Abstract: The knapsack has a bottom wall resting against a user's back when supported by shoulder straps. The bottom wall is larger than a bicycle luggage carrier or rack. The knapsack has a frame structure for reinforcing the bottom wall and the side wall of the knapsack so that when the knapsack is placed with the bottom wall horizontally on top of the carrier, loose articles in the knapsack do not cause the bottom wall of the knapsack to sag over the sides of the carrier. The shoulder straps are attached to the bottom wall or sandwiched between the bottom wall and the carrier to prevent the shoulder straps from dangling down and possibly interfering with movement of the bicycle wheels. The knapsack has fastening straps for securing the knapsack to the luggage carrier. In one embodiment, the frame structure is a removable insert for a conventional knapsack.