A method for designing a fixed cutter drill bit includes simulating the fixed cutter drill bit drilling in an earth formation, determining a dynamic centerline trajectory of the drill bit, and adjusting at least one design parameter based upon the graphical display of at least the dynamic centerline trajectory. In particular, the battery holding portion may include a battery having an anode andor a cathode formed in a shape corresponding to a shape of an outer surface of the battery holding portion and a connector connecting the battery to the circuitry of the mobile unit. The light subtraction unit generates the first output light by reducing the light intensity of the second output light from the light intensity of the input light in accordance with a control voltage. The tabs can be positioned on the horizontal surface that supports the cooking grate of the grill. In such manner, even if multiple ECUs are installed as the vehicle control system, a vehicular data recording apparatus is enabled to record required data at a time of occurrence of a predetermined event. The complex pneumatic tool can be conveniently used to facilitate service operation of a vehicle.