This is where the bees live, the heart of the Chris King RingDrive™. These parts are compatible with Boost, ISO, Superboost, Classic, Tandem, Single Speed, Uni-Disc, and BMX hubs. PHB525 - 72T Drive and Driven Ring Set PHB977 - 36T Drive and Driven Ring Set The 72T Drive and Driven Ring Set is the standard in all our MTB hubs. It's designed for immediate engagement (read: traction, agility, and power), and is best for XC, trail, and really any pedal-intensive riding. The 36T Drive and Driven Ring Set is gravity-focused. While still providing reasonable engagement, it's really focused on minimizing drag and noise (read: downhill speed). This set is best for downhill racers and shuttle-junkies.