The term 'stuffing it up the inside' might raise some eyebrows in polite company.

But MX riders know this is one of the biggest buzzes to be had on an MX track.

Because cornering is where races are won and lost.

For 2009, the YZs have been developed with a particular focus on quick cornering performance in tight curves.

YZs continue to dominate on all fronts with the 2007 AMA-MX championship closely followed by a win in the 2008 AMASX title. They also won the world MX championship in 2007 and the YZ450F is currently leading both the world and Australian series in 2008.

By dominating supercross with its consecutive tight turns and motocross with its large number of high-speed turns, the YZs have proved that their development direction is spot on.

Tight curve performance is an important development issue for motocross machines that requires the ultimate in rider-machine unity. In the YZ series we have worked continuously to reduce weight to a minimum, optimise rigidity balance and maintain traction to maximise balance during braking on bumpy surfaces, ensure lightness in leaning and give the rider maximum freedom in the choice of riding lines.

The capacity to turn all of the abundant power of the 4-stroke engine into forward drive with optimum traction and no waste is the key to increasing the speed that will be achieved on the straight waiting at the exit of the curve. And that means better lap times over the full course.

Riders who ride the YZ series models will find that after they come through a tight corner they can enter the straight with more speed than ever before and take the jumps with greater height than ever.

Advances in analytical simulation and measuring sensing technologies have increased the speed and accuracy of machine development. This means that a prototype already attaining a certain level of performance can be prepared before entering the test-riding stage. This makes a big contribution to the development process for the YZ series, which undergo further maturation each year.

At the same time, Yamaha has always placed special importance of the evaluations made by the test riders, and this is true for the YZ series models as well.

What the press said:

2009 YZ450F

"Our affection for the YZ450F was instant. After two turns we knew this was a better bike than before."
Scott Bishop, Dirt Action

"It's the easiest 450 I've ridden, it makes good power without tearing my arms off."
Ben Foster, Dirt Action

2009 YZ250F

"I'm amazed at the amount of power a little engine like this can generate. It is easy and almost effortless to ride and has changed my opinion about 250cc four-strokes."
Ben Jones, Dirt Action

"I love it, it really gels with me. I love the way you can rev the beejeezus out of the engine and it still makes power. It's a bike you can ride aggressively and still be rewarded for it."
Ben Foster, Dirt Action

2009 YZ250

"Everything about the YZ feels firm and agro... I love twostrokes!"
Ben Foster, Dirt Action

Main changes and features of the YZ450F

1) New lightweight swingarm
To help create more linear cornering performance through improved concentration of machine mass, a new-design rear arm is adopted.
The cross-section of the new rear arm is asymmetrical in its upper and lower portions and it achieved greater rigidity in the lateral direction compared to the existing unit. Also the cast parts of the pivot end have been redesigned for lighter weight. As a result, an additional 350 g has been removed from the rear arm compared to the existing model.

This combines with optimised suspension settings to bring cornering performance to a new level.

Since the 2002 models, the aluminium rear arms of the YZ series models have used parts manufactured with the hydro-forming method.

This is a method that was chosen to provide greater "forgiveness" in the longitudinal direction and stiffer rigidity in the lateral direction and in torsional force resistance. In this method, hydraulic pressure is applied to the inner surface of the tubular aluminium stock to shape it to the optimum form. It is a method that takes advantage of the inherent metallurgic structure of the aluminium to maximise strength while achieving a smooth, highly precise part shape at the same time. For production motorcycle parts, this method was first used for the 2002 YZ models and has since then its use has continued on the YZ450F/250F/250/125, and it is now used again for the 2009 models.

To accommodate this new rear arm, the bracket that holds the rear brake caliper and the seal guard are also newly designed.

2) New-design rear hub and large diameter (25 mm) rear axle adopted

In order to improve cornering performance by reducing un-sprung weight, a new-design rear hub that is 180g lighter than the existing unit is adopted. At the same time, a new large-type Φ25 mm rear axle has also been adopted to achieve a more rigid feeling when running.

This new rear hub is newly manufactured by a special high-vacuum die casting method. (The existing unit was a regular die-cast part) In this high-vacuum die casting method a vacuum is created in the interior of the die in order to draw the molten aluminium in under optimum conditions that reduce the occurrence of oxidised elements in the metallurgic structure, which increases the strength of the aluminium parts that are cast. This added strength makes it possible to design a thinner walled hub. For this new hub, the hub bearing spec has also been changed. The outer diameter of the bearing has been reduced and the number of bearings increased from two to three to decrease weight while maintaining the same level of reliability.

Plus this new rear hub contributes to more efficient use of space around the rear axle. The diameter of the rear axle has been increased from the 22 mm of the existing unit to 25 mm to contribute to improved rigidity balance. This translates directly into greater rigidity.

Also, in order to optimise the balance of the wheel and the abovementioned newly designed rear hub, the angles of the spokes with regard to the hub have been changed to achieve a suitable amount of "forgiveness" in the wheel which aids traction.

3) New front suspension settings

The settings of the front suspension have been changed with the aim of improving feel. In particular, the specification of the valve that creates the force has been optimised to provide a sure damping effect and a smooth, constant feel when entering a curve at high speeds.

4) New rear suspension settings

As with the front suspension, the settings of the rear suspension have been changed with the aim of improving operation by optimising the change rate of the link lever ratio and the valve that creates the cushioning force. This provides a sure damping effect when braking and a smooth feeling when entering a curve at high speeds. These changes in the front and rear suspensions combine with the effects of the lightened rear assembly to contribute to more linear controllability when cornering.

5) Adjustable bars (four positions)

Since the '06 models, an offset axis is adopted for the handlebar bracket on the YZ450F/250F. This made it possible to select between two different (forward and backward) handlebar positions by turning the bracket handle bar holder forward and backward.

For the '09 model, two attachment holes are provided on the handlebar crown (holder). This makes it possible to select between four different handlebar positions by combinations of the bracket handle bar holder side (forward and backward) and the crown side (forward and backward). This increases the forward and backward range of adjustment to enable settings that accommodate a wider range of rider preferences, course layouts and track surfaces.

6) New seat adopted

The base material of the new seat is a smooth nylon base material and composite vinyl chloride leather-look cover with optimized chemical makeup for just the right balance of hold and ease of sliding. Also the shape of the grain of the seat cover has been changed. The result is a seat that has both the ease of slip when the rider starts to shift his/her weight and the right amount of grip when it's necessary to hold the machine firmly. The shape of the seat surface has also been changed to reduce build-up of sand or mud in the crevices in the seams and leaves the seat cleaner after riding.

7) Gold DID chain adopted
For the drive chain a lightweight, compact and highly reliable DID (non-seal) 520DMA2 chain is adopted. For this model gold type with a high-shine plating finish has been chosen. Besides outstanding durability it also provides a quality look that adds to pride of ownership.

8) Aluminium front brake hose bracket

An aluminium bracket has been adopted for the front brake hose for further weight reduction.

9) Adjustable clutch lever adopted

A new clutch lever holder that includes an adjustment mechanism is adopted. With a simple bolt operation the rider can adjust the lever to fit his/her taste. Also, the lever itself can be short-levered for lighter operability.

10) Special Edition colour available

In order to further heighten the pride of ownership, a white version has been added as a second colour option. This white provides a sharp and refined image that adds value and includes a pair of trick black rims.

11) Euro spec muffler adopted

YZ450F now features full power low decibel muffler (Euro spec type) which together with power bomb pipe produces high levels of power while keeping within MA noise limits

Main changes and features of the YZ250F

Light clutch operation

The construction of the primary driven gear and housing for the clutch have been changed to improve operation, with the damper spring being replaced by rubber. Also the judder spring has been eliminated and other specs altered to lighten the feel.

All other features from 1 to 10 of YZ450F above are shared with YZ250F. Note that the revised suspension settings for YZ250F are specific to this model.

Meanwhile, the two-stroke YZs continue to be the machines of choice for junior racers and freestyle tricksters.

Take the YZ250 for example, the ultimate twostroke race bike and king of the jump ramps featuring a lightweight long-stroke engine that delivers an instant hit of freerevving power. And for 2009 the bike's high-tech aluminium chassis comes with new lightweight wheel rims that reduce unsprung weight for enhanced handling and suspension performance. New speedblock graphics give that factory look - and a special new seat design delivers just the right balance of grip and rider mobility.

With its punchy 2-stroke power, superb agility and low weight, the latest YZ250 is built to deliver a unique and exciting riding experience.

Main changes and features of the YZ125 and YZ250

Lightweight rims adopted front and rear (YZ250 only)

The same lightweight rims used on the YZ450F since the 2007 model are adopted front and rear. The combined weight reduction is about 150g
YZ two-strokes also share the following changes with their 4-stroke cousins:

• Aluminium front brake hose bracket
• Gold DID chain
• New seat

Note: 2009 YZ85/85LW and 125 will be released in Australia at the
beginning of September 2008.

For further information contact: