In the old days, one of the hallmarks of a really outstanding off-road vehicle was that the body was shaped by practical considerations. Vehicles such as the early Jeep, and even the later Land Rover Defenders and Mercedes-Benz Gelandewagens, had a timeless beauty because every part of the body was shaped to perform its function rather than just look pretty.
At present, the search for ways and means to reduce fuel consumption has resulted in body shapes whose most important feature is a low drag coefficient and a small frontal area. The grille is often just an air passage covered in a few ugly chrome strips, to make you think the company actually employed a stylist.
These modern shapes are also practical, but in a very different way. For example, the improvement in aerodynamic drag has resulted in a modern five-litre V8 using the same amount of fuel at 100 km/h as a 1,5-litre four-cylinder engine used 30 years ago.
The increased slipperiness of modern cars has made it possible to employ a high top gear ratio. This not only means that less power is needed to maintain a steady speed, but that speed can be achieved at lower engine revs.
The nature of drag
Some of the power reaching the driving wheels is used to overcome a small amount of friction drag between the tyres and the road, but most of the power by far is absorbed by the aerodynamic drag. This is the force that resists motion and is created by the car’s movement through the air. It grows as the square of the vehicle speed, meaning that every time the car’s speed is doubled the air resistance is multiplied by a factor of four.
A car can only accelerate while the engine’s power exceeds the power absorbed by the sum of the above two drags. The moment the engine’s power is equal to the total drag, acceleration will stop and the car will have achieved its maximum speed.
The numerical value of the aerodynamic drag is the product obtained when one-half of the air density is multiplied by the frontal area of the vehicle, the drag coefficient and the square of the vehicle speed.
Measuring the drag coefficient
This coefficient is a shape factor and will have a desirable low value if the car body is smooth. It is usually determined in a wind tunnel, where the same air is circulated around inside a huge smooth-cornered rectangle and the air temperature and humidity can be controlled.
The test section has to be instrumented in such a way that the true forces acting on the car can be measured. When the test involves a scale model, an arm is often used to locate the model, and the forces acting on the arm are then measured. If a full-size car is being tested, each wheel rests on a measuring pad, which in a modern tunnel is able to measure not only the rearwards drag, but also the magnitude and direction of any vertical, or even sideways forces on the wheels. In some tunnels, the car can also be angled to the wind so that the effect of gusts from the side can be studied.
Pioneers in the aerodynamic field were forced to use scale models, because the first full-scale tunnels were only built in the mid-1930s. At present there are enough big tunnels around for most manufacturers to test full-scale cars, and most of them do.
Scale models are still being used, either to save cost in the early stages of a car’s development, or because some sophisticated new technique can be tried only in a small tunnel.
This has always been a problem. In the early days, tufts of wool were taped to a car, and this technique is still being used. However, it is being supplemented by a stream of smoke lines, or unexpanded polystyrene balls, or the inspection of a deliberately dirtied car under ultraviolet light.
Another technique is to test a scale model in a water tunnel where an electric current generates regularly spaced hydrogen bubbles that make the flow underneath the car and in the wheel housings more visible.
Most modern companies either own wind tunnels or are able to use tunnels at some research organisation. The result is that some of the latest German models have drag coefficients between 0,26 and 0,3, and many others have coefficients not much over 0,3.
Techniques for designing the main body shapes are well established, and the attention has now shifted to optimising the smaller details. The latest cars have side mirrors that not only generate little noise but also help to keep the side windows clean. Sunroofs have a notched wind deflector to reduce noise.
Wheel arches have received much attention because they can generate a lot of turbulence. Subtle spoilers on the boot lid keep rear-end lift within narrow limits. The grille and front apron are designed to direct air smoothly over and around the body. – Jake Venter