Every car in motion has to force its way through the air, which opposes its progress with a force that grows the faster the car goes. That force is known as aerodynamic drag and, together with rolling resistance and transmission friction, it represents the work the engine must do simply to keep the car moving. Its expression is a fairly simple one:
Fa = ½ · ρ · v² · Cd · S
where ρ is the density of the air, v the relative speed between car and air, S the frontal area and Cd the drag coefficient. This last figure, written as Cd in English-language literature but as Cx in Italian and French usage and as Cw in German, is a dimensionless number: it does not depend on the size of the body, only on its shape and on the way the flow leaves it. A low Cd identifies a body around which the air closes back in an orderly fashion, without leaving a broad wake of vortices behind it — and it is precisely that wake which accounts for the largest share of a car’s drag.
The Cd cannot be worked out on paper, it has to be measured. The classic check is carried out in a wind tunnel, where purpose-built balances record the forces exchanged by the car as it is struck by an airflow of known speed, while a moving belt beneath the floorpan reproduces the flow under the car. Alongside it sits computational fluid dynamics (CFD), which simulates the flow on a computer and makes it possible to assess dozens of variants before a physical model is ever built.
Drag coefficient and frontal area: two figures to be read together
Alongside the coefficient, the formula also contains the frontal area S, that is the surface of the outline the car projects when viewed exactly head-on. Drag depends on the product of the two terms, written as CdA (or SCx) and expressed in square metres. It follows that a tall, wide car with an excellent Cd can present the air with more resistance than a low, narrow car with a mediocre one: this is why SUVs, for all the considerable progress made on their shape, remain at a disadvantage compared with saloons.
As an order of magnitude, modern saloons sit between 0.28 and 0.32 Cd and sports cars between 0.25 and 0.30, while boxy-bodied vehicles comfortably exceed 0.35. Cars designed around efficiency drop below 0.25: the Mercedes EQS was presented as the first production car with a claimed figure starting at 0.20, while the Lucid Air claims 0.197 in its aerodynamic-wheel configuration.
Why the drag coefficient matters so much
In the formula, speed appears squared, so doubling the pace means quadrupling the resisting force. And since power is the product of force and speed, the power absorbed rises with the cube of speed: overcoming the air at 200 km/h takes roughly eight times the power needed at 100 km/h. Four consequences follow from this.
The first two concern energy. On fuel and energy consumption, rolling resistance dominates at low speed, but at motorway pace the aerodynamic contribution becomes the prevailing component of the total resistance to motion, and on electric cars range depends on it to a considerable degree. On top speed, reached where the power at the wheels equals the power absorbed by the resistances, gaining a few km/h at the top end demands ever larger increases in power: working on the shape is almost always more worthwhile than working on the engine.
The other two concern behaviour. Stability, because the flow running over the bodywork travels a longer path than the one passing beneath the floorpan, and the resulting pressure difference generates an upward vertical force, lift, which lightens the axles and reduces grip; and if that force is then distributed unevenly between the two axles, steering precision and behaviour in a crosswind both suffer. And noise, since the vortices generated by mirrors, A-pillars and window seals produce a wind rustle that at high speed exceeds the noise of the engine and of the tyres on the road.
To manage all of this, designers have at their disposal a set of devices that act on the flow at different points of the car:
- Front splitters and spoilers
- Rear spoilers and wings
- Flat floor and diffuser
- Active aerodynamic elements
Front splitters and spoilers
At the front end the flow divides between the part running over the bodywork and the part channelled beneath the floorpan. The splitter is a horizontal blade projecting from the base of the bumper: the air striking it head-on is slowed almost to a standstill and creates a zone of high pressure above it, while the air passing underneath is accelerated and sees its pressure drop. The difference between the two loads the front axle downwards.
On production cars the same job is entrusted to a simpler front spoiler, which reaches down towards the tarmac to limit the air directed under the car, where the underbody and the exposed mechanical components generate a great deal of drag. The same logic lies behind active grille shutters, which partially close off the engine bay air intakes whenever cooling requirements allow.
Rear spoilers and wings
It is worth distinguishing between two devices that everyday language tends to conflate. The spoiler is an element integrated into the bodywork, typically a raised lip on the edge of the bootlid or tailgate, whose job is to establish a clean separation point for the flow, preventing the air from following the sloping tail and generating lift and disordered vortices. Correctly sized, it reduces lift and drag at the same time, because it makes the wake more compact.
The wing, on the other hand, is an aerofoil section separate from the bodywork and mounted upside down with respect to an aeroplane wing: its function is to produce downforce, a force that presses the rear axle onto the tarmac, increasing grip in corners and under braking. But every profile that generates downforce also generates drag: choosing the angle of attack is always a compromise between the two effects.
Flat floor and diffuser
The underbody of a conventional car is an irregular surface, populated by exhaust, suspension and crossmembers, which slows the flow and causes it to separate. Covering it with underbody panels is today among the most effective ways of lowering the Cd without touching the visible shape of the bodywork, so much so that it is now adopted on mid-range and budget cars too.
A smooth floor also makes the diffuser possible, the diverging channel formed in the rearmost part of the floorpan. The air travelling through the narrow gap between floor and tarmac is forced to accelerate and its pressure drops, following the same principle that governs a constricted duct; the diffuser then gradually returns the outgoing flow to ambient pressure, drawing further air out from beneath the car in the process. The result is downforce distributed along the entire floor without adding surfaces exposed to the wind: this is why the floor is the most efficient way of producing aerodynamic load, and why in motorsport it is the primary element.
Active aerodynamic elements
A car’s requirements change constantly: on the motorway minimum drag is what matters, in corners and under braking maximum load is. A fixed profile cannot satisfy both conditions, and from this comes active aerodynamics, the use of moving surfaces managed by a control unit according to speed, steering angle and pressure on the brake pedal. On the Porsche 911 the extending rear spoiler takes up different positions depending on speed and driving mode; on the Bugatti Chiron the hydraulically operated rear wing rises to generate load, drops when top speed is being chased, and sets itself almost vertical under braking, acting as an air brake. The ALA system (Aerodinamica Lamborghini Attiva) on the Huracán Performante works differently, using valves that open and close ducts running inside the splitter and the wing.
Two philosophies compared
In the design of a road car the overriding aim is to reduce drag: work goes into clean surfaces, underbody panelling, wheel design and the flow around the wheel arches. Downforce is not sought for its own sake; the target is rather to cancel out lift and to balance it evenly between the two axles, just enough to guarantee stability and safety.
In a high-performance or competition car the priority is reversed. Since the force a tyre can transmit grows as the vertical load pressing it into the ground increases, generating downforce means carrying more speed through corners and braking later: the designer therefore knowingly accepts a higher Cd in exchange for load, in the knowledge that over a timed lap the time gained in the corners outweighs the time lost on the straights. Once this is understood, a specification sheet can be read correctly too: a high Cd is not necessarily a design flaw, nor is a very low Cd any guarantee of performance in the corners.
Photos: © Car-Shooters