Gearbox Confidential

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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/)

In going through the parts for the 1965 Yamaha TD1-B two-stroke roadracer I am assembling, I’ve spent a lot of time counting gear teeth and generally staring at transmission components.

Modern motorcycle gearboxes are usually of the all-indirect type, in which power enters on one shaft and exits from a second, always passing from shaft to shaft via one of the several gear pairs (one pair for each transmission ratio).

Because the TD series were production based (derived from the 250 YDS-2 that embarrassed many a Honda Super Hawk 305 in those days), the racer was at first given gears and ratios not greatly different from the stocker’s. That meant a low 2.5-ratio first gear, inspired by the streetbike’s need to heave machine, rider, and passenger away from uphill stoplights. Such a low first, in the early TD1 and TD1-A, essentially made it usable only at the starts of races, meaning that those bikes had just four track-usable speeds.

Related: Free Horsepower For The Taking!

With the stock 25 hp YDS-2′s moderate port timings, that was OK; its lower top speed and wider power range worked fine with just four speeds. But as the racebike was tuned more sharply, its useful rpm range narrowed, delivering rideable power from 7,500 rpm and pulling strong from 8,500. When an ornithologist was once asked how a tiny songbird’s call could be heard from so far away, the answer was that it puts all its energy into a narrow frequency range. That’s how it was with these two-stroke engines as well; resonant exhaust pipes (aka “expansion chambers”) could pump a lot of air through engines, but the more they were made to pump, the narrower the frequency range across which they worked.

Modern motorcycle transmissions have the power entering through one shaft and exiting the other. (Jeff Allen/)

When the TD1 of 1962 was redesigned as TD1-A the following year, only one change was made to its gearbox: first gear grew taller, changed from 2.5:1 to 2.267:1, while second through fifth ratios remained unaltered. The intended purpose was clear: to make first tall enough so it was actually useful for accelerating off the slowest corners. This taller first gear turned what had effectively been a four-speed into a five-speed the rider could actually use on the track.

The fewer speeds your gearbox has, the greater the compromises it imposes on the engine. I once rode a bus from flat Watertown, Massachusetts, to hilly Newton. As the bus encountered the first grade, the load pulled engine revs down, forcing its two-speed automatic to downshift. But now the engine was revving so high that the transmission immediately upshifted. That pulled the revs down again, forcing another downshift. And so we climbed, the engine alternately lugging and screaming. It really needed at least one extra ratio in between.

Something similar happened to the single-cylinder Manx Norton racer, still produced in the 1950s with a four-speed gearbox. As the engine was each year tuned to make a bit more power, its power range narrowed, putting it into a situation much like that of my bus ride. The aftermarket got busy and offered first a five-speed and then the Schaffleitner six-speed transmission. These gearboxes allowed improved acceleration: Since the ratios were spaced closer together, the engine’s rpm was pulled down less by each upshift. That DOHC engine, with its sharply tuned short megaphone, could pull from about 5,800 to its peak near 7,000, giving it a 1,200-rpm powerband. The four-speed with its wider-spaced ratios pulled engine revs down too far in the upshift from first to second, putting it in its “megaphonitis zone” below 5,800, where torque was weak.

Knowing the Rev Drop for Each Shift

It is instructive to work out how far down each upshift pulls engine revs. On a TD1-A, the upshift from first to second, if made at 10,000 rpm, brings the engine down to 6,316 rpm. The TD1-B engine, with its longer exhaust and intake port timings, began to pull at about 7,500 and pulled strongly from 8,500 to peak at 9,500–10,000. Clearly, that upshift rev drop to 6,316 rpm would make for slow acceleration. But with the taller 2.267 ratio, the rev drop to second became 6,965. A useful improvement, yet still some 500 revs below the beginnings of the engine’s best torque.

Bigger Gear Teeth vs. Smaller Gear Teeth

The early TD1-B got an even taller first gear, and for later-built TD1-Bs and the 1967 TD1-C model, everything was changed—top to bottom—and the torque capacity of the gearbox was greatly increased. What I saw, through the gears in my hands, was that early gearboxes had greater numbers of finer-pitched teeth, but the redesigned 1966-67 boxes had fewer but much more robust teeth. Why? For streetbikes, a greater number of teeth on each gear makes the bike quieter, but for racing, gears with fewer but chunkier teeth are stronger.

Although increased engine power must have dictated part of this change, there’s another factor. If you listen to sound recordings of 1960s racebikes at the Isle of Man TT, you can hear the result of rough pavement and hard, short-travel suspension: The engine’s exhaust note frequently spikes as the rear tire skips into the air across the irregular surface, allowing revs to shoot upward. When the rear tire hits the pavement again, kinetic energy in the engine’s crankshaft is suddenly converted into a severe torque shock to the gearbox. This use of fewer-but-stronger gear teeth is also common in auto racing.

Yamaha now made first gear taller yet again, and with the new 2.000 first gear, engine revs dropped down to 7,665, safely in the bottom of the pulling range. This was real progress. Before, to get the engine to accelerate off a first-gear corner, the rider might have to slip the clutch after upshifting to second to keep the engine revving in its best range.

The TD2 of 1969 was another big step forward, for it had four full-height transfer ports per cylinder and carbs. Those carburetors had also jumped 23 percent in flow area, from the previous 27mm to 30mm. Power had climbed from the 32 hp of the TD1-A to 35 in the B-model, 38 in the TD1-C, and to 44 ponies in the TD2.

It’s normal in motorcycle gearboxes for the ratios to be wider spaced at the bottom and narrower spaced toward the top. This is because aerodynamic drag, which is almost nonexistent in lower gears, requires more and more power as the machine moves faster. When upshifting out of fourth and into fifth at 10,000 rpm, the revs dropped down only to 9,022, right on the engine’s torque peak and well suited to keep accelerating.

In 1972 Yamaha decided to share a common horizontally split crankcase between its 250 and 350 twins, a sensible production economy. This also added the possibility of a six-speed gearbox. The new production bikes were the well-loved R5, RD250, and RD350 twins. The new TD3 250 racer now jumped from the TD2′s 30mm carbs to 34mm, a flow area increase of 28 percent.

The Concept of Gearbox Range

This brings us to another aspect of gearbox-ratio choice: range, which is first gear divided by top gear. Computing range gives us an idea of the ratio of top speed to speed in the slowest corner. In the case of the late TD1-B, C, and TD2, range was about 2.1. Even back in the 1920s, during the days of three-speed hand-shift motorcycle transmissions, range was close to this. Streetbikes need more range so they can pootle through slow traffic without chain snatch. More gearbox speeds become necessary only to allow higher-tuned engines to stay in their pulling range.

Range on the 1972 TD3 was wider, now 2.37 instead of 2.1, and the rev drops at each upshift, although nicely spaced, were not smaller than before. Why?

Yamaha’s 250 production racer had been getting faster all along, beginning with the TD1 at about 100 mph, the TD1-B at maybe 115–120, and the TD2-B and TD3 pushing 140. With top speed increased that much, yet with speed through slow corners unchanged, racers needed increased range just to keep the engine from over-revving to reach top speed.

Where the Gear Pairs Live

Another interesting change came with the new six-speed gearbox: the placement of the ratios along their shafts. In the past, gears on the input shaft had been small at one end and progressively bigger toward the other, but in the TD3, first gear was at one end of the gearbox and second was at the opposite end. Why this difference?

If the width of each gear remains nearly constant, each time another speed is added the shafts must become longer, and therefore more easily deflected by the wedging-apart action of the gear teeth. The smaller the pinion on the input shaft, the greater its leverage to wedge its meshing partner on the output shaft away from itself at peak torque. Therefore the “wedgiest” ratios, first and second, were placed at opposite ends of the gearbox, next to the strong support of the gearbox shaft bearings. This minimized gear-shaft bending and the improper tooth action it permits.

Before rule book changes, in 1967 Suzuki famously fielded the 14-speed RK67 50cc GP bike whose gearbox was substantially wider than its engine. (Public Domain/)

During the Japanese invasion of European GP racing from 1959-67, makers were constantly adding more gearbox speeds to get best performance from ever-more-narrowly tuned engines. Eventually gearbox shafts had to be so long to make room for all those ratios that in some cases the shafts had to be supported in the middle by extra bearings.

This concludes my broadcast day.

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