Outside idler wheels help keep track in proper alignment They also help spread load evenly across the entire track Adds two idler wheels to the outside of the rail, for a total of 4 rear wheels Includes 2 wheels, 2 spacer blocks, mounting hardware, and instructions Recommended when adding studs or for aggressive riding. Fits Models: 2016-2018 ZR 137" 2014-2018 XF 137" 2012-2015 XF 141" 2013-2015 XF CrossTour 141" 2012-2019 XF High Country 141" & 153" 2016-2019 M 141" 2012-2019 M 153" & 162" (Excludes 2019 ZR & XF 137") (Excludes 2016-2018 XF CrossTour 146") (Excludes 2019 M Hardcore, rail brace interferes) (Excludes 2019 M Alpha One)