For most solar cars the frontal area is 0.75 to 1.3 m2. While "C""d" as low as 0.10 have been reported, 0.13 is more typical. This needs a great deal of attention to detail. The vehicle's mass is also a significant factor. A light vehicle generates less rolling resistance and will need smaller lighter brakes and other suspension components. This is the virtuous circle when designing lightweight vehicles. Rolling resistance can be minimized by using the right tires, inflated to the right pressure, correctly aligned, and by minimizing the weight of the vehicle. The design of a solar car is governed by the following work equation:  which can be usefully simplified to the performance equation  for long distance races, and values seen in practice. Briefly, the left hand side represents the energy input into the car (batteries and power from the sun) and the right hand side is the energy needed to drive the car along the race route (overcoming rolling resistance, aerodynamic drag, going uphill and accelerating). Everything in this equation can be estimated except "v". The parameters include:  Note 1 For the WSC the average panel power can be approximated as (7/9)×nominal power. Solving the long form of the equation for velocity results in a large equation (approximately 100 terms). Using the power equation as the arbiter, vehicle designers can compare various car designs and evaluate the comparative performance over a given route. Combined with CAE and systems modeling, the power equation can be a useful tool in solar car design. The directional orientation of a solar car race route affects the apparent position of the sun in the sky during a race day, which in turn affects the energy input to the vehicle. This is significant to designers, who seek to maximize energy input to a panel of solar cells (often called an "array" of cells) by designing the array to point directly toward the sun for as long as possible during the race day. Thus, a south-north race car designer might increase the car's total energy input by using solar cells on the sides of the vehicle where the sun will strike them (or by creating a convex array coaxial with the vehicle's movement). In contrast, an east-west race alignment might reduce the benefit from having cells on the side of the vehicle, and thus might encourage design of a flat array. Because solar cars are often purpose-built, and because arrays do not usually move in relation to the rest of the vehicle (with notable exceptions), this race-route-driven, flat-panel versus convex design compromise is one of the most significant decisions that a solar car designer must make.