This is the third installment of our look at aerodynamics in cars. Part 1 was about the front splitter, Part 2 was about the rear wing, and today’s article will focus on the rear diffuser. The rear diffuser is used to shape and decelerate airflow from underneath the car. It was the cause of much controversy at the beginning of the 2009 F1 season, with Toyota, Williams, and Brawn GP having a so-called double diffuser that was assumed by the other teams to be outside of the rules. The double diffuser was allowed and made the 2009 spec F1 cars produce significantly more down force than the previous year’s spec. It is used in most forms of racing, on both prototype and GT class Le Mans cars and most formula cars, in addition to high performance sports cars.

The diffuser works in two ways. First, it reduces the turbulent airflow.  Second, it slows down and pressurizes the air as it comes off the car.

Turbulent airflow is always problematic to aerodynamic efficiency. The diffuser reduces this turbulence. When the air hits the front splitter and is forced underneath the car it speeds up and decreases in pressure. When it enters the diffuser the air is allowed to expand and slow down increasing pressure to equate to the normal atmospheric pressure of the surrounding air. This is where the airflow is smoothed to decrease turbulence. If the air were to blast into the normal airstream at its high velocity, low pressure state, parasitic drag is created. This is air needing to fill the void left by the moving object it follows. The diffuser serves to eliminate the creation of parasitic drag and the air is replaced in the same state as the surrounding air.

The second function of a diffuser is a suction effect created by the pressure difference between the fast moving air under the car and the slow moving air in the diffuser. As the air slows, it decreases in pressure. This low pressure area will create a vacuum effect. This vacuum sucks the rear of the car closer to the ground (seeing as the it cannot really move the ground up to meet the car, all relativistic physics aside). It should be noted that most of the effectiveness of the diffuser is lost if there is too much clearance between the road and the underbody.

Come back next week for a look at the rest of the miscellaneous canards and fins that are also common in aerodynamics today.