Stiffness of a close coiled helical spring in terms of wire diameter d, modulus of rigidity G, number of turns n and mean coil radius R is given by:
1
\(\frac{{G{d^4}}}{{16n{R^3}}}\)
2
\(\frac{{G{d^4}}}{{32n{R^3}}}\)
3
\(\frac{{G{d^4}}}{{64n{R^3}}}\)
4
\(\frac{{G{d^4}}}{{128n{R^3}}}\)