2008 Yamaha YZ 250 & 450Fs. September 2007
YZ450F engine
The engine of the 2008 model maintains the outstanding characteristics of the existing five valve powerplant while achieving further improvements in power output and reliability. This has been achieved by improving combustion efficiency by optimising the shapes of the intake ports and cam profiles. In addition, the intake valve timing and overlap is changed to create a good balance of intake and exhaust. All of these changes help improve torque character in the low to mid speed range.
Micron level adjustments have been made to the external diameter of the piston to optimise the clearance between the piston and the cylinder. This reduces piston (lateral) movement to a minimum and thus reduces vibration and mechanical noise.
Adjustment of the backlash of primary gears (in the clutch assembly) and balancer gear further reduces mechanical noise by enabling better meshing of the gears.
The engine breathes via a larger diameter titanium exhaust pipe and compact muffler that saves weight and improves balance. Adjusting the shape of the exhaust pipe taper helps achieve exhaust efficiency. The tapered silencing element known as a "punched cone" is used for the first time ever on a production motocrosser. This has enabled around a 35% reduction in the length of the muffler (505 mm to 333 mm) and a 270g weight reduction.
Chassis
To further develop the lightweight feel that is a trademark of the YZ models, detail changes have been made throughout the chassis design. As a result, the new model has lighter handling and exhibits outstanding front end response during cornering plus improved riding comfort.
In order to achieve this, while also reducing weight, the steering assembly has been redesigned. The frame's head pipe is now 5mm shorter compared to the existing model. The steering stem is also 5mm shorter. The shape of the forged aluminium under bracket has been changed (to an "H" cross-section shape) to reduce weight and enable a further tuning of the rigidity balance.
The rigidity balance of the aluminium frame's pivot assembly is also changed. This 'lightens' the handling, or the ease with which the machine can be leaned into a turn. In the front assembly a lighter front fork protector (120g lighter than the existing part) is adopted. This combines with the lighter brake parts to achieve an excellent rigidity balance in the front end. In addition to a change in the shape of the outer tubes of the front fork, further improvements have been made in the damping characteristics and ride comfort on both YZ models.
In order to further improve balance, a lightweight titanium spring has been adopted in the rear suspension. The shaft and collar have been re-designed and peripheral parts lightened. At the same time the link ratio has been optimised to achieve excellent cushioning characteristics and bring out optimum traction performance.
A new lightweight type (less 220g) front brake caliper has been adopted. The diameter of the piston has been reduced from 27mm to 22.65mm and the shape and material used for the pads have been optimised. The diameter of the master cylinder piston has also been changed from the existing 11mm to 9.52mm (less 45g).
Furthermore, a lighter wave type disc is adopted with new elongated slotting holes to help reduce weight while improving braking force.
New lightweight parts have also been adopted for the front axle and wheel collar (less 47g). The combined effect of these changes creates an even lighter feeling while providing outstanding stopping power. The rear brake retains the existing spec.
To provide even better traction characteristics, a wide 120/80-19 (D756) size tyre is adopted for the rear wheel (compared to the existing model's 110/90-19/D756). The front tyre has also been changed from a D739FAL to a D742FA.
To increase riding comfort, the wide type stainless steel footrests proven popular on the WR450F have been adopted (previous footrests were titanium).
YZ250F
Running a higher 13.5:1 compression ratio, and offering lower reciprocating weight and reduced frictional losses, this 2008 model GP developed engine allows the high revs to run even harder and faster. And to handle the extra performance is an uprated clutch with an increased oil supply. New lightweight forks deliver quicker turning - and a revised linkage and new shock settings ensure superior handling and even quicker lap times.
Engine
As with the YZ450F, the engine of the 2008 model YZ250F brings additional refinements to engine character and reliability.
A piston with a new top surface design has been adopted that enables an increase in the compression ratio from 12.5:1 to 13.5:1. Changes have also been made to the carburettor setting and the ignition timing.
Meanwhile, adjustment of the backlash of primary gears (in the clutch assembly) and balancer gear reduces mechanical noise by enabling better meshing.
Chassis
The chassis has been lightened with changes made in the shape of the front fork outer tubes.
As per the YZ450F, the front fork performance and rear suspension link ratio have been optimised and a new titanium spring employed (these are shared features of YZ250F and YZ450F).
Other changes include a new brake pedal shape to reduce weight. The same YZ450F wide-type stainless steel footrests are used for greater comfort.
Bridgestone tyres are now fitted as standard, replacing the previous Dunlops.
For more details, see your Yamaha dealer, or visit www.yamaha-motor.com.au.
Junior's weapon
Dick Johnson Racing's Steve Johnson has made sure he's looking after his sponsors when he's not on the racetrack by plastering his Pro Yamaha-supplied Yamaha WR450F with custom Jim Beam signage. Johnson says he couldn't imagine a training regime that didn't include regular dirt bike sessions, attributing much of his physical race fitness to cross-training aboard his WR.
"As V8 Supercars grow, so does the level of professionalism and competitiveness. If you're going to be a serious challenger you need to adapt your fitness to suit the confines of a racecar and riding my Yamaha I find it is the perfect way to develop the required strengths," said Johnson.
