The Reserve Rack II provides a simple, aerodynamic solution to carrying water & inflation devices. It mounts behind the saddle for enhanced aerodynamics, & if used only for water, it can hold up to two bottles with the included Rib Cage II bottle cages. & if used with our Sitero Wing, its capable of holding one bottle & a CO2 inflation device. Say goodbye to sacrificing aero performance for the sake of carrying your essentials.