| You can use the linear stiffness of a rod with constant diameter in tension. K = AE/L
A is the cross sectional area of the spring
E is the elastic modulus of the spring
L is the free length of the spring without any deflection
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You can use the linear stiffness of rod with constant diameter in torsion. K = JG/L
J is the polar moment of inertia = 2I
I is the moment of inertia = (Πr4)/4
r is the radius of the coil
G is the modulus of rigidity = E/(2(1+ν))
E is the elastic modulus of the spring
ν is the Poisson's ratio
L is the free length of the spring without any deflection
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