A wheel is a stiff elastic body with a consistent motion that passes through its center, perpendicular to the wheel’s plane. Due to elastic force, every particle of the wheel receives a centripetal acceleration. Due to this, it is directed towards the axis of rotation. When the wheel is half-turned, the mass distribution is similarly half-turned. There is no symmetry in the wheel when the mass is halved. As a result, the angular momentum and the angular velocity do not match. Hence, external torque is needed.