TRANSMISSIONS

TRANSMISSIONS

FUNDAMENTALS

The origin and evolution of the motorcycle transmission, from no-clutch belt drives to modern six-speeds and more

KEVIN CAMERON

Motorcycles need the multiple gear ratios provided by their transmissions because their spark-ignition internal-combustion engines have too little range to cover operation from zero to maximum vehicle speed.

Early motorists were pleased that their single-speed motorbikes even ran. They were pleased that, on that single speed, they might effortlessly reach 25 mph—the speed of an athletic bicyclist. They were not so pleased to have to supply “light pedal assist” in climbing hills or, when even that failed, to jump off and push until exhausted.

Rules for the 1911 Isle of Man TT races stimulated motorcycling to adopt amenities that had become standard on automobiles—the “free engine clutch” that made it unnecessary to restart the engine after every stop, and two or more ratios in a gearbox, making it possible to have one ratio for starting and hill climbing, and a higher ratio for enjoying the open road. Indian swept that event, taking first, second, and third with clutch-equipped bikes with two-speed gearboxes.

Alternative means of varying the drive ratio between the engine and rear wheel, such as variablediameter belt-and-pulley systems, were also tried. Gear drives, because of the durability of steel gears, became the majority choice. As more powerful engines increased vehicle top speed, three speeds became necessary, giving way in the late 1920s and early ’30s to the four speeds long traditional on English and European bikes. In the US, where fuel was cheap and abundant, slow-turning large-displacement engines prolonged the usefulness of three-speed transmissions.

As engine power in racing narrowed with tuning, four speeds were no longer enough. The Italians moved on to five or more speeds, and aftermarket fiveand six-speed clusters were made to replace classic four-speeds.

Then came the “one-note samba” of racing two-stroke power: engines so dependent upon exhaust resonance that by the mid-1960s, as many as 12 speeds were necessary to stay within a 500 rpm powerband. When that classic era of Japanese participation in GP racing ended after 1967, the FIM decided to limit gearbox speeds to six, which has since become the norm for production bikes. This now makes us chuckle because automatic transmissions in cars, in order to keep their engines on their “island of minimum fuel consumption” at all times, are being given eight or even 10 speeds.

“Most promising as the best of both worlds is the DCT, or dual-clutch transmission. ”

All of these transmissions shift gears by passing out of one gear, through a neutral, and then into the next gear, a function performed sequentially by incremental rotations of a shift drum or plate controlling axial movements of the shift forks that move the gears. Unlike the gate shift long used in cars, such sequential gearboxes cannot be shifted directly from any gear to any other.

Because the number of manualshift autos has steadily declined, numerous attempts have been made to interest the motorcycle market in some form of automatic gear changing. For a long time, the response was negative due to the apparent conviction of buyers that “you have to shift gears to be a real motorcyclist.” Today that attitude may be changing, especially since auto and bike racing have embraced alternate transmission types.

Back in the 1960s, Fuji’s Rabbit scooter had a three-element torque converter (aka “slushbox”), while Mitsubishi’s Silver Pigeon revived the variable pulley drive that is common today on snowmobiles. Both of these lacked the distinctive shift sound, instead allowing their engines to efficiently operate at a constant rpm while the vehicle accelerated.

Many small autos are built with a variety of the CVT, or continuously variable transmission, but although convenient for those without manualshift skills, they are criticized for an engine sound often described as “a sickly moan.”

So strong is the public perception of how an accelerating sports vehicle should sound that when Honda gave its Foreman four-wheeler an optional hydrostatic drive that efficiently held its engine at constant peak performance during max acceleration, buyers insisted it be reprogrammed to simulate the traditional sound of an engine accelerating through multiple gears.

Most promising as the best of both worlds is the DCT, or dualclutch transmission. It “sounds right” yet eliminates the need to pass through a neutral during shifting by having two clutches, one for the odd-numbered gears and another for the evens, located on a concentric but independently spinning shaft. To shift, one clutch engages the next gear while the other releases the previous ratio. It can operate as a full automatic if desired.

Because DCTs are strangely not legal in MotoGP, yet another form has been developed—the so-called seamless-shift transmission. In this type, when the next ratio is selected, the previous ratio is kicked out of engagement, thereby preventing a lockup. The attraction of seamless is not the saving of shift time but the increased smoothness of its upshifts, which is less upsetting to rear-tire grip.

Try the available options and pick something you like.