On August 17, 2004, at 2030 central daylight time, an amateur-built experimental Bowles Questair Venture, N39SB, sustained substantial damage when it nosed over after it veered off the left side of runway 17 (7,101 feet by 150 feet, asphalt), at the Liberal Municipal Airport (LBL), Liberal, Kansas. The pilot and passenger received minor injuries. The 14 CFR Part 91 personal flight departed LBL at 2010 on a local flight. Visual meteorological conditions prevailed. No flight plan was filed. The pilot reported that he returned to land at LBL after 20 minutes of flight and that the airplane operated normally during the flight, including the touchdown. He reported that when the rudder lost its effectiveness during landing rollout, the airplane started to drift to the left. He applied right brake but without effect. He reported the airplane exited the left side of the runway and the nose gear "dug in" and the airplane nosed over about 2,000 feet from the touchdown point. An inspection of the airplane revealed that the left and right brakes operated. The nose gear fork was found broken with numerous fracture surfaces. All the fracture surfaces were consistent with overstress fractures. The nosewheel tire exhibited scuffing on the right half of the tire around its entire circumference. Grass was found embedded in between the tire and wheel rim on the right side of the tire. The left side of the tire had no scuff marks. The inspection of the nose wheel steering linkage indicated the nose wheel linkage was positioned in the full left direction. The inspection of the accident site revealed three tires skid marks approximately 1,600 feet in length extended from the airplane wreckage back to the touchdown zone on the runway. The tire marks veered to the left after touchdown. When the tire marks neared the side of the runway, they turned back to the right, paralleling the runway for approximately 20 feet before veering back to the left and off the runway. The airplane was equipped with an electric/hydraulic nose wheel steering system. A toggle switch located in the cockpit turned the system on and off. The system is turned on during ground taxi. It is turned off for takeoff and landings. The system is turned on during landing roll when a safe taxi speed is obtained. The pilot reported that the nose wheel steering system was off during the landing. The airplane was equipped with the airplane's originally designed nose landing gear. The airplane was also equipped with the airplane's originally designed nose gear steering system's hydraulic actuator/shimmy dampner and steering linkage.