The spokes of cycle wheel increases its moment of inertia (I) because maximum mass due to rim which is way from the axis of rotation. If it is a disc some mass will be very near to axis of rotation. Due to spokes maximum mass for rim leads to more moment of inertia I, which gives greater stability to the rotational motion. As a result the cycle runs smoother and steadier. If the cycle wheels had no spokes from rim to hub, the cycle would be driven in jerks and hence unsafe.So maximum cycles will be prepared with spokes.