Race Car Aerodynamics Basics and Design Tips
Aerodynamics is the science of how air flows around and inside objects. More generally, it can be labeled “Fluid Dynamics” because air is really just a very thin type of fluid. Above slow speeds, the air flow around and through a race vehicle begins to have a more pronounced effect on the acceleration, top speed and handling.
Therefore, in race car design we need to understand and optimize how the air flows around and through the body, its openings and its aerodynamic devices.
- 1 Aerodynamic Principles
- 2 Aerodynamic Devices
- 3 Aerodynamics Tips
No matter how slowly a race car is going, it takes some energy to move the car through the air. This energy is used to overcome a force called Drag.
Drag, in vehicle aerodynamics, is comprised primarily of three forces:
- Frontal pressure, or the effect created by a vehicle body pushing air out of the way.
- Rear vacuum, or the effect created by air not being able to fill the hole left by the vehicle body.
- Boundary layer, or the effect of friction created by slow moving air at the surface of the vehicle body.
Between these three forces, we can describe most of the interactions of the airflow with a race vehicle body.
Frontal pressure is caused by the air attempting to flow around the front of the vehicle as shown in diagram D1 below.
As millions of air molecules approach the front of the car, they begin to compress, and in doing so raise the air pressure in front of the car. At the same time, the air molecules travelling along the sides of the car are at atmospheric pressure, a lower pressure compared to the molecules at the front of the car.
Just like an air tank, if the valve to the lower pressure atmosphere outside the tank is opened, the air molecules will naturally flow to the lower pressure area, eventually equalizing the pressure inside and outside the tank. The same rules apply to any vehicle. The compressed molecules of air naturally seek a way out of the high pressure zone in front of the vehicle, and they find it around the sides, top and bottom of the vehicle as demonstrated in diagram D1.
Rear vacuum is caused by the “hole” left in the air as a vehicle passes through it. To visualize this, let’s take a look at our demonstration car in diagram D2 below. As it drives down a road, the blocky sedan shape of the car creates a hole in the air. The air rushes around the body as described above.
At speeds above a crawl, the space immediately behind the car’s rear window and trunk is “empty” or like a vacuum. These empty areas are the result of the air molecules not being able to fill the hole as quickly as the car can make it. The air molecules attempt to fill in to this area, but the car is always one step ahead, and as a result, a continuous vacuum sucks in the opposite direction of the car.
This inability to fill the hole left by the car is technically called Flow detachment.
Flow detachment applies only to the “rear vacuum” portion of the drag forces and has a greater and greater negative effect as vehicle speed increases. In fact, the drag increase with the square of the vehicle speed, so more and more horsepower is needed to push a vehicle through the air as its speed rises.
Therefore, when racing reaches high speeds it becomes important to design the race car to limit areas of flow detachment. Ideally, we give the air molecules time to follow the contours of a car’s bodywork, and to fill the hole left by the vehicle, its tires, its suspension and its protrusions (i.e. mirrors, roll bars).
If you have witnessed the Le Mans race cars, you will have seen how the tails of these cars tend to extend well back of the rear wheels, and narrow when viewed from the side or top. This extra bodywork allows the air molecules to converge back into the vacuum smoothly along the body into the hole left by the car’s cockpit, and front area, instead of having to suddenly fill a large empty space.
The force created by the rear vacuum exceeds that created by frontal pressure, so there is very good reason to minimize the scale of the vacuum created at the rear of the vehicle.
When the flow detaches, the air flow becomes very turbulent and chaotic when compared to the smooth flow on the front of an object.
If we look at a protrusion from the race car such as the mirror in diagram D3 above, we see flow detachment and turbulence in action. The air flow detaches from the flat side of the mirror, which of course faces toward the back of the car.
The turbulence created by this detachment can then affect the air flow to parts of the car which lie behind the mirror. Intake ducts, for instance, function best when the air entering them flows smoothly. Wings generate far more downforce with smooth flows over them as well. Therefore, the entire length of the car really needs to be optimized (within reason) to provide the least amount of turbulence at high speed.
To enable the comparison of the drag produced by one vehicle versus another, a dimensionless value called the Coefficient of Drag or Cd was created. Every vehicle has a Cd which can be measured using wind tunnel data. The Cd can be used in drag equations to determine the drag force at various speeds. In his comprehensive book “Race Car Aerodynamics: Designing for Speed“, Joseph Katz provides a table of common vehicles and their Cds and Frontal Areas. Here is an excerpt from that table:
|Vehicle Type||Drag||Frontal area|
|Coefficient Cq||A[m2]||CDA [m2]|
|Ford Escort 1.3 GL||0.39-0.41||1.83||0.71-0.75|
|Nissan Cherry GL||0.39-0.41||1.83||0.71-0.75|
|Volvo 360 GLT||0.40-0.41||1.95||0.78-0.80|
|Honda Accord 1.8 EX||0.40-0.42||1.88||0.75-0.79|
|Nissan Stanza SGL 1.8||0.40-0.42||1.88||0.75-0.79|
|Mazda 323 1.5||0.41-0.43||1.78||0.73-0.77|
|Talbot Horizon GL||0.41-0.44||1.85||0.76-0.81|
|Alfa Romeo Giulietta 1.6||0.42-0.44||1.87||0.79-0.82|
|Toyota Corolla 1300 DX||0.45-0.46||1.76||0.79-0.81|
|VW Golf Cabrio GL||0.48-0.49||1.86||0.89-0.91|
|Renault 25 TS||0.30-0.31||2.04||0.61-0.63|
|Audi 100 1.8||0.30-0.31||2.05||0.62-0.64|
|Mercedes 190 E (190 D)||0.33-0.35||1.90||0.63-0.67|
|Mercedes 380 SEC||0.34-0.35||2.10||0.71-0.74|
|Mercedes 280 SE||0.36-0.37||2.15||0.77-0.80|
|Mercedes 500 SEL||0.36-0.37||2.16||0.78-0.80|
|BMW 518i (520i, 525e)||0.36-0.38||2.02||0.73-0.77|
|Citroen CX 25 Gti||0.36-0.39||1.99||0.72-0.78|
|Alfa Romeo 90 2.0||0.38-0.40||1.95||0.74-0.78|
|Mazda 929 2.0 GLX||0.39-0.44||1.93||0.75-0.85|
|Saab 900 Gli||0.40-0.42||1.95||0.78-0.82|
|Volvo 740 GLE||0.40-0.42||2.16||0.86-0.91|
|Volvo 760 Turbo w/intercooler||0.40-0.42||2.16||0.86-0.91|
|Peugeot 505 STI||0.41-0.43||1.97||0.81-0.85|
|Peugeot 604 STI||0.41-0.43||2.05||0.84-0.88|
|BMW 728i (732i/735i)||0.42-0.44||2.13||0.89-0.94|
|Ford Granada 2.3 GL||0.44-0.46||2.13||0.94-0.98|
|Porsche 944 Turbo||0.33-0.34||1.90||0.63-0.65|
|Nissan 300 ZX||0.33-0.36||1.82||0.60-0.66|
|Mazda 626 Coupe||0.34-0.36||1.88||0.64-0.68|
|Opel Monza GSE||0.35-0.36||1.95||0.68-0.70|
|Renault Fuego GTX||0.34-0.37||1.82||0.62-0.67|
|Honda CRX Coupe||0.35-0.37||1.72||0.60-0.64|
|Audi Coupe GT 5E||0.36-0.37||1.83||0.66-0.68|
|Chevrolet Camaro Z 28 E||0.37-0.38||1.94||0.72-0.74|
|Toyota Celica Supra 2.8i||0.37-0.39||1.83||0.68-0.71|
|VW Scirocco GTX||0.38-0.39||1.74||0.66-0.68|
|Porsche 911 Carrera||0.38-0.39||1.78||0.68-0.69|
|Mitsubishi Starion Turbo||0.38-0.40||1.84||0.70-0.74|
|Porsche 928 S||0.38-0.40||1.96||0.74-0.78|
|Porsche 911 Carrera Cabrio||0.40-0.41||1.77||0.71-0.73|
From this table and our knowledge of the body shape of some of these vehicles, we can conclude that the best Cd is achieved when a vehicle has these attributes:
- Has a small nose/grill, to minimize frontal pressure.
- Has minimal ground clearance below the grill, to minimize air flow under the car.
- Has a steeply raked windshield (if any) to avoid pressure build up in front.
- Has a “Fastback” style rear window/deck or sloped bodywork, to permit the air flow to stay attached.
- Has a converging “Tail” to keep the air flow attached, and to minimize the area against which flow detachment eventually occurs
If it sounds like we’ve just described a sports car, you’re right. In truth though, to be ideal, a car body would be shaped like a tear drop, as even the best sports cars experience flow detachment. However, tear drop shapes are not conducive to the area where a car operates, and that is close to the ground. Airplanes don’t have this limitation, and therefore teardrop shapes work.
The best road cars today manage a Cd of about 0.28. Formula 1 cars, with their wings and open wheels (a massive drag component) manage a minimum of about 0.75.
If we consider that a flat plate has a Cd of about 1.0, an F1 car really seems inefficient, but what an F1 car lacks in aerodynamic drag efficiency, it makes up for in downforce and horsepower.
Drag coefficient, by itself is only useful in determining how “Slippery” a vehicle is. To understand the full aerodynamic effect of a vehicle’s body shape, we need to take into account the frontal area of the vehicle. The frontal area defines the size of the hole the vehicle makes in the air as it drives through it.
In diagram FA1 below, the sedan car makes a far smaller hole in the air than the semi-trailer tractor.
So then it is by combining the Cd with the Frontal area that we arrive at the actual amount of drag created by a vehicle.
Downforce is the same force as the lift experienced by airplane wings, only it acts to press down instead of lifting up. Every object travelling through air creates either a lifting or downforce situation. Race cars use aerodynamic devices such as inverted wings to force the car down onto the track, increasing traction. The average street car however tends to create lift. This is because the car body shape generates a low pressure area above itself.
According to Bernoulli’s principle, for a given volume of air, the higher the velocity the air molecules are travelling, the lower the pressure becomes. Likewise, for a given volume of air, the lower the velocity of the air molecules, the higher the pressure becomes. This applies to air in motion across a still body, or to a vehicle in motion, moving through relatively still air.
In the Frontal Pressure section above, we said that the air pressure was high as the air rammed into the front grill of the car. What is happening is that the air slows down as it approaches the front of the car, and as a result more molecules are packed into a smaller space. Once the air stagnates at the point in front of the car, it seeks a lower pressure area, such as the sides, top and bottom of the car.
Diagram LD1 below demonstrates this effect using arrows to indicate the air velocity and density.
As the air flows over the hood of the car, it’s loses pressure, but when it reaches the windscreen, it again comes up against a barrier, and briefly reaches a higher pressure. The lower pressure area above the hood of the car creates a small lifting force that acts upon the area of the hood (Sort of like trying to suck the hood off the car). The higher pressure area in front of the windscreen creates downforce. This is akin to pressing down on the windshield.
Where most road cars get into trouble is the fact that there is a large surface area on top of the car’s roof. As the higher pressure air in front of the wind screen travels over the windscreen, it accelerates, causing the pressure to drop. This lower pressure literally lifts on the car’s roof as the air passes over it.
Worse still, once the air makes its way to the rear window, the notch created by the window dropping down to the trunk creates a vacuum (or low pressure space) that the air is not able to fill properly. The flow is said to detach and the resulting lower pressure creates lift that then acts upon the surface area of the trunk. Prior to the use of aerodynamic devices to reduce these effects, race car drivers would feel the car becoming “light” in the rear when travelling at high speeds.
Not to be forgotten, the underside of the car is also responsible for creating lift or downforce. If a car’s front end is lower than the rear end, then the front end restricts the air flow under the car and the widening gap between the underside and the road creates a low pressure area. If there is neutral or higher air pressure above the car, then we get downforce due to the difference in the pressure above and below the car. See the diagram LD2 below:
So, as you can see, the airflow over a car is filled with high and low pressure areas, the sum of which indicate that the car body either naturally creates lift or downforce.
Aerodynamic devices provided a means of taking advantage airflow around a vehicle. Some devices increase the effectiveness of airflows within the body of the vehicle, such as those feeding a radiator or engine. Other devices create downforce to increase traction.
Scoops/Positive pressure intakes
Scoops, or positive pressure intakes, are useful in providing a mild “Ram Air” or “Supercharging” effect to a combustion engine. They work on the principle that the air flow compresses inside an “air box” when subjected to a constant and oncoming flow of air. The air box has an opening that permits an adequate volume of air to enter, and the expanding air box itself slows the air flow to increase the pressure inside the box. The faster the vehicle travels, the greater the pressure increase and air volume through the air box. Diagram AD1 below shows a scoop and air box:
NACA ducts are useful when air needs to be drawn into an area which isn’t exposed to the direct oncoming air flow the scoop has access to. Quite often NACA ducts will be used along the sides of a vehicle. The NACA duct takes advantage of the Boundary layer, a layer of slow moving air that “clings” to the bodywork of the car, especially where the bodywork flattens, or does not accelerate or decelerate the air flow. Areas like the roof and side body panels are good examples. The longer the roof or body panels, the thicker the layer becomes (a source of drag that grows as the layer thickens as well).
The NACA duct scavenges this slower moving area by means of a specially shaped intake. The intake shape, shown below in diagram AD2, drops in toward the inside of the bodywork, and this draws the slow moving air into the opening at the end of the NACA duct. Vortices are also generated by the “walls” of the duct shape, aiding in the scavenging. The shape and depth change of the duct are critical for proper operation.
Typical uses for NACA ducts include engine air intakes and cooling.
Spoilers are used primarily on sedan-type race cars to provide downforce, but also to counteract the natural tendency of these cars to become “light” in the rear due to lift generated by the rear body shape.
Spoilers act like barriers to air flow, in order to build up higher air pressure in front of the spoiler. This higher pressure acts upon the area of the trunk/deck to provide downforce. Diagram AD3 below shows how the flow is manipulated to increase pressure.
Front Air Dam
A Front air dam is used to prevent air from flowing underneath a racing vehicle. It does this by creating a “dam” or wall across the front of the vehicle that extends close down to the road and usually along the sides to some extent. This creates an area of vacuum or low pressure underneath the car as shown in diagram AD4 below. This low pressure area, in combination with the higher pressures above the front and top of the vehicle, generates downforce at the front of the vehicle.
If we extend the air dam along the sides of the vehicle to become “skirts”, we can extend the vacuum or low pressure area generated under the vehicle by the air dam as well.
Probably the most popular form of aerodynamic device is the wing. Wings can perform very efficiently by generating a lot of downforce for a small penalty in drag. Spoilers are not nearly as efficient, but because of their practicality they are used a lot on sedans where wings are less efficient.
The wing, as shown in diagram AD5 above, generates downforce by using the difference in air pressure between the top and bottom surfaces. This air pressure difference results from the way the air flows around the wing shape.
According to Bernoulli’s principle, the higher the speed of a given volume of air, the lower the pressure that air will have. Therefore to create lower air pressure, we need to speed up the air flow.
A wing does this by making the air molecules travel different distances from the leading edge to the trailing edge. The longer underside of the wing requires the air flowing on that side to move at a higher speed (lower pressure) to meet up with the air flowing at a lower speed (higher pressure) over the top side of the wing.
The lower pressure area under the wing allows the higher pressure area above the wing to “push” down on the wing, and hence the vehicle it’s mounted to. The angle of attack or wing angle can be increased to enable even larger pressure differences, but eventually the wing will stall and lose downforce. Drag also increases with higher angles of attack.
Downforce can be increased even more without stalling the wing by using multi-element wings that position one or more small wings behind a larger wing. In his book “Race Car Aerodynamics: Designing for Speed“, Joseph Katz provides a Coefficient of pressure plot with an overlaid multi-element wing profile, shown in diagram AD6, below. The positioning of the elements is critical, with gaps between the wing elements “feeding” the low pressure side of the smaller wings.
NOTE: The wing in the diagram is shown “upside down” compared to how it would be mounted on a race car. The diagram is meant to plot negative pressure coefficients from the front to the rear (the Chord) of the wing (x/c)
Wings can be coupled with endplates to prevent high pressure air spilling over the ends to the low pressure underside. Diagram AD7 below shows a wing with endplates:
Venturi tunnels, much the like a venturi tube observed in a laboratory, use the constriction of a flow to generate high speed, low air pressure areas under the race car. In diagram AD8 below, we show a car with a venturi tunnel, and below that a similar venturi tube you might see in a lab.
On race vehicles, the venturi is usually formed by making the undertray of the vehicle shaped like an inverted wing. The distance between the undertray and the road forms a constriction and then expands to enable the low pressure created by the constriction to act along the middle and rear and of the vehicle. Venturis are very efficient devices but are susceptible to changes in vehicle ride height.
A diffuser, shown in diagram AD9 below, is used to generate downforce at the rear of a race vehicle. Similar to a venturi tunnel, it forms a curvature similar to the underside of a wing immediately before the low pressure area behind the vehicle. By doing so, the air flowing under the vehicle increases in speed and drops in pressure, creating downforce. Diffusers and venturi tunnels leverage the low pressure area behind the vehicle, and can sometimes leverage high speed exhaust gases ejected into the diffuser to create even lower pressure.
Cover Open wheels
Open wheels create a great deal of drag and air flow turbulence, similar to the diagram of the mirror in the “Turbulence” section above. Full covering bodywork is probably the best solution, if legal by regulations, but if partial bodywork is permitted, placing a converging fairing behind the wheel provides maximum benefit.
Minimize Frontal Area
The smaller the hole your race car punches through the air, the better it will accelerate the higher the top speed it will have. It is usually much easier to reduce FA (frontal area) than the Cd (Drag coefficient).
Converge Bodywork Slowly
Bodywork which quickly converges or is simply truncated, forces the air flow into turbulence, and generates a great deal of drag. As mentioned above, it also can affect aerodynamic devices and bodywork further behind on the vehicle body.
Spoilers are widely used on sedan type cars such as NASCAR stock cars. These aerodynamic aids produce downforce by creating a “dam” at the rear lip of the trunk, raising the air pressure over the trunk. Where a notch left by the rear window exists a spoiler can help restore pressure to the void behind the window.
Wings are the inverted version of what you find on aircraft. They work very efficiently, and in less aggressive forms generate more downforce than drag, so they are loved in many racing circles. Wings are best placed in areas that have clear airflow to them. Placing a wing behind an obstruction reduces the downforce the wing can produce.
Use Front Air Dams
Air dams at the front of the car restrict the flow of air reaching the underside of the car. This creates a lower pressure area under the car, effectively providing downforce. In many cases, the air dam also reduces the Cd of the vehicle.
Use Aerodynamics to Assist Vehicle Operation
Using vehicle bodywork to direct airflow into openings, for instance, permits more efficient, smaller openings that reduce drag penalties. Quite often, with some forethought, you can gain an advantage over a competitor by these small dual purpose techniques.
Another useful technique is to use the natural high and low pressure areas created by the bodywork to perform functions. For instance, Mercedes, back in the 1950s placed radiator outlets in the low pressure zone behind the driver. The air inlet pressure which fed the radiator became less critical, as the low pressure outlet area literally sucked air through the radiator.
A useful high pressure area is in front of the car, and to make full use of this area, the nose of the car is often slanted downward. This allows the higher air pressure to push down on the nose of the car, increasing grip. It also has the advantage of permitting greater driver visibility.
Keep Protrusions Away From The Bodywork
The smooth airflow achieved by proper bodywork design can be destroyed quite easily if a protrusion such as a mirror is too close to it. While it is important to design an aerodynamic mount for a mirror, the mirror itself needs to be placed far enough away from the bodywork to avoid adverse affects.
Rake the chassis
The chassis, as mentioned in the aerodynamics theory section above, is capable of being slightly lower to the ground in the front than in the rear. The lower “Nose” of the car reduces the volume of air able to pass under the car, and the higher “Tail” of the car creates an expanding space where a vacuum effect can form. This lowers the air pressure beneath the car, creating downforce.
Cover or streamline Exposed Wishbones
Exposed wishbones (on open wheel cars) are often made from circular steel tube to save cost. However, these circular tubes generate turbulence. It may be worth considering the use of oval tubing, or a tube fairing that creates an oval shape over top of the round tubing. See diagram AT1 below: