Yamaha’s Crossplane Crankshaft | Ask the Geek

I’m an ex-aircraft mechanic, so I understand piston engines. I was wondering, though, what is a “crossplane crank,” as in the Yamaha FZ-07? Robert Millette Laval, Quebec, Canada

A crossplane crankshaft is one that eliminates what Yamaha refers to as "inertial torque," and the "crossplane" term was coined by Yamaha with the introduction of the 2009 YZF-R1. Inertial torque is produced in an engine when the crankshaft doesn't spin at a constant speed over the course of a single revolution but rather speeds up and slows down at different points in each piston stroke. This torque, Yamaha contends, can mask the feel of combustion torque to the rider, and eliminating it gives better traction and feedback. This can be accomplished by balancing the engine's primary and secondary forces so that the crankshaft turns at a more steady speed over the course of each revolution.

In a piston engine, the weight of the piston going up and down in the cylinder bore is what causes primary imbalance. This motion repeats with every rotation of the crankshaft and can partially be offset by a counterweight on the crank itself. Any weight added to the crank, however, introduces its own fore/aft imbalance. A counterweighted shaft can also be used, spinning in an opposite direction to the crankshaft at the same speed. Another way to improve primary balance is to add another piston and cylinder beside the first; as one piston goes down, the other going up will offset that motion, and fore/aft motion is also nicely countered. What remains, however, is a rocking couple due to the distance between the two cylinders—like a kayak rocks as you row.

Because the piston’s downward motion is not precisely matched by the upward motion of the counterweight (consider that the piston is not at the midpoint in the stroke when the crankpin is in the 90-degree position), a secondary imbalance results. This repeats twice for every rotation of the crankshaft and, like primary imbalance, can be offset with another cylinder and piston or partially with a counterweight rotating at twice engine speed in the opposite direction of the crankshaft.

Most engines are designed with perfect primary balance, as that gives the least vibration. The twin-cylinder Kawasaki Ninja 650, for example, has a 180-degree crankshaft with one piston going down when the other rises. This gives perfect primary balance, and Kawasaki uses a balance shaft to offset the rocking couple. The engine does not have perfect secondary balance, but in a small engine this can be ignored. A traditional four-cylinder engine with a 180-degree crankshaft likewise has perfect primary balance, and now the rocking couple from one pair of pistons offsets the other, so no primary balance shaft is required. Some larger four-cylinder engines will have a single counterbalancer to partially offset the secondary imbalance inherent in this layout.

The key phrase here is “partially offset.” To fully eliminate secondary imbalance in a traditional four-cylinder engine, two counterbalancers would be required, and they would have to be in specific locations relative to the pistons and crankshaft. The Yamaha four-cylinder crossplane crankshaft with its 90-degree throws (when viewed from the end the crankpins form crossed planes, hence the name) has perfect primary and secondary balance inherent in its design, so secondary counterbalancers are not needed. The engine does require a balance shaft to offset the primary rocking couple, but this need only spin at crankshaft speed.

Yamaha’s three-cylinder FZ-09 engine has three evenly spaced crank throws for the same effect and likewise needs a balance shaft to offset the rocking couple. The twin-cylinder FZ-07 (and Super Ténéré) has a crankshaft with the two throws 90 degrees apart, giving perfect secondary balance but at the expense of a primary imbalance in addition to the rocking couple. In the Super Ténéré this is offset with two balance shafts, while the FZ-07 makes do with a single balance shaft.