Over Kill
OVER KILL
RACING THE OVER/YAMAHA OV-15 EUROTWIN
YEP, IT TAKES JAPANESE ENGIneers and designers to make Japanese motorcycles-factory guys inculcated with the One True Corporate Way. What's interesting is that the engineers and designers involved in bringing Yamaha's new TRX85O to reality weren't all factory guys.
One of them was Over Racing's Kensei Sato. His dramatically styled TDM-powered OV-15A Eurotwin roadbike, with its oval-tube alloy spaceframe, not only caused a stir on the show circuit a few years ago,
but also pointed the way toward the Yamaha TRX850 (CW, June).
Sato’s Eurotwin racer is very closely based on his Eurotwin streetbike, but it’s a bunch lighter. Where the streetbike weighs 412 pounds, the racer weighs 341, thanks to carbon bodywork and titanium bits. The already sophisticated specification of the OV-15A roadbike has been further improved through use of components like six-piston Nissin calipers and a British-made Quantum rear shock. Best of all, its 10-va1ve engine now delivers a claimed 120 horsepower at the gearbox.
It’s a bike that needs to be ridden. So 1 went to Japan to do just that. And once aboard the OV-15 at the tight, twisty Tsukuba circuit near Tokyo, I found myself able to run with all but the best of the Ducatis. That’s not to say the tuned TDM motor is an easy ride, though, because to extract 120 bhp from this engine, Sato narrowed the powerband quite notably and delivered a
motor that is peaky rather than punchy. It accelerates smoothly from low revs, but there isn’t big-time power available until 7000 rpm. This gives you 2500 rpm to play with until the power curve trails off quite sharply after 9500 rpm. Sato says it’s safe to turn past 10,000 rpm, but there’s little point in revving it that high, because it just runs out of breath. This makes the six-speed gearbox that Sato has concocted to replace the stock wide-ratio five-speed all the more critical. You need to stir the lever around vigorously to keep the engine in its narrow sweet spot.
The trick is persuading this engine to rev. In spite of a lightened and rebalanced crank, twin 41mm Keihin flatslides and a 12.5:1 compression ratio, the 868cc, 10-valve engine picks revs up quite slowly until it’s turning 7000 rpm—probably because of the flywheel effect from the twin balance shafts, retained to minimize vibration.
The Achilles’ heel for the dry-sump TDM850 engine in racing has been insufficient oil supply to the bottom end. Sato resolved this by converting the engine to wet-sump lubrication,
increasing the oil supply by 50 percent, and by opting for just one oil pump instead of the two on the stock bike.
The chassis for the Eurotwin racer is similar to that of the OV-15A roadbike, but it uses a wheelbase that’s about a half-inch shorter and a head angle that’s about 2.5 degrees steeper. Sato’s design employs cross-braced oval tubing and weighs 19 pounds.
The Showa fork is sourced from Yamaha’s race kit for the YZF750SP, and the head angle is adjustable with eccentric inserts. This very stiff fork didn’t deflect under the amazing stopping power of the 12.6-inch cast-iron rotors gripped by six-piston Nissin calipers. In fact, this bike is better composed under hard braking than a Ducati, which without the benefit of the works sprag-type clutch will get its rear wheel hopping around under heavy braking. With its 54.5-inch wheelbase and fairly radical steering geometry, the Over TDM was also more nimble than Ducatis in Tsukuba’s tighter turns, only losing out on midrange punch.
The start of my race was wet, and at the green light, the TDM’s smooth power delivery and docile midrange actually became an asset as I scrimmaged up through the field.
After a brief dice with another Over rider, I climbed up to fifth place, first nonDucati, as we started the last lap. Then I crashed as the bike’s front wheel tucked at the slowest corner on the track....
In spite of this non-fairytale ending, my splash around Tsukuba on the OV15 convinced me that a sportbike based on the TDM engine is a viable concept. Judging by the TRX850, Yamaha was convinced, too. -Alan Carhcart
