Yamaha's world championship winning 2007 model four-stroke YZ450F MX bike features a host of changes designed to keep it at the top of the podium.

Less weight and more useable power are the main targets for 2007. And to achieve this, the five valve, five speed engine runs a new balancer shaft with a revised ratio. The reduced mass of the new balancer improves engine power characteristics and also enhances throttle response. Plus the reduced gyroscopic forces produced by the lighter rotating mass of the new balancer makes for easier directional changes.

Carburettor settings are also revised for 2007 aimed at boosting low to mid-range power. These combine with exhaust valve lift mods and a remapped CDI to further improve power delivery.

New high rigidity clutch pressure plates help get that power to the ground and offer greater durability and lighter operation. Plus the number of clutch and friction plates is increased and the overall thickness of each plate is reduced.

For 2007 a new lightweight titanium exhaust pipe replaces the stainless steel design previously used, and the diameters of both the exhaust and silencer are increased. These changes help to boost low to mid range power quality and meet new noise regulations.

Keeping things running cool, the YZ450F's large capacity radiators are now equipped with a reinforcement bracket for added strength, and the louvred plastic panel design is modified for improved airflow.

Both the upper and lower triple clamps are new for 2007, and are now manufactured from forged aluminium instead of the cast design used in 2006 to reduce steering head rigidity and improve handling. Revised damping in the front forks and modifications to the rigidity balance of the USD Kayaba units further improves handling.

Increased weight savings are made by the fitment of shorter fork clamp bolts in the upper and lower triple clamps, and the handlebar mounts feature a smaller diameter fastening bolt. The forged clamps and associated parts reduce overall weight and contribute to the feeling of lighter steering on the 2007 bike.

New wave type front and rear discs offer a double benefit. They reduce unsprung weight to give lighter steering and handling performance. But they also offer enhanced braking performance, and can give improved pad cleaning properties in muddy conditions.

New Pro Taper handlebars are used to achieve an improved riding position (8mm higher at the grips) and give an enhanced appearance. The wide middle section also features useful reference marks allowing the rider to experiment with different bar positions.

The innovative rear shock with its titanium spring uses new aluminium components in its construction to reduce overall weight and measures 1.5mm longer than the previous unit to sharpen handling. The seat is also lighter (and features an improved density foam that prevents distortion). Further weight reductions have been made by lightening over fifty bolts.

Just how good are these new YZ450Fs? The best on the market according to respected US dirt bike magazine Transworld Motocross. Top US privateer Joe Oelhof tested all the 450 MX bikes back-to-back in the January issue and claimed the YZ450F to be the best of all. In summing up, this is what he had to say:

First Place: Yamaha: "I had a hard time putting the Yamaha first over the KTM and Honda, simply because those motors were a little bit stronger. With how well the Yamaha did everything else - from handling, cornering, braking, jumping, and the way the suspension worked - I felt comfortable right away. Because of that, I had no choice but to put it in the top spot. I felt like I had been riding it all year."

The 2007 bikes are available in two colour schemes, traditional Team Yamaha Blue/White with an RRP of $11,499 and Special Edition White/Silver priced $11,599.

2007 YZ450F
Key Features:

• For 2007, the bike that started the four-stroke revolution benefits from significant chassis and engine refinements that ensure the five-titanium-valved YZ450F remains at the head of the open-class pack.
• Modifications to the light and strong 'controlled flex' aluminium frame and suspension mean light, precise, confident handling at any speed.
• Five-speed gearbox provides maximum flexibility and the right gear for a wide range of riding environments.
• Ready to race straight out of the crate - with a ProTaper aluminium handlebar in adjustable clamps, titanium shock spring, ti footpegs, gripper seat, etc.

New for 2007:

• New balancer weights, carb settings, valve lift specs and remapped CDI offers improved power characteristics.
• A more rigid clutch pressure plate helps put power to the ground and dissipates heat more efficiently, especially during partial-clutch operation. Shorter lever stroke with 12-percent reduced effort means instant response.
• New, titanium exhaust pipe fattens low- and mid-range power while simultaneously adhering to strict 2007 noise regulations.
• Replacing the upper cast triple clamp with a light and strong forging, slightly reducing steering head rigidity, and the use of lighter outer fork tubes (with revised damping); lighter wheel rims and light wave-type brake rotors add up to super-accurate, super-responsive steering.
• Increasing the length of the rear shock by 1.5mm sharpens handling. Replacing steel damping adjusters with aluminium ones removes even more weight from the four-way adjustable, titanium-sprung rear suspension unit.
• This year's ProTaper aluminium handlebar bend raises the grips 8mm for an expanded cockpit. Forged bar clamps reduce weight for superior handling.
• New lightweight seat with anti-distortion foam reduces rider fatigue

Engine:

• 449cc five-valve DOHC liquid-cooled engine is super-powerful yet tractable.
• 39mm Keihin FCR flat-slide carburettor with throttle position sensor (TPS) and leak jet delivers precise fuel/air mixture and instant throttle response.
• Revised CDI ignition keeps that amazingly broad and deep powerband hard-hitting, yet controllable.
• Lightweight, titanium valves and load-reduced springs minimize reciprocating mass for fast-revving, hard-hitting power delivery across the rev range.
• Additional weight-saving engine features include: small-design cam chain tensioner and oil pump, aluminium oil hose, all-titanium exhaust header, flange, bracket and heat shield and light CDI ignition unit.
• Primary kickstarting with automatic decompression and convenient handlebar-mounted hot start lever ensures easy restarts.
• Kick pinion gear is separate from the primary driven gear for smoother clutch engagement.
• Large-capacity radiator is reinforced for greater strength.

Chassis/Suspension:

• Aluminium frame uses a mix of alloy casting, forgings and extrusions to produce a tuned-flex frame that makes the YZ the finest handling motocross bike ever.
• Speed-Sensitive System 48mm Kayaba fork places transfer control valves (TCV) above the fork springs, so that damping force is controlled by piston speed; Eliminating mid-stroke damping force changes provides a smoother, more precise ride. Rigidity and damping revised for 2007 model.
• Titanium-sprung four-way adjustable rear shock with Kashima-coated internals reduces friction for fantastic compliance and great handling.
• More weight savings come via two-piece (part aluminium, part plastic) engine protector plates and thin-walled engine guard construction.
• Removable aluminium rear subframe constructed of square-section tubing for light weight and easy maintenance - lighter fasteners for 2007 enhance weight distribution and handling.
• Lightweight front wheel hub for reduced unsprung weight and increased durability.
• Premium Dunlop knobbies provide unbeatable, competition-spec traction and great wear.
• Lightweight 21-inch front and 19-inch rear Excel rims are exceptionally durable and reduce unsprung weight.

Additional Features:

• "Gripper" style seat cover gives excellent feel and control, and the seat is lighter for 2007.
• Wide, cleated, folding titanium footpegs are light, strong, and provide excellent comfort, grip and ground clearance.
• Works-style clutch lever features oversize barrel adjuster for easy, on-the-fly adjustability of lever/cable play.
• Cleated rear brake lever and folding shift lever tip offer superb grip and excellent durability.
• Durable, comfortable handlebar grips.
• Stainless steel clutch cable for longer life.