Drag Reduction and flow control

The overall shape of a vehicle is measured as the Coefficient of Drag, (Cᴅ). By enhancing the aerodynamic shape of a truck and therefore reducing the drag resistance, improves the flow control around the vehicle. Drag reduction devices do this, saving engine power and therefore fuel whilst give the vehicle a steadier ride.

Typical Cᴅ figures in the automotive world are as follows:

Vehicle Type Typical Cᴅ
Formula 1 Racing Car 0.7 – 0.8
Best in class production car 0.22 +
HGV without Truck Aerodynamic Devices 0.7 – 0.9
HGV with Truck Aerodynamic Devices 0.6 – 0.7


Adding aerodynamic devices will reduce the Cᴅ, and this is typically measure in counts, (1 count = 0.001 Cᴅ). In the car world, aerodynamicist’s will look seriously at improvements with 1 or 2 counts, this could be a result of very small detail e.g. the wheel or tyre design. In comparison, well designed HGV aerodynamic devices can reduce the Cᴅ by 10 – 20 counts, providing a significant improvement.

Types of truck aerodynamic devices

There are many types of truck aerodynamic devices available, below you will find the most common ones.

Fitted to top of cab

This helps the flow control from the front of the cab and lifts it up over the body or trailer. Available in fixed or adjustable height options

Fitted to rear vertical corners of cab

These help push the air out around the width of the body as well as fill in the cab to body gap which is particularly effective when the vehicle is in a yaw


Otherwise known as a FULL KIT, AIR MANAGEMENT KIT OR AMK. This solution gives the ultimate cab to body / trailer front aerodynamics

Fitted under the body / trailer and between the wheels on a tractive unit

Whilst also giving great styling they prevent drag under the vehicle / chassis and keep the flow control regular as it travels rearwards from the cab

Normally fitted at the rear of the body / trailer roof

These aerodynamic devices, consist of a small vane usually attached to a lifting surface at the rear of the vehicle and modify the boudary layer, creating less air drag as it leaves the rear of the vehicle.

What forces impede motion?

Assuming a steady speed on a level road, the truck needs fuel to overcome the following forces:

Drivetrain Friction

Drivetrain Friction

Tyre Rolling Resistance

Tyre Rolling Resistance

Aerodynamics Drag

Aerodynamics Drag

Drive train friction is usually the smallest force consuming only about 15% of the engine power. Tyre rolling resistance (what you feel when you push a car from stand still) consumes power proportional to road speed. So double your speed and you need twice the power to push the vehicle along. The real shocker is aerodynamic drag (what you feel with your hand out of a car window at speed), requiring minimal power at low speeds it rises exponentially as the vehicle increases speed. The power required to overcome it is proportional to the cube of the speed (v³).

For example… Double the speed and you need eight times the engine power to overcome the air drag. In this context, the heavy demand on engine power to overcome air drag means a rapid usage of fuel at high speeds.

The concept of drag - dominant speed (DDS)

Drag-Dominant Speed

The graph here shows that as speed rises air drag eventually takes over from tyre rolling resistance to become the chief fuel consuming force. This point is called the ‘Drag-Dominant Speed’. Aerodynamic drag has become very significant by the time we reach this speed, and as speed rises further, the effects of drag on fuel consumption will be dramatic. The graphs also demonstrate that good fuel savings can therefore be made at surprisingly low speeds on light and medium trucks (e.g. 20-35 MPH and above). You don’t have to be trunking on the motorway with such vehicles to save fuel.

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