Tdc

TDC

Big Bang Theory

Kevin Cameron

FOUR YEARS AGO, 500cc GP BIKES exited turns in a series of lurid, grip-slip-grip leaps and weaves, forcing riders into a defensive, bronc-riding style. So many riders were injured in turn-exit highside crashes that it seemed there would be none left riding at season’s end.

Successful tire development caused this. Michelin tires, in particular, acquired a dizzying peak of unbelievable grip-then let go with suddenness when pushed that tiny bit more. Nature is a zero-sum game. You can have a moderate amount of something spread across a wide range, or you can push it up into a tall, narrow spike.

Riders accelerating out of turns twisted the throttle, feeling for traction, teetering at the peak of that spike. Because the peak was so high and sharp, many fell off its far side and crashed. Street tires are intentionally designed to avoid this, but in racing, grip is (nearly) everything.

As quickly as the crisis had come, though, it was over, and bikes were again squirting out of corners without drama. At the time, it seemed that tire grip and engine power had been tamed by a new technology of automatic torque control-a variety of devices that limit or modulate engine thrust during lower-gear acceleration.

Now it turns out that this is only part of the story. Another, far older and subtler effect has been used, as well-something I will call “Big Bang” technology.

As the rider turns the throttle to increase thrust during acceleration, he asks the rear tire for more and more grip, pushing it toward its maximum capability. With old bias-ply tires, cornering stress distorted the contact patch, causing partial sliding to begin. As the rider called for more, sliding spread over more of the patch, causing the back end of the bike to hang out more and more. This told the rider how rapidly and closely he was approaching peak grip. Progressive sliding had two effects: (1) It reduced the peak grip and (2) it widened the warning zone in which the rider could “feel the edge.”

With the coming of successful radial tires, the peak was raised (good), and the warning zone was narrowed (bad, but inevitable). A well-designed radial tire lays its contact patch down on the pavement very uniformly, so that all parts of it are under equal stress. Rather than beginning to slide progressively at the edges like that of a bias-ply tire, the radial’s contact patch holds on everywhere until the whole thing is at the limit, ready to snap loose. And then it does, often faster than human reaction. Even traction-control devices can’t always anticipate and prevent it.

Something was needed to re-establish a warning zone-even at the possible sacrifice of a part of the tall traction peak.

That something turns out to be abandonment of “rational” engine firing order. A rational order is one that gives a smooth flow of power and assists in balancing the engine. To use an electrical analogy, power should flow like direct current, with some unavoidable ripple in it, the result of the combustion events in the cylinders. Four-cylinder two-stroke 500cc GP engines of the 1976-86 era fired two cylinders together, then the other two 180 degrees later. This order gave good balance and a high-pitched exhaust note. Not any more.

Today’s winning engines sound more like 500 Singles-a deeper, flatter sound. The reason? Instead of 180 degrees between the firings of the two cylinder pairs, some of today’s engines fire all their cylinders within 90 degrees or less, creating violent torque pulses with nothing between. This is convenient in the twin-crank Yamaha, which can be balanced as a pair of V-Twins. It would be hard to do in the single-crank Honda, which would become as horribly unbalanced as a gigantic Single.

Why commit this seemingly crude mechanical crime? When you change torque delivery from a smooth flow to a series of big bangs, you cause (at least in the lower gears) an instant of rear-tire slippage once per engine revolution. As the rider turns up the throttle exiting a corner, the duration of this slippage phase will increase in proportion gradually-and the rider will feel it. He will feel it because any rear-tire slippage will be partly sideways, causing the back of the bike to hang out more and more.

Instead of a slipless rise of grip to a needle-like maximum, followed by catastrophic release, the rider will again have the warning he needs to tell him how close the maximum is. And therefore he will get marvelous, controlled drives off corners and leave the opposition shaking their heads in disbelief. He may even be able to remove or disable some of his torquecontrol devices.

And the downside? Well, gears and cases will have to be reinforced to survive the increased torque-pulsing. Top speed will drop a bit because energy is wasted in slippage. Tire temperature at the end of a long straight may rise slightly. Yet the gain in the rider’s ability to control the motorcycle will be enormous.

Now think of the ramifications. This is why dirt-track Harleys work on the mile, and it's why the old Yamaha 750 four-stroke Twin, with its evenly spaced firings and extra power, did not. A narrow firing order doesn't improve traction or raise average torque; it improves the rider’s ability to sense traction so he can ride closer to the limit in safety. And do those World Superbike Ducatis actually accelerate harder than the four-cylinder bikes off corners, or is their narrow firing order working to improve the rider’s ability to locate the traction limit?