How to Make Your Motorcycle Stay In Gear
Motorcycle gearboxes are said to be “sequential shift,” which means you must pass through all ratios in sequence to get from first to top gear. You can’t, for example, shift from second to fifth. The device that holds a bike’s gearbox in gear is called a detent, and today it usually has the form of a spring-loaded roller pressing radially against a multi-pointed “star” that turns with the shift drum—a cylindrical cam that typically drives three shift forks, which actually move the gears to engage and disengage.
In each gear position, the detent roller is down in the valley between star “points,” which means that to rotate the drum something must rotate the star, lifting the roller up the next point, compressing the detent spring. Because this takes some force, the detent is able to prevent the shift drum from rotating except when the rider applies enough force to the shift pedal that, in turn, rotates the shift drum by one position—from second gear to third, for example. As neutral is between first and second, the point of the star corresponding to neutral has a shallow cavity machined into its tip. Next time you struggle to find neutral, know that you are trying to rotate your shift drum just enough to make its detent roller perch on that shallow cavity.
But in earlier days, shifting was accomplished differently. British bikes employed a rotatable shift plate instead of a drum, and the wiggly groove that drove the shift forks were machined into one face of this plate. When you made a shift, the internal mechanism translated your dab at the shift pedal into the motion of a claw that rotated the shift plate by one position, to the next higher or lower gear. That plate or some part that rotated with it had the equivalent of the previously described star, with a spring-loaded detent to hold the plate in the desired position, keeping the transmission in the gear you want.
My 1965 Yamaha TD1-B 250cc road racer was based on the twin-cylinder, two-stroke YDS-2 street bike, whose strong acceleration had been an embarrassment to so many Honda 305 and Triumph 500 owners in its day. Being at the transition between the British-European tradition and a fast-coming future of all-Japanese design, it used a shift plate instead of a drum, and instead of a detent based on a pivoted finger carrying a roller, it began with a detent consisting of a hard steel ball in a steel tube, backed by a spring. The ball pressed against the rim of the shift plate, into which there were machined six detents, one for each of that engine’s five speeds, plus a neutral.
Modern roller-shift detents are designs such that, unless the spring-backed roller is sitting exactly on one of the points of the detent star, it is exerting a force tending either to return the shift drum to the previous year or to advance it to the next one. Balancing on the tip of a star point is unlikely because engine vibration quickly jiggles the detent off the tip and into the next detent “valley.”
But in my TD1-B, because the detents were machined into the edge of a shifting plate over 80mm (3-1/4 inches) in diameter, there were no points. Instead, once the plate had rotated enough to push the detent ball back against its spring, it had to scrape along a full half-inch of not-very-smooth shift-plate edge to reach the next detent. Not only was this scraping producing resistance, but the ball was not even free to rotate because of the fairly stiff coil spring pressing it against the shift plate.
No wonder early magazine testers often found that some bikes had “notchy shifting” and even “tended to hang up between gears.” When the rider pressed on the shift pedal and the connection mechanism tried to rotate the shift plate, the initial motion pushed the detent ball diagonally against the inside of its guide tube. Because the contact between a ball and a close-fitting tube is a circular line, no oil wedge could form between ball and tube, meaning that initial resistance to the motion was high. This produced more resistance to shifting, making shifting “notchy.”
t didn’t take long for engineers to identify this as a problem, for the next model had a hemispherical-ended plunger in the guide tube instead of a ball. Now an oil wedge could form between the cylindrical plunger and cylindrical guide tube to ease the movement of the detent plunger as the shift plate tried to lift it.
Ah, but now, if a wear particle got into the small clearance between plunger and tube, the plunger could stick. Now the shift plate could rotate at random, engaging any gear or remaining in neutral. More lousy criticism from those magazine testers! Why do they have to be so picky? Why can’t they just wax poetic about the riding experience?
Looking at the detent plunger in the next model revealed the engineers’ solution: to cut into the outside surface of the cylindrical plunger a series of steeply spiraling shallow grooves, looking a bit like rifling marks on a fired bullet. Now, should a wear particle, possibly from the wet clutch, get between this grooved detent plunger and its tubular guide housing, any in or out the movement of the plunger would push the particle into one of the grooves, freeing the plunger and assuring normal function. Shift quality up, up.
This reminded me of a related matter: the 60-degree crosshatch marks left in engine cylinder bores by the finishing process called honing. One function of such marks is to provide shallow surface features that retain oil, but another is to stop scoring or seizure between piston and cylinder promptly. This was explained to me by a piston engineer named Havel. Imagine that in a tiny area of the piston’s surface, lubrication has broken down and the hot piston metal presses hard against the bare cylinder wall, momentarily welding to it. If the damaged area and wear particles broken from it by further piston movement continue being dragged down the cylinder wall, a streak of damage called a “score” may result, and that damage can spread. But thanks to the presence of all those fine crosshatch hone marks, our area of micro-damage is very quickly pushed off into one of those hone marks—which, under a microscope, looks like a ditch—stopping it from propagating further. Engine oil washes away the particles to the filter (in four-stroke engines, anyway) and continued piston movement polishes the damaged area back to smoothness.
The shift-plate problem came to an end in 1967 when Yamaha adopted drum shifting on its new models. Just such a machine, the five-speed “R” series 350cc two-stroke twin, ridden by Yvon Duhamel and Art Baumann, finished second and third in the 1968 Daytona 200 despite being handicapped by an archaic AMA rule limiting transmissions to four speeds.
BLOG SOURCE: CYCLE WORLD | Why Does Your Motorcycle Stay In Gear?—50 Ways To Get It Wrong