YZguy15,

One reason to have a spring with different diameter coils is to have a progressive rate spring. The larger diameter coils will deflect first, at a 'softer' rate. The smaller diameter coils will have a higher rate.

The 'G' term is a property of the material. It's actually the modulus of rigidity, also called the shear modulus. Not to be confused with the modulus of elasticity, E, which is a more commonly used property for engineering calculations.

E, the modulus of elasticity, also called Young's modulus, is the slope of the straight line portion of the stress/strain diagram, derived by pulling on a sample of the material.

G, the modulus of rigidity or shear modulus, is similar, but for shearing stress and strain, derived by putting the material in shear, not tension. For most materials they are related by Poisson's ratio, nu, something like

G=E/2(1+nu)

For example, for common 300-series stainless steel, E is about 29 million psi, while G is about 11 million psi.

References:

MIL-HDBK-5F - "Metallic materials and elements for aerospace vehicle structures"

"Mechanics of Materials", Higdon, et. al., 4th edition

"Mechanical Engineering Design", Shigley, 5th edition.

Of course, this is all just engineering garbage. To get my suspension working it's best, I ask my friend the fast desert racer to give me advice. There's no substitute for experience...

Burt