PCA Southeast Michigan Region

From the Blog


Car manufacturers have long taken advantage of the gullibility of the typical owner.   In the old adage of win on Sunday and sell on Monday the car maker takes advantage of the enthusiasts desire to relate to the excitement of racing.   The knowledgeable car enthusiast knows the difference between a “stock” race car and a true production street model.  That does not preclude adding so called racing parts to a street driven car on the guise that performance is enhanced.  This is visually demonstrated in the various added on aerodynamic appendages such as spoilers, splitters, wings and diffusers.

The science of harnessing the energy of wind passing over the surface of a wing is usually based on the work of Swiss mathematician Daniel Bernoulli’s 1738 theorem of fluid dynamics.  The basic idea is that fluid; in this case air behaves as a fluid, speeds up over a curved surface relative to the opposite flat surface creating lift energy over the curved surface.    But it took until 1935 for another mathematician, Pistolesi to describe what happens when a wing shape is in close relationship to the ground.

Early studies of the effects of air on automobiles were all centered on the drag effect of wind to limit speed.   The 1929 Opel Rocket land speed record car was the first to adopt wings to the body shape.  But it was 1956 when Swiss mathematician and amateur racer Michael May mounted an inverted wing over the cockpit of his own Porsche 550 Spyder for the Nurburgring 1000 race.  When Porsche race boss Huschke von Hanstein saw that May’s creation was faster than the rest of the Werks Porsches he filed a protest and Mays car was banned.  May then later worked for Porsche designing fuel injection.   He was promised a formula car test ride if he could increase engine power through fuel injection but von Hanstein reneged fearing Michael would embarrass the Werks drivers.  May left Porsche to develop fuel injection for Ferrari and led them to install wings to the 1968 model 312 Formula 1 car.   Leading aero development in the US was Jim Hall with his winged Can Am cars from the original 2C in 1965 and the variable pitch rear wing through the true ground effect 2G sucker car in 1968, cars so effective they got banned.    Dan Gurney did create the Gurney flap during an Indy car test of his 1971 AAR Eagle race cars.  The intent was to reduce drag of the rear wing to improve top speed.  The actual result was that the small lip at the trailing edge created a turbulence that increased the effective length of the lower wing surface and increased down force.

Aero development in race cars is credited with the huge leap in cornering forces from about 1.5 G to as much as 4 G at speeds well over 100 mph.   To achieve that the combined wings and ground effects of a modern Indy car create as much as 5,000 pounds of down force at 200 mph.   Since down force also creates speed reducing drag the winning design is the best compromise of wing size to produce the optimum lap times.

Porsche came out in 1973 with the very first street model car with a racing spoiler in the Carrera RS.   Called the Duck Tail or “Burzel” it was a tall flat panel attached to the rear deck lid at right angles.  The true effect of this spoiler was not to create downward pressure on the rear tires but to negate the lift by 75% created by the wing shape of the 911 roof and to reduce the drag effect of the roof.  Porsche measured the speed increase of .5 kph with just the rear wing and 2 kph with the introduction of a squared off front bumper valence.  In combination the two aero pieces increased top speed by 4.5 kph.   Naturally I had to adapt these pieces to my own 1972 911 with the only noted effect that the back side of the duck tail is continually smashed with dirt and exhaust film.   The duck tail spoiler was gone by 1976 with the introduction of the whale tail spoiler on the Turbo model.   Versions of rear spoiler have continued on the present 911 model.  On the 996 model Porsche revealed that the rear spoiler reduced front lift from 64 kg to 5 kg and rear lift from 136 kg to 14 kg but this was measured at 157 mph.   Aero devices were insignificant below speeds of 80 mph but do contribute to gas mileage robbing drag which is why Porsche developed the disappearing rear spoiler.  The vast majority of street cars equipped with rear spoilers are strictly cosmetic or at best an attempt to reduce the aerodynamic drag of air as it leaves the trailing edge of the body work.   For a rear wing to create down force on a car it would have to be mounted significantly high above the rear body to receive clean air flow.

This leads me to the latest fad of the rear lower bumper valence in the shape of a racing diffuser.   The diffuser has the curved shape of a wing as the lower surface of the bumper.   The 1978 Lotus Formula 1 car is credited with advancing the science of automotive ground effect.   The Lotus employed a diffuser in each side pod of the race car with a sealing edge that scraped on the ground.  As with many racing advances it was so effective it was banned.  Based on aerodynamic theory the wing shape applied to the underside of a car has the effect of creating negative pressure, effectively a suction effect between the ground and the bottom of the car.   The effectiveness of ground effects and diffusers depends on the aero property called boundary layer.  This is the principal called the Coanda effect that fluids (in this case air) have a viscosity or thickness that tends to stick to a curved surface.    Many factors contribute to the end result of control of air flow under a car.  Chief of which is the gap between the diffuser and the ground.   To control air flow strakes or end plates guide the air across the curved surface of the diffuser.  To be truly effective the gap to the ground and the length of the diffuser have to be in a ratio of .05 which is why race cars have a gap between the diffuser and ground often measured in fractions of one inch.  On a street car this is totally impractical.  Also for boundary layer to be effective the flow of air must have minimal interruption.   On a street car this would call for a totally smooth bottomed car, also impractical.  This makes virtually all street car rear diffusers just a visual tack on and more of an aero drag reducing device than effectively creating down force.

So there you have the straight skinny on the wing, spoilers, splitters, diffusers and the rest of the kit we would love to believe links our street car to its racing brethren.   Yes it is marketing hype for the most part but it is a modern day essential styling feature of almost all of our cars.