A method for determining a braking torque resulting from the actuation of a braking system for vehicle, by means of detection performed at at least one fixing interface (i, ii, iii, iv) between a brake caliper body and a respective support, or between an axle and a wheel suspension, the method comprising the steps of: - inserting at least one washer device at said at least one fixing interface (i, ii, iii, iv), so that the washer device is fixed and pressed between the brake caliper body and the brake caliper support or between the brake caliper support and a head of a clamping element (63, 64), or between the axle and the wheel suspension, wherein said washer device is subject to deformation when exposed to forces, so that the deformation and/or strain locally present at each point of the washer device is representative of the forces acting at said at least one fixing interface, dependent on the braking torque, wherein said washer device incorporates at least one deformation and/or strain sensor, at a respective detection position, arranged to detect a deformation and/or strain representative of the three spatial vector components of the force acting on the washer device at the detection position, said at least one deformation and/or strain sensor being a fiber-optic strain sensor of the fiber Bragg grating type; - detecting, by means of each of said at least one fiber-optic strain sensor, the local deformation and/or strain acting in the respective detection position, and generating a respective at least one photonic signal representative of the detected deformation and/or strain; - receiving said at least one photonic signal, by an optical reading/interrogation unit, optically connected to said at least one fiber-optic strain sensor; - generating, by the optical reading/interrogation unit, at least one electrical signal representative of the local detected deformation and/or strain, on the basis of said at least one received photonic signal; - determining the braking torque on the basis of said at least one electrical signal representative of the deformation and/or strain; wherein the one or more fiber-optic strain sensors are comprised in an optical fiber arranged within a tangentially peripheral band of the washer device, and each of these fiber-optic strain sensors is obtained in a respective optical fiber portion inclined with respect to a radial plane of the washer (p) at an acute or obtuse inclination angle different from each of the values 0°, 90°, 180° and 270°, to detect a strain in a direction different from a normal direction and from a tangential direction, with respect to the washer, so that the detected strain depends both on the normal component and on the tangential component of the force acting on the sensor.. - inserting at least one washer device at said at least one fixing interface (i, ii, iii, iv), so that the washer device is fixed and pressed between the brake caliper body and the brake caliper support or between the brake caliper support and a head of a clamping element (63, 64), or between the axle and the wheel suspension, wherein said washer device is subject to deformation when exposed to forces, so that the deformation and/or strain locally present at each point of the washer device is representative of the forces acting at said at least one fixing interface, dependent on the braking torque, wherein said washer device incorporates at least one deformation and/or strain sensor, at a respective detection position, arranged to detect a deformation and/or strain representative of the three spatial vector components of the force acting on the washer device at the detection position, said at least one deformation and/or strain sensor being a fiber-optic strain sensor of the fiber Bragg grating type; wherein said washer device is subject to deformation when exposed to forces, so that the deformation and/or strain locally present at each point of the washer device is representative of the forces acting at said at least one fixing interface, dependent on the braking torque, wherein said washer device incorporates at least one deformation and/or strain sensor, at a respective detection position, arranged to detect a deformation and/or strain representative of the three spatial vector components of the force acting on the washer device at the detection position, said at least one deformation and/or strain sensor being a fiber-optic strain sensor of the fiber Bragg grating type; - detecting, by means of each of said at least one fiber-optic strain sensor, the local deformation and/or strain acting in the respective detection position, and generating a respective at least one photonic signal representative of the detected deformation and/or strain; - receiving said at least one photonic signal, by an optical reading/interrogation unit, optically connected to said at least one fiber-optic strain sensor; - generating, by the optical reading/interrogation unit, at least one electrical signal representative of the local detected deformation and/or strain, on the basis of said at least one received photonic signal; - determining the braking torque on the basis of said at least one electrical signal representative of the deformation and/or strain;