A coil spring is to be designed for use in the suspension of an experimental trailer. Specifications are as follows: static load per spring = 3500 N, rate per spring = 40 N/mm, jounce = 150 mm (i.e., compression beyond static load position is limited to 150 mm by a rubber bumper), rebound = 58 mm (i.e., extension beyond static load position is limited to 58 mm by a rubber bumper). Infinite fatigue life is required, using a safety factor of 1.3 applied to the maximum load only. (A safety factor of 1.0 is applied to the minimum load.) A metallurgist from the spring manufacturer has advised that for the size range and surface finish of the spring, fatigue strength can be represented by a line between tmax = 600, tmin = 0, and tmax = tmin = 900MPa. Determine a suitable combination of d, D, and N.