Section 2 - Basics of aerodynamic devices

2.1 Why use aerodynamic devices?

At highway speeds, much of the energy used to move a truck goes to pushing air out of its way.

When a vehicle moves, the air exerts a force on the vehicle that resists its motion, making it more difficult to move. This force is known as aerodynamic drag and it has a significant impact on the amount of fuel a vehicle uses. Consequently, a non-aerodynamic tractor-trailer requires more power (i.e. more fuel) than an aerodynamic one. The impact of poor aerodynamics on fuel consumption increases as the speed increases.

Aerodynamic devices reduce the amount of drag by creating a more streamlined shape to the tractor-trailer. Figure 1 illustrates how aerodynamic devices can reduce the amount of drag on a tractor-trailer.

2.2 What are aerodynamic devices?

Aerodynamic devices improve the streamlining of the tractor-trailer. The devices reduce the surface area that the air hits, so less drag is produced, and consequently, less fuel is used.

Research on aerodynamic devices has been improving fuel efficiency in tractor-trailers for many years. These older cab over engine photographs in Figure 2 still accurately show how the air movement changes when a streamlining cab roof deflector is added. The change in the flow of the green coloured smoke indicates the improved air flow.

(1) When the cab does not have a deflector, air hits the front of the trailer and creates drag in the tractor-trailer gap.

(2) When a cab deflector is added, the air moves over the tractor-trailer gap, which reduces the area being hit by the air. (Photos: National Research Council Canada)

As a tractor-trailer moves, there are four main areas of aerodynamic drag: the tractor frontal area, tractor-trailer gap, undercarriage and the trailer rear area (see Figure 3).

You can achieve significant fuel savings by streamlining the tractor-trailer’s aerodynamics. Technologies that target these problem areas:

• are mounted on or are components of the tractor, such as fuel tank side skirts, aerodynamic mirrors and rounded grills and bumpers
• improve the aerodynamics of the tractor-trailer gap, such as cab roof fairings and deflectors, trailer gap reducers and cab extenders
• are mounted on the trailer, such as trailer rear fairings and trailer side skirts

Although there are many aerodynamic devices in the aftermarket, this guide focuses on the following technologies because they offer the most proven fuel savings and can be used by anyone who owns a tractor or a trailer (see Figure 4):

• cab roof fairings
• adjustable cab roof deflectors
• trailer side skirts
• trailer rear fairings