Yamaha Motor Australia has released the latest incarnations of
the four-stroke motocross bikes that invented a category, the race
winning YZFs. The 2008 models come with significant improvements
aimed at keeping the class leaders at the top of the podium, as
demonstrated by YZ450F World MX Series leader Josh Coppins and
YZ250F World MX Series leader Antonio Cairoli. Both models come in
two guises, standard Yamaha Racing Blue/White and Special Edition
White/Red.
YZ450F, the undisputed MX1 class leader
With Josh Coppins leading the '07 MX1 series - and the bike's
unrivalled track record from '06 that includes winning 27 out of 30
heats - the YZ450F is officially today's most successful MX1
machine.
For 2008 season the undisputed class leader's engine gets a
number of significant improvements - including reshaped intake
ports and cam profiles, reduced piston tolerance and modified
transmission backlash and shorty muffler. And the YZ450F's
lightweight aluminium chassis runs with a shorter headpipe, lighter
forks, compact new front caliper, and modified rear suspension -
which makes the 2008 bike accelerate better, turn quicker and feel
lighter, for an even more dominant package.
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 character and
reliability. This has been done 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 new larger-diameter titanium exhaust
pipe and super compact muffler that saves weight and improves
balance. Adjusting the shape of the exhaust pipe taper helps
achieve excellent 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.
YZ450F chassis
To further develop the lightweight feel that is the trademark of
the YZ models, detail changes have been made throughout the chassis
design. As a result, the new model is even lighter handling and
exhibits outstanding front end response during cornering plus
greater riding comfort.
In order to achieve excellent rigidity while also reducing
weight, the steering assembly has been re-designed. The frame's
head pipe is now 5mm shorter compared to the existing model, and to
accommodate this, 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 to improve overall balance. 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 the comfort of the ride on both YZF
models.
And in order to achieve a further reduction of front assembly
weight and good balance, a lightweight titanium spring has been
adopted in the rear suspension. The shaft and collar have been
re-designed and the 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.
And to increase riding comfort, the wide type stainless steel
footrests proven popular on the WR450F have been adopted (the
existing footrests were titanium).
YZ250F, racing further ahead in MX2
Tony Cairoli's phenomenal
record in the 2007 MX2 series underlines the YZ250F's awesome
unbeatable potential.
Running a higher 13.5:1 compression ratio, and offering lower
reciprocating weight and reduced frictional losses, this 2008 model
GPdeveloped engine allows the highrevs to run even harder and
faster. And to handle the extra performance we've fitted 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.
YZ250F engine
As with the YZ450F, the engine of the 2008 model YZ250F
maintains the outstanding characteristics of the existing model
with 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.
This further brings out the inherent potential of the five-valve
combustion chamber design for excellent power development
characteristics due to faster combustion. Also, in order to
accommodate the higher compression ratio, changes have been made to
the carburettor setting and the ignition timing has been optimised
(and new CDI unit adopted).
Meanwhile, adjustment of the backlash of primary gears (in the
clutch assembly) and balancer gear reduces mechanical noise by
enabling better meshing.
The shapes of the boss clutch and the primary assembly have been
changed to promote better oil supply to the friction plate(s) and
clutch plate(s) and contribute to better competitiveness at starts
and coming out of turns.
YZ250F chassis
Weight reduction measures have been taken in order to lighten
the handling of the controlled flex chassis. Changes have been made
in the shape of the front fork outer tubes, a lightweight front
fork protector is adopted (less 120 g compared to existing part)
and a new shaped (H cross-section) forged aluminium under-bracket
is adopted.
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 and the
same YZ450F wide type stainless steel footrests are used for
greater comfort (width increased compared to existing part from 45
mm to 55 mm on both sides).
To match these chassis changes, Bridgestone tyres are fitted
(front: M403, rear: M404) where previous models have used
Dunlops.
Download the attached PDF document for YZ250F and YZ450F Technical Specfications.
For further information contact:
Sean Goldhawk, advertising and PR co-ordinator
Yamaha Motor Australia,
489-493 Victoria Street, Wetherill Park, NSW 2164
Office: (02) 9757 0068
Mob: 0438 423844
Fax: (02) 9757 1384
Website: www.yamaha-motor.com.au