Yamaha Shows Wonder Thumper!
YAMAHA SHOWS WONDER THUMPER!
ROUNDUP
TAKING ADVANTAGE OF A new rule allowing a manufacturer a one-shot, single-season opportunity to run a four-stroke prototype instead of a production-based racebike, Yamaha has introduced the OW-H2, a works motocrosser that will compete in the 1997 AMA 250cc outdoor Motocross Championship. But unlike almost every other machine in that series, the OW is neither a 250 nor a two-stroke; instead, it’s a 400cc four-stroke.
Thumpers are nothing new in motocross. Indeed, they dominated the 500cc world championship well into the 1960s, when the Japanese invasion began. It was then that two-strokes came to prominence, and they remained dominant until 1993, when Jacky Martens won the world title on a Husqvarna. In 1995, Belgian Joel Smets again topped the world on a four-stroke, this time a Swedish Husaberg. Now, the Japanese are re-evaluating the feasibility of four-stroke MXers.
Yamaha USA recently let Cycle World look at one of the two existing OW racebikes, which will be called YZM400Fs in America. However, company spokesmen weren’t exactly forthcoming with details. According to National Communications Manager Bob Starr, the bike currently “displaces at least 400cc, has a four-speed gearbox and makes more than 50 horsepower.” But
whether that was measured at the crank, the countershaft or the rear wheel, Starr couldn’t say.
Fortunately, the new powerplant’s appearance told far more. To start with, it’s extremely compact-less than 14 inches tall and only about 9 inches from the front of the engine to the countershaft, making it slightly taller than a current 250cc two-stroke engine but not as long. You can almost overlook the OW’s tiny liquid-cooled cylinder, as it extends only 1.5 inches above the machined-from-billet cases. Assuming a normal connectingrod length, the stroke of the new Thumper would have to be less than 60mm. For a 440cc displacement, this would put the bore size in the 97 to 100mm range, yielding cylinder dimensions closer to those of twincylinder Superbikes than traditional Singles. If so, the OW should safely rev past 10,000 rpm, indicating the 50-
horsepower rating may be overly conservative.
The sand-cast cylinder head houses two camshafts driven at least in part by a chain from the crank. The two separate exhaust ports and head pipes argue for two exhaust valves, though rumors of this machine have hinted that it has “lots of valves.” Perhaps it’s one of Yamaha’s fivevalve Genesis designs, but there’s no outward indication of that. A single Keihin flat-slide pumper carb feeds it fuel and air, and a turn of the quick-acting twistgrip indicates a smooth, light throttle action. A water pump is driven off the left end of the intake cam, and a hose plumbed into the intake tract presumably lets cool air enter to aid hot starting.
Most tellingly, there is no external oil reservoir, and the cases can’t be seen to house a traditional sump; like Husaberg and Husqvarna four-strokes, this engine likely has a minimal oil system, relying more on two-stroke quality bearings than traditional fourstroke oil-system bulk.
Housing the new engine is a frame very like that of a current
YZ250, though the titanium subframe is obviously nothing like a production part and wraps around the right-hand exhaust megaphone differently than it would an expansion chamber. In any case, the compact engine, aluminum fuel tank, works Kayaba fork, carbon-fiber muffler and liberal use of magnesium and titanium are said to keep the OW-H2 down at the class 216-pound weight limit.
Showing how seriously Yamaha believes in four-stroke possibilities, the company has contracted former 125cc national champion Doug Henry to ride the bike starting in Gainesville, Florida, on March 3. The program is clearly in its very early stages: As this is written, Henry has only tested the bike twice in Japan, and Yamaha officials are anxious to note that qualifying and mid-field placements are more the early goals than race wins. But no matter what, the deep-throated boom of a big four-stroke Single will
soon resound again at U.S. motocross races. -Steve Anderson
