Thursday, 3 September 2009

What’s a Cross-plane Crankshaft?

The cross-plane crankshaft has its pistons arranged 90 degrees apart from each other. As high-performance motorcycle engines have grown more powerful, it’s become difficult to harness that power through the small contact patch on the rear tire. A typical four-cylinder engine uses a flat-plane crankshaft, and the two outer and two inner pistons rise and fall in pairs, firing 180 degrees apart. Torque is applied to the crank during combustion, of course, but also from inertia as the crankshaft rotates. This inertial torque is seen as noise to engineers, and it has the effect of confusing a rider about the amount of traction available from a bike’s rear tire.

The same holds true in the world of MotoGP racing, so Yamaha engineers introduced in 2004 an uneven firing interval for its inline four-cylinder M1 with what’s called a cross-plane crankshaft. In this new design the pistons are arranged 90 degrees apart from each other around the crank, which eliminates the inertial torque fluctuation of a typical four-cylinder mill. A balance shaft keeps vibration at tolerable levels. Yamaha has adapted this configuration to its R1 streetbike, providing enhanced traction and a very distinctive exhaust note we’ve been hearing from Valentino Rossi’s racebike.

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