Speedfil Z4/Z4+ Between-The-Arms (BTA) Water Bottle Cage with Computer Mount. Speedfil Z4 Cage is the first cage optimized for BTA (Between-The-Arms) aero hydration The Z4 Cage works with standard water bottles, as well as the Speedfil A2 to offer several unique and distinct advantages Versatile: The Z4 is lightweight, easily configurable and can be used on any front, frame and rear saddle mounts. Direct Aero Mount: The Speedfil Z4 Cage is designed with numerous ports for a direct BTA mount into the aerobar extensions with a minimum of 4 nylon zip/cable ties (included). No additional mount or hardware is required.