the center of the cam follower 36 the function of this arrangement will be more clearly brought out in the following discussion of the operation of the device assume that the apparatus is in the condition shown in fig1 and 2 with the shoes 32 in radially extending ground engaging position as the wheel rotates each portion of the periphery of the shoes 32 will sequentially engage the road surface and the force exerted by the road on the ground engaging shoe will pass through the point of contact and the center of the wheel when this line of force acts along a radius extending through the cam follower 36 the shoe 32 is restrained from moving radially inward by engagement of the cam follower 36 with the cam surface 38 when this line of force is coincident with any other radius it will urge the edges of the slots 46 against the abutments 48 and such engagement will be effective to prevent radially inward movement of the shoe 32 thus the abutments 48 and cam follower 36 provide a support for the shoe 32 which will maintain the same in radially extended position and which will resist any force which tends to rock the shoe or move the same radially inward to insure retention of the various parts in their relative positions as shown in the drawings the nut 30 is drawn up against the plate 24 to move the same toward the disk 18 thereby clamping the shoes 32 firmly therebetween and providing a positive locking arrangement when it is desired to position the shoes 32 in a non operative position the nut 30 is loosened and the plate 24 is rotated in a counterclockwise direction as shown in the drawings the angle 44 will.