In mounting one of the most impressive comebacks in Grand Prix history (even if it did come up a little short), Valentino Rossi demonstrated that the '06 version of Yamaha's inline-four YZR-M1 still had the necessary performance to win in MotoGP. Winning five races over the course of the season (more than any other rider), plus teammate Colin Edwards II coming within 300 feet of winning the Dutch GP, was ample proof of the bike's potential. Only some bad luck and a certain rider by the name of Nicky Hayden prevented Rossi and Yamaha from notching up their third consecutive MotoGP crown.
After five years of a 990cc displacement limit for the MotoGP category, 2007 sees the MotoGP machines shrink to 800cc. Although many are predicting that the overall power and speeds won't decrease that much in the switch to the smaller displacement, the fact that '06 was the curtain call for development of the liter-class machines made the opportunity to try out the last of these racebikes before they disappear something that couldn't be missed. Thankfully, yours truly got the call to fly out to Valencia's Ricardo Tormo circuit for a 15-minute test session on Rossi's M1 racebike. It was truly an eye-opening experience-and no, not because of the sheer speed and claimed 250 horsepower.
Development Of The Yzr-M1 Since this was the last of the 990cc-generation machines, Yamaha was much more open about many mechanical aspects of the M1 without fear of industrial espionage. This included many details regarding the M1's development process, from its debut years in '02 and '03 with riders Max Biaggi and Carlos Checa to the then-present form at Valencia with Rossi and Edwards.
Although the inline-four engine architecture was used throughout the M1's lifespan (and continues onward to the 800cc era), continuous evolution enforced by the intensely competitive arena of MotoGP resulted in numerous changes to both engine and chassis in an effort to maximize handling agility, power and the ability to fully use that power. As power outputs inexorably rose higher with every year (from its '02 debut to the final version of the '06 M1, power increased a total of approximately 35 horsepower, with a corresponding 3000-rpm increase in rev limit), tire technology increased as well, leading to ever-higher levels of grip. However, that increased level of traction led to chatter from the increased loads, requiring constant changes to the chassis. It's also interesting to note that the engine's overall size shrank every year despite the ever-increasing power output, enabling Yamaha engineers to lengthen the swingarm for improved handling and traction feedback.
The original '02 M1 used the chassis from the previous YZR500 two-stroke in order to gauge which direction to take with that area of development. The engine used a conventional even firing pattern that unfortunately made the bike difficult to handle accelerating off corners due to the engine's 210-horsepower output. Biaggi managed to score two wins that year and finish second in the points standings, but Rossi, then on the RC211V, had easily sewn up the title with several races to spare.
In '03, Yamaha tried to soften up the M1's power by using electronic fuel injection for the first time (the previous version actually used flat-slide carburetors), as well as adding the ICS (idle control system) to reduce the engine braking effect into the corners. However, power was also increased by about 10 horsepower via a more oversquare engine configuration, and all four of Yamaha's riders that year complained of the engine still being difficult to handle, as well as the ICS being inconsistent in performance.
Big changes ensued for '04, with Rossi's sensational defection from Honda to Yamaha overshadowing the major revisions to the M1's engine. The signature Yamaha five-valve cylinder head was dropped in favor of a four-valve unit, the bore/stroke ratio was once again made even more oversquare, and the crankshaft firing order was changed to an uneven pattern that was easier on both rider and rear tire. The chassis also underwent some subtle modifications that reduced lateral and torsional rigidity with the goal of quelling the midcorner chatter problems that were encountered with the new breed of ultra-grippy racing slicks in MotoGP. But it was the Long Bang firing format that was credited with improving traction and tire life enough to enable Rossi to garner yet another world title.
The following season brought with it a new 22-liter fuel-capacity regulation (down from 24 liters) that required improved fuel economy, in addition to a necessary increase in power to address the specter of Honda looking for some payback for losing the '04 title. Increasing power meant-once again-shortening the stroke for more rpm (resulting in a gain of more than 1000 rpm on top). In order to keep the engine's overall dimensions as compact as possible, the hybrid chain/gear cam drive was converted to a complete gear-drive assembly and moved to the center area behind the cylinders. The chassis also underwent further revisions aimed at reducing lateral and torsional rigidity even more in the pursuit of improved cornering feel. Engine electronics also reached new heights in power and capability, with traction control now enabling even better late-race tire management.