ASK KEVIN: Would You Please Explain a “Seamless” Gearbox?

QUESTION: Could you explain the seamless gearbox that seems to have become essential in MotoGP? Indeed, I read that Nicky Hayden believes that the poor performance of the customer RCV213 is partly due to the fact that the bike was designed to use a seamless gearbox but that is not available. I understand that dual-clutch transmissions as used in many cars today are banned but does the seamless box essentially provide the same capabilities?

Duncan Laidlaw

CycleWorld.com

ANSWER: On race bikes, upshifts are generally made without the clutch. To soften the "clunk" as the shift goes through, a shifter switch built into the shift linkage cuts the engine's ignition for a programmed instant. This is valuable because a conventional gearbox must pass through a neutral before the next-higher gear is engaged. If the ignition were not cut, the engine would accelerate sharply during this neutral, magnifying the clunk as the next-higher gear engages. Even with a shifter switch, there is engine speed variation during the neutral period.

As shown by data from onboard Inertial Measuring Units (IMU), even with a shifter switch, the upshift gear-engagement clunk causes the motorcycle to lurch forward, making the bike pitch slightly to the rear. Some of this motion is in the suspension and some takes place within the flexibility of the tires.

If the bike is leaned over, accelerating out of a turn, this pitch impulse continues as a slight fore-and-aft rocking, which causes the loads on the two tires to vary. Variations in tire loading reduce grip, so the bigger the gear-engagement clunk, the greater the potential upset to tire grip.

A seamless gearbox is one that does not go into a neutral between disengaging the first gear pair and engaging the next pair. It engages the next ratio while the current ratio is still driving, and disengages the previous pair before the gearbox can lock up from a double engagement. This means that (1) the engine continues driving the rear wheel at all times during the upshift and (2) the clunk caused by the upshift is reduced to its minimum possible value—just the difference in engine rpm between the two ratios. You can bet that this is further softened by some electronic trickery.

When the seamless gearbox first appeared in 2011 on the factory Hondas, we all reflexively first thought of the time saved by eliminating the neutral between gears. If there are, say, 30 upshifts per lap, and shifter-switch cut-off period is 0.012 seconds, a seamless box ought to give 30 X 0.012 = 0.36 seconds more time when the engine is driving, which in turn ought to improve lap time.

But that’s not what riders perceived. What they perceived was smoother, more confident acceleration off corners, because the size of the upshift clunk was reduced to the minimum possible. In many cases, riders with conventional gearboxes find the upshift so upsetting to the bike in corners that they either short-shift or overrev their engine to move a given upshift to a less upsetting place on the track. Riders talking about this have said the seamless gearbox gives them more options.

The seamless gearbox is also of advantage in improving braking stability (which doesn't need any great clunks to cope with). Yamaha in reply to Honda's gearbox was to develop one of its own, but as it was at first upshift-only, the company continued to suffer a deficit in braking stability until the engineers were able to add seamless downshift capability as well.

To learn more, Google “Honda seamless gearbox” and look particularly at the patent line drawings that come up when you select “images.”

In Honda’s design, the engage/disengage mechanism is entirely inside one of the two gearbox shafts, and operates by means of rocking pawls actuated by multiple axially sliding cam-rods. As these small parts must transmit very large forces, I suspect replacement is frequent. There is one mechanic detailed to this work, which is performed away from all but his eyes. Annual lease cost for the seamless gearbox was at one time 300,000 euros.

The other gearbox shaft is splined and its gears are solidly engaged to it at all times.

Send your “Ask Kevin” questions to cwservice@cycleworld.com. We cannot guarantee a reply to every inquiry.