A wind tunnel experiment has been conducted to determine the changes in drag and side force due to the presence and position of cab extenders on a model of a commercial tractor-trailer truck. The geometric variables investigated are the cab extenders angle of incidence, the tractor-trailer spacing and the yaw angle of the vehicle. Three cab extender angles were tested-0°, 15° (out) and -15° (in) with respect to the side of the tractor. The cab and trailer models have the same width and height.
The minimum drag coefficient was found for the tractor and trailer combination when the cab extenders were set to 0° angle of incidence with respect to the headwind. This result holds for all yaw angles with moderate gap spacing between the tractor and trailer.
This study suggests that commercial tractor-trailer trucks can benefit from adjustable cab extender settings; 0° when using a trailer and -15° when no trailer is used. When no trailer is present, at 0° yaw, a -15° setting has approximately 5% less drag compared to having extenders at 0°, and 18% less drag when compared to a similar truck with no cab extenders installed.
Increasing the drag coefficient is also possible by deflecting the cab extenders in either direction. Similar to engine braking, this effect might be desired to reduce the vehicle speed without the application of brakes. At moderate tractor-trailer spacing, the 15° (out) and -15° (in) settings can increase CD by about 10 - 20%, depending on yaw angle. At larger separations, the -15° setting can increase CD by as much as 90%.
The cab extenders angle of incidence can also affect the side forces acting on the tractor and trailer. Setting the cab extenders to -15°, at 6° yaw, CSF for the trailer increased almost 200% in magnitude, and CSF for the tractor reduced to zero for moderate tractor-trailer spacing.