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Aero lab · experimental version

Aerodynamics and CdA calculator

Explore your frontal area, adjust a 3D posture and compare equipment scenarios. A transparent estimate with assumptions you can change.

Local photos · no account

Start with a front photo or try the example. Tracing is manual; the 3D model is generic. This is not a scanner or a fluid simulation.

1Photo and frontal area

Photograph rider and bicycle from the front in the actual riding position. Keep the camera centred, level and far enough away; avoid wide-angle lenses. Put a known-length reference in the same plane as the body.

Trace the combined outline of body and bicycle. Close it and subtract visible openings (for example, between arms and torso). Openings must not touch or cross the outer boundary.

Your photo will appear here. You can also start with the synthetic example.

Photo ready. Mark a reference and trace the outline.

Click the photo to add points. Keyboard: arrows move the crosshair, Shift for fine adjustment, Enter adds a point and Esc cancels.

2Calculation assumptions

Cd is not derived from the photo. 0.60–1.00 is an initial demonstration interval, not a validated reference for your posture. Replace it if you have measurements using the same frontal-area convention.

3Posture and equipment

Use the side photo to adjust torso angle relative to horizontal. The photo is not analysed automatically. Current torso width and angle are the reference for comparing changes.

Drag to rotate. Geometric mannequin: a 62 × 24 cm ellipsoidal torso; other dimensions are generic. Only the change in torso projection modifies scenario area.

Modified scenario · Power change · central assumptions

−3%, −2% and −1% are editable examples, not measured savings. Enabled adjustments combine multiplicatively. Positive values increase CdA. Results depend on posture, product and wind; their interactions are not modelled.

Current vs. modified

Modified scenario

Complete a valid measurement and check posture and equipment assumptions.

Sensitivity interval, not a statistical confidence interval. Still air: aerodynamic drag only. Rolling resistance, gradient, drivetrain and crosswinds are excluded.

Your scenarios

Up to four scenarios, for this session only. Watts are recalculated at the current speed and density for a like-for-like comparison.

No scenarios saved yet.

ScenarioA (m²)CdA (m²)WRemove

How the estimate works

The reference length converts pixels to metres. Outline area minus openings is multiplied by the square of that scale. We then apply your assumed minimum and maximum Cd and area variation. Reference-length errors affect area quadratically: poor calibration dominates the result.

CdA = A × Cd
Paero = ½ × ρ × CdA × (v / 3,6)³

For posture changes, the torso is approximated by an ellipsoid. Its frontal projection is π·(width/2)·√((0.31·sin θ)²+(0.12·cos θ)²). We add the projection difference to traced area, keeping the rest of the body and bicycle fixed. This does not calculate changes in wakes, head, arms or drag coefficient as you move.

Physical basis: NASA Glenn drag equation. The posture model and equipment percentages are unvalidated simplifications used by this tool. NASA Glenn — Drag Equation

Can a photo measure my actual CdA?

No. It helps estimate projected area, not drag coefficient. Depth, posture, clothing and airflow all matter. Validate results with controlled field tests or a wind tunnel.

Does an aero helmet always help?

No. Percentages on this page are scenario assumptions, not purchase recommendations. A helmet may work well in one posture and worse in another. For a useful comparison, enter measurements from comparable conditions.

What happens to my photos?

They are opened and processed locally in this tab. They are not sent to a server or included in the results download. Reloading or closing the page clears photos, traces and comparisons.

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