These spring preload adjusters allow on-the-fly fork spring preload adjustment. With the use of a 5/16” hex wrench you can easily increase or decrease preload within a 30 mm range to alter the handling characteristics for a softer or stiffer ride. Replaces the OEM top fork caps and are available for 39 and 49 mm forks. Installation is fairly easy with no need to remove forks or top triple tree from bike.