Skateboards get put through a lot of use and abuse by riders. Riding over uneven pavement, being dropped from heights, having people jump on them or throwing them, they get beaten up quite easily. Riser pads, also known as shock pads, can help reduce some of the damage from daily use and abuse.
Reducing Wheel Bites
Wheel bites, also known as wheel rashes, are the primary issue risers are meant to minimize. Wheel bites happen when you make a turn or do a trick on the skateboard. As you turn or put pressure on one side of the board, the wheels can come into contact with that side of the board. Not only can this cause difficulty steering the board, it can cause you to fall off the board. Bites also wear away the underside of the board. Risers increase the space between the board and the tops of the wheels.
Improving Board Handling
Skateboard trucks connect to the underside of the board and allow wheels to be attached. Trucks have a central bolt that lets you adjust how much "turning" occurs on the board. Loosen this bolt on each truck and the board will turn more easily; tighten the bolt and the board will not make turns as easily. While looser bolts mean quicker, more noticeable turning, they also increase the risk of wheel bite, which actually interferes with turning. A riser reduces this problem.
Risers can also make your ride a bit smoother by absorbing some of the shock when landing tricks or riding on rough surfaces.
Reducing Stress on the Board Material
Trucks are held to the board by four heavy screws on each truck, connected to pre-drilled holes that go through the board. The screw goes through the top of the board into the trucks. This means the hard metal of the trucks are directly pressed against the softer material of the board. Over time, this can cause stress cracks in the board, weakening the board, letting in moisture and eventually ruining the board. Risers can slow this process by putting a soft buffer between the metal trucks and wooden boards.
Disadvantage of Risers
While more of a personal preference, board risers make the board higher off the ground. This, for some skaters, can make technical tricks more difficult to do, especially if they are used to a board without risers and without extra height.