At the recent Intermot show in Germany, Yamaha introduced a concept model with a three-cylinder engine using the crossplane concept from the YZF-R1 and YZR-M1. It wasn’t actually a complete motorcycle, but rather an engine suspended between two wheels. A YZF-R1 engine was suspended above and a YZR-M1 MotoGP engine behind, intended to indicate the three-cylinder’s design roots. According to the press material, “This new concept model represents an important and significant development with regard to Yamaha’s plans for the company’s future direction in the motorcycle marketplace. Yamaha firmly believe that this new engine concept will open up new horizons in riding enjoyment.” The implication is, of course, that this new engine will be used extensively in the company’s model lineup — perhaps including sportbikes.
The associated press material — a terse, four-paragraph press release and several images of the concept model — confirms the main goal of the three-cylinder crossplane engine is to eliminate the inertia forces. “[The YZR-M1 and YZF-R1] have become famous for their linear driving power which is a direct result of the ‘clean’ torque output created by the elimination of inertial torque to nearly zero. And it is this crossplane concept of eliminating the unwanted inertial torque to create clean torque that forms the basis of the new three-cylinder Yamaha engine.”
One logical extrapolation of the crossplane concept for a three-cylinder engine is a crankshaft with its journals offset by an evenly spaced 120 degrees, just as the R1’s four-cylinder crank journals have an even spacing of 90 degrees. This spacing, as it does on the R1, balances the primary and secondary inertia forces to practically eliminate inertia torque. A balance shaft would be required to deal with the primary rocking couple created, similar to what’s used on the R1.
"…the new triple “has the potential to bring race-inspired performance to the street" "
So far, this is nothing new. Triumph and MV Agusta use evenly spaced crankshaft journals on their three-cylinder middleweight engines, along with balance shafts. The original patents for Yamaha’s crossplane crankshaft, however, provide some insight. The patents describe how the crossplane concept in a four-cylinder engine results in no net primary or secondary inertia forces. One method is to use evenly spaced journals, as on the R1. But a second method details a crankshaft journal spacing that results in natural cancellation of only the secondary inertia forces; the primary forces are then eliminated by two balance shafts, one spinning opposite the crankshaft direction, and one in the same direction (both at the same speed as the crankshaft).
Yamaha’s Super Ténéré has such an arrangement. The twin-cylinder’s crankshaft has a journal spacing of 90 degrees, cancelling the secondary inertia forces, while the primary forces are eliminated using two balance shafts. In a three-cylinder engine, the three journals would be spaced at 60-degree intervals to achieve a similar result, and the engine would likewise need two balance shafts to deal with the primary inertia forces. This solution would also give some interesting options for firing order, and definitely distinguish a Yamaha triple from others currently on the market.
The patents also detail a design where specifically shaped crankshaft counterweights can be made smaller and lighter, with the resulting imbalance accounted for in the balance shafts. The realization of the patent is seen in the R1’s crankshaft, which has uneven counterweights for each cylinder and smaller counterweights on the outside cylinders than on the inside cylinders. With smaller and lighter counterweights, the engine in turn can be lighter and more compact. Assuming this part of the crossplane concept carries over to a triple, it would help counter some of the additional weight and complexity of the two balance shafts.
Yamaha has plenty of experience with three-cylinder engines. On the motorcycle side, the company manufactured the XS750 and XS850 in the late ‘70s and early ‘80s. More recently, its Nytro line of snow machines utilizes three-cylinder four-stroke engines. Introduced several years ago, the Nytro Genesis engine is essentially three FJR1300 cylinders on a 120-degree crank, even sharing some parts with the sport-touring machine.
That said, Yamaha claims that the new triple “has the potential to bring race-inspired performance to the street,” indicating that the company’s sportbikes will switch to the three-cylinder engine at some point in the future. This will certainly bring a new twist to the market, especially in the literbike class, where there hasn’t been a triple since the Triumph 955i Daytona 10 years ago.