A moving truck encounters resistance from the air. This drag is made up of pressure drag and skin friction drag. The oncoming airflow pushes against the front of the tractor, creating a high-pressure region, just as it does on the wheels and the front of the semi-trailer. The truck moving forward in the airflow creates a low-pressure region behind the tractor and the semi-trailer: these areas ‘suck’ the vehicle backwards, as it were. Interestingly, the high-pressure region at the front contributes just as much to drag as the low-pressure region at the rear: with or without crosswind, each accounts for about 1/3 of the overall drag. The remaining 1/3 of the overall drag is created by the vehicle’s under-body.
The source of the skin friction drag is the contact between the airflow and the bodywork. Because of the viscosity of air, a layer of air around the vehicle, known as the boundary layer, is dragged along with it, creating shearing forces. The sum of the shearing forces over the entire surface produces the skin friction drag, with the sides and top making the largest contributions. It should be noted that skin friction drag increases with vehicle length. To keep the skin friction drag low the surfaces need to be smooth.
The correlation between pressure drag and skin friction drag for different types of vehicle is ilustrated. This shows that pressure drag is by far the main drag component in the case of heavy goods vehicles.
The drag D can be represented by the following formula:
D = ½ Cd rho V2 S
where CD stands for the coefficient of drag, ρ the air density, V the speed and S the frontal area. The drag is thus determined by the aerodynamic shape expressed in terms of CD, the wind pressure ½ρV2 and the size of the vehicle S.
Air density ρ
The air density depends on the temperature, the pressure and the height above sea level. The air density at sea level at 0ºC is ρ = 1.293kg/m3.
Vehicles travelling at high constant speeds have to cope with large aerodynamic forces. The power needed to overcome this drag increases with the cube of the speed. It is worthwhile, therefore, to modify trucks that are run at higher speeds (above 50 kmph). The most efficient way of saving fuel is to drive more slowly.
Frontal area S
Another major factor in drag is the frontal area of the vehicle: the larger the frontal area, the greater the drag. It is definitely worthwhile, if at all possible, to use a smaller cab and a lower semi-trailer or body (load permitting), as this directly reduces the drag.
Drag coefficient CD
A vehicle’s drag is determined to a substantial extent by its shape. To express the quality of a vehicle’s aerodynamic design with a single number we use what is known as the ‘drag coefficient’ CD. The figure gives an indication of the drag coefficient of different types of vehicle. A tear-drop shape offers the lowest pressure drag.
As CD is always related to the frontal area S, the product of CDS (known as the drag area) is critical to the drag and its contribution to fuel consumption. Thus a vehicle with a high CD and a small frontal area S can have a lower drag than a large vehicle with a low CD and vice versa. To compare the drag of two vehicles correctly, then we need to consider their respective drag areas CDS.
The sharp edges of the cab create a separated turbulent airflow that adversely affects the drag. On the other hand a cleaner flow pattern owing to the rounded corners.
The second basic rule is that the cab should be the same height as the semi-trailer. The correlation between drag coefficients and cab and semi-trailer height is shown in the figure.
The effects of a rounded (blue) and non-rounded (green) cab roof edge. It shows that the cab with a sharp roof edge has a lower drag coefficient as the height ratio (semi-trailer/cab) increases, up to a certain point. The sharp edge causes flow separation that generates less drag with a higher semi-trailer. With a rounded edge, on the other hand, the drag coefficient only increases with the height ratio (semi-trailer/cab), as the attached airflow pushes against the semi-trailer, creating a larger high-pressure region. In many cases it is not possible to match the height of the cab and the semi-trailer because of the load capacity the semi-trailer needs to have, so a roof fairing or deflector is fitted to ensure that the airflow lines up with the dimensions of the semi-trailer. The fairing/deflector must be set to the correct height, of course.
The third basic rule is that the gap between the cab and the semi-trailer should be minimised. Tractor side panels can be used to guide the airflow across the gap in the case of an articulated vehicle. In the case of a rigid vehicle a ‘collar’ can be used to bridge the gap, so as to guide the airflow rearwards. The correlation between the gap and the height of the semi-trailer: the bigger the gap, the greater the drag.
Tip: The basic requirements for truck streamlining are:
- Rounded corners
- Cab and body of equal height
- Minimal gap between the cab and the semi-trailer
An important aspect of airflow around a truck is crosswind. The wind rarely comes constantly from the front, so it is important to consider this situation. In a crosswind situation the airflow will be nicely attached on one side, but on the other side of the cab or semi-trailer it will become detached, resulting in a higher drag coefficient, hence more drag. On top of this, the gap between the wheels and the under-body of the vehicle disrupts the airflow in crosswind conditions.
Aerodynamic enhancements to trucks yield more advantages than just less drag and lower fuel consumption. Along with the fuel consumption there is of course the effect of reduced emissions, which in turn benefits the environment. Also, some external components that can be fitted to the tractor or semi-trailer may have other advantages, e.g.:
- Less road spray (i.e. better visibility for other road users)
- Reduced dirt deposition on the tractor and semi-trailer
- Reduced sensitivity to crosswinds, hence better steering
- Stability, less tyre wear and tear and improved driving comfort
- Better safety (additional crumple zones, better protection against blind spot accidents)
- Lower noise