Crossbow limb's stiffness
Let's estimate the stiffness of the crossbow limb. Will consider the limb as a bending beam, which is clamped on the one side and free on the other (console beam). The force F is applied orthogonal to the free end.
The stiffness of the spring in general is a ratio of applied force to deformation. Say, the spring was loaded with 100 Newton force and it got deformation 1 cm (0.01 meters), then the stiffness is: