Abstract: A helical torsion spring is provided having an adjustable spring rate. The torsion spring includes a main spring formed from a coil of wire having a first end and a second end. The wire extends away from the first end to form a first arm and away from the second end to form a second arm. The main spring is loaded torsionally by angular displacement of the first arm with respect to the second arm about a central axis passing through a geometric center of the coil. An inside diameter of the coil defines a core. An adjustment spring is oriented within the core. The adjustment spring is formed from adjustment spring wire configured in a manner similar to that of the main spring. The adjustment spring exhibits an exterior diameter not less than the inside diameter of the main spring. The adjustment spring is rotatable to threadably adjust a proportion of the core of the main spring that includes the adjustment spring therein.