Back shock towers are redesigned to allow for the Horizontal Split Bulkheads (HS), and have denser upper shock mount roles to get more accurate set-up changes. A corner surprise tower features additional roll center jobs for even more fine set-up modification also additional wing mounting roles. Back shock towers are machined from premium-grade 3.5 mm graphite and they are lightweight & strong, with numerous surprise jobs and roll center adjustment roles Fits XB4'21