YZ 125
ENGINE

To improve performance and power delivery at the same time, the 1.7 kg lighter new power plant is implementing many new features. This new engine is extremely compact and is not much bigger than a last generation 80cc engine.

What the rider will realise most obviously is the new six-speed-gearbox. As you may recall, during the last 2 years the YZ 125 was equipped with a 5-speed-gearbox, which proved more efficient at the time. However, to transmit the increased power most efficiently on the track a closer gear ratio was needed. The higher frequency of shift actions is now clearly compensated by the new engines performance, and lap times are likely to improve. In particular the acceleration out of corners gives the new YZ125 its leading edge.

YZ 125 New piston

The new piston has a flat head to adopt to the redesigned cylinder head. The inlet window has been eliminated and the side wall design has been changed. This resulted in a 4 gram lighter weight. Although just a few gram seems very little, at a higher revolution this few gram make a big difference in load on bearings and in engine response. As a result the weight savings and the changed shape of this new piston helped to increase performance.

YZ 125 Further tilted cylinder

The new 125 cc engine is a most compact design. In order to increase performance, the expansion chamber needed to be straightened (see red arrow). Therefore the cylinder had to be tilted forward by an additional 7.5 degrees (arrow B). Due to a more compact layout of crankshaft and gearbox assembly, the engineers could still manage to reduce engine length (arrow A) from previously 195.9 mm to 188.7 mm.

YZ 125 Port timing

Port timing and port shapes are modified for higher engine performance. The total height of the cylinder body itself is reduced for the same reasons by about 6 mm. The width of the body has also been reduced by about 6 mm too.

YZ 125 New crankshaft

A lot of detail elements have been modified on the crankshaft. The oilseal diameter for example has been reduced from 30 mm to 22 mm. This results in lower friction of the oilseal. Together with the cylinder and piston modifications the connecting rod was shortened by 3 mm in order to increase primary compression. Other dimensions like shaft diameters, big end bearing and pin etc.- have also been reduced in size. This all contributes to engine performance and throttle pick up.

YZ 125 Six speed transmission

To optimize transmission ratios, a six speed transmission was applied instead of the five speed before. The closer ratio in combination with the increased performance allows to accelerate faster out of corners. The additional shift action required will be offset with the homogeneous new package of gear ratios and available power band.

YZ 125 Straight exhaust port

The engineers set a high priority at gas flow management of this new 2stroke. A consequent example is shown above: Intake manifold (1) has been straightened to allow the gas to speed up without hurdles. Furthermore the cylinder port was straightened (2) as well as the exhaust outlet pipe (3). Naturally this increases power output and engine running smoothness.

YZ 125
CHASSIS

For better handling and lighter weight (app. 2kg lighter), Yamaha uses an aluminium frame for the first time on a Yamaha production Motocross bike.

Unlike for road racing and supersports machines, where the use of Yamaha Aluminium Deltabox construction has been state of the art since decades already, the YZ range has never utilized Aluminium material for good reasons: in spite of steel being heavier, it is more resistant to bending. The harsh shocks encountered during Motocross riding, in particular when landing after jumps, impose a very high stress on the frame. Since the steel construction however has now reached its lower weight limits, the use of a new aluminium chassis was the next logic step.

In order to cope with the stress on the frame as mentioned above, Yamaha designed a completely new frame thanks to the knowledge gained in recent years from some of the factory racing machines. This new aluminium frame has similar flexibility like its steel counterpart. This was possible due to the shape and material thickness on certain parts of the frame. And unlike a Deltabox chassis, with its characteristic thick and rigid profiles surrounding the engine, the YZ chassis looks pretty much similar to what we are used from the previous models.

The new chassis required new suspension: front fork is new and rear arm and shock absorber also.

Frequent visitors of the Design Café know how important riders' voices are for Yamaha's product development and our product planners and engineers could positively answer the requests of customers and dealers. Additional desired improvements on the technical side focused on: front fork, braking, rear suspension and handle bar. All four points were taken into account during the developments of the new bikes. And of course there are a fair amount of other detail changes, which will not be mentioend on this site.

YZ 125 Aluminium handlebar

The new aluminium handlebar is not only lighter it looks also great. It was requested by many customers in recent years.

YZ 125 Aluminium frame

The shape of the new aluminum frame is similar to that of the 2004 steel frame. However it is 1,8 kg lighter and has a similar flexibility than the steel frame. The biggest difference is around the headpipe area, where the aluminum frame is shaped differently and features other wall thickness. The rear sub frame is of course detachable like before.

YZ 125 Rear shock absorber

The rear suspension unit saw a small but efficient improvement: to increase comfort and a smoother absorption of shocks a full rebound oil lock system (red circle) is used.

YZ 125 Front fork

The new front fork was developed in line with the characteristics of the new aluminum frame. The difference between 2004 and 2005 fork are significant, as shown in the illustration.

• Transfer control valve adopted for improved middle-to-full stroke performance.

• Separated the oil chamber to improve air –tightness for stable damping.

• New front fork oil is used.
  

YZ 125 Modified swingarm linkage

The illustration shows the rear swingarm linkage. Changes are as follows:

• Redesigned the separate flanged collar to one-piece straight collar for increased rigidity.

• Collar thickness increased from 2mm to 2.5mm.

• Redesigned the bearing to accommodate to the new-design collar.
  

YZ 125 Swing arm 

In order to suit the characteristics of the new aluminium frame the rear swingarm needed increased rigidity. At the same time its weight could be reduced by half a kilogram.
Following parameters have been changed:

• Arm diameter increased from 37mm to 42mm.

• Reduced cast metal portion (grey colouered area) for reduced weight.

• Modified strength balance for better riding comfort and feeling 
  
   

ENGINE

The proven 250 cc power plant has seen some minor changes for 2005:

• A newly designed exhaust pipe
• A newly designed chain cover

Both to accommodate the new chassis.

CHASSIS
The basic layout being similar to YZ125, however to accommodate the bigger 250 cc engine certain parts of the frame needed to be changed. Suspension units are also same basic construction, of course set-up again is different to cope with the increased 250 cc performance and weight.

Photo Gallery 2005-YZ125

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Photo Gallery 2005-YZ250

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