The Basics of Downforce
Complete with Hand Drawn Sketches on Notebook Paper - Try to Ignore the Lines
This tutorial will explain how an external flow interacts with a car to affect its dynamics. The drawing below shows a representation of the path of air moving around a car (or more correctly, the path of air as the car moves through it).
At the front of the car, the oncoming air goes through a process of stagnation, slowing down and increasing in pressure. This is represented by the shaded area at the front of the car. The airflow parts and some goes over the car while some goes below it. The air moving over the car follows the hood, but when it reaches the sharp direction change at the base of the windshield, it stagnates again. It then continues up over the roof to the rear windshield. At this point, the momentum of the air and the friction of the car's surface are usually not enough to keep the airflow attached, and flow separates over the rear windshield. This phenomenon creates an area of low pressure above the rear windscreen and over the trunk. This effect further continues to the rear of the car as the airflow is subjected to another drastic direction change when the trunk drops off to the rear bumper.
The flow that goes beneath the car is subjected to many obstacles along its path. Underneath the car, the air strikes the oil pan, steering rack, and transmission, causing it to stagnate. It runs into the tires, also inducing stagnation. As it continues along the length of the car, it further runs into the driveshaft, exhaust, rear end, and gas tank, all of which cause the flow to stagnate. The air finally exits at the rear of the car into the low pressure wake at the rear bumper.
The stagnation zone at the front of the car creates an area of high pressure, while the flow separation at the rear creates an area of low pressure. This pressure differential creates a force pushing opposite the car's direction of travel, called drag. Similarly, the stagnation below the car coupled with the separation above the trunk creates a net upward force, called lift. Both of these forces increase with vehicle speed - as the car goes faster, the effects increase. In the world of automotive performance, both drag and lift are bad. Drag holds the car back, limiting both acceleration and top speed. Lift reduces the weight of the car on the tires, lessening steering response, stability, and overall grip.
To combat the effect of these forces, several devices have been developed over the years.
A spoiler is simply a device that slows and collects air, causing it to stagnate. The most common spoilers are placed at the rear of the car, over the trunk, as shown below.
The spoiler creates an area of high pressure to replace the usual low pressure over the trunk. This helps reduce the pressure differential between the bottom and top of the car, reducing lift. The only drawback to spoilers is that they usually increase drag. Because all of the air collects in front of the spoiler, there is very little behind it. The spoiler itself contributes to the pressure differential between the front and back of the car, increasing drag. However, in rare cases, the increased pressure drop behind the spoiler can help pull air from beneath the car to fill in the void at the rear bumper, causing a net decrease in drag, but this is a rare circumstance.
One simple way to combat stagnation on objects underneath the car is to block air from going there in the first place. Airdams, sometimes referred to as "chin spoilers" are spoilers placed at the front of the car at the bottom of the bumper.
Chin spoilers also collect air, superficially increasing drag. However, upon further examination, by preventing the drag induced by stagnation on the bottom of the car, an airdam reduces the overall drag of the car. The low pressure area just behind the airdam also creates a negative pressure differential between the top and bottom of the front of the car, creating downforce to aid the front tires. This effect increases steering response, and cornering grip.
A splitter is a device that uses the effect of an airdam to create extra downforce. As the air stagnates at the airdam, it creates an area of high pressure. A splitter takes advantage of this high pressure by jutting out from the front bumper (as shown below). This provides a surface for the pressure to act on, creating downforce. The function of a splitter can be two-fold. If it is sufficiently close to the ground, the splitter will squeeze air between it and the ground, accelerating it. This raises the velocity of the airflow and reduces its pressure. Now the splitter has a larger pressure differential and, as a result, more downforce is developed. Splitters also have the added advantage of blocking the pressurized air from flowing down beneath the airdam, amplifying its effectivness and blocking even more air from flowing beneath the car.
The low pressure created by an airdam won't last long if air can simply fill the void by flowing in from the sides. Sideskirts are devices that block airflow from entering beneath the car from the side. A side skirt hangs down from the side of the car between the front and rear tires, as shown in the picture.
Side skirts preserve the low pressure area below the car as well as prevent air from entering that could stagnate on objects underneath the car and increase drag.
Another way to prevent air from stagnating on objects underneath the car is to cover the objects and prevent air from reaching them completely. A smooth undertray fitted to the bottom of a car keeps the air flowing beneath it traveling fast and at low pressure. This reduces the top to bottom pressure differential on the car, reducing lift. However, an undertray alone cannot prevent air from entering the underside and ruining the low pressure area there. For this reason, undertrays are usually used along with sideskirts and airdams.
Airfoils are one of the most well known devices for producing downforce. More commony called wings, airfoils use a pressure differential to create downforce. Exactly like the wing on an airplane, an airfoil creates a pressure differential by forcing the flow above and below it to move at different speeds. The slower air is at a higher pressure than the faster moving air, creating a pressure differential. While an airplane uses this effect to create lift, by simply flipping the airfoil upside down, downforce is generated.
Front of Indy Car showing front airfoil.
For further information, see the Airfoils Tutorial.
Quite possibly the most prolific downforce generation device is the diffuser. It is so effective that many racing bodies heavily regulate their use and design; some have even banned them completely.
A diffuser is a device that expands in the direction of airflow. This expansion forces the air moving through it to fill the void. Contrary to what you might think, this expansion does not reduce airflow pressure. Rather, the expansion slows the air, actually increasing the pressure of the flow. How does this make downforce you ask? It is a phenomenon called "Pressure Recovery". As the air slows, the molecules in it must get closer together in order to increase pressure. This requirement forces air to be pulled in to allow this pressure increase to occur. The most effective diffusers are closed on the sides by walls that extend close to the ground, in effect forming a sealed tunnel (sometimes referred to as a "ground effect tunnel") only open at the ends. Because air cannot come in from the sides, it is forced to accelerate through the entrance of the diffuser to fill it. This acceleration decreases the pressure of the air at the inlet of the diffuser creating downforce. If a smooth undertray is present at the entrance to the diffuser, this low pressure has more area on which to act, creating even more downforce. It is not actually the diffuser that creates downforce, it is the area in front of it. According to wind tunnel tests, the optimum diffuser angle is approximately 9.5° above horizontal.
A diffuser located at the rear of the car can also serve as a pressure reservoir to fill the void at the back of car. Diffusers can not only produce huge amounts of downforce, but they can also significantly reduce drag at the same time!
Diffusers are usually located at the rear of a car, where the low pressure they create can be applied to a large area, namely the entire underside of the car. However, diffusers are also used right behind splitters, where they can increase the pressure differential above and below them, thereby amplifying the effect.
Underside of Indy Car showing rear diffuser with side plates and smooth undertray.
Vents and Ducts
Vents and ducts serve many purposes on cars. They are used to direct air for cooling and ventilation. However, they can also be used to reduce lift and create downforce by relieving unwanted pressure differentials. Pressure differentials can occur anywhere that a fluid flow is allowed to enter an area that is difficult to exit. If this pressure differential contributes to lift, or reduces downforce, it is obviously unwanted.
One area where a large pressure differential can develop is the engine bay. The pressure differential is created in two ways. Firstly, as the air flows into the engine bay it strikes the engine, accessories, and firewall, stagnating and raising the pressure in the engine bay. The air flowing over the hood is moving quickly and is at a lower pressure, creating a pressure differential. Because the pressure below the hood is higher than above it, lift is created.
The effect is amplified by the fact that the air entering the engine bay is heated by the radiator and the engine itself. Buoyant forces (the same forces that cause a hot air baloon to rise) are created because the hot air has a lower density than the cool air above the hood. This effectively turns the hood into a hot air baloon, compounding the problem. The problem is again made worse because the engine bay is sealed on the sides and top. This allows the air only one escape path - down. The air is forced to flow down and beneath the car, increasing drag and reducing downforce. Obviously these conditions are detrimental to a car's performance.
However, simply placing a vent in the hood can help reduce the pressure differential in the engine bay, reducing the effect of these phenomena. This solution can be seen on many race cars including JGTC and DTM cars and also on very high performance road cars like the Ferrari F50, Jaguar XJ220, and Lotus Elise.
It is important to note that if the vent is placed too far back on the hood, then it may cross into the stagnation zone at the base of the windshield. If the vent is open to this high pressure area, it may actually force air into the engine bay, compounding the problem that it was installed to relieve.
However, placing the duct in this high pressure area can be used as an advantage. On older muscle cars, "induction cowls" were used to tap into this high pressure area and provide more air to the engine, making more power. For this to be most effective, the cowl should be sealed so that the air is forced into the engine and none is allowed to collect under the hood.
The wheel wells are another area where an undesired pressure differential can develop. Air flowing beneath the car stagnates on the tires, creating an area of high pressure. This effect is compounded by the brake cooling ducts that many teams use, which force even more air into the wheel wells. All of this air is also heated by the brakes, creating the same buoyant forces as in the engine bay. This high pressure area also exerts a drag force on the car. For this reason, many race cars and high performance road cars also incorporate vents at the top or rear of the wheel wells.
Vortex generators are aptly named devices; they generate vortices. A vortex is a swirling mass of air that has a very high rotational velocity, meaning that it spins very fast. This gives the airflow a high local velocity and momentum. Vortex generators are structures that protrude into the air flow and induce local flow separation, causing a tight swirling pattern to develop in the flow as shown below.
This increased momentum can help keep airflow attached over sharp direction changes. This effect can reduce flow separation at areas like the rear windshield and reduce the low pressure area developed there. This in turn reduces the drag and lift penalties incurred by the presence of the lower pressure areas.
One of the most prominent applications of vortex generators was their use on early jet powered fighter aircraft. Arrays of vortex generators were placed along the upper surface of the wings, reducing flow separation and allowing the aircraft to function at higher angles of attack without the wings stalling. One current automotive example can be found at the rear windshield of the Mitsubishi Evolution MR.
Dive plates, sometimes called "canards", are angled plates, usually at the front of a car, that are used to fine tune its aerodynamics. While these plates will develop some amount of downforce by functioning similar to a spoiler, the amount will be very small. Further, if the angle of the dive plate exceeds 45° above horizontal, it will actually create significantly more drag than downforce. As a result, dive plates are usually used for secondary purposes, such as vortex generation to amplify the effect of some other downforce producing device.
Review of Aerodynamic Devices
References and Further Reading
Competition Car Downforce by Simon McBeath. Haynes Publishing, 1998.
Vortex Generators. www.aerospaceweb.org
The Aerodynamic Performance of Automotive Underbody Diffusers #980030. Cooper, Bertenyi, et al. SAE Publishing, 1998.
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