Now that a ruling has been handed down regarding the controversial rear diffusers designed and used by Brawn, Toyota, and Williams, a basic review of the aerodynamics involved will help explain why these drivers had what the other teams considered an unfair advantage.

The FIA changes for the 2009 season, the greatest number in the history of the sport, included a new rear wing design consisting of a higher wing with a smaller surface area. That higher location helps the wing work for effectively in the clean air, and the smaller surface area reduces downforce, as well as speed. All of this is meant to aid in overtaking, something the FIA has been focused on for some time.

Teams also had to alter the configuration of the rear diffuser, which is basically a shaped extension of the floor of the car. Its purpose is to organize the fast-moving air under the car and slow it, thus reducing flow separation when the faster air hits the slower air at the rear of the car. Filling the space behind the car with the air coming from underneath reduces drag and increases downforce, essentially sucking the car to the track. It works like an airplane wing, only in reverse. The pressure over the top of the car is greater than the pressure underneath it, creating what is known as aerodynamic grip.

Here's where the trouble began. The McClaren, Ferrari, Renault, and Sauber teams abided by the letter of the law and designed diffusers according to the dimensions stated in the FIA rule, calling for a smaller, less complex design. However, the Brawn, Toyota, and Williams teams used some creative interpretations of that same rule to design diffusers that were much larger, more complex, and ultimately more effective. By employing a double-decker system, these designs moved more air through, creating more downforce potential, thus giving the drivers an advantage on the track.

The controversy surrounding these teams centered on the legality of their designs. The rule stated that the height of the three symmetrical channels that make up the diffuser should not exceed 175 mm. Williams and Brawn got around this by redesigning the rear crash structure of the car itself and creating a second central section over the first, which exceeded the 175 mm limit. Toyota created a triple decker design. The legal debate was whether to consider the three channels as one unit, thus violating the rule, or declare the center section completely separate and not in violation. The FIA took that latter position and declared the diffusers used by Williams, Toyota, and Brawn to be legal. This left the other teams scrambling to redesign their systems in order to compete.

One year later, it appears the FIA has made a final, final decision on the matter. Double diffusers like the ones declared legal last year are now banned for 2011, requiring yet another redesign by all the teams. This will level the field once again, but as to improving overtaking, nothing has changed. Besides, given the talent of the drivers coupled with the challenging track designs, finding the right combination will always be a challenge.