Japanese invention of the year: Yamaha’s cross-plane crankshaft
Bang 270º bang 180º bang 90º bang 180º
If Japan has been known for one thing when it comes to cars and motorbikes, it is innovation. This year, Yamaha’s cross-plane crank engine, which appears in their R1 Superbike is sure to make some (asymmetrical) waves.
It’s going to be a good year for Japanese inventiveness. We’ve got Showa’s Big Piston Fork (fitted to the Suzuki GSX-R1000K9 and Kawasaki ZX-6R); Honda’s electronic braking for the CBR6 and Blade; and Yamaha’s ‘just like Rossi’ cross-plane crank. If you want useful, the Honda brakes are the easy winner. Trouble is, they’re so clever you don’t know they’re working. Same goes for the new Showa forks, unless you’re braking hard into a bend.
But you can’t ignore the R1’s new crank. It looks different, it feels different, and boy does it sound different. Although Yamaha patented a cross-plane crank in the 1960s, this one dates back to 2003. They’d just tempted Valentino Rossi away from Honda, and they needed something special to turn their dog of an M1 into a MotoGP winner. Of four experimental ‘growler’ engines at the Sepang winter test, one stood out.
At the time most commentators called it the ‘Big Bang’ engine. We now know that a better description might be ‘Big Grip’. That’s what Yamaha are claiming, at any rate.
So what’s so great about uneven firing pulses? They are certainly nothing new. For more than 100 years singles, V-twins and some parallel twins and triples have delivered uneven pulses to the rear wheel, while fours have stayed regular. But the last 20 years have seen a growing consensus (first in 500s, now in MotoGP) that uneven firing intervals are superior.
The traditional explanation is that an engine with an irregular beat gives the rear tyre more time to recover between pulses, so the rider can use more throttle before it spins up. But if you cluster all the pulses together, who’s to say they won’t make the tyre let go more easily? With the new R1, Yamaha offer a more credible reason. They say their system creates a more direct feel for the rider between throttle and rear tyre. And it does that by stripping out the undesirable ‘momentum effect’ of a traditional four-cylinder crankshaft.
To get your head around this idea, think about what an engine actually deals out to the rear wheel. Firstly, it transmits the combustion force. Open the throttle, the engine gulps more fuel and air, the burn does its thing, and you get a bigger ‘whomp’ acting on the tops of the pistons. Simple and controllable. But there’s also a secondary effect. As a conventional four-cylinder crankshaft rotates, it creates a stop-start signature. All four pistons (and their crank web balancing weights) reach their highest and lowest speeds at the same time. Yamaha call this ‘inertial torque’, and describe it rising and falling in a sine wave.
This fluttering, say Yamaha, gets in the way of you feeling what’s going on. And there’s nothing you can do about it. Enter the crossplane crankshaft. Viewed from one end, there’s a crankpin every 90 degrees (north, south, east and west). So as the crank spins there are always two pistons going flat out when the other two are stopped. The inherent architecture of Ducatis creates the same effect.
Is it really any use? In MotoGP, it’s contributed to three world titles in five years. On the road, the advantage isn’t so clear, unless you yearn to own a straight four that sounds like a Vee. The 2009 R1 is a peaky beast, and low-speed running is – well, lumpy. The Suzuki and Honda rivals are tough competition, too. We’ll have to wait for hot summer tarmac to know for sure.
Doh. What is a crankshaft again? A crankshaft is just a device that turns up-and-down motion into round-and-round – for example, the pedals on a bicycle. Naturally, the pedals are spaced opposite each other, so that one of your legs is pushing when the other one can’t. Most four-cylinder engines use the same principle: the pistons (the things that transmit the ‘shove’ derived from burning fuel and air in a confined space) rise and fall in pairs. Because an individual cylinder only fires once every two revolutions, the crankshaft as a whole receives a one-cylinder pulse of power once every half revolution, or 180 degrees. Yamaha’s new crossplane crankshaft is different. There’s no direct comparison with a bicycle because the bike has four pistons, and you’ve only got two legs. But imagine the angle between your pedal cranks being something like 100 degrees, rather than 180. It would feel horrible. But the R1 revs 200 times faster than your legs, so it’s not bothered.
