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I agree that wheel spoilers and side skirts most likely decrease drag, but the latest 2005 edition of “Aerodynamics of Road Vehicles” (in German) takes a different view of the front air dam. According to the book (pages 236 and 521) the wind resistance of cars with rough underbodies goes down if you increase the depth of the front air dam until the point that the air dam is almost the lowest point of the underbody. If you make the air dam deeper than that, overall wind resistance will start increasing again even before the frontal area is impacted although front lift will keep going down. It also states that if the car has a smooth underbody, a deeper front air dam will increase drag, period.That’s surely a generalization that doesn’t apply to every car ever made, but to decrease the aerodynamic drag of an NSX, it’s probably a better idea to make the bottom of the car smooth rather than fit a deeper front air dam.In "Aerodynamics of Road Vehicles" as well as in "Race Car Aerodynamics", there’s an interesting chart regarding the downforce and drag created by diffusers with different angles. As would be expected, downforce increases the steeper the diffuser as long as the flow stays attached. Drag is minimized at around 3-5° (5° is what Honda used for the first generation Insight) and it goes up again at steeper diffuser angles. As goldNSX said, a properly designed underbody and diffuser can get you downforce effectively for free and at diffuser angles of 3-5°, Mother Nature actually pays you well to take it. The diffuser speeds up the flow in the throat of the car's underbody and at the same time, decreases the effective base area at the rear of the car.
I agree that wheel spoilers and side skirts most likely decrease drag, but the latest 2005 edition of “Aerodynamics of Road Vehicles” (in German) takes a different view of the front air dam. According to the book (pages 236 and 521) the wind resistance of cars with rough underbodies goes down if you increase the depth of the front air dam until the point that the air dam is almost the lowest point of the underbody. If you make the air dam deeper than that, overall wind resistance will start increasing again even before the frontal area is impacted although front lift will keep going down. It also states that if the car has a smooth underbody, a deeper front air dam will increase drag, period.
That’s surely a generalization that doesn’t apply to every car ever made, but to decrease the aerodynamic drag of an NSX, it’s probably a better idea to make the bottom of the car smooth rather than fit a deeper front air dam.
In "Aerodynamics of Road Vehicles" as well as in "Race Car Aerodynamics", there’s an interesting chart regarding the downforce and drag created by diffusers with different angles. As would be expected, downforce increases the steeper the diffuser as long as the flow stays attached. Drag is minimized at around 3-5° (5° is what Honda used for the first generation Insight) and it goes up again at steeper diffuser angles. As goldNSX said, a properly designed underbody and diffuser can get you downforce effectively for free and at diffuser angles of 3-5°, Mother Nature actually pays you well to take it. The diffuser speeds up the flow in the throat of the car's underbody and at the same time, decreases the effective base area at the rear of the car.
