How (most) Motorcycle Transmissions Work
For you 'super wrenches' out there who can split an engine case and rebuild a motorcycle transmission blindfolded this page is going to seem like a nearly criminal over simplification of how the gearbox functions. You'll be correct, it is. This page is only meant to provide a basic understanding of how a motorcycle transmission is operating so riders know what's happening when they hear 'gears' (we'll sort that out in a moment) grinding and the transmission is doing quirky things.
Now the meat and potatoes of this page.
Most manual transmissions are called "constant mesh" which simply means all of the gears in the box are constantly in contact with each other. When you shift gears you aren't actually moving any gears. You're moving a plate or a cylinder that locks into the side of a gear engaging the output shaft with that gear.
Check out the animation below
Animation courtesy Mike Challenger, Haydndesign ltd.
What you're looking at:
The violet shaft is the "input" shaft from the engine. This isn't actually the crankshaft. The input and crank shafts are separated by the clutch. Notice the blue gear attached to the input shaft is turning a gray gear. At this point that gear is not 'engaged' so the bike is in neutral
The green cylinder is a barrel shaft that's rotated by a ratchet mechanism (what you're actually moving when you raise or lower your shift lever) Notice as the animation starts that shaft rotates and moves the fork. As the fork moves it pushes the gold colored disk (with holes in it) toward the gear (with dogs that fit into those holes) and drive is engaged.
Once engaged the yellow output shaft turns and you're now moving down the road.
In a transmission with more than one forward gear that shift fork would move back and forth, alternately disengaging from one gear and engaging another so you'd have a 1-2 shift. For a 3-4 shift the fork would move to a central position, disengaging the 1-2 gears and another fork would engage 3rd and then 4th gears.
The animation below is actually a 5 speed automotive transmission which has a few extra parts (like synchronizers to prevent grinding) but click on the gear shift and watch how the forks move back and forth engaging the various gears.
Now that you understand what's happening inside the transmission take another look at that first animation. Notice the 'pegs' on that gray gear? Those are called "dogs" for reasons not transmitted (ha, see what I did there?) the 'dogs' as shown in the picture (left) are a part of the driven gear just as shown in the animation. The holes are in the slider shown in the picture (right). When that slider is moved by the shift fork the holes slide over the dogs and the output shaft begins spinning.
If you're shifting properly, matching engine/transmission speeds and shift quickly those dogs can slip right into the slots no muss no fuss. If, however, you are a little lazy with a shift and take too long or don't put much pressure on the shift lever those dogs will just skitter over the top of the slots causing what many riders misinterpret as grinding 'gears'.
There are two common problems that develop with motorcycle transmissions.
1. Each time the dogs are allowed to grind the rider is wearing just a little bit off of them. Those 'pegs' get shorter and shorter (or the holes become more elongated) until the transmission will no longer stay in a particular gear or it pops out of a gear. This is 'most' common between 1st and 2nd gear for some reason.
2. The rider forces the transmission to shift too quickly and/or puts too much pressure on the shift lever. When this happens the dogs might be pressed hard against the gear in the solid space between slots. Look at the top animation again and notice the green shift fork. That fork can be bent and, as you can see from the animation if the fork is bent backward (to the right in this picture) it probably isn't going to completely engage the dogs. Result, the transmission will pop out of gear. If the dogs just barely release you'll not only be back in neutral but could hear a lot of grinding with the dogs rubbing against the slots.
Either way the fix is the same. You have to go inside the transmission case and replace the broken parts. In the case of most metric motorcycles that means pulling the engine and splitting the case to gain access to the transmission. Most Harleys, custom bikes and some BMW's have a separate transmission which can be removed from the bike independent of the engine and serviced.
Now that you have the 'theory' here's a picture of an actual (BMW) motorcycle transmission so you can pick out the parts discussed above.
Photo thanks to Ted Porters Beemer Shop
See how the shift forks are moved back and forth by grooves in the shift cam? That cam is ratcheted one direction or the other based on whether you are up shifting or down shifting. The cam is the reason you can't skip a gear and just go from first to third or third to fifth as you can in most cars. Again, I have oversimplified to an extreme degree but hopefully riders now have some idea what's going on inside the gearbox.
Now, go ride.