Indian Created The Motorcycle Gearbox

Until the 1911 Isle of Man TT races, motorcycles were very simply driven, usually by belt or chain to a pedal sprocket, then by belt or chain to the rear wheel. There was no clutch, so starting was achieved by lifting the exhaust valve, pedaling furiously, then dropping the valve and hoping the engine fired.

Cars—too big to be set in motion by such athletics—already had free-engine clutches and multi-speed gearboxes. To start, you put the transmission in neutral, adjusted the manual throttle lever, retarded the spark timing (because normal ignition timing would produce a kickback), then trotted around to the front to pull up smartly on the engine’ starting crank. Why up? Because it gave some protection from a potentially arm-breaking kickback.

Compression ratios were low to avoid the knock that early gasoline so easily provoked, making it possible to hand-crank big engines. The presence of a clutch allowed easy starting from rest or stopping with the engine running.

TT authorities decided it was time for motorcycles to achieve driveline parity with cars, so pedaling gear was banned for the 1911 event. Makers then had to provide a better way to extend the engine’s limited range of pulling. A variety of schemes were tried, such as variable-ratio belt pulleys or hub gears. The American-made Indians arrived with free-engine clutches, two-speed gearboxes, and all-chain drive. And they were fast; they swept the event 1-2-3. Gearboxes were the future.

Why did this transition take so long? Bikes having only one or two cylinders, their power came in big thuds that quickly fatigued and broke roller chains, even though they were an early application of stronger nickel alloy steel. Belt drives, on the other hand, just slipped a bit at each thud and kept on working.

Once some form of protection from the engine’s banging was provided, chains and gears became more reliable. Good quality gears already existed on the automotive side—heat-treated steel alloyed with nickel chrome vanadium had the toughness to resist breakage and case-hardening provided hard tooth surfaces to resist wear.

Indian's two-speed was bone-simple, providing two possible paths by which engine power could reach the output sprocket. The clutch drove the gearbox main shaft, while behind the clutch a sleeve gear free-spinning on that shaft carried the output sprocket. Movement of a dog ring splined to the main shaft either connected the sleeve gear to the shaft at a 1:1 ratio (direct drive) or, by connecting to a second gear on the main shaft, sent power to a countershaft that was geared back to the sleeve gear at a ratio in the range of 2:1 or 3:1 (low gear).

Indian’s later three-speed dropped the dog ring in favor of a system that selected first and second by physically crashing the teeth of the gears into or out of mesh. Making a smooth shift called for finesse. Henry Ford so disliked this process that he called such sliding-gear transmissions “crunch gears” and instead equipped his famed Model T auto with a “crunchless” two-speed that used planetary gears engaged by brake bands.

Indian’s original two-speed gearbox design is called “direct” because in top gear power goes directly from the clutch (on the main shaft) to the output sprocket (on the sleeve gear). The countershaft transmissions later developed for British bikes continued this direct feature, but German and then Japanese gearboxes took another form: indirect drive with constant mesh; no crashing different sets of gear teeth into each other. In indirect transmissions, the clutch drives the main gearbox shaft but the output sprocket is on the countershaft, with power always being transmitted from one shaft to the other by a gear pair (one pair for each ratio). In the constant-mesh system, all gear pairs are always in mesh, but only in the case of the one ratio selected are both of its gears locked to their respective shafts by either splines or engaging dogs.

Indian continued with its "crunch" three-speed but Harley-Davidson, when it designed the OHV V-twin E and EL models of 1936–'37, chose to give it a much more civilized gearbox that looks quite modern to this day. Harley continued to use this basic design until 1986—50 years!

Motorcycle gearboxes switched from two-speed to three-speed, then to four, five, or six ratios because, over the years, bikes went faster on America’s fast-improving roads. Just going faster with a two-speed meant buzzing the engine at high revs and feeling vibration. Riders wanted another gearbox speed because that would bring down engine revs in cruise, making riding more comfortable. That same impulse exists today, as shown by the aftermarket’s offering “overdrive” top gears that bring engine revs down a bit. The usual ratios are 0.86 or 0.8 (rather than the usual 1:1 top gear), bringing interstate cruising engine revs down by a useful few hundred.

If you took an interest in the mechanically supercharged, 300-plus-hp, 998cc Kawasaki H2R, you may remember it has what is called an “all-dog-ring” gearbox. This means that when you shift gears you are not sliding the dogs of a splined gear into engagement with the dogs on the facing side of a free-spinning gear in order to lock it to the shaft. Instead, you are sliding just a narrow-splined ring with engaging dogs on both its faces. This can make a gearbox faster shifting because it is much easier to rapidly slide lightweight dog rings than it is whole gears. Because the gears themselves do not move along the shaft, they don’t have to be made wide enough to keep each pair in mesh. This translates into a narrower gear cluster with less give in the middle of its shorter shafts.

The surprise is that both Indian’s first two-speed and the gearbox designed for Harley’s 1936 EL are 100 percent dog-ring shifted. Thoroughly modern.

That 1936 gearbox has another modern feature: Its dog rings are moved not by British-style flat cam plates but by a compact cylindrical rotary shift drum that looks perfectly familiar to anyone who has serviced modern Japanese gearboxes. The shift drum has cam slots in its surface, into which driving pegs on the shift forks fit. As the shift drum is rotated from one position to the next by the shift mechanism (held in each position by a spring-loaded detent), the cam slots move the shift forks and the dog rings they drive into positions that successively select first, second, third, and fourth ratios.

As so often in mechanical matters, what is new is old.