What are the Benefits of a Flat-Plane Crankshaft?

Reasons to think twice about your crank

flat-plane crank

QUESTION: Would you please have Kevin Cameron comment on the benefits of a flat-plane crankshaft? (Thanks for the best technical explanations in print today.)

Gordon Banks

US Army, Retired

Madison, AL

ANSWER: The original reason for a flat-plane crankshaft is that it gives an even 180-degree firing order (four-stroke) and has inherent primary balance (primary or first-order means at crankshaft rotation frequency). Inline-fours with a flat crank do have unbalanced secondary shaking forces (at twice crank speed), arising from con-rod angularity having an effect on piston height. In larger engines with pistons heavy enough to make this annoying, secondary balance shafts are added to restore smoothness. The little Chevy Cobalt I'm driving has these shafts down in its crankcase, driven by chains.

When Yamaha's vibration specialist Masao Furusawa was put in charge of that company's MotoGP racing program in May of 2003, the first problem he tackled was the question of why race bikes with V-4 engines were accelerating harder off corners than bikes with flat-crank inline-fours.

At first, even the question seems crazy, but Furusawa identified one effect that looked possible. With a flat crank, the inline engine’s pistons are all stopped together every 180 degrees, imposing a considerable speed fluctuation on the crankshaft. But in a V engine with two rods per crankpin, when one piston is motionless at TDC or BDC, the other piston is at or near its maximum velocity. Thus, instead of moving energy from the piston inertia into and out of the crankshaft as the inline does, a V engine trades this energy from piston to piston, producing no fluctuation in crankshaft speed.

Yamaha YZF-R1 cross-plane crank

Furusawa was not sure whether this inline crank-speed fluctuation was directly reducing grip by “jiggling” rear tire speed, or whether the variation was affecting the rider's ability to feel and use maximum grip.

He therefore had his prototype department manufacture a crankshaft with its crankpins at 90-degree spacing instead of 180, and to build also the cams necessary with such a crank. When Valentino Rossi tested a bike with the 90-degree crank against one with the 180-degree crank on January 6, 2004, he was able to accelerate faster off corners with the 90. All Yamaha M1 MotoGP bikes have used the 90-degree crank ever since.

Because the 90-degree crank was not in primary balance as a 180-degree crank is, a balancer shaft had to be provided for use with it. When the production R1 was changed to a 90-degree crank, it too had to be given a balance shaft. I suspect the production R1 wasn’t given a 90-degree crank to compete with on-the-limit VFR riders. Rather, I think the marketing department liked the unique V-8ish sound that the 90-degree firing order has, making it stand out from the higher-pitched wail of GSX-Rs and Ninjas.

The 90-degree crank is called “cross-plane” because its crankpins are in two planes at 90 degrees to each other, rather than in a single plane as are the crankpins of a flat, or 180-degree, crank. Thank you for taking the time to write.

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