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Rear Diffusers | A Lesson in Aerodynamics

It is not enough that your car makes ludicrous amounts of power and torque. Going fast does not rely on sheer power alone. There is also an unseen property called aerodynamics. 

Aerodynamics is the study of the properties of moving air and the interaction between the air and solid bodies moving through it. One component of aerodynamics in – or – outside the car is the rear diffuser.

In automotive terms, a diffuser is a shaped section of the car’s underbody, which improves the vehicle’s aerodynamic properties. This happens by enhancing the transition between the high-velocity airflow underneath the car and the freestream airflow of the ambient atmosphere. In simpler terms, the purpose of a rear diffuser is to reduce air turbulence as your car drives at high speeds.

Picture this: As a vehicle is driven down the road, high-speed air is passing underneath the vehicle at low pressure. As the air passes to the rear of the vehicle through the diffuser, the air expands through an expansion chamber. Through this expansion, air speed is reduced and pressure is increased - through what is called the Venturi effect. This pressure differential between the low-pressure air under the car and the high pressure at the rear creates a vacuum sucking air out from under the car. With high-pressure air above the car and low pressure under the car, high downforce is created while reducing aerodynamic drag.

But to better understand drag, it is the force of air pushing in the opposite direction of the vehicle, reducing efficiency.

Imagine the comparison between the front of a semi-truck which is mostly flat and tall compared to a sports car’s low and pointy front. The semi-truck has a greater surface area pushing against the air as it moves forward, preventing the vehicle from efficiently moving through the air. More power is needed to overcome this negative force creating inefficiency.

By decreasing drag, rear diffusers make your car more stable at high speeds and reduce the amount of wind resistance it must overcome. Less wind resistance means that your vehicle will be able to utilize its existing horsepower and torque more efficiently. A rear diffuser also adds an even more aggressive style to your car which helps it stand out from the crowd. 

Having a rear wing on your car also affects the function of the diffuser. When the rear wing is mounted low and close to the diffuser, the low pressure under the wing helps suck air through the diffuser.

As the front of a vehicle slows down the air without a diffuser, this is the ideal place to install an inlet. A splitter is commonly used and it serves to increase the amount of downforce (lift inverted) at the front of the car. The airstream is brought to stagnation above the splitter by an air dam, which results in an area of high pressure. Below the splitter, the air is redirected away from the stagnation zone and accelerated, causing the pressure to drop. The decrease in pressure in combination with the high pressure over the splitter area creates downforce. Consequently, the larger the area of the splitter, the more downforce is generated.

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