Cd Value

The Cd value is the drag coefficient and is the figure generally quoted in publications in relation to a vehicle's aerodynamic qualities. It is used as a comparative value to assess the aerodynamic efficiency of the shape of a body, regardless of the body's size. A number of variables influence the Cd value, including the vehicle shape (saloon, estate or fastback). Narrow gaps and joints reduce the buffeting effect, thereby helping to keep drag low. The design of the underbody also has an effect. The lower the Cd value, the more aerodynamically efficient the vehicle's design. A precise statement regarding the vehicle's aerodynamics can only be made once both the Cd value and the vehicle's frontal area (the projection of the vehicle's front outline onto a plane surface) are known. A small vehicle may have a higher Cd value than a large saloon but its aerodynamic drag could still be lower due to the smaller frontal area.