MRP Progressive Springs are the only non-linear shock springs on the market for trail and all-mountain/enduro riding. Progressive Springs increase in rate late in stroke to provide additional bottom-out support and open up the use of coil shocks on frames previously only suited to air shocks. these springs are offered in a single free length per rate that fits most shocks from 200mm (7.875") to 230mm.* They accomodate up to 65mm of shock stroke. A 1.5" / 38mm ID fits our Hazzard natively and spacer kits are available to use with other makes and models. Inner Diameter = 1.5" / 38mm If you don&';t run an MRP Coil Shock, you&';ll also need a MRP Coil Spring Spacer / Adapter Kit Use our Spring Calculator to find the right weight spring for you, your bike, and your shock!