[ Despite the unfortunate result in Jerez (where he was taken out by Ducati’s Valentino Rossi), Repsol Honda’s Casey Stoner has shown incredible speed this season. Honda’s new gearbox is one new advantage among many that has help the Australian and other Honda riders perform so well during testing and in the early season. ]
Honda has returned to competitiveness in MotoGP, and one of the means to this end is a new type of six-speed gearbox. The familiar drum-and-forks shift mechanism used through the past five decades must first disengage the gear pair that is driving, then engage the new gear pair. To ease disengagement and minimize damage as dogs engage, the engine is briefly cut off by a shifter switch (the common ignition-kill quick-shifter). If the shifter switch is set to a delay of .06 second and there are 15 upshifts made under power on a given track, conventional shifting wastes 15 x .06 or 9/10 of a second per lap. If this were the actual yield (it may be less), it is a huge time advantage, and one that comes at almost zero risk to the rider.
The new Honda gear selector mechanism eliminates this shift delay by engaging the new gear while the previous gear is still engaged. Forward drive is essentially constant.
Some clue to how this might work is provided by a common drag-race-only transmission modification. In it, the non-drive face of each gear-engaging dog is cut off at something close to 45 degrees, and a modified shift drum is provided. During an upshift, the new gear is engaged without disengaging the previous gear. As the new gear takes up the drive, the previous gear’s engaging dogs are driven backward, causing their angled faces to contact each other. This kicks the previous gear out of engagement. This simple form of zero-shift-delay transmission is impractical outside of drag racing because closing the throttle causes immediate disengagement of whichever gear is engaged.
Honda engineers have found a way to implement this same function—zero-delay engagement of the new gear, followed immediately by disengagement of the previous one—during both acceleration and deceleration. As pictured in the patent application, the countershaft is of normal diameter and splined. Six gears are stacked on it at close spacing, as there are no engagement dogs and therefore no need for space to allow any gears to move axially along the shaft. The gear stack is therefore only 4 to 5 inches long—much more compact than a conventional transmission.
The output shaft is considerably larger in diameter and is not splined. Six gears, each at all times in mesh with its “ratio-mate” on the countershaft, can spin freely on this shaft. A given gear may be locked to the shaft by means of pegs or tabs, moving radially in holes in the shaft. The pegs or tabs engage recesses in the inner diameter of the gear.
The interesting stuff is inside the output shaft. What it accomplishes is to allow a set of pegs to engage the next gear, then cause the pegs of the previously engaged gear to disengage. There are separate means of engagement for torques in the forward and reverse directions.
This may remind older riders of the “ball-lock” transmissions tried by certain makers in the 1960s. A shift rod with an enlarged end was inserted into a hollow gearbox shaft, and to engage a gear, the enlarged end was moved axially to push out balls, carried in radial holes in the gearbox shaft, to engage recesses in the inner face of the desired gear. There were two major problems with ball-lock transmissions. First, because the engagement of balls-and-recesses took place at a small diameter, forces were large and parts could soon become damaged enough to interfere with shifting. Second, the shift rod had to move the full length of the gear stack, resulting in its sticking out of the shaft to an inconvenient degree at one end of its travel.
Honda’s invention seeks to avoid the first problem by making the output shaft larger so that engagement takes place at a larger radius and so at lower forces. Rules that limit each MotoGP rider to six engines per year do not prohibit gearbox replacement (ratio changes are made at each event), so if necessary the whole assembly could be changed for every practice.
It avoids the second problem by moving the control rod through a distance approximately equal to the total width of six sets of engaging tabs, and staggering the positions of features on the internal cam rods which operate the tabs. Thus, only a small axial movement brings the next set of tabs into engagement. In this system, the “shift drum” is very small, with a single spiral groove able to move the control rod in or out over a total travel of just over one inch.
The precise details of what is inside that output shaft are for the moment obscure to me!
Although some pundits are already announcing an “Era of Honda Dominance,” I think that all that has happened is that Honda, having dozed for a number of years, has now awakened and begun to seriously address its problems in the sport. The zero-shift-delay gearbox is what we have learned to expect from Honda—innovation. The rest of Casey Stoner’s and Dani Pedrosa’s lap time improvement comes from Honda’s finally catching up in the electronics race.
