Speed Science
Thermal dynamics
Understand how surface temperature reaches the boundary layer, wheels, and axles, using the tested claims and the debunked myths that Derby Tech published in 1984.
HEAT IS A DRAG TERM
At derby speeds a warm skin helps, and the reason is the boundary layer.
The thermal argument in gravity racing was laid out by Tony Carlini Sr. in Derby Tech, May 1984, in an article called "Color Me Fast". His case rests on two aerodynamics references rather than on shop lore: Abbott and Von Doenhoff's Theory of Wing Sections, and Schlichting's Boundary Layer Theory.
The mechanism he describes is heat transfer across the surface changing where the laminar boundary layer trips into a turbulent one. Schlichting notes that at very high speeds, Mach 2 and above, heat moving from the surface into the air destabilises laminar flow. Carlini's point is that at low speeds the reverse holds, so a racer running at 30 miles an hour with a surface warmer than the surrounding air gets a more stable boundary layer, less air drag, and a faster car. From Theory of Wing Sections he takes the comparison that skin friction for a surface absorbing no heat runs about 1.330 against 1.420 for a soap box at a quarter of the free-stream absolute temperature at 50 miles per hour.
Since the sun is the only heat source available on a race hill, and black absorbs the most of it, his conclusion is that the surface should be as hot as possible and therefore black. He also credits the idea's origin: "In 1949, Fred Derks, of Akron, figured it all out with a complicated system of calculating heat, pressure, and resistance. He raced a black car and won handily", reported in Popular Science, May 1952.
HOW BIG IS THE EFFECT
Small. Carlini said so himself.
This is the part that keeps the page honest. Asked how much faster a black car physically is than a white one, Carlini answered plainly: "The answer may surprise you by its smallness, about six inches." He adds directly that "colour selection alone is not enough to make you win, but it helps".
Six inches at the finish line is a real effect at championship level and an irrelevance if the car has a binding wheel. Treat thermal work as the last quarter inch of a setup, never as a substitute for load, geometry, alignment, and a driver who can hold a line.
MYTHS DERBY TECH KILLED
Four thermal theories that sound right and do not work.
Carlini listed these specifically because they circulated in pits and none of them survive testing. They are reproduced here as the debunk, not as advice.
| The claim | Why it fails |
|---|---|
| A black car cooks the resin and fibreglass inside, costing resiliency | Softening those materials needs roughly 145 degrees Fahrenheit and up. Those temperatures are never reached inside a racer. |
| White front half, black back half, so cool air contracts ahead and hot air expands behind and pushes the car | "It does not work on hill tests, wind tunnels, or anywhere." |
| A black car heats surrounding air, thinning it so the car penetrates more easily | Black is not faster for this reason. |
| A white racer reflects heat onto the wheels and axles and speeds them up | Wheels and axles do run faster hot, but a white racer does not radiate enough heat to achieve it. |
WHAT DID HOLD UP
Two effects Carlini said he tested.
RADIATE TO THE ROLLING PARTS
Black axle trees
A black racer, or any colour racer with black axle trees, builds heat from the sun and radiates it into the wheels and axles. Carlini reported measuring slightly higher wheel and axle temperatures, which made them slightly faster.
WARM THE STRUCTURE
A warm interior
"The inside of your racer should be warm. The heat adds resiliency to all component parts, and causes them to use less energy during deflection." Less energy lost to deflection is the same currency as everything else on this hill.
Both claims are testable and both are worth re-testing on modern wheels and materials rather than accepted from a 1984 newsletter. The correct experiment measures wheel and axle temperature directly, in the sun, on the day, against a paired baseline.
BEFORE YOU PAINT ANYTHING
Finish is a rules question first and a physics question second.
Colour, coatings, films, waxes, tapes, and surface treatments are governed by your current ISBD or NDR rule book, your division build plan, its amendments, and the inspector at your race. A thermal argument from 1984 authorises nothing. The Masters plan in particular controls surfaces and prohibits aerodynamic tape treatments outright.
Two further cautions. Carlini's article spends its second half on colour as psychological warfare against opponents and officials, which is period colour and not a claim this site endorses or repeats. And a dark car sitting in the sun before a run is a heat problem for the child inside it long before it is an aerodynamic advantage: check seat, shell, and helmet temperature, and keep the driver and the car shaded until they are called.
SOURCES & FURTHER READING
Check the primary material.
- Derby Tech, May 1984 - Newsletter for the Technical Enthusiast Primary source held as a photographed page set. Editor and publisher Ollie Brower, Newbury Park, California. Facts, measurements, and survey results are reported here with short attributed quotations; the issue itself remains under its own copyright and is not republished in full.
- Abbott and Von Doenhoff, Theory of Wing Sections The aerodynamics reference Derby Tech treated as standard reading. Cited in the May 1984 issue for skin friction and for the NACA 6-series profiles behind its airfoil data sheet.
- Schlichting, Boundary Layer Theory Cited by Tony Carlini Sr. in Derby Tech May 1984 for how surface-to-air heat transfer shifts laminar-to-turbulent transition.
- 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 Rule Book - linked for the 2026 World Championship Official PDF currently linked from Race Week resources; the file itself says revised June 2025.