Although it is perfectly true that "constante de raideur" is literally stiffness constant (and by extension, as it were, spring constant), I feel that the occurrence of "ressort" a few words previously is sufficient to drop the spring later on, on the grounds of redundancy. Hence, "vertical springs whose stiffness....".
Note that "spring constant" is not quite the same thing as "stiffness constant", the former being a property of an object (eg a spring, and measured in N/m), the latter being a property of the material, and measured in N/m^2 (often also referred to as Young's modulus)