The Yamaha WR450F is an off-road motorcycle made by Yamaha Motor Company. It currently has a 450 cc (27 cu in) liquid-cooled single-cylinder engine. First offered in 1998 at 400cc, it shared many components and design concepts with the YZ400F motocross model. It is basically the racing YZ450F detuned slightly for more controllable power, with a headlight and lighting coil, softer suspension, a kickstand, lower noise specifications, larger radiators and lower emissions. The WR in the name indicates a wide-ratio gear box common to most enduro or trail bikes and stands in contrast to the close-ratio gearbox essential to a motocross racer. Over the years the WR has benefited from the advances made in the YZ motocross version gaining displacement and advancements such as an aluminum frame and improved suspension. Over much of its life the weight of the WR450F has remained fairly constant ranging from 244 to 249 pounds dry weight.
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First generation: WR400F - 1998-2000
Yamaha introduced the WR 400F in 1998. The 400F is a four-stroke off-road motorcycle produced for three years, beginning in 1998 and ending in 2000 (only the YZ was upgraded to 426 cc in 2000). The WR400F is related to the YZ400F, a motocross model. Like the YZ400, it was considered to be a groundbreaking model in motorcycle history, ushering in the four-stroke era which ended the dominance of two-stroke engines in motocross and offroad racing. While many modern performance four-stroke dirt bikes have been criticized for excessive rebuild costs and short motor lifespans, the Yamaha WR400F has an impressive record of reliability, often attributed to its steel valves and generous oil capacity.
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Second generation: WR426F - 2001-2002
In 2001 the WR400F's engine was uprated and it became the WR426F increasing the displacement to 426 cc (26.0 cu in) for greater power and throttle response. A beefier YZ style clutch basket and plates than those on the WR400 improved clutch performance. Frame geometry was identical to the 2002 YZ250 and YZ426F with a 58.7 inch wheelbase and 14.7 inches of ground clearance. This was the last WR to make use of a manual compression release for starting.
Third generation: Yamaha WR450F - 2003-2006
For 2003, the engine displacement was increased to 449 cc, the fuel tank was reduced to 2.6 gallons and the bike came with an electric starter as standard equipment. The 2005 WR weighed in at 244 pounds dry, the seat height was reduced to 38 inches and the fuel tank was decreased to 2.1 gallons. The 2006 WR weighed in at 249 pounds dry and produced 42 horsepower.
A limited production version of the WR450F was the WR450F 2-Trac, which featured 2-wheel drive. The gearbox output sprocket drove a short, fully enclosed chain to a pump, which in turn pushed the oil to the front hub via the pipe and back via the parallel pipe. A maximum of 15% of the power was transmitted to the front wheel at any time. Yamaha planned on building 250 2-Tracs in 2004, but only a few were built, which were used for enduro racing.
Fourth generation: Yamaha WR450F - 2007-2011
For 2007, the WR gained restyled plastics and an aluminum frame dropping the weight from 249 to 246 pounds. In addition to the aluminum frame, the 2007 WR450F saw a revised dry-sump engine, with a new balancer, cylinder head, and camshafts with less lift & duration than previous generations. 2009 saw new graphics, and 2010 was not sold in the USA.
Fifth generation: Yamaha WR450F - 2012-2015
For 2012, the WR450F was re-designed with fuel-injection and a compact YZ250F-based chassis to provide a lighter feel and more manueverablity.
Sixth generation: Yamaha WR450F - since 2016
For 2016, the WR was given the reverse slant, 4-valve, fuel injected motor previously available in the YZ450F since 2010. Yamaha also introduced a new model, the YZ450FX, a more Enduro competition oriented version of the off-road 450 utilizing more design elements of the motocross model such as stiffer suspension and a close ratio gear box while maintaining electric start and the 18" rear wheel.
https://www.yamahamotorsports.com/off-road/models/2016/wr450f
Source of the article : Wikipedia
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