R1 MotoGP Wizardry
You may think your connection to MotoGP World Champion Valentino Rossi is distant, but with the 2009 Yamaha YZF-R1, that bond is tighter than you think. And you can credit Yamaha’s latest technological advance in four-cylinder engine design to none other than ‘The Doctor’.
No, the charismatic Italian superstar, who possesses an unnatural mastery of motorcycle control, hasn’t hung up his leathers to sit at a drawing table designing engines. Instead, he selected the odd-firing crossplane crankshaft over the traditional single-plane crankshaft when he first tested for the Yamaha MotoGP team. That crankshaft design is the latest performance-enhancing innovation in the new R1.
Part of Rossi’s motivation to switch race teams in 2004, after wining three consecutive titles with Honda, was Yamaha’s insistence that he take an active part in the development of the race bike. This appealed to Rossi, who, in his autobiography, What If I Had Never Tried It, described Honda as insensitive, even cold, in their relationship.
During initial testing with Yamaha, Rossi rode the YZR-M1 MotoGP machine with four different inline-four engine configurations: a five-valve head design with a conventional crankshaft and firing order, a four-valve head with conventional crank/firing order, a five-valve head with a crossplane crankshaft, and a four-valve head/crossplane combination. Rossi’s seat-of-the-pants impression dictated that the version with four valves per cylinder and the odd-firing crankshaft worked best.
Although this engine configuration did not produce the highest output, Rossi described its power characteristics as “sweet” to engineers. That sweetness instantly translated into race wins, as Rossi won the first race of the 2004 season on the bike’s competition debut. He went on to win the championship that year, and took the MotoGP title again in 2005 and last year. Although chassis and engine development have evolved since that first championship-winning M1, one thing has remained constant-the engine/crankshaft layout.
Yamaha had been using five-valve Genesis cylinder heads on street bikes since 1984, as it did on the R1 since its introduction in 1998. After development of the four-valve M1 proved one less valve per cylinder was more efficient, Yamaha went to a four-valve head on the R1 in 2007.
Two years later, the new R1 gets that sweet crankshaft. An engine using a crossplane crankshaft is not to be confused with what is commonly known as a big-bang engine, which is mostly associated with odd-firing V-fours. Some race teams have tried big-bang inline fours-using a regular single-plane crank with cams and ignition reconfigured to fire the cylinders in pairs effectively acting like a 180-degree parallel twin-though this layout proved ineffective and produced excessive vibration.
A crossplane crankshaft is not intended to boost power output; there are other means of accomplishing that goal. The main reason Yamaha chose this layout is because of its odd firing order. On an inline four using a traditional single-plane crankshaft, pistons rise and fall in pairs: cylinders 1 with 4, and 2 with 3. Combustion intervals are therefore separated evenly every 180 degrees of crank rotation, with a 1-2-4-3 firing order-this is what gives an inline four its humming exhaust note. In the crossplane design, crankpins are set at odd intervals, with combustion arriving at 270, 180, 90 and 180 degrees and a firing order of 1-3-2-4. If you were to look lengthwise at both crankshafts with the crankpin for cylinder 1 facing up, on the single-plane crank you would see only crankpins 1 and 2 splayed vertically; on the crossplane crank all four crankpins would be visible and forming a cross.
All of this research into odd firing orders is critical because it affects drivability and feel. These abstract concepts are better described as the connection between throttle hand and rear tire-not so critical on small-displacement machines, but of paramount importance as power output climbs to open-class levels. Odd firing orders are naturally produced in the V-4 and V-5 engines found in the championship-winning Ducati and Honda MotoGP bikes.
To date, no traditionally firing inline four-cylinder MotoGP machine has won a race, as evidenced by the Kawasaki ZX-RR, whose pilots often complained of poor corner-exit traction. Corner exit speed is a vital component of MotoGP racing. The harder you exit a turn, the higher the speed you will attain at the end of the following straight, and the quicker you will reach the next turn-it is all about lap times.
The way in which power pulses transfer to a crankshaft greatly affects the way power transfers to the rear wheel. There are two kinds of torque applied to a crankshaft: combustion and inertial. Combustion torque is produced on the downward stroke of the piston as the cylinder fires, while the momentum of the spinning crankshaft produces inertial torque. Inertial torque interacts with combustion torque, producing composite torque, and it is this composite torque that Yamaha claims affect drivability and feel.
On a traditional single-plane crankshaft, the evenly spaced firing intervals counteract with a crank that accelerates and slows every 180 degrees-all four pistons come to a stop at the same time twice per revolution at the top and bottom of the cylinders. This causes composite torque to fluctuate greatly, which is detrimental to drivability and makes it challenging for a rider to feed the throttle when exiting from a corner. Because of a crossplane crankshaft’s odd firing intervals, inertial torque is more constant and composite torque more stable, closely matching the engine’s power pulses. Yamaha engineers claim the resulting smoother composite torque produces a more manageable powerband, improving feel and traction at corner exit.
Despite what Yamaha engineers might believe in theory, all this engine wizardry is meaningless if it does not work. The crossplane crankshaft design earned Rossi’s stamp of approval, and we have seen what he has accomplished with this engine layout.
I had the chance to sample Yamaha’s crossplane crankshaft a few years before it became available in the R1. In 2006, I was offered the rare opportunity to ride Rossi’s 990cc M1 following the final MotoGP round in Valencia, Spain. That year he struggled with early season handling problems. Excessive wheel chatter was eventually traced to an incompatibility of the chassis with newly developed Michelins. Changes in the frame to alter its flexibility eventually cured the chattering, and Rossi rebounded from a mid-season 51-point deficit to lead the championship by the Valencia round. A rare unforced crash by Rossi in that race assured Nicky Hayden the MotoGP title.
Riding the M1 was an imposing proposition, as only ten were in existence-four for Rossi, four for teammate Colin Edwards, and two test mules. Once fired, the M1 sounded more like a V-four than an inline-four, the first hint that things were different in the engine.
Having only previously “ridden” the Valencia circuit virtually in a video game, my first real lap was taken cautiously. At my modest pace, I was immediately struck by how this purebred, hand-built race bike at the pinnacle of motorcycle performance-with a machined-from-billet engine using the most advanced electronics, in a handcrafted, fully adjustable frame-felt remarkably similar to an ordinary street bike.
Gone are the days when viciously temperamental two-strokes terrorized riders-machines on which throttle control required the dexterity and finesse of a microsurgeon. At moderate throttle openings, the M1’s fuel injection operated flawlessly; the engine accelerated smoothly and without stumbling. The bike steered with a slight vagueness, not quite reacting to handlebar input as I had become accustomed to on a Supersport. It required higher effort, but turned in quicker. Then, when I turned onto the front straight for the first time, I twisted the throttle full and the M1 transformed into the world-class race bike it most certainly is.
With a claimed power output of “around 240 horsepower” and a published claimed dry weight at the FIM-mandated minimum of 326 pounds, the M1 had an absurdly power-heavy power-to-weight ratio. Its vision-blurring acceleration was like nothing I had felt before. The engine pulled hard from a relatively low rpm and pulled progressively harder-much harder-as revs increased. However, despite this explosive delivery of power, the bike was more manageable than anticipated, no doubt partially due to the bike’s fly-by-wire throttle.
As speeds increased a couple of laps into my five-lap session, the handling that I had grappled with earlier began to come around.