What Is A Vertically Stacked Gearbox?

To achieve maximum acceleration, motorcycles with short wheelbases need all the help they can get

If you accelerate very hard on a fairly powerful motorcycle, opening the throttle generously in first gear, the front wheel will lift. If you were to instrument that motorcycle, you would find the front wheel starts to lift at a certain rate of acceleration. Driving force is applied at ground level by the back tire but the mass of the motorcycle acts as if it were concentrated at the center of mass, which is somewhere 20 to 22 inches above ground. The combination of force acting at ground level and the resistance of the mass of the machine being at a higher level means a torque is created that lifts the front wheel off the ground.

The point at which the front wheel lifts in acceleration would determine your performance at the dragstrip. If you were making one run after another, you would soon find yourself leaning forward on the machine to put more weight on the front end so you could accelerate harder before the front wheel lifted. When you went home to your shop, you would say, “I’m going to make a longer swingarm to put even more weight up front.”

The roadrace people can’t do that because they need a short wheelbase so their bike can turn quickly. They would say, “Okay, we are going to build a new frame. We are going to move the engine forward.” Well, hold that thought.

This 500cc Kawasaki engine is built the old way: Everything is lying in an open clamshell—crankshaft, gearbox, input shaft, output shaft. Behind all that is the kickstarter shaft. Lining everything up in a single plane makes the engine very easy to assemble, but it also makes for a long engine. With a long engine, it is harder to concentrate more mass of the engine forward to increase the rate at which you can accelerate on the motorcycle before the front end lifts. If you keep on opening the throttle after the front end lifts, it lifts higher, and the higher it goes, the easier it is to lift it more, until finally you are standing at the parts window ordering another taillight lens because you have wheelied over backward.

Back in 1922, Moto Guzzi in Italy built a motorcycle with its one cylinder horizontal. That made the engine quite long. So instead of having the gearbox shafts one ahead of the other, the engineers put one above the other and saved that amount of length. That allowed them to make the wheelbase shorter. That was the origin of the vertically stacked gearbox.

In 1984, Yamaha applied vertically stacked construction to its two-stroke YZR500 Grand Prix racer. On this Honda CBR600RR, you can see the gearbox input shaft and, almost directly below it, the output shaft. This engine has vertically stacked gearbox shafts. This results in the clutch being higher than it was in older engines. So when you look at the right side of a late-model Japanese sportbike engine, you will see that the clutch seems unusually high.

Vertically stacking the gearbox shafts allows the engine to become shorter so more of its mass is concentrated forward to enable the motorcycle to do what it does best, which is accelerate.