Kevin Cameron explains why you should buy a junk motorcycle and “play” with it—in this case, the transmission.

Most of the problems with poor shifting are traceable to causes outside the engine. It’s quite common for people, attempting to adjust the height of the shift pedal to suit the limits of their ankles, to get the linkage in such a state that it cramps against itself when you make the shift and doesn’t push the shift claw far enough to fully engage the gear. So if you have a shifting problem, it’s a good idea to look first in the external linkage, especially if you’ve recently made a change.

Inside the engine, engagement of one gear to another takes place when one set of dogs is pushed into engagement with another—the two gears now rotate as one. The space into which the engaging dogs can fit determines how much time it takes to complete a shift. So it is better for rapid shifting to have fewer, rather than more, dogs. Wide spacing between the dogs allows more chance that the gears will promptly engage. For this reason, eager racers of an earlier era used an abrasive cutoff disc to saw out every other dog.

On the other hand, touring riders—and other people who are going to spend time on and off the throttle in traffic—are annoyed by the large “clunk” of backlash from dogs that have a lot of space between them. One way in which some designs have dealt with this is to have six dogs but every other dog cut down to half its original height. The initial shift is made in a condition in which three dogs are engaging three dogs. Once the engagement is made, the backlash is quite small because of the halfheight dogs.

I have spoken before about the shift from first to second, which is a difficult one because of the speed difference being larger than from second to third and so forth. Quite often the engaging dogs on second gear get rounded off by spirited shifting. All it takes to fix it are new parts. If you want to shift that way—bang ’em through, clutchless shifting—there may be a cost. But if we’re real enthusiasts, we don’t mind replacing gears, do we?

Another set of problems comes about from either pushing the shift drum too far, in which case the gearbox jumps into a neutral of the ratio you were selecting or kicks back out of the ratio to which you were shifting. There’s an eccentric pin in the assembled engine that determines how far the shift arm can move. Sometimes I found that adjustment was not correct. When I corrected it, I could see the claw pushed the drum almost into the next position. The rule I found was that the shift claw should stop pushing about a millimeter before the shift drum drops into its next locked detented position.

At one point in my early days, I had a bike with a Burman gearbox, which, instead of having dogs, had fine splines. That was valued by some riders because it had almost no backlash. But it was the scourge of people who wanted to shift quickly because there was so little space between one spline and the next, which offered very little time for a rapid engagement. In general, those Burman gearboxes with splined dogs were not for the sporting rider.

All of this stuff makes sense to you after you’ve played with it a lot. For those of you who are interested in doing this sort of work, get yourself a junk motorcycle and play with it. It’s the only way you’re going to get familiar with what the parts actually look like.

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