This purchase is for two pairs of axle sliders (one front and one rear), designed to help protect your bike expensive swing-arms and front fork in case of a low-side. Installation is a simple bolt on with the bar going through the axle and one plastic puck on each side. The slider pucks are made of UHMW-PE (ultra high molecular weight polyethylene), a tough plastic with good strength, wear resistance and dimensional stability. It is more dimensionally stable than Nylon and used in many industrial ball bearing and friction reducing applications. They can be inexpensively replaced in case of a low-side. The long bar and inserts inside the sliders are made 6061-T6 aluminum for strength. Rest assured that you are getting the best material commercially available.