|Manufacturer||Yamaha Motor Company|
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.
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. Additionally, most four strokes produced very little power (the 1996 Honda XR400 made 32 bhp (24 kW), compared to the 40 bhp (30 kW) produced by most 250 two-strokes of the time).
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. Yamaha engineer Yoshiharu Nakayama first came up with the idea of creating the first competitive four-stroke race motocross bike. 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. The YZ400M was the predecessor of the production YZ400F, which was released the next year.
|Engine||400 cc single-cylinder, water-cooled, four-stroke, DOHC five-valve|
|Weight||250 lb (110 kg) (dry)
First Generation: YZ400F - 1998-1999
Yamaha introduced the YZ400F in production in 1998 after a successful AMA season in 1997. It was "the first modern production four-stroke motocrosser that was directly competitive against two-strokes."  Initially, Yamaha targeted a dry weight of 233 pounds (106 kg) (on par with the 250 two-strokes of the time), but by production, the bike weighed 250 pounds. The bike had an 11,600 rpm redline  power and torque close to its 250 cc two-stroke rivals. It benefited from engine compression braking, which allowed the engine to slow the bike down during deceleration, giving the brakes a rest.
In 1998, Doug Henry won the AMA National Motocross Championship aboard the YZF, becoming the first rider to win a championship on a four-stroke motorcycle. This victory is considered by some to be the major turning point in the motocross world—for the first time, four strokes were considered a competitive racing machine.
|Engine||426 cc single-cylinder, water-cooled, four-stroke, DOHC titanium five-valve|
|Weight||231 lbs (105kg) (dry)
|Fuel capacity||2.1 gallons (7.95 L)|
Second Generation: YZ426F - 2000-2003
In 2000, Yamaha updated the YZ400F, increasing the displacement to 426 cc for greater power and throttle response. In addition, the carburetor and jets were updated to ease the YZ400F's starting woes.
The next year, in 2001, Yamaha replaced the previous steel valves with titanium ones. The new valves were more than forty percent lighter than the previous valves, allowing for lighter and softer valve springs, in turn allowing a quicker revving engine, improved throttle response, higher rev ceilings, and more power. The crankshaft was also reshaped and the whole assembly was redesigned for quicker throttle response and, Yamaha claimed, "less high-end horsepower loss." In addition to motor modifications, a few changes were made to the transmission to help contain the power and ensure longevity. The suspension also received a bit of an overhaul with the goal being reduced weight and smoother action throughout the stroke. Yamaha also designed a new exhaust pipe design so that exhaust header removal would no longer be required when replacing the oil filter. Carburetor tuning was modified to deal with starting and off-idle take off difficulties.
Also in 2001 the subframe was changed from a steel to a blue painted aluminum style. In 2002 the blue painting was stopped and left to a bare aluminum look. Motorcycle.com says that "The gas tank is reasonably thin and allows good forward and backward movement while providing something nicely shaped to hold onto with your knees. In fact, the entire ergonomic package on this YZF is well thought out. The handlebars have a nice bend to them and are well-placed for good rider control and legroom. The footpegs are well-made units with a wide platform and sit in a position that keeps them from dragging in ruts without cramping a rider's legs."
In 2002 Yamaha remapped the digital CD ignition system which delivers a more precise spark and optimal timing for faster, stronger response during hard acceleration, and less kickback during starting. Also an all new swingarm which is lighter and stronger for reduced weight, greater rigidity and more compliant rear suspension action. While an anodized finish gives the bike a tricked out look. The 426 also includes a larger pivot shift for increased durability. A larger rear brake disc was also added which means greater stopping power.
|Engine||449 cc single-cylinder, water-cooled, four-stroke, DOHC titanium five-valve|
Third Generation: Yamaha YZ450F - 2003-2005
For 2003, the YZF received the biggest update in its history. First, the engine displacement was increased to 449 cc, producing over 52 horsepower, in compliance to the new AMA Motocross maximum displacement rule. Next, the frame and plastic were all updated for a new, sleeker look. Additionally, the YZ's weight was engineered from its original 250 pounds down to 233 pounds. The new YZF also had a gearing change, going from a traditional 5-speed to a 4-speed transmission. The bike made tremendous power; however, many thought that the bike had too much power for a motocross track.
Fourth Generation: Yamaha YZ450F 2006-2009
|This section requires expansion. (September 2009)|
The YZ450F received a major update in 2006, with over 300 parts changed and improved. The power was smoothed out, and the bike was made easier to ride, putting to rest the complaint that the YZ had too much power. The four-speed transmission was replaced with a five-speed. An all new aluminum single backbone frame shaved another 10 pounds off the weight.It also was the most advanced bike of its time
Fifth Generation: Yamaha YZ450F 2010
September 8, 2009, Yamaha introduced its all new YZ450F with fuel injection . The engine is now rear-ward mounted, the crank rod angle has been changed to provide a more complete combustion, the 5-valve head is now a 4-valve head, the gas tank is located under the seat, the exhaust pipe is now a tornado exhaust pipe that lies under the seat, and the air intake is now in the front of the bike. They have a new bi-lateral frame which allows for new improved steering and the new engine components. The entire package comes together to create a bike that is mass-centralized like no other, and allows the bike to out-turn any other in its class. The bike was endorsed/introduced by James Stewart, and the bike is currently being tested by magazines.
Yamaha yz450f 2011, Yamaha updated the 2011
New for 2011: New clutch lever and pushrod for more consistent clutch feel during long motos http://www.yamaha-motor.com/sport/products/modelfeatures/209/0/features.aspx Dimensions
New for 2012: Revised fuel injection settings (mapping for injection and advance) offer smooth, controllable engine response in the low-to-mid-rpm range. New, longer 94 dB muffler. ECU programming is updated to optimize performance characteristics with the new-spec muffler. New front and rear suspension settings and valving deliver exceptionally smooth suspension performance. Dunlop® Geomax® MX51 tires provide great all-round traction. New seat foam material has an excellent cushion feel. Both color options feature rich-looking black rims and a gold-color drive chain.
2011 YZ450F:1.63 gal
2010 YZ450F:1.6 gal
||This article has an unclear citation style. (September 2009)|
- Lewis, Jimmy (January 2002), "The Main Event: Honda CRF450R lines up against KTM 520 SX and Yamaha YZ426F", Cycle World 41 (1): 70 ff (8)
- Hoffman, Scott (February 2002), "Concept 2003 YZ450F imminent future", Dirt Rider (Primedia Enterprises): 28ff (3), "It was less than a year ago that innuendos regarding 'an all-new Yamaha YZ450F' circumnavigated the motocross world. The gossip was far less shocking compared to when Yamaha first unveiled its works YZ400M in '97 and its plans to build a competitive motocross four-stroke, although it still held merit. This was also mixed in with Honda's breaking news of its project four-stroke motocross bike, the CRF45OR. Nevertheless, Yamaha forged its name into the hooks five years ago and its revolutionary machine will forever be remembered as the catalyst that sparked the fire under the smoldering world of four-stroke motocrossers."
- "Titanium spring improves motocross motorcycles", Advanced Materials & Processes 164 (7), July 2006: 9, "For model year 2006, the Yamaha YZ125 and YZ250 two-stroke and the YZ250F and YZ450F four-stroke motocross motorcycles all carry titanium shock springs, marking the first time production motorcycles from any manufacturer have been fitted with titanium suspension springs as original equipment, reports Timet Automotive, Morgantown, Pa. Yamaha claims the new springs offer "outstanding strength and fatigue tolerance" and reduce unsprung weight for improved suspension performance."
- 2001 Yamaha YZ426F: MD Ride Review
- MX 2006 Rulebook Final.pdf
- YZ426F: Miracle Machine? – Motocross News, Race Reports, Videos, Photos, Interviews, How-Tos
- Motorcycle Hall of Fame: Doug Henry
- GT Thunder Test Bench
- "Year 2001 Yamaha YZ426F". Motorcycle.com. 2000-10-27. Retrieved 2011-12-27.
- Year 2001 Yamaha YZ426F
- "New Yamaha Dirt Bikes For 2002 : Off-Road.com". Dirtbike.off-road.com. 2005-11-01. Retrieved 2011-12-27.
- 2003 Yamaha YZ450F: MD First Ride
- First Impression: 2004 Yamaha YZ450F – Motocross News, Race Reports, Videos, Photos, Interviews, How-Tos
- 2006 Yamaha YZ450F - First Ride - MotorcycleUSA.com
- The Long History of Reverse-Cylinder Engine Designs - motocrossactionmag.com