Gettin' down with downforce.
From the November 2013 Issue of Car and Driver
The same principles that loft a 618-ton Airbus into the sky also keep a Formula 1 race car glued to the track. Downforce is lift inverted, the net result of the pressure differential between the air flowing over and under a front splitter, a front or rear wing, or an entire car body. In a modern Formula 1 car, the diffuser can contribute more than 1000 pounds of downforce. The problem is that this extra stick also raises aerodynamic drag, so engineers have to balance their need for cornering grip and stability against their car’s targeted maximum velocity. What makes a diffuser a preferred aerodynamic device is its efficiency. In terms of beneficial downforce versus parasitic drag, a diffuser is vastly superior to any freestanding wing.
Finessing the Flow
The diffuser acts as an expansion chamber to manage the air as it exits from underneath the car and reintegrates it with higher-pressure ambient air. Smoothing this transition reduces turbulence and drag in the car's wake and improves airflow under the car.
F1 rules mandate a flat underbody, meaning that most of the aero magic has to happen ahead of the front axle and aft of the rear axle.
Bernoulli's Principle
The laws of fluid dynamics state that as the velocity of air increases, its pressure decreases. An F1 car is low to the road, and thus its flat bottom acts as a venturi to pinch the air and force it to move faster relative to the air above the car. This pressure differential manifests as increased load on the tires.
Optimum Angle
The angle of the diffuser relative to the ground plays a critical role in its effectiveness. Too steep and the flow separates from the underbody, increasing turbulence and drag. A too-shallow angle lessens the diffuser's ability to create low pressure under the car.
Rake Matters
When the rear of the car rides higher than the front (rake angle), the entire underbody can act as a diffuser. But, as the rear of the car gets farther from the road, the under-car flow is more likely to be disturbed by air entering from the sides and around the wheels.
Fence Me In
Vertical fences, called strakes, guide airflow into and out of the diffuser to maximize its efficiency.
Hot Wind
Carefully placed exhaust pipes can improve a diffuser's efficiency. Formula 1 cars use fast-moving exhaust gases to seal the diffuser sides from the influence of turbulent air coming off the tires. Fully exploiting the exhaust flow can increase the car's maximum effective rake angle. New rules for 2014 permitting only one exhaust exit located higher and roughly centered in the car will force engineers to rethink how they can use the fast-moving exhaust stream to their advantage.
Faux ’Fuser
A functional diffuser requires a flat underbody and a low ride height, attributes not often found on mainstream cars. The diffuser you get on, say, a Kia Forte Koup is just for looks.
Eric Tingwall
Print Director
Eric Tingwall holds degrees in mechanical engineering and journalism, a combination he pursued with the dream of working at Car and Driver. While living his dream, he has cut car parts in half, driven into a stationary dummy car at 50 mph, lapped Virginia International Raceway in the hottest performance cars, and explained the physics behind the wacky, waving, inflatable, flailing-arm tube man.
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