A wheel is a rigid body. The centripetal accelerations of the particle of the wheel arise due to the internal elastic forces which cancel out in paries.

In a half wheel, the distribution of mass about its centre of mass (through which axis of rotation passes) is not symmertical. Therefore, the direction of angular momentum of the wheel does not coincide with the direction of its angular velocity. Hence, an external torque is required to maintain the motion of the half wheel.