For decades, rear diffusers have brought Formula 1 cars back to earth, and now these clever bits of aerodynamic trickery have found an everlasting home adorning today's hypercars. I love the idea of a rear diffuser, especially since aerodynamics are still considered to be somewhat of a dark art; this means that no automobile manufacturer will dream up the same design. Each hypercar manufacturer has different ideas on how to craft their diffusers for the greatest aerodynamic effect.
Lets first discuss how these rear diffusers work. Rear diffusers provide downforce in the opposite way that the main element of a rear wing would work. A wing creates a high pressure area on the top of a wing and a low pressure area on underneath the wing; the high pressure area forces downward pressure on the wing, much like pushing down on it yourself, and the low pressure area on the underside of a rear wing creates a vacuum, pulling the wing down. All of this downward pressure forces the car toward the asphalt improving grip on the tires which improves the speed a car can carry through a corner.
A rear diffuser acts much like the underside of a wing. the air travels underneath the car approaching the diffuser, the floor then curves upward creating more physical space for the same amount of air; this creates a vacuum like effect on the underside of the car, sucking it towards the earth. Below is an illustration by Craig Scarborough of the Red Bull Renault RB7's Monza iteration rear diffuser.
Rear Diffusers in Formula 1
The diffuser world holds many types, each vying for maximum performance extraction from the dynamic regulations. The double diffuser came with Brawn and lead them to a championship, but was eventually banned. The double diffuser, or double deck diffuser, was allowed through a loophole in the rules. I'll let Craig Scarborough do the rest of the talking:
Since 2009 the regulations regarding the floor have been interpreted in a literal sense to allow the double deck diffuser (DDD). Indeed the very same rules were exploited to a lesser extent under the previous rules, but this only produced small extra channels in between the outer and middle diffuser tunnels. With the major cut in aerodynamic aids for 2009, several teams sought to find a way to gain more expansion ratio from the smaller diffusers. In essence the loophole exploited the definition of surfaces formed between the step and reference planes. Multiple surfaces allowed fully enclosed holes, which fed the upper diffuser deck that sat above the 175mm lower diffuser. This allowed diffuser to be significantly larger in order to create more downforce. Notably Brawn, Williams and Toyota launched 2009 cars with DDDs. Other teams soon followed suit in 2009 and last year every car exploited the same loophole. Over the winter the FIA acted to close the loophole, by enforcing a single continuous surface across a 90cm span under the floor. In a stroke this banned the double diffuser, there being no scope to create any openings in the floor to feed the upper deck.
Blown Exhaust Diffusers
Blown diffusers have caused a stir among Formula 1's governing body, the FIA. An exhaust blown diffuser works by running the exhaust exit either below or above the diffuser/floor to increase the surrounding airflow underneath the diffuser. The faster the air flows at the rear of the floor, the more that the air is stretched thin, creating a lower pressure area and pulling the car down. An added benefit of these blown diffusers were the sealed off airflow around the diffuser that would otherwise be disrupted by the turbulence of the airflow in contact with the tires. Rear blown exhaust diffusers won't ever be fully banned now that F1 engineers and aerodynamicists have knowledge of their immense benefits.
Rear Diffuser Implementation in Production Cars
After all of the years of these diffuser solutions propelling Formula 1 into the next corner, hypercar manufacturers are now starting to saturate their offerings with performance enhancement diffusers (haha!). Lets start by looking at what McLaren, Bugatti, and Ferrari have brought with them.
McLaren's diffuser may not be as innovative and flashy as a certain elitist F1 and hypercar constructor, lets call them 'Team F', but it looks damn mental! The downforce produced by the P1 is 5 times greater than their own MP4-12C.
Bugatti Veyron Super Sport
The Bugatti Veyron Super Sport looks to support a form of double diffuser. The importance of a low coefficient of drag in attempting a top speed record eliminates the might of a rear wing; however, the Veyron still needs downforce at those speeds, so it must extract an optimum out of its diffuser.
I shall dub thee an Active Double Diffuser. Active aerodynamics have been banned in Formula 1 and have lost the perpetual engineering development that F1 brings, so the revolutionary active rear diffuser underneath the new Ferrari LaFerrari turned heads. The silver colored flaps are able to rotate depending on what the computer system dictates, subtracting downforce and improving the drag coefficient for long straights, or adding downforce for the twisty bits.
The Plastic Rear Diffuser Design Trend
You've all seen them, and I hope you've scoffed. They do absolutely nothing, or at the very most, relatively nothing. The floor of these cars aren't even flat. What good is a diffuser on a FWD Prius doing 45 to go get groceries? Absolutely nothing. I'm not saying that cheap road cars shouldn't have rear diffusers, but I'd like it more if they were actually there for something other than aesthetics. I'd like to think that the toyobaru twins' diffuser worked; it looks as if it does something, though very minimally.