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Downforce is a term used to describe the downwards force created by airflow as it passes the exterior of a moving object, namely cars in this case. The purpose of downforce, in an automotive sense, is to counteract lift (basically the opposite, where airflow passing a moving object causes it to lift off the ground). It is commonly used to ensure a car does not lose traction at high speeds and to allow a car to travel faster through a corner by increasing the vertical force on the tires, thus optimizing grip.
A Diffuser is a great example of a downforce generating automotive component. A diffuser is a shaped section of the car’s under-body, usually on the rear bumper, which improves the car’s aerodynamic properties by improving the transition from the high-velocity airflow underneath the car to the much slower free-stream airflow of the ambient atmosphere. It works by providing a space for the underbody airflow to decelerate and expand so that it does not cause excessive flow separation and drag. Thus the diffuser causes a more efficient flow of air, which in turn accelerated the underbody airflow which means more downforce.
Basically what all that means is the following:
- Airflow when it hits the front bumper can either go above or below the car, either way the airflow accelerates as it compresses, which for the underbody causes a low air pressure and creates a “suction effect” thus causing downforce
- A rear diffuser allows the underbody airflow to efficiently expand and decelerate back into the atmosphere
- By improving this transition the diffuser allows more airflow to decelerate, allowing the underbody to have increased airflow which in turn increases underbody airflow acceleration
- Increasing underbody airflow acceleration means lower air pressure in the underbody and thus more downforce