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The Yamaha YZ450F is a four-stroke racing motocross bike built by Yamaha Motor Corporation. It was the successor to the previous YZ426F which was discontinued in 2003. 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. The WR450F is the enduro version of the YZ450F. ==Introduction== For many years, the motocross world almost exclusively used two-stroke engines. AMA racing classes had two classes: 125 cc and 250 cc two-strokes, with no provision for four strokes. Most riders considered four-stroke engine technology to be largely utilitarian and uncompetitive, relegated solely for trail riding.〔(2001 Yamaha YZ426F: MD Ride Review )〕 In 1996, the AMA changed racing rules to allow 450 cc four strokes to compete in the same racing class as 250 cc two-strokes.〔(MX 2006 Rulebook Final.pdf )〕 Yamaha engineer Yoshiharu Nakayama first came up with the idea of creating the first competitive four-stroke race motocross bike.〔(YZ426F: Miracle Machine? – Motocross News, Race Reports, Videos, Photos, Interviews, How-Tos )〕 The Yamaha YZ400F was developed to fit into this category. It solved the power dilemma by borrowing superbike technology and giving the YZ a five-valve head, liquid cooling and a 12.5-1 compression ratio. In 1997, Yamaha rocked the motocross world with the introduction of the YZ400M prototype, a concept motorcycle which borrowed much technology from road racing. The YZM was far ahead of all competition among four-stroke motocross bikes. Doug Henry piloted the YZ400M to its first victory in 1997 at the Las Vegas Supercross. This was the first time any four-stroke had won an AMA event.〔(Motorcycle Hall of Fame: Doug Henry )〕 The YZ400M was the predecessor of the production YZ400F, which was released the next year. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Yamaha YZ450F」の詳細全文を読む スポンサード リンク
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