The present invention discloses a clip retainer which includes a top surface having a groove and a bottom surface which has stopping ribs and a barb formed between the stopping ribs. The hydraulic control system may control a first brake operated at a first forward speed, a first clutch operated at a second forward speed, and a second brake operated at a reverse speed. The user may then lever the presser rod about the pivoting location towards the side so as to store the presser rod at its horizontal position. This configuration prevents damaging contact between the tread and the undersurface of the snow flap andor the chassis. Rotation of the pinion in response to the drive force of the driving unit is converted into a linear movement of the rack gear, and a distance of the linear movement of the rack gear is enlarged by means of the travel-enlarging member, so that the second body fixed to the second hinge is slid to an enlarged travel distance.