For the front suspension systems of 4WD vehicles, foreign materials such as sand caught between the front coil spring and spring lower sheet may cause the paint of the spring to peel off, because the shape of the spring lower sheet is not well designed, and as a result of it, rust may occur. If this problem is left unattended, a snow-melting agent may cause the rust to progress, and the spring could eventually break. If such breakage occurs, the tyre may burst and the vehicle could stall.