Bontrager Versatile race saddle designed for the needs of Posture 2 riders with the ability to support Posture 3 Contour Relief Zone Plus - Cutaway design eliminates contact points on targeted soft tissue areas Oversize Carbon Rails - For the ultimate in lightweight strength, requires Oversize rail-compatible seatpost clamp. Bontrager seatposts use 421564 or 436658 Carbon Reinforced Shell - Compliant carbon fiber-reinforced shell strikes a balance between strength and light weight 148MM/155G