What Is The Difference Between A Flat And A Crossplane Crankshaft?

Choose the exhaust note you prefer, a ripping bedsheet or a V-8

This Honda CBR600RR crankshaft is a "flat" crank because two of its four crankpins are up and two are down. One hundred and eighty degrees later, those relationships are reversed. If we were to diagram this crankshaft as a zigzag, it would all be in one plane. So we call it a flat crankshaft.

Based on an experiment devised by Masao Furusawa, an engineer who is now retired, Yamaha decided it might be better to orient the crankpins at 90 degrees to each other. That type of crankshaft is referred to as “crossplane” because the four crankpins are now in two planes. Those planes are crosswise to one another, hence the term.

If you have listened to Yamaha’s YZF-R1-based Superbikes on the racetrack, you may have noticed that they have a sort of V-8 sound. Part of the reason for that is production V-8s have 90-degree crankpins.

Honda’s flat-type crankshaft gives an exhaust sound that is Suzuki GSX-R-like; it makes that ripping-bedsheet sound so many of us find thrilling. The crossplane that Yamaha is using on its R1—and which Suzuki has adapted on its MotoGP bike—adds a second, deeper sound, which also has charm. I like to listen to both.