A small, round or oval piece of abrasion-resistant material, often
plastic, but possibly also leather or metal; frequently shaped like a
hockey puck or a
bar of soap. The slider is attached with
Velcro to the outside of the knee area of a
motorcycle road racer's leathers. Its purpose is to allow the rider to drag his or her knee on the ground while negotiating corners, without damaging the leathers.
In a turn, a skilled rider will slide to the inside edge of the seat, or even hang off to the inside, with only his thigh on the seat, in order to move his center of gravity towards the center of the turn radius. This allows the bike to travel around the turn with less lean angle (thereby taking advantage of more of the tire tread), or (more commonly) to travel around the turn at a higher velocity for the same lean angle.
By touching the ground with his knee, the rider can judge his lean angle effectively, and furthermore may be able to push the bike back upright if his tires begin to slide and he would otherwise be in danger of a low-side.
Needless to say, one must be fairly skillful, and ride fairly aggressively, to touch one's knee to the ground in a turn. Therefore, for the less skillful or aggressive rider, some stores offer pre-scuffed knee sliders, which said rider can attach to his leathers to give the impression of greater skill and/or larger cojones, without the commensurate risk to his person or vehicle.