The spring constant for a helical coil spring is predicted by: where G is the material modulus, d is the wire diameter, D is the mean diameter of the coil and na is the number of coils. Assume the following spring properties and measurements and determine k in N/m. G = 82 kN/mm2 d = 1300 microns OD = 21 mm na = 45 coils Recall the coil mean diameter D is slightly different from the outside coil diameter OD.