The crossplane I4
Yamaha is the only company running an I4 in the MotoGP series, a layout to which it has been loyal since the inception of the current four-stroke formula; all the other engines are now V4s. Having the cylinders in a vee, rather than in a line, allows the crankshaft to be shorter and stiffer, with fewer main bearings, and the engine to be narrower. This is, however, perhaps not such a great advantage.
The reasoning can be found in the width of the human knee. Complete with protective clothing, a biker is at least 350 mm across the knees, even with them clamped tightly together. With a chassis in between them, the rider's knees are a lot wider than most engines. Using similar reasoning, ankles are about 125 mm wide each, and they have to go each side of the swinging arm (which, in turn, must be wider than the rear tyre) so there's no point in trying to make an engine much narrower than this. (In the previous Grand Prix motorcycle racing formula, for 500 cc two-stroke engines, Honda made both V2- and V4-engined bikes, and the difference between them in overall width amounted to only a few millimeters.)
Perhaps surprisingly, having a light crankshaft of low inertia seems to give no particular advantage in bike racing. Riders are usually happier with the bigger flywheel effect of a heavy crankshaft. In other forms of bike racing, like motocross or flat-tracking, it is common for riders to optimise the crankshaft inertia with bolt-on flywheel weights. Bizarrely, the motorcycle crankshaft is thus now a piece of sports equipment which, like a shotgun, cricket bat, or golf club, has an optimum inertia that must be tuned to individual competitors.
A current MotoGP bike spends a surprisingly small proportion of a race developing full power. Wide-open throttle (WOT) is commonly used for less than 20% of a lap, even on high-speed circuits. Critical to improved lap times is best possible use of available grip when accelerating away from the apex of a corner. To this end, MotoGP bikes rely on a combination of advanced traction control systems and measures to improve the riders' 'feel' of the tyre grip.
A racing motorcycle can bank to more than 50º from the vertical, and its movements in yaw and pitch are much more significant than those which a racecar experiences. The inertial and gyroscopic effects of the crankshaft are certainly large enough for riders to notice, and they prefer the feel of a heavy crankshaft because the flywheel effect of a high-inertia crank slows down its rate of response and makes it easier to control.
In trying to improve the performance of their I4, Yamaha engineers speculated that an inherent (and hitherto unnoticed) advantage of the vee-configuration engine was the constant crankshaft speed, due to one piston being somewhere near to maximum velocity as its partner (which shares the same crankpin) is at standstill. By contrast, the cyclic speed variation of the crankshaft of a conventional I4 engine is relatively large, because all the pistons change direction - that is, come to a halt - at the same time, and all of them reach their maximum speeds at more or less the same time.
Yamaha's solution, the 'crossplane' layout of the latest I4 MotoGP crank designed to mimic the V4's low 'inertia torque', gave rise to a more constant crankshaft speed, and also to a lop-sided exhaust note due to the irregular firing intervals.
As is often the case in race engine design, an elegant theory has failed to make much impact in the real world. The Yamaha MotoGP engine is down on power compared to its V4 rivals. Yamaha released a production sports bike with a crossplane crank, which was hailed with great fanfare by journalists for its uncanny grip out of corners, and yet in racing for modified production bikes - where the crossplane crank competes with conventional 'two up, two down' I4s - the Yamaha is only tolerably competitive in the World and British SuperBike series. It also made no impression on the leader board at the 2011 Isle of Man TT races, a 'real roads' event where one might expect the Yamaha's supposed improved traction to be most significant. All the TT races this year were won by conventional flat-crank I4s.
Fig. 1 - Yamaha's crossplane I4 crankshaft
Written by Ian Cramp