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Issue link: https://magazine.cyclenews.com/i/128171
2003 Yamaha YZ450F Tech Ticker ENGINE • Increased stroke from 60.1 to 63.4mm (bore remains g5mm) • Decreased flywheel weight • Valve system miniaturized • Cutaway is added to cylinder skirt to decrease pumping loss, and fastening bosses are eliminated from cylinder • Smaller cylinder head reduces weight and optimizes valve system (valves are more vertical) • Titanium valves are shorter and 18% lighter, and valve springs are lighter for quick throttle response • Crankshaft assembly is lighter (20% less inertial mass) for quick throttle response and reduced weight • Piston is 7% lighter (I mm smaller pin and 1mm shorter skirt) for quick revving • Gearbox is now four speeds instead of five • New Keihin carburetor has sealed throttle cover, and hot-start actuator is now handlebar-mounted • Head pipe is now titanium (including flange, bracket and heat guard) • Exhaust silencer has titanium pipe and end-cap (were stainless); pipe is 20mm longer and body is 50mm longer (for increased displacement) • Automatic decompression system replaces manual system for easier starting • Cam-chain tensioner is smaller for reduced weight • Primary driven gear has different clutch damper (rubber instead of coil spring) to smoothen clutch engagement • COl magnet rotor and boss are now unified for less weight and inertial mass • Ignition coil is integrated with spark-plug cap for reduced we.ight • Smaller oil pump reduces weight • Oil hoses are shorter and material is now aluminum (instead of steel) for reduced weight • Airbox has new, lighter design at joint w/air boot SUSPENSION • Revised settings for Kayaba fork CHASSIS D _pit. the 1'lIIatIv~ high-speed nature of . . . Lake ~JNck, Sprenkel nel!W mluedhawll'l8 a fifth gear (the VZ45QP .. a feu........., for '031. something that's leaps and bounds better than anything else out there," explains Terry Beal, Yamaha USA's P.R. manager. "The YZ400F was stalwarts Doug Dubach, Don Dudak and erstwhile Yamaha race-team manager Steve Butler). The crew's pride can arguably be leaps and bounds better than a Honda XR400R. People figure that if justified even before one throws a leg you can build the best four-stroke, you can build the best of anything. It shows your engineering prowess and innovation, so it's just something we hang our hat on." somewhat of an engineering and manufacturing marvel. The most over the bike, since the YZ450F is obvious change was the addition of 23cc of displacement (the 450 is actually a 449), bringing the bike up to the 2003 FIM and AMA limit for PROUD PAPAS the 250cc division. Fortunately, Obviously pleased with its new creation, Yamaha wanted a press introduction that would suit the occasion, so they invited the American media to Lake Whitney Motocross Yamaha already had reams of displacement data, since the European team had experimented extensively with the factory machines of Andrea • Frame has .2mm-thicker main pipe and .3mm-thinner cross pipe to improve handling and reduce weight • Different location for oil plug (lower on frame), and oil volume is smaller (1200cc instead of 1700cc) • New bodywork (plastic and seat) is flatter and more streamlined • Fuel tank is smaller (1.85 gallons instead of 2.1 gallons) • Upper-rear motor bracket is smaller and thicker, w/larger bolts (3 instead of 4) • Skid plate is now 2 pieces (plastic front, aluminum rear) • Right engine guard is thinner (3mm instead of 4mm) • Steering stem is now tapered for improved rigidity and reduced weight • Swingarm is tapered (80 grams lighter), a small hollow is added near chain adjuster, and surface is anodized • Chain slider is 25 grams lighter • Rear-brake master cylinder has reserve tank integrated into design, and piston diameter is reduced (l1.6mm instead of 12.7mm) • Rear-brake caliper has smaller piston (25.4mm instead of 27mm) for increased rigidity and different brake-pad area • Rear-brake-caliper protector is smaller and 14 grams lighter • Front-brake-caliper piston is plastic (instead of aluminum) for less weight and better feel • Front-brake master cylinder has new design and is lighter Bartolini, Marniq Bervoets and Everts Park, a picturesque facility on central Texas' WB Ranch - just down the road from President George W. Bush's Crawford ranch. Yamaha also before settling on that bike's current 500cc size (Hayashi insists that, starting - just four goals, but their fuel - still 13.5 less than the old bike's "real" number and one pound less than the Honda CRF450R (we have yet to back up these numbers ourselves). although U.S. riders like Button and attainment necessitated an incredible acknowledged the occasion's significance by sending several important Japan-based staff members, includ- Ferry were restricted to the stock engine configuration by AMA production-based rules, they also helped 185 item changes (112 to the motor alone). ing YZ project leader Yasuo Hayashi, motor engineer Taisuke Sakurai, chassis engineer Takehisa Katsura, and assistant to vice president Max Nakamura (also on hand were Americans such as head U.S. engineer Mark Porter, and testing-department immensely, with testing information on details like power characteristics, car- complete reworking of the bike, cutting excess weight and using lighter buretion and transmission ratios). Having settled upon a displace- materials wherever possible (see Tech Ticker). Although company lit- ment, Yamaha then hoped to bless the new YZF with tight weight, quick erature puts the weight at 222 pounds, Yamaha admits the real would tend to make the bike slower- response, light handling and easy number is actually 232.5 wet with no revving (Yamaha didn't want to 44 SEPTEMBER 11, 2002' cue • • n • _ is The first target alone required a In order to achieve quick engine response (goal number two), some of that aforementioned lightening was focused on the drive train, important since the move to a longer stroke