A coil spring comprises a continuous wire forming a lowermost convolution, an uppermost convolution, and a plurality of helical intermediate convolutions. The coil spring is divided into an upper portion and a lower portion. The intermediate convolutions of the lower portion have a diameter greater than or equal to a lower minimum diameter, and the pitch between each of the convolutions of the lower portion is greater than or equal to a lower minimum pitch. The intermediate convolutions of the upper portion have a diameter greater than or equal to an upper minimum diameter, and the pitch between each of the convolutions of the upper portion is less than or equal to an upper maximum pitch. The upper minimum diameter is greater than, less than, or equal to the lower minimum diameter, and the upper maximum pitch is less than the lower minimum pitch to provide a non-linear and variable response.