A spring comprising a spring itself and end seat plates and a mount. In the spring seat plates are mechanically connected (constantly, variably, or actuable to connect) to each other and to the mount, so when a force or moment is applied to one of those seat plates its movement induce the movement of other seat plate as well, changing the total movement of the spring. Furthermore, there are provided mechanical means (a cams, a gears, a levers etc.) for changing the linkage between the end seat plates and the mount, which result in increasing, remaining the same (constant spring) or decreasing the spring constant. So, the spring constant of the spring can be controlled as to be adjustable.