DO you notice that nowadays car manufacturers no longer proudly expound their cars’ drag coefficient (Cd)? It used to be that some would pronounce their car to be more slippery through the air than their rivals. Of course, having competitors who design their cars along the lines of a brick really helps.
It seems that nowadays most manufacturers have come to the same conclusions when it comes to Cd. When this happens, their cars will tend to have similar Cds and funnily enough, look similar as well. Of course, when your Cd is no longer special it doesn’t really help marketing it any longer. So, other unique selling points come into the picture.
Still, Cd is an important aspect of car design. A car that meets as little resistance as possible from the air it travels through it will be more fuel-efficient (and faster, too). The more aerodynamically efficient it is, the less fuel it will use to travel along at any given speed. The faster the car, the more important it is to keep the air resistance to a minimum.
The Cd figure is calculated and measured in a wind tunnel. Most passenger cars will have a Cd of around 0.3. As an example, a flat plate held at right angles to the airflow has a Cd of 1.25, whereas the best production car shapes at the moment have a Cd of about 0.28.
However, this Cd figure is not a total aerodynamic drag figure because it does not take into account the car’s frontal area. The frontal area is the total amount of space it occupies when viewed from the front. The figure then becomes CdA where A is the total frontal area.
In a wind tunnel, the car (either full scale or a scale model) is anchored down and air is blown past it to simulate the real-life conditions.
The car is connected to instruments that record how much down-force and how much lift is being generated by the car. Attaching small tufts of wool to the car’s body or by blowing smoke trails past it will reveal air characteristics at the top and the sides.
The model in the wind tunnel can be turned round at various angles to the airflow so that the engineers can see how their various body shapes behave in side winds.
Cd is important to designers and engineers because the amount of power needed to propel a car at high speed rises with the cube of the speed. This basically means, the faster you want to go, the more power it will take to reach that speed.
For example, a car developing 100bhp will reach about 115mph, so you can work out how fast a similar car with twice the power should go (ignoring rolling resistance). However, a 200bhp car will not go 230mph. The cube root of 2 (from 200bhp) is 1.26, so the second car should reach 115x 1.26 = 145mph. Thus, with the same Cd figure, roughly doubling a cars’ bhp will not double its speed.
Many a car designer will have produced a prototype that looks as if it will slip through the air easily, but a scale model without items such as air intakes, wipers and door handles will skew the Cd figures.
Therefore, designers put in features that help to smooth the airflow such as windscreens and side windows that are almost flush with the bodywork and wheel trims or wheels with careful contours.
Details such as recessed door handles and streamlined side mirrors help to reduce aerodynamic drag. Windscreen wipers which tuck under the scuttle panel or bonnet, pop-up headlamps and eliminating gutters around the edges of the car’s roof will allow air to flow smoothly. Careful design will even keep the rear light lenses clean. Some cars even had concave rear windows to help keep it clean.
Interestingly, the car’s overall design impacts how stable it will be on the road. This is where the engineers and designers come together in order to design a stable car.
Basically, the engineers will pinpoint the centre of pressure of the car (the effective point on the car where the wind acts) in relation to the centre of gravity (the point inside the car through which gravity effectively acts).
The relative positions of a cars’ centre of pressure and its centre of gravity are critical in determining the stability of the car. For example, if the centre of pressure lies well in front of the centre of gravity, a side wind would tend to deflect the car to the side easily.
A car is most stable when the centre of pressure lies slightly in front of the centre of gravity, as is the case with a front-wheel-drive car in which most of the weight is towards the front.
The relative heights of these two points are also important. If both the centre of pressure and the centre of gravity are high, then a vicious side wind could make the car unstable or even turn turtle.
However, the location of the centre of pressure shifts with changes in the car’s speed, and in some cases can even shift so that it is in front of the car itself.
The solution is to first ensure that the car’s centre of gravity is well forward. A car which has a forward weight bias (and therefore a forward centre of gravity) tends to be more stable. A greater area of bodywork towards the rear of the car will also keep the centre of pressure close to the centre of gravity.
So, are you ready to trade in that Hi-Lux for a DS?