Speed Science
Aerodynamics
Understand where air resistance takes energy from a gravity car, then improve legal driver presentation, fit, and finish without reshaping a regulated class.
ILLUSTRATED DRAG MODEL
The air sees relative velocity and yaw.
vehicle energy-loss rate Pvehicle = Fᴅ·vground
- Fᴅ
- drag force in newtons
- ρ
- air density in kg/m³
- Cᴅ, A
- drag coefficient and its stated reference area in m²
- vᵣₑₗ
- car velocity relative to air in m/s
- vground
- car velocity along the ground in m/s
Illustrative still-air example, not a target
For ρ = 1.20 kg/m³, Cd = 0.35, A = 0.45 m², and vrel = vground = 15 m/s, Fd ≈ 21.3 N and vehicle energy-loss rate ≈ 319 W. In wind, compute force with vrel but multiply by vground for the vehicle's mechanical-energy loss rate.
TEST A LEGAL AERO HYPOTHESIS
Separate posture from weather and steering.
- State the legal change and prediction before testing.
- Record wind and ground speed; derive relative airspeed with sign and units.
- Photograph driver posture, helmet, steering neutral, and body state.
- Randomize paired baseline/change runs while controlling lane and wheels.
- Use multiple pairs; reject a claim smaller than ordinary timing spread.
At fixed ρ, Cd, and A, a 10% rise in relative airspeed raises modeled drag about 21%. That arithmetic still does not isolate the cause of a track-time change.
GARAGE WORKSHEET
The minimum record needed to act on this guide.
QUESTION THIS STATION ANSWERS
Does one legal posture or assembly change reduce repeatable time loss without adding steering correction or control risk?
BRING
- Front, side, and top-view camera with a repeatable scale/reference
- Timing system plus wind speed/direction, temperature, and pressure record when available
- Driver posture marks, wheel/lane map, and randomized A/B run sheet
LOCK BEFORE READING
- Legal shell, foam, fasteners, helmet, and driver fit
- Sight line, steering reach, brake access, safe exit, and posture repeatability
- Wheel set, lane strategy, release method, load state, and weather window
WRITE THE RAW RECORD
- Posture photos, projected-width/height references, steering corrections, and driver notes
- Ground speed or elapsed time, head/tail/crosswind component, lane, wheel IDs, and run order
- Multiple randomized A/B pairs plus within-condition spread
DECISION GATE
Call a result unresolved when it is smaller than ordinary timing spread or disappears after order reversal. Reject any posture that reduces control, visibility, braking access, or plan compliance.
VERIFY NEXT
Replicate the legal change on another day or lane before preserving it as the team's baseline.
THE DRAG BILL
Air resistance rises quickly with relative airspeed.
A gravity car has no engine replenishing lost energy. Every bit spent pushing air, scrubbing a wheel, or correcting a line is gone. Aerodynamic drag can be approximated by:
Relative airspeed is the car's velocity relative to the moving air, expressed in consistent units. A headwind raises it; a tailwind lowers it. Because the term is squared, a small shape difference matters more late in a fast outdoor run than it does while the car is barely moving off the gate.
LEGAL AERO FIRST
The plan defines the shape envelope.
Stock, Super Stock, and Masters are regulated constructions. Sanding a shell thinner, filling an unapproved gap, adding a fairing, taping an opening, changing driver placement outside the plan, or attaching a surface because it looks fast can fail inspection. The current Masters plan specifically controls surfaces and prohibits aerodynamic tape treatments.
Legal gains usually begin with exact assembly: seat and foam installed as directed, shell seated correctly, required hardware flush where the plan allows, driver compact and repeatable, helmet presented consistently, and nothing loose enough to flutter. Ask the inspector before adding any film, filler, coating, seal, or edge treatment.
TEST THE WHOLE CAR
A low-drag posture must still be drivable.
- Photograph the driver from front, side, and above in full race gear.
- Verify sight line, steering reach, brake access, helmet clearance, and quick exit.
- Mark the legal posture so it can be repeated at scales and on the ramp.
- Use coast-down or paired timed runs; compare multiple passes, not one lucky number.
- Reject any posture that causes steering corrections, head movement, or reduced braking control.
A narrow silhouette can lose to a slightly larger but calmer one if the driver cannot hold it. Stability and alignment are aerodynamic tools because yaw and steering corrections increase drag.
SOURCES & FURTHER READING
Check the primary material.
- Mid-Missouri Soap Box Derby physics program Historical educational material on energy, drag, frontal area, and alignment; current rules still control.
- ISBD Rule Book - linked for the 2026 World Championship Official PDF currently linked from Race Week resources; the file itself says revised June 2025.
- ISBD Masters build plans, July 2026 Current official build plan linked from the division hub. Use it directly; this site does not reproduce it.
- New Hampshire Soap Box Derby: Tips from the locals Licensed local-program notes on alignment, weights, tuning, and driver position.
- NASA Glenn: Drag coefficient and the drag equation Primary educational reference for drag force, density, velocity, reference area, and drag coefficient. It explains physics, not Derby legality.
- NIST Engineering Statistics Handbook: Uncertainties of calibrated values NIST guidance on uncertainty in future results corrected by a calibration curve, including calibration-curve and future-measurement contributions. It does not characterize the whole Derby measurement process by itself.