You’ve probably noticed that on the majority of late-model sportbikes the projecting round cover for the clutch is located surprisingly high on the right side of the engine. This is because the two gearbox shafts—the input shaft carrying the clutch and the output shaft carrying the drive sprocket—are “vertically stacked” one above the other rather than one ahead of the other as they used to be.
When I was a very young man and first took apart a Japanese-made motorcycle engine, I was impressed by its rationality and ease of assembly. Once the lower half of its horizontally split crankcase was removed, all parts were in plain view—crankshaft at the front, then the gearbox input shaft, the output shaft, and behind them, the kickstart shaft—neatly sandwiched between the two crankcase halves. This simple layout, I would soon find, also allowed easy transmission setup (how deeply the various dog sets engaged each other).
That remained the usual way to build engines for many years, but eventually came a series of revolutions forcing tires (from 1972 on), suspension (1974 through the 1980s), and chassis (1980s) to evolve rapidly. The revolutions were driven by the fast-rising power of two-stroke racing engines, at first in AMA 750cc roadracing and then as the Japanese makers shifted their attention to winning European Grand Prix.
A motorcycle’s swingarm rear suspension makes it tricky to keep the bike from squatting at the rear when power is applied. When squat occurs in a corner, it disturbs front/rear grip balance, causing front end “push” and the bike refuses to hold line. Therefore engineers found ways to use other existing forces (such as the tension in the top run of the chain) to oppose and cancel squat. Trouble is, the relationships making this balance of forces possible change as the swingarm moves through its suspension travel. The longer the swingarm is made, the smaller these changes become and the easier it becomes to keep squat/anti-squat under control over the full suspension travel.
In the early 1980s, Yamaha decided to make the swingarm on its YZR500 GP bike longer. Engineers couldn’t just put in a longer swingarm because: 1) The longer the wheelbase, the slower a bike steers (those super-long-chassis hook-and-ladder fire trucks need steerable rear wheels); and 2) extending the swingarm length to the rear shifts weight to the front wheel, making the back tire spin more easily. That left only one alternative: to move the whole swingarm forward, allowing it to be made longer without changing the wheelbase. But the engine is in the way, making that impossible.
Back in 1923, Moto Guzzi had a problem: The long horizontal single-cylinder engine it chose to build pushed the crankcase and gearbox to the rear. The obvious path to an acceptably short wheelbase was to shorten the engine/gearbox unit by stacking its gearbox shafts one atop the other, so that’s what Guzzi did. And so, when Yamaha needed room to lengthen the swingarm of its YZR500, its engineers did the same. Because the clutch is on the end of the gearbox input shaft, putting that shaft atop the output shaft raised the position of the clutch (the output shaft could not be lowered because the geometry required to prevent chassis squat during acceleration dictates how high the output sprocket has to be).
At first, this had perceived value only in racing, but as competition intensified in production-based superbike and supersport racing that once upon a time did so much to promote sportbike sales, one model after another was given vertically stacked gearbox shafts at its next redesign.
That’s why modern sportbikes now have such high-mounted clutches.