Science Fair Project Encyclopedia
The same principle that allows an airplane to rise off the ground by creating lift under its wings is used in reverse to apply force that presses the race car against the surface of the track. This effect is referred to as "aerodynamic grip" and is distinguished from "mechanical grip," which is a function of the car's tires and suspension. The creation of downforce can only be achieved at the cost of increased aerodynamic drag (or friction), and the optimum setup is always a compromise between the two. The aerodynamic setup for a car can vary considerably between race tracks, depending on the length of the straights and the types of corners; some drivers also make different choices on setup. Because it is a function of the flow of air over and under the car, downforce typically rises with the speed of the car and requires a certain minimum speed in order to produce a significant effect. But some cars have had rather unstable aerodynamics, such that a minor change in angle of attack or height of the vehicle (for example, caused by a bump on the track) has caused the car to experience lift, not downforce, sometimes with disastrous consequences.
The amount of downforce that can be created is typically much greater for an open-wheeled Formula One or Indy car than for a full-bodied touring car or stock car because of its enhanced aerodynamic characteristics and the use of wings rather than spoilers.
Two primary components of a racing car can be used to create downforce when the car is travelling at racing speed:
- the shape of the body, and
- the use of airfoils.
Most racing formulae have a ban on aerodynamic devices that can be adjusted during a race, except at pit stops.
The rounded and tapered shape of the top of the car is designed to slice through the air and minimize wind resistance. Detailed pieces of bodywork on top of the car can be added to allow a smooth flow of air to reach the downforce-creating elements (i.e., wings or spoilers, and underbody tunnels). The underside of the body is similar in shape to an inverted wing and creates an area of low pressure between the car and the track, pressing the car to the road. This is sometimes called a ground effect and has been the subject of many rule changes over the years in different racing series.
The amount of downforce created by the wings or spoilers on a car is dependent primarily on two things:
- The shape, including surface area, aspect ratio and cross-section of the device, and
- The device's orientation (or angle of attack).
A larger surface area creates greater downforce and greater drag. The aspect ratio is the width of the airfoil divided by its depth. Also, a greater angle of attack (or tilt) of the wing or spoiler, creates more downforce and more drag.
The rear wing of a modern Formula One car, with three aerodynamic elements (1, 2, 3). The rows of holes for adjustment of the angle of attack (4) and installation of another element (5) are visible on the wing's endplate.
The function of the airfoils at the front of the car is two-fold. They create downforce that enhances the grip of the front tires, while also optimizing (or minimizing disturbance to) the flow of air to the rest of the car. The front wings on an open-wheeled car undergo constant modification as data is gathered from race to race, and are customized for every characteristic of a particular circuit (see top photos). In most series, the wings are even designed for adjustment during the race itself when the car is serviced.
The flow of air at the rear of the car is affected by the front wings, front wheels, mirrors, driver's helmet, side pods and exhaust. This causes the rear wing to be less aerodynamically efficient than the front wing, Yet, because it must generate more than twice as much downforce as the front wings in order to maintain the handling balance the car, the rear wing typically has a much larger aspect ratio, and often uses two or more elements to compound the amount of downforce created (see photo at left). Like the front wings, each of these elements can often be adjusted when the car is serviced, before or even during a race, and are the object of constant attention and modification.
Competition Car Downforce: A Practical Handbook by Simon McBeath (2nd edition), SAE International 2000 ISBN 1859606628
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