In fact, the wheels used for buses are convex and concave at the front and back. The concave part can make more room for installing the brake and steering system, which is why the concave surface is inward.

As for what the rear wheel looks concave, this is the viewing angle effect. Because in order to facilitate steering, only one tire is installed in the front wheel, two tires are installed in the rear wheel to carry the load, and the outer tire fixes the convex tire and the inner tube, so it is visually a concave tire.