A helical compression spring is to be designed for infinite life when subjected to a load that fluctuates between 55 and 110 lb. Steel spring wire having Su = 180 ksi, Sus = 144 ksi, Sy = 170 ksi, Ssy = 99 ksi, and a zero-to-maximum torsional endurance limit of 80 ksi is to be used. (These values are assumed to apply to the size range and surface finish of the spring.) For a value of C = 7, determine the wire diameter theoretically required (safety factor = 1), (a) if presetting is not used and (c) if presetting is used to maximum advantage.