Bicycle suspension refers to the system or systems used to suspend the rider and all or part of the bicycle in order to protect them from the roughness of the terrain over which they travel. Bicycle suspensions are used primarily on mountain bikes, but are also common on hybrid bicycles, and can even be found on some road bicycles. Bicycle suspension can be implemented in a variety of ways: Suspension front fork, Suspension stem (although these have fallen out of favor), Suspension seatpost, Rear suspension, Suspension hub or any combination of the above. Bicycles with suspension front forks and rear suspensions are referred to as full suspension bikes. Additionally, suspension mechanisms can be incorporated in the seat or saddle, or the hubs. Besides providing obvious rider comfort, suspensions improve both safety and efficiency by keeping one or both wheels in contact with the ground and allowing the rider's mass to move over the ground in a flatter trajectory.