Hot Rod Sled Shop 4 Spring "Drop-In" Kit--This complete Rear suspension 4 Spring Kit for Ski-doo SUmmit models wit T-motion 2015-2021 models includes our HD rear torsion springs and our dual rate center spring setup with a Delrin spring separator. The dual rate center spring setup is initially softer than oem ski-doo allowing the sled to get on top of the powder while offering a stiffer rate under hard compression for big hits and bottoming, The center dual rate springs are only for ski-doo shocks with a diameter under 2" and fits Sport and "x" models.. The oem rear torsion springs are vextremely soft and take a set quickly. We get calls every day with guys bottoming and cranking the adjuster all stiff to no avail.....this is the cure with superior rear springs and a dual rate center spring combo that rides smooth but doesn't bottom AND works well without valving work. These are not Ski-doo's "heavy" springs listed in the catalog which generally ride too stiff for most guys. It was tested and designed for a more agressive ditch banging style of riding where the stock setup is bottoming excessively. It still retains the smooth ride and still works very well with the stock shock valving or we offer our own valving package for individual needs. It also works really well for heavy riders (up to 300lbs.). Spring kits are offered for all 2015-2021 models with the T-motion suspension and a center shock diameter under 2" in the dropdown menu. Please call about our revalve package for more agressive riders that includes this spring kit.