ERGY
                    this.stiff = 1.053*(v0*r*y^2*sqrt(m))^(2/5);
                case this.OVERLAP
                    this.stiff = 1.053*(v0*r*y^2*sqrt(m))^(2/5) * exp(-atan(beta)/beta)^2;
                case this.TIME
                    this.stiff = 1.198*(v0*r*y^2*sqrt(m))^(2/5) * (1+1/beta^2);
            end
            
            % Unloading stiffness coefficient
            switch this.unload_stiff_formula
                case this.CONSTANT
                    this.unload_stiff = this.stiff 