Yamaha YZ450F

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The YZ450F is a four stroke big bore racing motocross bike built by Yamaha. It is considered by most to be the starter of the four stroke dirt bike revolution. It is a replacement for the YZ250, which is slowly being phased out.

The WR450F is the enduro version of the YZ450F.

Contents 1 Introduction 2 First Generation: YZ400F - 1998-2000 3 Second Generation: YZ426F - 2000-2002 4 Third Generation: YZ450F - 2003-present 4.1 2006 update 5 References

[edit] 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 antiquated and uncompetitive. ^[1] 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. ^[2] Yamaha engineer Yoshiharu Nakayama first came up with the idea of creating the first competitive four stroke race motocross bike. ^[3] 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. ^[4] The YZ400M was the predecessor of the production YZ400F, which was released the next year.

First Generation
Manufacturer	Yamaha Motor Corp.
Production	1998-2000
Class	Motocross
Engine	400 cc single-cylinder, water-cooled, four-stroke, DOHC five valve
Transmission	5-speed
Weight	250 lb (110 kg)

[edit] First Generation: YZ400F - 1998-2000

Yamaha Motor Corporation introduced the YZ400F in 1998. It was "the first modern production four-stroke motocrosser that was directly competitive against two-strokes." ^[5] 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 ^[6] and more power and torque than its 250 cc two stroke rivals. ^[7] It benefited from engine compression braking, which allowed the engine to slow the bike down during deceleration, giving the brakes a rest. ^[8]

Though the YZ 400F had a wider powerband than its two stroke counterparts, the bike had some problems. It would stall far more easily than two strokes, and had a very difficult time starting. ^[9]

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 turing point in the motocross world--for the first time, four strokes were considered a competitive racing machine. ^[10]

Second Generation
Production	2000-2003
Class	Motocross
Engine	426 cc single-cylinder, water-cooled, four-stroke, DOHC titanium five valve
Transmission	5-speed

[edit] Second Generation: YZ426F - 2000-2002

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 intake valves with titanium ones. The new titanium valves are forty percent lighter than the previous steel valves. ^[11] This simple material update allowed the engine to rev faster and higher. 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. ^[12]

Third Generation
Production	2003-Present
Class	Motocross
Engine	450 cc single-cylinder, water-cooled, four-stroke, DOHC titanium five valve
Transmission	4-speed

[edit] Third Generation: YZ450F - 2003-present

For 2003, the YZF received the biggest update in its history. First, the engine displacement was increased to 449 cc, 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 new YZ was much easier to start as well. ^[13] The bike made tremendous power; however, many thought that the bike had too much power for a motocross track. ^[14]

[edit] 2006 update

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. ^[15] The much-hated four speed transmission was replaced with a five speed. An all new aluminum single backbone frame shaved another 10 pounds off the weight.

[edit] References