An assembly comprising a tire, a rim and a coupling unit (10, 10') for coupling the tire to the rim, the coupling unit comprising: an upper surface, a lower surface, and side surfaces (13, 14), wherein a maximum length of the coupling unit (10, 10') is larger than a distance (L R) between inwardly facing ends of hooks (21a, 21b) provided at radially upper ends of opposite rim flanges, wherein the side surfaces (13, 14) include sliding areas that are slidable with respect to the hooks (21a, 21b) of the rim such that the coupling unit (10, 10') can be easily inserted into the rim, wherein in a state in which the coupling unit (10, 10') is completely inserted into the rim, at least a part of the upper surface is in surface contact with lower surfaces of the hooks (21a, 21b) to prevent the coupling unit (10, 10') from being separated from the rim, wherein the maximum height (h) from the upper surface to the lower surface of the coupling unit (10, 10') is less than or equal to the height (H R) of the inner walls of the rim flanges, characterized in that the following equations are satisfied: L ≥ L R + P − 1 2 h 2 + H R + R 2 wherein, L denotes the maximum length of the coupling unit (10, 10'), L R denotes a length between the hooks (21a, 21b) of the rim, P denotes a maximum length, by which the hooks (21a, 21b) protrude from inner walls of a rim flange, R denotes 1/2 of the thickness of the hooks (21a, 21b), h denotes a maximum height from the upper surface to the lower surface of the coupling unit (10, 10'), and H R denotes a height of the inner walls of the rim flange; and l ≤ L R + P − b 2 + H R + R 2 wherein l denotes a minimum length between the side surfaces (13, 14), and b denotes a larger one of the maximum height from the upper surface to the lower surface of the coupling unit (10, 10') and 0.5* (the maximum length of the coupling unit (10, 10') - the minimum length between the side surfaces (13, 14)). an upper surface, a lower surface, and side surfaces (13, 14), wherein a maximum length of the coupling unit (10, 10') is larger than a distance (L R) between inwardly facing ends of hooks (21a, 21b) provided at radially upper ends of opposite rim flanges, wherein the side surfaces (13, 14) include sliding areas that are slidable with respect to the hooks (21a, 21b) of the rim such that the coupling unit (10, 10') can be easily inserted into the rim, wherein in a state in which the coupling unit (10, 10') is completely inserted into the rim, at least a part of the upper surface is in surface contact with lower surfaces of the hooks (21a, 21b) to prevent the coupling unit (10, 10') from being separated from the rim, wherein the maximum height (h) from the upper surface to the lower surface of the coupling unit (10, 10') is less than or equal to the height (H R) of the inner walls of the rim flanges, characterized in that the following equations are satisfied: L ≥ L R + P − 1 2 h 2 + H R + R 2 wherein, L denotes the maximum length of the coupling unit (10, 10'), L R denotes a length between the hooks (21a, 21b) of the rim, P denotes a maximum length, by which the hooks (21a, 21b) protrude from inner walls of a rim flange, R denotes 1/2 of the thickness of the hooks (21a, 21b), h denotes a maximum height from the upper surface to the lower surface of the coupling unit (10, 10'), and H R denotes a height of the inner walls of the rim flange; and l ≤ L R + P − b 2 + H R + R 2 wherein l denotes a minimum length between the side surfaces (13, 14), and b denotes a larger one of the maximum height from the upper surface to the lower surface of the coupling unit (10, 10') and 0.5* (the maximum length of the coupling unit (10, 10') - the minimum length between the side surfaces (13, 14)). wherein, L denotes the maximum length of the coupling unit (10, 10'), L R denotes a length between the hooks (21a, 21b) of the rim, P denotes a maximum length, by which the hooks (21a, 21b) protrude from inner walls of a rim flange, R denotes 1/2 of the thickness of the hooks (21a, 21b), h denotes a maximum height from the upper surface to the lower surface of the coupling unit (10, 10'), and H R denotes a height of the inner walls of the rim flange; l denotes a minimum length between the side surfaces (13, 14), and b denotes a larger one of the maximum height from the upper surface to the lower surface of the coupling unit (10, 10') and 0.5* (the maximum length of the coupling unit (10, 10') - the minimum length between the side surfaces (13, 14)).