2000_YZ426F_Press Release

2000 model Yamaha YZ426F Motocrosser
Introduction
When Yamaha launched the original YZ400F 4-stroke motocross machine it astounded
the entire dirt bike world. Until then the typical 4-stroke motocrosser was a large capacity
heavyweight ‘thumper’ that appealed to a minority of riders.
The YZ400F changed all that overnight!
Looking, sounding and performing like no other 4-stroke dirt bike, the ground-breaking
YZ400F was constructed using a free-revving, highly compact engine housed in a light
and agile 250-class chassis.
This winning formula has proved itself to be a great success at all levels in the last few
seasons. In the 500cc Motocross World Championship series, Yamaha’s factory bikes
have stormed to many GP wins since their debut race in early 1997 – while Doug Henry’s
historic win on his YZ400F in Las Vegas two seasons ago was the first time that a 4stroke had won a US Supercross round.
In 1998, YZ400F-mounted Henry went on to win the AMA 250 Motocross title, and after
5 rounds of the ‘99 500cc Motocross World Championship season, Yamaha YZ400F
factory riders Andrea Bartolini and Alessandro Puzar are placed 1st and 4th overall
respectively. Currently there are four Yamaha riders in the top 10 of the World 500cc
Championship, with privateers Francisco Garcia Vico lying 8th, and Miska Aaltonen
10th – an impressive statistic that underlines the great impact that the YZ400F is having
at GP level.
Not only has the Yamaha YZ 4-stroke proved that it can beat the best 4-strokes in the
world – its US Supercross and AMA 250 Championship victories demonstrate beyond
any doubt that in the right hands the YZ400F can beat the world’s best 250 2-strokes!
In just two years the Yamaha YZ400F has made a massive impact on the world of
motocross from privateer through to World Championship level. Now, for the 2000
season, Yamaha are to launch the YZ426F, a second generation 4-stroke motocross
machine that features a larger capacity powerplant as well as an extensive range of engine
and chassis upgrades developed directly from our GP-winning factory bikes.
Yamaha YZ426F Engine Profile
Bigger bore, larger capacity
For the new race season the YZF 4-stroke gets a highly modified bigger-bore engine
which, as the name illustrates, now displaces 426cc. This has been achieved by increasing
the bore from 92mm diameter to 95mm diameter, while the short 60.1mm stroke remains
unchanged. The net result is an engine that delivers more low to mid range power, while
at the same time retaining its free-revving character that has made this one of the most
competitive machines – 4-stroke or 2-stroke – in today’s MX scene.
New internal components
Also new for 2000 is a 19mm diameter piston pin, 1mm larger than the ‘99 design, as
well as a modified connecting rod running with a beefed-up big end bearing whose width
is increased by 1mm to 16.8mm. And for reduced vibration and enhanced engine
response the crankshaft design is modified and now features a reshaped web.
Redesigned Keihin FCR carburettor
Another important change for next season is the fitment of a new Keihin FCR carburettor
which is designed to offer even better throttle response in a range of conditions. This new
carburettor now features an integral red-coloured hot start knob situated next to the blackcoloured cold start knob, unlike the ‘99 model whose hot start knob was mounted
remotely. Another change on the intake for 2000 is the use of a lighter resin joint between
the carburettor and air box which replaces the previous aluminium component.
New heavy-duty clutch
To handle the new YZ426F’s increased low to mid range torque output the machine is
equipped with an uprated clutch. Friction plate diameter is increased from 148mm to
155mm for the 2000 model, and the number of clutch plates goes up from 7 to 8. To
accommodate the new heavy-duty clutch the bike’s right-side engine case is redesigned
and now features a 157mm diameter cover.
Revised 1st and 2nd gear ratios
Significant changes have also been made to the YZF’s compact transmission, and the
latest model now runs with revised 1st and 2nd gear ratios for improved acceleration
performance. Yamaha’s engineers have also enhanced the shift feel and speed by the use
of additional chamfering on various gears.
Enhanced cooling efficiency
Special attention has been paid to the cooling system on the new big-bore engine, and the
YZ426F’s increased power output will be matched by the fitment of new larger-capacity
radiators. Similar to the system used on the WR400F enduro machine, the new radiators
are 10mm wider at 121.8mm, and incorporate 11 core tubes each – 1 more than the 1999
model. Supplied in bare, unpainted aluminium, they improve the cooling efficiency of the
new YZF for consistent performance throughout the longest race.
Free-flowing silencer
The final engine modification is the use of a new free-flowing silencer which now
employs a larger-diameter 50.8mm pipe feeding the muffler – a design that enables the
new larger-capacity engine to achieve higher levels of performance across the rpm range.
Modified CDI and smaller CDI rotor
The new YZ426F is also equipped with a modified CDI designed to offer enhanced
throttle response, and the diameter of the CDI rotor comes down from 98mm to only
83mm for reduced weight and instantaneous pick-up.
YZ426F Engine
Technical Highlights
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Increased capacity of 426cc
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Bigger bore of 95mm (1999=92mm)
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Uprated connecting rod, piston pin and big end
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New low-vibration crankshaft
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New FCR carburettor with integral hot start button
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New resin joint between carburettor/air filter box
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Larger 155mm diameter clutch with 8 clutch plates
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New right side engine case
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Revised 1st and 2nd gears
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Smoother shifting transmission
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New larger capacity radiators
•
•
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Free-flowing big-bore silencer
New CDI
Smaller diameter CDI rotor
Yamaha YZ426F Chassis Profile
When the YZ400F production machine was first launched for the 1998 season its 250class chassis made it the most compact production 4-stroke motocrosser ever
manufactured. For the 2000 season our designers have not only produced a higher-torque
big-bore powerplant – they have also succeeded in creating an even more agile handling
package whose total engine and chassis performance has the potential to defeat any
existing motocrosser.
New high-tensile steel frame
YZ426F’s new main frame is manufactured from stronger HT-780 steel tubing which
utilises new-technology brazing in its construction to create a lighter and more rigid
assembly that ensures precision handling performance. In addition to the use of higher
tensile steel tubing the 2000 frame features a relocated head pipe which is moved back to
give a compact 1490mm wheelbase, 5mm less than in ‘99. And for further weight savings
the new chassis is equipped with a fully detachable aluminium subframe.
Uprated front forks
Yamaha’s suspension has always been regarded as being one of the best original
equipment systems in today’s MX scene, and to ensure that things stay that way we have
made a number of important modifications to the design of the upside down front forks
for 2000.
For even more responsive characteristics the new YZ426F runs with lightweight
aluminium piston rods instead of the steel components featured last season. And for
smoother suspension action – especially over less severe terrain – new anti-stiction dust
seals are fitted and the internal faces of the outer tubes have a new low friction treatment.
All-new triple clamps
Both the triple clamps have also come under close scrutiny by our engineering team
during the last season. To increase front end rigidity for precision steering the thickness
of the lower clamp where it grips the fork tube has been increased by 5mm to 45mm deep
– and the underside of the lower triple clamp now features an extra strengthening rib.
Also modified is the upper triple clamp, where the handlebar mounting brackets have
been shifted forwards by 10mm so that the centreline of the ‘bars now sits 5mm in front
of the steering pipe’s centre – a move designed to allow the rider to get more weight over
the front end in tight corners.
With the exception of spring rates and settings the 2000 YZ426F forks are the same as
those fitted to the latest YZ250 and YZ125.
Lightweight rear suspension
Like the YZ426F’s new front forks the latest model’s rear shock is also featured on the
2000 model YZ250 and YZ125.
As with the front forks we have looked at ways of saving weight, and in the case of the
rear shock this has been achieved by replacing the ‘99 model’s steel damper rod bearing
with an aluminium component. And for easier suspension tuning a new-style and
redesigned compression damping adjuster is fitted.
Improved controls
As well as the major changes seen to the YZF’s engine, frame and suspension for 2000
the latest machine has undergone numerous detail changes which underline Yamaha’s
total commitment to offering one of the best overall packages to today’s motocross rider.
For reduced fatigue we have redesigned both the clutch lever and bracket. The new
lever’s dogleg bend is modified so that the rider can use all four fingers to operate the
clutch, and the effort required to pull the clutch lever is also reduced. A new-design
mechanism now allows quick and easy one-touch cable adjustment without using any
tools – and a new stainless steel inner clutch cable is stronger and smoother.
Changes have also been made to the rear brake master cylinder aimed at improving
overall feel. This has been done by reducing the bore of the cylinder from 14mm to
12.7mm, a move which should help prevent accidental wheel-locking and stalling.
Other detail changes
Final detail changes for the new YZ426F include a new-shape front mudguard, as well as
modified radiator air scoops and louvres which offer enhanced cooling efficiency.
YZ426F Chassis
Technical Highlights
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New stiffer frame manufactured from HT-780 tubing
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Head pipe moved 5mm back
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Shorter 1490mm wheelbase (1999=1495mm)
•
New detachable aluminium subframe
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Uprated low-friction front forks
•
New stronger lower triple clamp
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Modified upper triple clamp
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Lighter rear shock absorber
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Redesigned clutch lever with one-touch adjuster
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New specification rear master cylinder
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New front mudguard
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Redesigned radiator air scoops and louvres
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New graphics
YZ426F Summary
Since its explosive debut in the GP scene in 1997 followed by the launch of the
production model in 1998 the Yamaha YZ400F has turned the dirt bike world upside
down. Single-handedly this light, compact and free-revving motocrosser has totally
transformed the image of the 4-stroke from that of a marginal specialist machine to that
of a mainstream dirt bike with the potential to beat anything from the latest 250 2-strokes
through to large-capacity 4-strokes.
With its new bigger-bore engine the new YZ426F delivers higher levels of low to mid
range power for explosive acceleration of the line and out of tight corners – while the
latest lightweight shorter-wheelbase chassis and uprated front and rear suspension
systems make for improved handling performance.
Without a doubt the 2000 model is even more competitive than ever, and backed up by
the excellent reputation established over the past two seasons by its ground-breaking
predecessor, the new YZ426F is ready to maintain and even extend Yamaha’s lead in the
rapidly expanding 4-stroke dirt bike market.