A bike adapter includes a tube composed of an inner tube and an outer tube which is slidably mounted to the inner tube. Two sleeves are respectively mounted to the inner and outer tubes. Each sleeve has an engaging member so as to release the inner/outer tube from the sleeve corresponding thereto. Two catchers are respectively connected to the inner and outer tubes. Each catcher has a catch end which is coated by a coat. A connection portion is formed to the catch end. Two insertions each have a connection end coated by another coat and has a slot. The insertion is securely inserted through the sleeve. The connection portion of the catcher is detachably engaged with the slot in the connection end of the insertion when moving the sleeves along the tube to form an opening between each catcher and each insertion.