A decade ago, the motorcycle off-road community was dominated by 2-stroke machines. To win competition, both in motocross and in enduro, the powerful and lightweight 2 strokes were superior to the heavier and bigger 4-strokes.
The 4-strokes were mainly used as hobby off-road bikes, and also for rallyes where they were still competitive.
But the future of the 2-strokes was under pressure. Environmental concerns in US and Europe combined with noise restrictions made it more difficult to maintain big capacity 2 strokes.

A few engineers in Yamaha took up the challenge and designed a 4-stroke machine combining competitive performance and low weight. The result was the YZ400F and the hard-enduro WR400F. It was a revolution: with the size of a 250cc, the power band of a 500 cc and a very controllable traction, the bike changed the world of off-road racing. Today, the off-road riders have turned largely towards these high power 4-stroke machines. They have proved to be easier to ride and they are less polluting compared to the 2-stroke generations before.

Yamaha’s WR-range follows the development of the YZ-motocross range and has undergone 3 major model changes since its debut. The latest 2007 model has a large number of improvements:

Handling improvements 
 “Like on the YZ-F motocross model previously, the WR250F and WR450F were about to improve their easiness to ride” explains Huub Forschelen, responsible for off-road competition machines at Yamaha Motor Europe. ”We have worked on engine performance at low and midrange. The handling has improved as well by a lower centre of gravity and improved chassis dimensions.”

New Aluminium frame
Most obvious is the frame change. The tubular steel chassis has been replaced by an aluminium die-cast frame. Huub Forschelen explains the background: “We took our time to introduce the new aluminium frame. Yamaha has quite some experience and knowledge on thin wall casting aluminium frame on road bikes like the R-series for example. We did lots of testing and tried different ways to make the best balanced frame for off-road. The new chassis has everything we have learned from racing. It means it has an ideal balance of rigidity, where it is needed and softer flexible construction in other areas. On top of that, the new frame is more compact and has a different geometry than before. The target was to improve the handling character of the WR- range. You can feel it straight away, when hopping from the previous machines onto the new ones.”

Both front and rear suspension settings have been altered to accommodate to the new frame rigidity.

The brake discs are wave type, to reduce weight.

Comfort improvements
Rider fatigue is an important issue in enduro competition.
“Normally on competition machines, the rider comfort has not had a great deal of consideration”, Huub continues. ”But on this new model, comfort was part of the concept.”
The seat has a more comfortable cushion and the rider position has changed to fight fatigue. Even the footrests have been widened by 5mm in diameter and are also located 5 mm higher.

The chassis and body changes apply to both the WR250F as well as the WR450F.

WR450F engine: Cylinder angle changed
To improve handling character and compactness, the 450 cc engine was part of the modification plan and saw some fundamental improvements: The cylinder angle was reduced from 8,5° to 4,5°. This is not only to make the engine more compact, with the new aluminium frame wrapped tightly around it. Also the centre of gravity was changed and this modification again contributes to an improved handling.

The separate dry sump oil tank has also disappeared. This oil tank is now integrated into the crankcase. The new dimensions of the engine and its components have helped to make the machine more compact.

To reduce vibration, a new more compact balancer shaft is used. The balancer weight is made of tungsten instead of steel, in order to reduce size.

New 5 speed gearbox
The new 5 speed gearbox and a taller primary gear ratio contribute to run the engine always in the appropriate rpm range. A modified cam profile in combination with a new mapping of the CDI unit provides better power delivery.

The new generator is identical with the WR250F unit, is lighter and will improve throttle response.

Huub explains another feature: ”When you look at the new radiator, you think it is smaller. That’s true at a first glance, but it is a new and interesting construction. It has more fins and in fact has a higher capacity than before (from 900 to 990 cc) with 27% improved cooling performance. This ensures to keep this compact high performance 4-stroke operating at safe temperatures in even extreme conditions.”

WR250F engine: detail improvements
The 250 engine has also undergone some detail improvements like cam profile and CDI-mapping optimization to improve engine response. The carburettor setting has been modified accordingly. The top gears (3-5) have a closer ratio to improve pulling power. To compensate a taller primary ratio was used.
To stabilize the oil flow, the strainer is now located in a lower and deeper position in the oil pan with the oil tank located in front of the engine.

The new radiator design like on the 450 version was applied on the 250 too.

The major features and changes
Most changes apply to both WR250F and WR450F:

Chassis
• New aluminum frame
• Suspension setting modified
• Fork triple clamps modified
• New high capacity radiator
• New air cleaner box
• New head light cowl and head light assembly
• New YZ-style exterior parts
• More comfortable single seat
• Wave type brake disk
• Pro-Taper handlebar
• LED tail light
• Wide footrests
• New engine guard
• 8 liter fuel tank

Engine WR450F
• Carburetor setting changed
• Camshaft profile changed
• CDI setting is changed  
• New 5 speed transmission
• Primary reduction gear ratio is taller
• New dry sump lubrication
• New tungsten balancer shaft 
• New Generator assembly (from WR250F)

Engine WR250F
• Carburetor setting changed
• Cam shaft profile changed
• Changed shape of the oil pan and strainer position
• CDI setting changed
• Closer ratio gears 3-5
• Primary drive taller ratio

Photo Gallery 1: 2007-WR450F

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Photo Gallery 2: 2007-WR250F

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