The Wahoo KICKR 18 and KICKR CORE smart trainers come with a standard Shimano/SRAM-compatible freehub body. However, if you have a bike with a SRAM XD or XDR cassette, you will need a separate freehub body to use it with the KICKR. This XDR/XD freehub body allows you to install an XD or XDR cassette onto the KICKR, giving you more options for your drivetrain. The installation process is straightforward and can be done with basic tools. It is important to ensure that you have the correct freehub body for your specific trainer model to ensure proper compatibility.