How a motorbike gearbox works?

Answer 1

while the XR was in pieces JAW grabs the camera and takes some happy snaps in order to share some understanding of how a motorbike gearbox works.
Power enters from the clutch onto the countershaft and leaves via one of the gear ratios out the sproket. "X" means this cog idles; ie is able to free spin. Cogs are numbered.

Power Flow.

Mumbo from the engine hits the gearbox via the clutch and spins the countershaft. You can see there are 5 cogs on the countershaft and these correspond to cogs on the mainshaft, each of the cog pairs produces a different ratio. This box is... yeah okay, you guessed it, a 5 speed ;)

The "X" crosses mean the cog can free spin. At the moment in fact, the gearbox is in neutral. You'll notice for every cog that can free spin it's mate cannot - it is locked to the shaft. It can move backwards and forwards, but it always spins with the shaft it is attached to.

Selection.

From the first photo you can see firstly how a cog is locked to it's shaft but can move from side to side - it is splined. Are ya with me? A shows the lumps on the shaft and B shows the inverted lumps in the cog.

C which we shall call a "locker" for want of a better term performs the selection of a gear. This mechanism is shown better in the next photo...

A cog with it's (B) spline connection and (C) "lockers".

Lumps and holes... when a gear that is locked to the shaft gets told to "swing left" or "swing right" as the case may be, the locker mates with it's next door neighbour cog. By an amazing coincidence, that cog is normally and idler, but once joined it too is locked to the shaft and therefore must spin with the shaft.

This how gears are selected - the rider essentially starts moving the cog across to it's next door neighbour. When the locker lines up with the corresponding hole, in she clunks.

A cog and it's neighbour - with (B) spline connection and (C) "lockers".
Choose a Ratio.

First lets just consider neutral again. When in neutral, no cogs are locked together, so all gear ratios have one cog that is free spinning, and one cog that is locked to the shaft. What happens? Nothing! Power is fed via the clutch to the countershaft and can't do anything! (Note - the first picture on this page shows the gearbox in neutral).

First gear is selected.

But now when you select 1st gear, the bottom neighbour has slid to the right. Look carefully at the picture, it's the lower cog on the 4th gear that has moved right and locked. Okay the top cog was already locked to the shaft, and thanks to lower 4th gear the bottom is too. Nothing has happened to 4th gear, the top of 4th is still free-spinning. All the other cogs remain the same too.

The 1st gear top cog has 12 teeth, the bottom cog has 28. So now when power enters the countershaft 1st gear is selected and for every one revolution, the mainshaft does 12/28 = 0.43 revolutions. Yah, we're moving!

Examine the next 4 photos, each one represents a gear selection. In each case one of the selection cogs - ie those ones that don't free spin - has moved across to lock up with it's neighbour. Note the when you change between gears the previously locked gear must first disengage before the new one can engage. Think about what would happen if two gears were selected at the same time... Ugly.

Second gear is selected.
Third gear is selected.
Fourth gear is selected.
Fifth gear is selected.

The selector forks in place.Selector forks.

Well this is all well and good JAW, but how does my left foot make it happen?

Look at this picture, what you can see are the selector forks. They slot in on the selector cogs and push them left and right, right?

Those little stumps on the top of the selector forks, at the very top are controlled by a programmed lump of metal known as the shift drum. Heh, there is a mechanical program running in your bike. Let's not refer to it as software though shall we...

Shift drum.

And this is it. The deeper worm holes are where those little stumps fit in. By rotating the shift drum clockwise or counter clockwise it will push the selector forks left or right, depending on what the drum is "programmed" with.

Lets bring it all together...

The brains of the situation - the shift drum.

How the shift drum fits together.
From a different angle - you can see on the end
of the shift drum the "stopper plate".From the foot...

The final part of the picture is the rotation of the shift drum. On one end of the shift drum, the end with the bolt hangin out, is the stopped plate. It has a "clicker" wheel (the shift pawl) that holds the drum in position when a gear is selected.

On the other end is the selector rod itself, with a little claw that basically connected to your foot. When you push down on the gear lever the little claw pushes the shift drum one stopper click clockwise, and vice versa.

So now that you've seen it, and how "clunky" it can be, you are gonna be nice to your gearbox and at least use the clutch in 1st and 2nd gear aren't you? ;)

The stopper plate arrangement.
The selector spindle with "claw" (far right out of picture is the drum). That shaft you see it the gear selector lever shaft itself. Notice here too the broken part and thus why I had the gearbox out in the first place...