Pre-loader allows use of a softer spring rate that is "pre-loaded" at ride height. The pre-loaded spring will have less rate increase as the suspension travels. For example, a 150 lb/in spring can be pre-loaded to 400 lbs. at ride height, but still only increases at a rate of 150 lb/in as suspension travels. The entire unit may also be exchanged with another unit with a different rate spring pre-installed to allow spring rate changes without affecting ride height. Commonly used on the rear coil-overs, pre-loader requires 3" I.D. spring, includes a thrust bearing set and fits most shocks. Note: Unit is typically installed with slotted inner half towards the shock body. Always check components for possible interference.