Ow01 Take Two

OW01 TAKE TWO

ROUNDUP

WE'VE WAITED AND we've hoped. Yamaha's World Superbike entry has long been the oldest design in the series, and the coming of the wonderful YZF-Rl and YZF-R6 hinted that a 750cc version couldn't be far behind. At last, Yamaha's new four-stroke racer is here. Made as a limited-production homologation special, the OW02/R7 features the super-compact engine architecture of the R1 and R6, with their desirably long swingarms. The 72 x 46mm engine features ultra-lightweight titanium con-rods and valves.

Bikes with short swingarms are hard to set-up for racing be cause they swing through a larger angle for a given amount of suspension movement. This makes it harder to prevent -` unwanted squat or jacking at the rear. In Grand Prix racing, swingarms have for some time been nearly half the wheelbase, but this is less easy to do on production models, which ` have bulkier engines, often inherited from previous models. On its new R-se J nes Yamaha has gone to the . J limit to make its engine radi cally short from front to back. The short engine and vertical ly stacked three-shaft gearbox make room for a long, GP-style swingarm-and its advantages. Ohlins suspension front and rear underlines the racing aim of this machine, which will be sold only to licensed racers with solid resumes.

Yamaha's ups and downs with its trademark five-valve engine have stimulated speculation that the new engine might have only four. A four-stroke's combustion chamber is really just the clear ance cutouts for the valves-the rest of the chamber has to be tight squish. The presence of one extra valve increases this cutout area, so to maintain com pression ratio, the chamber height must be correspondingly reduced. This vertical tightness leaves less room for the mixture turbulence essential for high combustion-flame speed. Slow combustion increases heat loss, and you may have noticed that Yamahas have the biggest radia tor array in Superbike racing.

Why has Yamaha retained five valves, and how is it making the concept work in a fourvalve world? The presence of five valves is Yamaha’s signature feature, and having made the effort to find performance in this design, the company is reluctant to start again with four. To get acceptable flame speed, engineers must concentrate every last tenth of a cc of chamber volume close to the central sparkplug. The rest of the bore area has to be a near-perfect fit of piston to head minus whatever clearance it takes to prevent contact. To control this fit absolutely, Yamaha is CNC-machining every combustion chamber. With no chamberto-chamber variation, combustion will be as good as it can be made. In any case, the engine’s share of responsibility for lap times has shrunk

while that of chassis, tires and suspension has grown. Yamaha’s achievement in compacting its engine to make room for the long swingarm is an important one. The price? Try $25,000 to $30,000 per copy. Do I hear a swift intake of breath? This isn’t so bad, considering you pay as much for a TZ250, the only production parts of which are its crankcases and flywheels. All of us who grew up with Yamaha roadracers have been sorry in recent years to see the crossed tuning-fork emblem too rarely on the podium. We have longed to see a fresh, no-holdsbarred design from this very experienced company. Here it is. Kevin Cameron