Concept Technology History

A front hood air duct:
the aerodynamic mechanism for creating downforce

At the rear, downforce is easily obtained using a wing-type spoiler. At the front, though, adding too big an aerodynamic device can negatively affect minimum ground clearance and/or the approach angle. Increased aerodynamic resistance resulting in reduced acceleration is also another example of the many problems associated with obtaining appropriate downforce in a road-going car.

The solution we chose was to design the underbody of the car as flat as possible to encourage smooth airflow under the car, maintaining airflow speed to create downforce. This method not only provides for adequate ground clearance and approach angle but also does not unduly increase the forward-protruding surface of the body. However, this led to a new problem: how to extract the airflow through the front radiator that had previously been channeled underneath the car?

Taking advantage of the car's mid-ship layout, an air duct was added in the front hood to provide the necessary extraction route. Longitudinal fins were also added to the outer left and right sides of the front under-cover tray to prevent the air passing under the car from entering the front wheel wells. Similarly, spats have been added to both sides of the air ducts to channel air passing through the ducts away from the wheel wells. The opening ratio under the front bumper has also been reduced to limit as much as possible the actual amount of air flowing through. All these innovations result in a smoother airflow both under the body and through the front hood, achieving the desired downforce.

No large aerodynamic appendage was required, helping maintain the original NSX's overall design and ensure a relatively low aerodynamic drag. Downforce was thus achieved without sacrificing top speed.

Wind tunnel tests have shown that when the car is at an angle relative to wind direction, the longitudinal fins of the front under-cover tray function in the same way as the chin spoiler, effectively reducing body lift and improving transient characteristics.

Wind tunnel testing
Wind tunnel testing

View of the underbody
View of the underbody
Cd (Drag coefficient): 0.32
Cl (Lift coefficient/overall): -0.100
Clf (Lift coefficient/ front): -0.040
Clr (Lift coefficient/rear): -0.060

Compared to the original NSX-R, this translates into an increase in vertical force acting on the front tires of 36.2kgf, and of 25.0kgf on the rear tires (test results measured at 180 km/h in both cases).
Lift reduction measures (effect measured piece-by-piece)

Lift reduction measures (effect measured piece-by-piece)
Extensive circuit testing to determine the optimum
equilibrium between downforce and front-to-rear balance

The air duct in the front hood could have been made simply by cutting an opening and trimming the edges with plastic. But because we wanted to maintain the beauty of the original design including the simplicity of line worthy of a car cut for speed like the NSX-R, and to reach the weight reduction target we had set for ourselves, we chose carbon fiber instead. The rear spoiler is similarly a single piece of carbon fiber designed to achieve the required downforce while maintaining a low drag coefficient in a simple shape embodying functionality and beauty.

Both parts are formed using an autoclave, a method more often seen in aircraft manufacture. Multiple layers of pre-pregs made of resin-impregnated carbon fibers are cured in a high-pressure oven to form the parts. The front hood is made of carbon Aramid fibers for added resistance to tearing. In the event of an accident, it is designed not to shatter into small pieces. During the laminating process, fibers are offset by 45 degrees, with each layer above and below being symmetrically angled to provide equal strength in all directions.

Nine to ten hours are required to complete the laminating process of each single part. After lamination, the whole lay-up is wrapped in a baking film, and a vacuum is applied to consolidate the laminate prior to curing for 2-3 hours in the autoclave at a pressure of two to three atmospheres. Once in the autoclave, it takes one hour to bring the part to temperature, while some five hours are required for the cooling down process. Air released from the resin when liquefying at high temperature is carefully bled off to form a strong CFRP (Carbon-Fiber Reinforced Plastic).

The front hood's outer skin is formed separately from the inner frame before being glued together. Glue thickness is strictly maintained at less than 0.5mm. The resultant strength is superior to that of the base materials. The rear spoiler is a hollow, one-piece molding made using a proprietary process developed in cooperation with a parts supplier.

Durability, a matter not normally emphasized in aerodynamic carbon fiber parts manufacture, has been pursued to the utmost. In all aspects of the product, durability on par with steel is achieved. The painting process has also been the object of painstaking attention, especially regarding the undercoating, with both parts undergoing a "5 coat/5 bake" process. For the front hood in particular, paint has been applied so as to let the roughness of the carbon fiber surface show through ever so slightly.

In order to endow the new NSX-R with outstanding high-speed performance, we turned our attention to aerodynamics and their effect on high-speed cornering power, braking, turn-in, and other aspects affecting vehicle controllability. This led us to a new technical approach called "aerodynamically-induced stability". In addition to increasing high-speed cornering power, we have also striven to improve vehicle control quality - the ease with which the driver can control the car, and thus exploit its full potential. This enabled the chassis to be tuned for reduced understeer at low to medium speeds. The resultant improved handling at both low and high speeds endows the New NSX-R with outstanding speed on all types of circuits.
The third advantage of downforce is that it helps reduce body roll as well as body pitch. This in turn helps reduce sudden variations in vertical forces applied to the tires at the limit, increasing vehicle stability in the wake of driver input. Vehicle behavior is also more linear near the limit of adhesion, contributing to increased driver control. In other words, creating downforce to press the vehicle onto the road as speed increases not only contributes to increased absolute cornering speed and thus absolute dynamic performance, but also significantly improves vehicle control quality as measured by response to driver inputs and vehicle stability at the limit. These are the fundamentals behind downforce and aerodynamic stability as a means to improved high-speed vehicle handling.
Downforce balance front to rear (Straight-line driving at constant speed)

By creating a downforce with the same front-to-rear balance as vehicle weight, changes in steering characteristics from low to high speeds remain well under control. At higher speeds this translates into a more linear response. More precise control of the vehicle helps the driver delve further into the car?s potential.
Testing at Honda's proving grounds in Takasu, Hokkaido
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