Designing for Downforce

Airflow is paramount for performance vehicles, which require aerodynamic enhancements to promote high-speed stability and greater cornering capability.

February 1, 2014

Ryan Gehm

The 2014 Camaro Z/28’s rear spoiler was modified with a “wickerbill” — a small, vertical tab at the edge of the spoiler — that allows the car to handle turns at higher speeds and have greater overall high-speed stability.

Aerodynamics currently is a hot topic for the transportation sector, from passenger vehicles all the way to Class 8 trucks. Along with advanced powertrain technologies and light-weight materials, aerodynamics is an area receiving considerable engineering attention due to its potential for improving vehicle fuel efficiency — a major goal as stricter regulations loom for cars and light-duty trucks as well as medium- and heavy-duty vehicles.

But for cars that easily surpass the 100-mph (161-km/h) mark and feel as at home on the racetrack as on the road, airflow takes on added significance. Their speed potential must be complemented with the appropriate aerodynamic solutions to keep such cars safely and firmly planted to the driving surface at high speeds.

The Z/28 features rocker moldings and unique wheelhouse extensions that help push air past the tires. Deflectors at the bottom-front corners of the front wheel flares take the place of a conventional air dam.

Following are details of two recently announced performance vehicles — the Ferrari 458 Speciale and the Chevrolet Camaro Z/28 — that place particular emphasis on design for downforce.

Active aerodynamics for special Ferrari

Ferrari revealed at the 2013 Frankfurt Motor Show a new special-series sports car, the 458 Speciale, to join stable mates 458 Italia and 458 Spider. Derived from the 458 Italia, the new berlinetta (i.e., sporty coupe) embodies an even-greater focus on performance and boasts an abundance of advanced technologies not just in the powertrain and chassis, but also in the finer details of its body.

“The engine, in which we have put a lot of Formula One technology, is not the only news in this car,” Ferrari Chairman Luca di Montezemolo said at the Frankfurt reveal. “We have a very innovative, patented active aerodynamics. This is Ferrari technology and is completely new on the market.”

Mobile aerodynamic solutions at the front and rear of the car ensure that different aerodynamic configurations can be adopted in cornering, where maximum downforce is required, and on straightaways, where drag must be minimized. The 458 Speciale has a downforce (Cl) value of 0.53, along with a 0.35 Cd.

Ferrari 458 Speciale’s performance specs are impressive: a weight/ power ratio of 2.13 kg/PS, acceleration from 0-100 km/h (0-62 mph) in 3 s, and maximum lateral acceleration of 1.33 g. Aerodynamic refinements were necessary to complement such performance.

At the front of the car are two vertical flaps in the center and a horizontal flap below them. The company explains that at relatively low speeds, the vertical flaps are closed, channeling air into the radiators for engine cooling; but at speeds in excess of 170 km/h (106 mph), the flaps open to reduce the volume of air flowing into the radiators, thereby cutting drag. Above 220 km/h (137 mph), the horizontal flap lowers to balance downforce between the front and rear axles, leading to a 20% shift in overall downforce toward the rear, Ferrari claims.

Turning vanes at either side of the front bumper are designed to slow the airflow and increase downforce, shifting the aerodynamic balance 4% over the front, according to Ferrari. Aerodynamic fins ahead of the rear wheels increase downforce in a similar fashion as the front turning vanes.

By repositioning the tailpipes, Ferrari engineers could design a new diffuser that optimizes the extraction capacity of the underbody, increasing downforce (Cl) value by 6.5 points.

Ferrari engineers developed a rear spoiler with a larger surface area and more pronounced shape, which improves underbody efficiency and also increases downforce by 7 points. And by repositioning the tailpipes, engineers could design a new diffuser that optimizes the extraction capacity of the underbody, increasing Cl by 6.5 points.

Flaps at the rear of the 458 Speciale have two configurations: raised for higher downforce and lowered to minimize drag. Sensors and a specific algorithm allow the flaps to be lowered by as much as a 17° angle, thereby stalling the diffuser and reducing Cd by 3 points, the automaker claims.

The Ferrari Style Center collaborated with Pininfarina on the car’s racing-inspired styling. Most of the body panels have been redesigned without modifying the passenger cell or the signature design features of the 458 Italia. Glass thickness has been reduced to cut weight, and the rear windscreen is now a Lexan (from SABIC Innovative Plastics) polycarbonate panel. The composite bumpers were redesigned, and the front hood features two deep air outlets to channel away the air exiting the radiator.

Aero-focused Camaro Z/28

The new Camaro Z/28 is the fastest factory-produced Camaro ever on a track, posting a lap time at Nürburgring that was 4 s faster than the Camaro ZL1’s and beating published times for the Porsche 911 Carrera S and the Lamborghini Murcielago LP640, according to General Motors.

Turning vanes at either side of the front bumper are designed to slow the airflow and increase downforce, shifting the aerodynamic balance 4% over the front.

To accomplish such a feat, GM engineers had to focus on more than just what was under the hood; they had to modify numerous exterior elements compared to the Camaro SS to improve downforce. These modifications help press the Z/28 to the ground with 440 lb (1.96 kN) more force at 150 mph (241 km/h) than the SS model, which generates slight lift at that speed.

“Most production cars are designed with some lift at speed in order to slip through the air for greater fuel economy, but the all-new Camaro Z/28’s track-oriented purpose dictated an entirely different take on airflow management,” said Tom Froling, Lead Development Engineer–Aerodynamics. “Its carefully tailored aerodynamics package generates downforce for greater handling stability at speed, and the grilles are optimized to meet stringent cooling flow and brake-cooling requirements.”

The Z/28 shares several racing-inspired aero features from various Camaro and Corvette racecars, Froling noted. A front splitter that provides downforce at the front of the car is designed to withstand 250 lb (1.11 kN) of force at its tip. The splitter is matched with an aero closeout panel under the front of the engine compartment, along with molded-in aero features forward of the front wheels.

To meet downforce requirements for Z/28, the rear spoiler used for Camaro SS was modified with a “wickerbill” — a small, vertical tab at the edge of the spoiler. The “minor” change adds about 28 counts of drag, improving rear lift performance by 70 counts, according to GM.

A functional carbon-fiber hood extractor — similar to the one featured on the Camaro ZL1 — not only increases engine cooling but also provides a path for air channeled through the grille to exit out the hood and over the car. GM notes that without the vent, the air would be pushed out the bottom of the engine compartment, possibly generating lift.

A functional carbon-fiber hood extractor not only increases engine cooling but also provides a path for air channeled through the grille to exit out the hood and over the car. Without the vent, the air would be pushed out the bottom of the engine compartment, possibly generating lift.

The Z/28 underbody incorporates a belly pan that helps reduce front lift and also contributes to drivetrain cooling. Modified NACA duct profiles draw air into the underbody tunnel area to provide extra cooling for components affected by the engine’s exhaust thermal energy. Unique wheelhouse liners with closeouts also assist with airflow.

The fog lamps, air dam, and upper grille found on the front fascia of the Camaro SS are replaced with covers, an air duct support bracket, an airflow-optimized upper grille for enhanced cooling, and a modified fascia lower inlet that helps funnel air from the splitter/lower grille through ducts to the brake rotors and calipers.

Engineers employed CFD, reduced-scale rolling wind-tunnel testing, full-scale clay models, and full-size prototypes tested in several wind tunnels, including GM’s facility in Warren, MI, for the aerodynamic development of the new Z/28.