Originally Posted by dddd

Every part of a structure has a resistance to deformation, whether it is in bending, in shear, in compression or in tension. Once the part has it's dimensions and material defined, it can be said to have a spring rate along whatever critical direction that the expected loading will be applied.
In the case of spokes, obviously this will be tension force along the length of the spoke, and the stiffness or spring rate of the spoke will be proportional to the cross-sectional area of the spoke (and inversely proportional to the spoke's length).
Any vertical or lateral force applied to the rim of a built wheel will cause a change in the individual spoke's tensions that will alter their lengths and allow the rim to move relative to the hub. This is wheel flex, which is reduced as the spokes become thicker (or more plentiful) and thus more resistant to changes in length.