Refers to the brake clearance automatic adjustment arm, referred to as automatic adjustment arm, can automatically and timely adjust the gap increased by wear, so that the brake clearance is always maintained within the design range. The angular stroke of adjusting arm due to the elastic deformation of force transmission parts after contact between brake shoe and brake drum will not be adjusted. When braking begins, the adjusting arm turns the camshaft around the clearance Angle and the excess clearance Angle, and records the wear caused precisely. At this time, the CAM angular stroke is in the clearance area, which is characterized by little change in braking torque. Continue to brake, CAM Angle stroke into the elastic deformation zone, braking torque rises sharply, until the car stops. Release pedal, brake return, brake torque down, CAM Angle stroke back to the gap area. According to the excess clearance recorded during braking, the internal adjusting mechanism drives the camshaft to turn a certain Angle through the worm gear, thus completing an adjustment. When there is excess clearance between shoe and brake drum, the camshaft adds excess clearance Angle "B" in the braking process. At this time, the rotation stroke of adjusting arm can be divided into three parts: normal clearance Angle "A", excess clearance Angle "B" and elastic Angle "C". In the braking process, the automatic adjusting arm can automatically identify these three strokes and adjust only the excess clearance.