PURPOSE: To reduce the extent of rigidity in a lug part, insomuch that tractive force will not go down, and give a vibro-absorbing capacity to a lug by embedding an elastic core, being softer than an elastic material forming the lug, in an inner part of this lug.

CONSTITUTION: An elastic core 11, being softer than an elastic material forming a lug 6 and similar form as this lug 6, is embedded in an inner part of the lug 6. As for the elastic core 11, for example, foaming material such as foam rubber, sponge rubber, urethane foam, etc., and soft rubber made into hardness 20-55° or so by adjusting those of type and quantity of a compounding agent of a thermoplastic elastomer of hardness 20-55° or so, a siftner or the like and their vulcanized degrees and so on are used. With this constitution, the extent of rigidity in a lug part is lowered, and thereby a rigid difference between the lug part and a lug spatial part becomes lessened, so amplitude is reduced, thus a vibration at time of traveling is reducible.