The Vertically Stacked Gearbox

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Kevin Cameron has been writing about motorcycles for nearly 50 years, first for <em>Cycle magazine</em> and, since 1992, for <em>Cycle World</em>. (Robert Martin/)

A well-informed reader has noted that the vertically stacked gearbox in Harley-Davidson’s new 1250 Pan America is nothing new. Right he is! In 1984 Yamaha introduced vertical stacking in its YZR500 two-stroke GP bike as a means of allowing use of a longer swingarm. Ducati, during its short-swingarm (19 inches) era, often resorted to overly stiff rear springs to prevent large changes in the squat/antisquat behavior of its Superbike racers. The longer the arm can be made, the smaller the angle through which it rotates over a given wheel travel, and the easier it is to achieve consistent rear ride height.

Another motivation to adopt vertical stacking is to shorten wheelbase. In the earliest days of motorcycling, the Belgian firm FN produced a bike with a longitudinal four-cylinder engine, noted for its smoothness. But the engine’s position and length, and the resulting long wheelbase, resulted in a motorcycle known for “the dreaded side-slip.” Get a long bike’s chassis swinging and it can be very hard to stop.

Aprilia’s RS 660 stacked gearbox is so compact it is nearly hidden behind the clutch when viewed from the right side. (Aprilia/)

When Italian flying corps vets Carlo Guzzi, Giorgio Parodi, and Giovanni Ravelli decided to build motorcycles in 1921, one of their primary design goals was cooling. Their solution was to orient the machine’s single cylinder horizontally, pointing the cylinder head, the engine’s hottest part, into the wind. This made the engine longer than a powerplant with a single vertical cylinder, yet the trio knew they must somehow avoid the FN’s overlong wheelbase. Their answer was to stack their gearbox shafts one above the other, rather than the more conventional one-behind-the-other layout.

Only a couple of years later, two young graduate engineers, Carlo Gianini and Piero Remor, corrected FN’s mistake by placing their in-line four-cylinder engine transversely, allowing the wheelbase to be much shorter. In time their prototype evolved into Gilera’s Grand Prix-winning four of the 1940s and ’50s, whose legacy shines through in every transverse inline-four today.

The 500 Gilera with Geoff Duke on the way to victory at the 1953 Assen GP. (WikiCommons / Fotocollectie Anefo/)

In the early 1970s, when Fabio Taglioni tackled the twin problems of making more power and getting rid of the numbing vibration of Ducati’s 450 desmo, he famously chose the self-balancing 90-degree V-twin architecture. But also, knowing the long history of overheating that has plagued the rear cylinders of conventional air-cooled V-twins, he placed his front cylinder almost horizontal, much as Guzzi had done, with the rear cylinder near vertical. In this way each cylinder could receive cooling air that had not been heated by the fins of a cylinder ahead of it.

The result was a very long engine, so Taglioni sensibly chose a vertically stacked gearbox arrangement to save length, just as Guzzi had done. This was not ideal, as the wheelbase was long and the steering slow, but it was good enough to take a 1-2 win at the 1972 Imola 200, establishing Ducati as a maker of large-displacement bikes to reckon with.

The vertically stacked transmission of the 1972 Ducati Imola racebike shortened what was an already long engine. (Seth DeDoes/)

Vincent engineer and revered technical writer Phil Irving notes in his autobiography that the competition potential of his company’s iconic postwar air-cooled V-twins was limited by rear cylinder overheating.

Another “taker” for the idea of vertical gearbox stacking was Guzzi’s great designer of the 1950s, Giulio Cesare Carcano. As he reached for 500cc Grand Prix success with his hugely ambitious Guzzi V8 it was only natural to give such a bulky engine a length-reducing vertically stacked gearbox.

After Yamaha’s success with the concept in GP racing, vertical stacking leapt to production, bringing the same suspension benefits of a longer swingarm. Today it no longer looks odd to see the clutch mounted up so high on the right-hand side—the prime indicator that the bike you’re looking at has vertically stacked gearbox shafts.

Now the question must arise: Why was it ever done any other way? There are several answers to that. For many of us, myself included, what has long looked “normal” is horizontally split engine cases with all the shafts set into them in a single plane. Crankshaft is up front, then the gearbox input and output shafts, and finally, maybe even a kickstart shaft. Although this creates increased engine length, mostly small-to-mid displacement engines were built this way, and it also simplified assembly on production lines.

The British tradition dictated a separate gearbox behind the engine, driven by chain and sprockets. Engine and gearbox were joined by a pair of fore-and-aft engine plates which in turn bolted to points on the frame. It took time for this system (which allowed a company to easily use gearboxes from different makers) to be replaced by modern unit construction, with the gearbox located in the engine cases.

Another point was gearbox lubrication. For many designers it was only natural to put both gearbox shafts in the same plane, as in popular two-strokes the gears could then be lubricated by filling up the gearbox to its shaft level with oil. When the substantial loss caused by gearbox oil churning was realized, progressive designers like Noriyuke Hata (who designed Yamaha’s first 125 GP engine for 1961) began to provide a small gearbox lube pump, supplying oil jets directed into each gear mesh. In our present four-stroke-dominated era it has become normal to lubricate gears from the engine oil pump, so their shafts can be positioned at will.

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