Speed Science
Wind-tunnel testing and composite legality
Run a defensible tare-corrected airflow experiment, then treat composite construction as a separate division-specific rules and safety question.
ILLUSTRATED WIND-TUNNEL CHAIN
Tare the fixture before trusting the balance.
Re = ρvL / μ
blockage = model frontal area / test-section area
- Re
- dimensionless Reynolds number
- ρ, v, L, μ
- kg/m³, m/s, characteristic length m, and viscosity Pa·s
- tare
- sting, mount, cable, and zero force without intended model load
Illustrative similarity example, not a target
At ρ = 1.20 kg/m³, v = 12 m/s, L = 0.40 m, and μ = 1.8 × 10⁻⁵ Pa·s, Re ≈ 320,000. A smaller model or slower test can have a different Re, so its Cd may not transfer to the full car.
EXPERIMENT SEQUENCE
Zero, tare, repeat, and disclose corrections.
- Calibrate the balance with known loads in the measurement direction.
- Record zero, fixture tare, speed uniformity, temperature, pressure, and geometry.
- Remove and reinstall the baseline model between repeated readings.
- Randomize baseline/change order and return to baseline to detect drift.
- Report blockage, Re, yaw, spread, and every correction.
- Compare effect with spread; smoke and tufts show flow but do not measure a gain.
Wind-tunnel data and composite skill do not create rule permission. Use carbon fiber only where the current division plan explicitly permits the full method.
GARAGE WORKSHEET
The minimum record needed to act on this guide.
QUESTION THIS STATION ANSWERS
Can a controlled flow test separate car drag from fixture tare, and is the tested construction method legal in this division?
BRING
- Calibrated force balance, fixture/sting, airspeed measurement, and documented test-section dimensions
- Baseline model or full car with fixed yaw and driver geometry
- Current division plan, material amendment, PPE, and resin safety data for any permitted composite work
LOCK BEFORE READING
- Balance zero, fixture tare, airspeed uniformity, temperature, pressure, blockage, and yaw
- Same installation and remove-reset procedure for every condition
- Material system, cure, inspection access, and rule permission treated separately from aero results
WRITE THE RAW RECORD
- Zero, tare, loaded force, corrected force, speed, Re, blockage, yaw, and repeated-installation spread
- Randomized baseline/change order and return-to-baseline drift check
- For composites: division authority, cloth/resin identity, PPE, cure record, and inspector access
DECISION GATE
Do not credit fixture drag to the car or transfer a coefficient across unmatched similarity conditions. A faster tunnel result never authorizes a material or shape the current plan does not permit.
VERIFY NEXT
Confirm any signal with controlled coast-down or track pairs and obtain written rules guidance before construction.
WHAT A WIND TUNNEL CAN TELL YOU
Measurement beats visual guesswork.
A low-speed wind tunnel can compare drag, pressure, tuft movement, and flow separation at controlled airspeeds. A useful derby test holds the same car angle, driver position, wheel exposure, and scale calibration while changing one thing. Smoke and tufts show where flow separates; a force balance shows whether the change matters.
Full-scale testing avoids uncertain scale effects, but it is expensive and sensitive to tunnel blockage, fixture drag, and repeatability. Coast-down testing is more accessible, though wind, temperature, wheel variation, pavement, and steering can overwhelm a small aero difference. Neither method makes an illegal shape legal.
CARBON FIBER IS NOT A UNIVERSAL UPGRADE
Material permission changes by division.
| Division | Current ISBD boundary |
|---|---|
| Stock / Super Stock | Build the official kit and plan. Do not add composite structure or skins unless expressly authorized. |
| Masters | The July 2026 plan prohibits fiberglass, carbon fiber, and similar material inside or outside, except authorized fiberglass repairs. |
| Legacy | The plan allows fiberglass or carbon-fiber-type cloth over permitted wood or foam construction, subject to its exact laminate, inspection, and amendment requirements. |
Carbon fiber can be stiff and light, but lightness only helps if the finished legal car uses the saved mass productively. Poor laminate design, hidden voids, sharp edges, heat, resin exposure, and hard-to-inspect structure introduce real safety and compliance problems.
THE ULTIMATE SPEED LESSON
Innovation history is not a current-class loophole.
Launched in 2004, the former Ultimate Speed Challenge encouraged high-technology cars while retaining size, weight, and safety boundaries. Reporting described Zero Error testing at Texas A&M, custom wheels, tight driver fit, and speeds approaching 40 mph. It was a laboratory for ideas, not a current Stock, Super Stock, Masters, Legacy, or NDR rule set.
Use old Ultimate Speed machines to study engineering questions. Do not copy their bodies, wheels, bushings, or materials into a current car without explicit authorization in today's division plan.
SOURCES & FURTHER READING
Check the primary material.
- Ultimate Speed and Zero Error interview 2010 reporting on the retired open-technology division and Texas A&M wind-tunnel work.
- NPR: High-tech Soap Box racing 2010 reporting on Zero Error, wind-tunnel testing, and the former Ultimate Speed Challenge.
- ISBD Masters build plans, July 2026 Current official build plan linked from the division hub. Use it directly; this site does not reproduce it.
- ISBD Legacy build plans Official plan describing the design-led class and its 275-pound combined limit.
- ISBD Legacy rule amendment, April 2025 Official amendment that updates mold and helmet-area foam requirements.
- NASA Glenn: Measuring aerodynamic drag Primary educational reference for calibrated force balances, tare measurements, and repeatable wind-tunnel drag tests.
- NASA Glenn: Similarity parameters and Reynolds number Primary educational reference for matching flow conditions; a small model can produce a different drag coefficient when Reynolds number is not representative.
- 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.