High Quality Content by WIKIPEDIA articles! The Yamaha YZ450F is a four-stroke racing motocross bike built by Yamaha Motor Corporation. It is credited by Cycle World and Dirt Rider magazines as the bike that started the four-stroke dirt bike revolution. It is a replacement for the YZ250, which is slowly being phased out. The 2006 YZ250F and YZ450F were the first production motorcycles equipped with titanium suspension springs.