1992-02-01

Aerodynamic Drag Implications of Exterior Truck Mirrors 920204

Until recently, the aerodynamic design of large transport trucks has often ignored an important contribution to drag caused by the accessory rearview mirrors.In this study, three commercially available truck mirrors are tested full-scale in a wind tunnel at highway speeds. The actual drag forces and the coefficients of drag based on frontal and glass areas are compared for various mirror angles, with and without a convex-mirror attachment. All three mirror types produced significantly high drag forces which were used to estimate the fuel consumption attributed to mirror aerodynamic resistance.A standard rearview mirror, with mounting brackets, set was found to exhibit a drag coefficient, based on the glass area, of 1.81, which would increase the drag of a typical (Cd = 0.7, Af = 4.5m2) truck by about 8.5 percent. A teardrop-shaped accessory mirror, although appearing more streamlined, actually produced a higher glass-based drag coefficient of 1.94, adding about 10 percent to the truck drag. A recent fully-faired mirror set performed best, with a glass-area-based drag coefficient of only 1.13, making up only 5.3 percent of the typical total aerodynamic truck drag.Comparing the energy implications of the best to the worst mirrors, the estimated saving in fuel based on 150,000 km of annual driving, could amount to 800 L of diesel fuel or 1250 L of gasoline. These results suggest that more attention needs to be paid to truck-mirror aerodynamic design and selection.

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