SKATE BOARD Technical Field [1] The present invention relates to a skate board and more particularly to a skate board which can freely turn its traveling direction by separately providing two decks and which can adjust a distance between the two decks by a user' selection and the balance of posture of which can be more stably and easily maintained against a centrifugal force using a eccentric shaft twisted by a predetermined angle while turning its traveling direction. Background Art [2] Conventional skate boards have a single deck, the bottom of which is provided with front and rear wheels. [3] The turnabout of the skate boards is performed by slightly lifting the front wheels with the weight load put backward or by applying a force to the front wheels. However, the conventional skate boards have problems that beginners have a difficulty in turning the skate board and a safety accident takes place by mistake during traveling. [4] In addition, skate board manias have a difficulty in exhibiting delicate and splendid movements with the skate boards. Disclosure of Invention Technical Problem [5] The present invention is contrived to solve the above-mentioned problems and it is an object of the present invention to provide a skate board which can freely turn the traveling direction thereof and adjust the distance between decks by a user's selection and the balance of posture of which can be more stably and easily maintained against a centrifugal force using a eccentric shaft twisted by a predetermined angle while turning its traveling direction. [6] According to an aspect of the present invention, there is provided a skate board comprising: two trucks rotatably fitted with wheels; two decks detachably coupled to the top ends of the two trucks, respectively; and a rotation member which is coupled to the bottom end of each deck and which is twisted and rotated by a predetermined angle in response to movement of the deck. Brief Description of the Drawings [7] The above and other features and advantages of the present invention will become more apparent by describing in detail exemplary embodiments thereof with reference to the attached drawings in which: [8] Fig. 1 is an exploded perspective view of a skate board according to the present invention; [9] Figs. 2 and 3 are a top plan view and a front view illustrating the assembled state of the skate board according to the present invention, respectively; [10] Fig. 4 is a cross-sectional view taken along Line B-B of the skate board shown in Fig. 2; [11] Fig. 5 is a cross-sectional view illustrating a rotation member of the skate board according to the present invention; [12] Figs. 6 and 7 are side views illustrating states of the skate board before and after feet are put on the decks of the skate board according to the present invention; [13] Figs. 8 and 9 are schematic views illustrating the turnabout of the skate board according to the present invention; [14] Fig. 10 is a perspective view illustrating a connection member at the time of the turnabout of the skate board according to the present invention; and [15] Figs. 11 and 12 are side views illustrating method of adjusting a distance between the decks of the skate board according to the present invention. Mode for the Invention [16] Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the attached drawings. [17] Fig. 1 is an exploded perspective view of a skate board according to the present invention, Figs. 2 and 3 are a top plan view and a front view illustrating the assembled state of the skate board according to the present invention, respectively, Fig. 4 is a cross-sectional view taken along Line B-B of the skate board shown in Fig. 2, Fig. 5 is a cross-sectional view illustrating a rotation member of the skate board according to the present invention, Figs. 6 and 7 are side views illustrating states of the skate board before and after feet are put on the decks of the skate board according to the present invention, Figs. 8 and 9 are schematic views illustrating the turnabout of the skate board according to the present invention, Fig. 10 is a perspective view illustrating a connection member at the time of the turnabout of the skate board according to the present invention, and Figs. 11 and 12 are side views illustrating method of adjusting a distance between the decks of the skate board according to the present invention. [18] As shown in Figs. 1 to 12, the skate board according to the present invention comprises two trucks 20 rotatably fitted with wheels 24, two decks 10 detachably coupled to the top ends of the two trucks 20, respectively, rotation members 30 which are coupled to the bottom ends of the decks and which are twisted and rotated by a pre¬ determined angle in response to movement of the decks 10, and a connection member 40 which has first and second crossbars 41 and 42 coupled to both sides of the rotation members 30 and which can adjust a distance between the two decks 10. [19] Each truck 20 includes a plurality of projected pieces 22 projected upward, screw holes 25 formed in the projected pieces 22, a fixing member 21 to which a wheel shaft 23 rotating in the length direction is coupled, and wheels 24 coupled to both ends of the wheel shaft 23. [20] A plurality of coupling holes 11 is formed in the decks 10 on the same line as the screw holes 25 of the fixing member 21 such that the coupling holes 11 and the screw holes 25 are coupled with bolts, etc. The decks 10 may be made of wood, plastic, synthetic resin, or the like. [21] A sliding prevention portion 12 including penetrated openings with various shapes is formed on the decks 10 so as to freely slide and rotate. [22] Each rotation member 30 includes an eccentric shaft 32 coupled to the bottom surface of the deck 10 and a housing 33 coupled to the eccentric shaft 32 with bearings 34 interposed therebetween. [23] The eccentric shaft 32 is coupled to the bottom surface of the deck 10 by using a direct welding method or the like to be eccentric by a predetermined angle. [24] The deck 10 and the eccentric shaft 32 are fixed to each other by a pivot pin 31, and the deck 10 is fixed perpendicular to the pivot pin 31. [25] It is preferable that an eccentric angle a of the eccentric shaft 32 ranges about 4°to 7°. [26] The housing 33 surrounds the outer circumferential surface of the eccentric shaft 32. A snap ring 35 and the bearings 34 are interposed between the eccentric shaft 32 and the housing 33. A fixing nut 36 fixing the bearings 34 is coupled to the lower end of the eccentric shaft 32. [27] One or more bearings 34 can be interposed between the eccentric shaft 32 and the housing 33. [28] The first and second crossbars 41 and 42 of the connection member 40 are sym¬ metrically fixed to the outer circumferential surface of the housing 33 and a distance adjusting portion 45 for adjusting the distance between the decks 10 is provided in both sides of the center of the first and second crossbars 41 and 42. [29] The first and second crossbars 41 and 42 have the same shape to form a pair. The first crossbar 41 is coupled to both sides of the housing 33 at the front side of the rotation member 30 and the second crossbar 42 is coupled to both sides of the housing 33 at the rear side of the rotation member 30. [30] A plurality of fixing holes 43 are formed at one end of the first and second crossbars 41 and 42 and a plurality of fastening holes 44 are formed at the other end at a predetermined interval. [31] Bolts and nuts are inserted into the fixing holes 43 of the first and second crossbars 41 and 42, which are fixed to both sides of the housing 33. [32] The distance adjusting portion 45 includes a distance adjusting plate 46 surrounding both side surfaces of the first and second crossbars 41 and 42 and a boss 49 provided in the inner surfaces of the first and second crossbars 41 and 42. The boss 49, the first and second crossbars 41 and 42, and the distance adjusting plate 46 are fixed to each other with bolts and nuts. [33] A plurality of coupling holes 48 having the same pitch as that of the fastening holes 44 is formed in the distance adjusting plate 46 and a coupling groove 47 for stably receiving the first and second crossbars 41 and 42 is formed on one side surface thereof. [34] Penetrated holes 50 are formed in the boss 49 with the same pitch as that of the fastening holes 44. [35] The fastening holes 44 of the first and second crossbars 41 and 42, the penetrated holes 50 of the boss 49, and the coupling holes 48 of the distance adjusting plate 46 are positioned in the same line and fixed to each other with bolts and nuts. [36] Operations and effects of the skate board according to the present invention will be now described. First, when both feet are stepped on the separated decks 10 of the stopped skate board, as shown in Figs. 6 and 7, the decks 10 are slanted by a phase difference "C" of the decks 10. In this state, by moving the decks 10 to left and right, the skate board travels forward and backward by means of operation the rotation member 30. [37] Specifically, as shown in Fig. 5, the eccentric shaft 32 of the rotation member 30 is eccentrically coupled to the bottom surface of each deck 10. Accordingly, the skate board is rotated by the eccentric angle "a" by the bearings 34 rotating about the eccentric shaft 32 and stably travels forward and backward while keeping the phase difference "c" of the decks 10. [38] The maximum radius of rotation of the decks 10 is 40°to the left and right sides about the pivot pin 31 and the eccentric shaft 32. [39] In this way, when the fixing members 21 of the trucks 20 rotates about the pivot pin 31 and the eccentric shaft 32 with rotation of the decks 10, the fixing members 21 collide with the side surfaces of the first and second crossbars 41 and 42 of the connection member 40, thereby preventing the rotation thereof by 40°or more. [40] On the other hand, when the skate board according to the present invention turns its traveling direction, the turnabout of the skate board causes the wheels 24 to have a phase difference "b" by the eccentric shaft 32, as shown in Figs. 6 to 9, but the wheels 24 is made to be parallel to the ground by the weight load and are slanted by an angle "c" about the plane of the decks 10. [41] It is preferable that the angle "b" ranges 4°to 7°and the angle "c" ranges 2°to 5°. [42] Therefore, since the centrifugal force Fc generated with the rotation is distributed into Fa and Fb by the angle "c" of the decks 10 as shown in Figs. 8 and 9, a rider can more stably maintain the balance of posture against the centrifugal force. [43] The relative phase difference generated when the respective decks 10 have different rotation angles causes the torsion Ft of the first and second crossbars 41 and 42 as shown in Fig. 10. However, since the first and second crossbars 41 and 42 have enough elasticity resistible to the torsion and the twisted first and second crossbars 41 and 42 have a restoring force, the respective decks 10 maintains equilibrium relative to the phase difference at the time of rotation of the eccentric shaft 32. Accordingly, the rider can stably ride the skate board. [44] Now, a method of adjusting the distance between the two decks of the skate board according to the present invention is described. The first and second crossbars 41 and 42 coupled to the distance adjusting plate 46 are moved inward, the fastening holes 44 of the first and second crossbars 41 and 42 and the penetrated holes 50 of the boss 49, and the coupling holes 48 of the distance adjusting plate 46 are positioned in the same line and fixed with the bolts and nuts. Then, as shown in Fig. 11, the distance between the decks 10 is reduced. On the contrary, when the skate board is adjusted as shown in Fig. 12, the distance between the decks 10 is increased. [45] As described above, the distance between the decks of the skate board can be adjusted by a user's selection. [46] Therefore, since the skate board can be freely turned, it is possible for beginners to easily use the skate board. In addition, since the distance between the decks of the skate board can be adjusted by a user's selection, it is possible to conveniently use the skate board. [47] Furthermore, since the load applied to the connection member of the skate board can be distributed, it is possible to keep the skate board stable in advancing or turning the skate board. [48] Although the exemplary embodiments of the present invention have been described in detail, the present invention is not limited to the exemplary embodiments, but may be modified in various forms without departing from the scope of the appended claims.