We used a long cable lock that went through the frames/wheels of the bikes and also around the car's bumper. One time we left the car and bikes parked at a museum in Chicago and a potential thief apparently didn't notice the cable. When we got back the bungie cords securing the bikes to the rack had all been cut and the bikes had been pulled partially off the rack until the cable stopped them. Fortunately the thief didn't have tools to cut through the cable.