A plurality of lugs 39 are coupled to ground contact sections 37 of wheels 5 and 6 so as to be aligned along a rotation direction A1 of the wheels 5 and 6 with a gap W1 therebetween. Each of the lugs 39 includes a first portion 41 in an oblique orientation relative to the rotation direction A1 of the wheels 5 and 6, and a second portion 42 obliquely oriented relative to the rotation direction A1 of the wheels 5 and 6, opposite to the oblique orientation of the first portion 41. The first portion 41 and the second portion 42 are provided such that ends of the first portion 41 and the second portion 42 in the rotation direction A1 of the wheels 5 and 6 are connected to each other.
