Does anyone know what the stiffness of the spring is?

1. spring characteristic curve
The relationship curve that represents the axial load F (or torque T) of a spring and its compression or extension deformation λ (or torsion angle) is called the spring characteristic curve.
1) Characteristic curve of equal pitch cylindrical spiral Compression Spring
When the spring is installed, a pressure Fmin is generally added in advance to make it reliably stable in the installation position. Fmin is called the minimum load of the spring (installation load). Under its action, the length of the spring is compressed to H1, and its compression deformation is λmin.
Fmax is the maximum operating load accepted by the spring. Under the action of Fmax, the spring length is reduced to H2, and its compression deformation increases to λmax. The difference between λmax and λmin is the operating stroke h of the spring, h=λmax-λmin.
Flim is the ultimate load of the spring. Under the action of this force, the stress in the spring wire reaches the elastic limit of the material. The spring length corresponding to Flim is H3, and the compression deformation is λlim. The maximum operating load Fmax of the spring is determined by the operating conditions of the spring in the arrangement. But it should not reach its ultimate load, and generally Fmax should be kept ≤ 0.8Flim.
2) The characteristic curve of the equal-pitch cylindrical spiral Tension Spring has the characteristic curve of the prestressed tension spring. For prestressed tension springs, Fmin>F0, F0 is the initial tension required to initially deform the prestressed tension spring. A prestressed tension spring is equivalent to a predeformation x, so under the action of the same load F, the deformation produced is smaller than that without prestress.
The characteristic curve of the spring should be drawn in the spring operation diagram as one of the basis for inspection and experiment. At the same time, when designing the spring, it is also more convenient to use the characteristic curve to analyze the relationship between load and deformation.
2. Spring stiffness
The load required to produce unit deformation of the spring is called spring stiffness, and its expression is:
kF=dF/dλ (or kT=dT/dφ)
In fact, the spring stiffness is the slope of a certain point on the spring characteristic curve. A spring that conforms to a linear characteristic curve has a constant stiffness. Its stiffness is:
kF=dF/dλ=F/λ=
The steeper the characteristic curve of this spring, the greater the stiffness, that is, the harder the spring, and vice versa.

For springs subject to dynamic loads or impact loads, it is advisable to use variable stiffness springs.