Speed & Sound: Performance – Diffusing the Situation
The aerodynamic impacts of a rear diffuser
Story by Budd Stanley
Aftermarket body kit companies mimic them and automakers place sporty looking replicas on the back of performance cars. They are diffusers, a curled piece of bodywork under the rear bumper used to give aerodynamic downforce to the rear wheels. But, do they work? To answer this question, first we have to learn the art of aerodynamics, and how a diffuser actually works.
Now if you have actually taken the time to read this, chances are you already know how a wing works. An aerofoil or wing is a curved surface pointing up (opposite of a airplane’s wing) connected to a flat surface on the opposite side. Air flowing over the curved surface is forced to speed up because the curved surface is longer than the flat. In turn, this creates a low-pressure area under the wing. This difference in pressure creates a downward force, which is measured as downforce. Effectively, this is how a rear wing works on the back of a race car. The differences in air pressures can be manipulated by the size and angle of the wing to give the desired amount of downforce to the back of the car. This increase in downward weight helps press the tires into the surface, giving you better traction.
Now we come to the actual diffuser itself. The diffuser is located under the car’s rear bumper, and operates as a mirror version of a wing foil. It takes air flowing under the car and uses it to create additional and much more efficient downforce to the rear of the car. The diffuser is a curved surface angling up and away from the ground. So the compressed area of air moving under the car must now expand into the now opening void the diffuser creates. Like the wing, air moves much quicker up the diffuser’s surface than along the ground, creating a drop in pressure. This translates into downforce. And again, downforce can be manipulated by the size and angle of the diffuser. Vertical fins, or strakes, are used to ensure only air traveling under the car travels this path, and not air sucked in from the side of the car.
The problem is, a diffuser requires a flat bottom car to work efficiently. The diffuser’s leading edge must be connected to a level plane that runs the length of the car in order to effectively create the negative air pressure. On top of that, the car needs to be low to the ground to regulate and compress the air flowing under the car. So if you’re thinking about putting a diffuser on your street car, think again. Without the flat bottom connecting to the diffuser itself, any device installed could have an opposite effect, literally catching air and holding it in behind your bumper, only being efficient as acting as an air brake. Building a proper system requires much custom work and fabrication to gain the desired effects.[PSGallery=179fdjw2jp]
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