Spin Doctors

Spin doctors

TDC

Kevin Cameron

FOR SEVERAL SEASONS NOW, PEOPLE have speculated that in World Superbike, Twins have a mysterious acceleration advantage. At times, this has verged on suggesting that V-Twin firing order is somehow analogous to the “Big Bang” phenomenon in 500 GP racing. In this view, Twins somehow get more grip out of their tires and so accelerate harder.

The answer to why WSB has turned into “Battle of the Twins” may be much simpler than that. Each of the V-Twin teams (Honda, Ducati, Aprilia) spends nearly $10 million a year in the class, which is surely far above what is spent by the remaining four-cylinder teams (Bertocchi’s and Harald Eckl’s Kawasaki teams, and the Batta Team Alstare Suzuki outfit). Less money spent means less of all the good things: power, acceleration, chassis setup.

Skeptics further point out that the four-cylinder bikes are still being ridden by the sort of rider traditional in WSB until recently-veterans gracefully finishing their careers, or second-level riders. The Twins-makers, on the other hand, are bidding directly against 500 GP to get top riders like Bayliss, Edwards and Corser.

In either case, nothing useful is likely to come of the recent rules-making decision mandating Twins carry 2 kilograms (4.4 pounds) more, and allowing Fours to weigh 3 kg. (6.6 lbs.) less. Four-cylinder lobbyists also argue for a bump in displacement up to, say, 850cc. England’s Superbike series will run under such rules in 2002.

All that aside, it’s interesting to consider a definite physical reason why Twins may accelerate particularly well. That is natural anti-spin. Years ago, I observed a series of dyno runs at Jerry Branch’s shop in California, the purpose of which was to establish criteria for limiting dirt-trackers’ engine power via intake restrictors. I was struck at the time by the unique torque curve of the unrestricted Harley XR750. Its maximum torque was given at the lowest rpm at which the engine would pull smoothly, about 5500 rpm. Peak power, on the other hand, came at something like 9300. Peak torque was thus at just under 60 percent of peak-power rpm, at the extreme bottom of the usable torque curve.

This struck me as odd because in typical roadrace engines, peak torque has to be located much higher. This is so

because to obtain high maximum speed, the rpm of best breathing (peak torque) has to be close to the engine’s rpm of peak power. Maximum breathing and maximum revs combine to make maximum power. For this reason, peak torque of roadrace engines typically comes at 85 percent or more of peakpower rpm.

At various times I wondered about that unique Harley dirt-tracker torque curve with its peak at the bottom, but since I didn’t know anything about dirttrack, I didn’t get anywhere.

Now come faint rumors about Yamaha’s development of its YZR-M1 Grand Prix bike. Everyone expected the new 990cc four-stroke GP engines to be fierce tire-eaters, and that’s just what the rumors reported about early Ml outings-critically short tire life.

Then the grapevine reported that Yamaha had, last spring, taken a new direction in its development. I had no idea what that might be until later, when I learned that recent versions of the YZR-M1 engine have their torque peak way down at something like 8000, with peak power still given up at 15,000. In order to push their torque peak down the rev scale this way, they’d had to give up some top power-early versions made a reported 225 horsepower, but these new engines were making only 180-190 bhp. More importantly, their peak torque was located at the bottom of the rev band-at something like 55 percent of peak-power revs.

A big lightbulb came on in my head. Here was a roadrace engine being given a classic dirt-track torque curve. Why would they give up 40 bhp to do this? It was suddenly obvious: to create a torque curve that fell instead of rising, as the revs rose. On a dirt-track-or anywhere traction is scarce-if the tire breaks loose and the engine revs up, torque falls, allowing the tire to hook up again. This is natural anti-spin, created not through electronic controls, but by means of a falling torque curve.

With a normal roadrace dyno curve, torque rises steeply in the bottom of the usable power range. If the tire breaks loose during off-corner acceleration, the engine revs up and torque rises, causing the tire to spin all the harder, provoking a slide. To prevent this, the rider naturally opens the throttle less for a margin of safety, keeping well away from triggering this sequence of events.

But with a falling torque curve, the rider can safely push right into controlled wheelspin and operate there. Even with less peak horsepower, a bike with a falling torque curve would outaccelerate one with a normal rising curve. This is apparently just what has happened with the new-version Yamaha Ml. Although less powerful, it accelerates faster.

Now another angle: As an engine cylinder is made bigger, it gains volume faster than it gains valve area because volume is proportional to dimension (bore, stroke) cubed, while valve area is proportional to dimension squared. Being thus deficient in valve area, the larger cylinder tends to deliver its peak torque at a lower rpm, and above that peak, torque then naturally falls slowly as the engine revs up to peak power. Could this be the reason why lOOOcc VTwins, even though often less powerful than 750 Fours at peak, nevertheless accelerate out of turns faster? Could it be that falling torque-natural anti-spinhas been Ducati’s great strength, recognized and reinforced by appropriate engineering along the way? And could it further be that, as Honda’s engine guys have increased the power of the 1 OOOcc RC51 Twin, hoping to gain some advantage on the Ducati, they have had to move its torque peak upward in rpm, creating a rising-torque characteristic that favors wheelspin rather than rapid acceleration?