Chapter 8 of 8
Below town speeds the air is barely worth mentioning. Above about 100 km/h it quietly becomes the strongest force on the car — the thing your fuel is really spent fighting, and, for racing cars, a resource worth more than engine power. The last chapter of the car track is about the fluid the whole machine swims through.
Air resistance grows with the square of speed — but the power to overcome it grows with the cube, because you are pushing a harder force at a higher speed. Drive twice as fast and the air demands eight times the power. This single law explains most of motoring: why fuel economy collapses on the motorway, why a car needing 20 kW to hold 100 km/h needs 160 to hold 200, and why top-speed bragging rights are bought in hundreds of horsepower for tens of km/h.
Watch the drag curve sink the tractive-force sawtooth at top speed
What the air charges depends on two things you can design: the car’s frontal area and its drag coefficient — how cleanly the flow closes behind it. The messy part is the back: air that separates off a bluff tail leaves a churning low-pressure wake that drags the car backwards like a parachute. That is why efficient cars taper, why hatchbacks carry little lips and diffusers, and why a small improvement in shape is worth real horsepower at every speed, forever, for free.
Racing asked a different question: chapter 6 said grip scales with load — so could the air be made to press the car down? Bolt on a wing from the planes track, invert it, and every km/h of speed buys clamping force that costs no weight. A modern Formula 1 car generates several times its own weight in downforce and could, in principle, drive on a ceiling. The bill is drag: every kilogram of downforce is bought with a slice of top speed, and wing angles are the compromise each circuit renegotiates.
The cleverest trick uses the road itself. Squeeze the airflow through the narrowing gap under the car and it speeds up, dropping its pressure — and the whole floor becomes a suction pad. The Lotus 79 sealed that channel with sliding skirts and dominated its era; its descendants do it with sculpted tunnels and diffusers. Ground effect is downforce almost without drag, which is why it was banned, re-invented, re-banned and finally re-embraced — the most regulated square metres in motorsport are the undersides of Formula 1 cars.