XLAB Aero Pouch 300 Rear Storage Bag for Delta/Sonic, Black Fits around the bottle cage on all metal XLAB single rear systems Velcro zipper pull stays down to avoid zipper flapping drag, XLAB focuses on the fine aerodynamic details Simple install by using the 2 bottle cage holes Aero pouch 3 actually helps the drag behind you by collecting the air into a smooth airflow