, crr=0.004)

for lap in calc.laps():
    print(f"Lap {lap.number}: CdA = {lap.cda:.4f} m²")

Requirements

Python 3.11+
fitparse
numpy, scipy
matplotlib or plotly
PySide6 (optional, for GUI)