Cycle News is a weekly magazine that covers all aspects of motorcycling including Supercross, Motocross and MotoGP as well as new motorcycles
Issue link: https://magazine.cyclenews.com/i/128160
~~WBike~ 2003 Yamaha Motocrossers YAMAHA INTRODUCES THE FIRST SECOND-GENERATION MX THUMPER. By CHRIS JONNUM '::i our-stroke motocrossers are not a .J fad. If you had any doubt, Yamaha's 2003 MX line-up should clear that up, highlighted, as it is, by a brand-new-from-the-ground-up, bigbore thumper. We've become accustomed to model overhauls in motocross, but if you think about it, the YZ450F has become the number-one priority year introduction of the YZ400F - is that we cover on this page represents the first MX-focused four-banger to for the manufacturers. Not only are big-bore four-stroke MXers a source the one to make this significant step. Oh yeah, the rest of the YZ line get a complete makeover (yes, the of substantial revenue, but they're an has been improved as well, but this YZF has been significantly improved over the years, but nothing like this). This is an important benchmark in opportunity for these companies to gain prestige by showcasing their technological abilities. How appropri- new 450 looked so trick at Yamaha's recent press introduction that you can surely forgive us for sometimes over- motocross, as it indicates an even firmer commitment to a segment that ate, then, that Yamaha - who got the segment started with the 1998 model- looking that detail. Can't wait for a test unit... eN First things first: weight. According to Yamaha, the new YZ450F is significantly lighter than its 426cc predecessor. Technicians gave us "real" numbers (232 pounds) and "claimed" numbers (222 pounds), but rather than speculate as to what the true figure is, we'll wait for a test unit and tell you for now that Yamaha says the bike is a pound lighter than the 2002 Honda CRF450R. You can be sure that this is no coincidence, as the Honda was the lightest big-bore moto-thumper ever produced, and a significant improvement on anything that came before it. So, how did Yamaha go about cutting the poundage? Actually, "cutting" is probably not the best word, since the tuning-fork guys pretty much replaced the entire bike, rather than try to shave weight here and there (there are 185 item changes on the bike, with 112 of those being to the motor). The engine is noticeably smaller (it's easy to mistake it for a YZ250F mill), and although it has the same bore as the 426 (95mm), the stroke has been increased to 63.4mm to achieve the new displacement. Nonetheless, the cylinder head is 7mm shorter, made possible partly because of the shorter valves and smaller springs (possible now that Yamaha uses titanium valves). Rod length is obViously increased, though a lighter piston (shorter skirt and smaller pin) means that the small end can be smaller (the crankshaft assembly is also lighter). In addition, the cylinder body has a new cutaway added at the bottom to decrease the pumping loss. 8 JUNE 26, 2002' cue I e n e _ s We were pleased to learn that Yamaha has followed the lead of Honda and KTM by going with an automatic decompression system, meaning you won't have to hassle with a handlebar-mounted compression-release lever. The cam-chain tensioner is smaller, as is the oil pump; the oil lines are now made from aluminum (they were steel), and they're shorter in length (the oil plug is now lower on the frame, and oil volume is decreased from 1700cc to 1200cc). The exhaust system is now 50mm longer and has a bigger silencer, but since the header pipe (including the flange, brackets and heat shield) and silencer tip are now made from titanium, the entire system is said to be 210 grams lighter. One big change is the switch to a four-speed transmission. Yamaha admits that this will make the bike less adaptable for off-road use, but feels the four-stroke MX wars have gotten serious enough to justify a pure-moto approach (if it makes you feel any better, Timmy Ferry's factory 426 is a three-speed). The clutch is now damped by eight rubber pads rather than coil springs, and the ignition coil is now Integrated in the spark-plug cap (the flywheel has a unified rotor and boss, as well). The carburetor has a seal on the throttle cover, and the hot-start button can now be actuated from the handlebar. The oil-filter element is now paper instead of steel. As for the chassis, the frame has less gusseting (but with a stronger material) by the head, and it utilizes a different tubing throughout (frame weight is said to be three pounds lighter). The engine guards have been modified to decrease weight (the front of the skid plate is now plastic, and the right guard is made from a thinner material), while the motor brackets are thicker (but smaller). Up front, the triple clamp is redesigned, and the steering stem has an "hourglass" shape for less weight. The big YZF, which sells for $6299, also gets new, flatter bodywork, along with a simplified airbox design.