Gremlin Not The Car asks: Say Kevin why did Yamaha in 2009 switch their production YZF R1 sportbike from a flat plane crankshaft to a cross plane crankshaft?

Kevin Cameron replies:

The original reason for building the 90-degree crank was that Yamaha's in-line four with flat (180 degree) crank could not accelerate with the V4s in the early years of MotoGP. Yamaha's eminent NVH engineer Masao Furusawa saw that twice per revolution, the flat-crank M1 Yamaha had to accelerate all four pistons from a dead stop (two at TDC, two at BDC). The energy for this had to come from the crank, creating an inertia torque that was either interfering with rear tire grip or with the rider's ability to sense that grip.


​Orienting the crankpins at 90 degrees ("cross-plane") changed that to pistons exchanging energy with each other, rather than with the crank, putting an end to the strong inertia torque formerly transmitted to the rear tire.

Furusawa also disagreed with Yamaha's 5-valve "Genesis" concept because its combustion was poor (with so many valves, little area was available as "squish").

He therefore had four bikes built for testing by the newly-hired Valentino Rossi (before the start of the 2004 season).

1) one with flat crank and 5 valves
2) one with flat crank and 4 valves
3) one with cross-plane crank and 5 valves
4) one with cross-plane crank and 4 valves

In track testing, Valentino liked best and went fastest on the cross-plane 4-valve. Using that configuration Rossi was able to take the MOtoGP championship for Yamaha.

When the production department tested with the cross-plane crank, they loved the V8-like exhaust sound but did not like having to include a balance shaft. It was decided to switch the street R1 to four valves and flat crank mainly because of the sound (just as a parallel twin with 270 degrees instead of 360 between right and left cylinder has the more exciting, syncopated sound of a DUcati V-twin, while the parallel-twin with both pistons moving together makes a boring drone (like a pre-1970 Triumph Bonneville twin).

Racing switched to cross-plane crank for performance reasons - it allowed the bike to accelerate as strongly as the V4.

Production switched to cross-plane crank because it had a high-performance sound - like a V8.