There's little understanding of how or why aerodynamic aids work; people tend to bolt them on to feel the downforce placebo effect. We've already discussed the basics of drag, lift and flow in Part 1 (SCC, July 2006, page 124), so in this second part of our auto aero series, we'll talk about how several common aero aids work to make a car slick and stick.
For cutting-edge aerodynamics on production-based cars, look no further than Germany's DTM and Japan's Super GT cars. Both series have rules allowing significant use of aero aids (such as multi-element wings, diffusers, super-low side skirts, air dams, and splitters) while keeping reasonably faithful to the cars' original lines. These components combine to produce prodigious amounts of downforce, helping to keep the car planted through corners at ballistic speeds. They also improve braking performance and acceleration thanks to the added traction.
There is a price for this amount of downforce. Drag. Redirecting the energy of the airflow to hold a car down creates more resistance for the car to push against. But that's OK. Although drag reduces top speed somewhat, the increase in cornering speeds makes for faster lap times, so some drag is acceptable. Many race cars have drag coefficients of over 1.1, while modern production cars hover around 0.35. The big difference is that race cars have enough horsepower to compensate.
The key is to produce just enough downforce to maximize the average speed around the track. Produce too much downforce and the increased drag will slow the car excessively, too little downforce will hurt cornering speeds. It usually takes some experimenting with wing settings and other components to find the sweet spot where performance is optimized.
Keep in mind that the basic shape of a production car generates a lifting force when moving through air. The lift characteristics increase exponentially (to the second power) as speed increases. As a result, traction at high speed is reduced and the car's handling can become vague and unresponsive, especially when cornering. By adding downforce, some or all of this unwanted lift can be reduced.
Several aero components are mounted nearer the front, others are usually located closer to the rear. For example, a wing is usually mounted on the trunk lid; an air dam is attached to the front bumper cover. The effect produced by the component will generally be concentrated near to where it's mounted. Consequently, the rear tires will 'feel' more of the wing's downforce than the front tires. Since a wing is usually mounted behind the rear wheels, there will usually be a decrease in the load acting on the front tires due to the fulcrum effect; downforce from the wing will actually lift the front of the car.
To improve high-speed handling, we would normally add downforce in proportion to the car's lengthwise weight distribution. In many front-wheel-drive cars, which have about a 60/40 split, adding downforce in the same percentages to the front and rear retains a balanced handling feel.
If a wing is added to an already 'balanced' car, then the tendency will be to increase understeer because of the slight front-end lift. Since most production-based cars are designed to understeer, the addition of a wing will make this tendency even worse at higher speeds. To correct understeer, the easy fix is to add downforce to the front. A properly designed air dam - with or without a splitter - will add some much-needed downforce.
When we want more downforce at both ends, a wing or spoiler can be used at the rear in conjunction with an air dam and/or splitter at the front. The size and design of the wing and the size and type of the air dam and splitter will determine how the high-speed handling will be affected. Usually the air dam is a fixed size and shape, whereas the wing can be adjusted for more or less downforce, depending on the wing's angle of attack relative to the oncoming airstream. Increasing the nose-down attitude will result in more downforce - up to a point. Altering the wing's angle of attack will fine-tune the high-speed handling balance.