Old Britts

Commando Gearbox -- How does it work?
by Fred Eaton

Summary:

This article is for Commando owners who want to know what is going on inside their gearbox. I will try to show how the Commando four speed gearbox works and provide some technical information.

The basics:

There are four sets of gears in the Commando gearbox, on two shafts. The main shaft (the long shaft) runs on two bearings. This shaft is splined into the clutch basket and driven from the engine. The 4th gear on the main shaft (called the sleeve gear) rides on bushings on the main shaft and has the counter shaft sprocket splined to it, which turns the rear wheel. The lay shaft (the short shaft) runs in a bearing next to the fourth gear and in a bushing inside the kickstart shaft.

The main shaft gears are:

  • 1st gear, 28 teeth, is splined to the main shaft.
  • 2nd gear, 18 teeth, rides on a bushing on the main shaft.
  • 3rd gear, 20 teeth, is splined to the main shaft.
  • 4th gear, 18 teeth, rides on bushings on the main shaft.

The lay shaft gears are:

  • 1st gear, 14 teeth, rides on a bushing on the lay shaft.
  • 2nd gear, 24 teeth, is splined to the lay shaft.
  • 3rd gear, 21 teeth, rides on a bushing on the lay shaft.
  • 4th gear, 23 teeth, is splined to the lay shaft.

As you can see by the above gear information, each set of gears has one gear splined to its shaft and its mate runs on a bushing. What this means is the gear that is splined to the shaft, must turn when the shaft turns where the gear riding on a bushing can spin free of the rotation of its shaft. You will also notice that each gear has the opposite attachment to its shaft than the gear next to it.

This combination of gears provides the following gearbox ratios:

  • 1st gear is 2.56 to 1
  • 2nd gear is 1.70 to 1
  • 3rd gear is 1.21 to 1
  • 4th gear is 1 to 1

What happens when you press on the shift lever:

When you press on the shift lever, it moves the gearshift pawl against the ratchet plate and moves the ratchet plate one position (from one gear to the next). The ratchet plate has a shaft that fits into the knuckle pin roller, which is attached to the quadrant. The quadrant is geared to the cam plate. So, when you press on the shift lever you are actually moving the cam plate. When the cam plate moves, the two selector forks move the 3rd gear main and the 2nd gear lay. By moving these two gears back and forth along their shafts, the different gear combinations are obtained.

Gearshift pawl & ratchet Plate

This picture is of the gearshift pawl and the ratchet plate bolted to the inside of the outer cover. The ratchet plate is being pushed into its upper most position (4th gear) by the gearshift pawl.

knuckle pin roller & quadrant
The inner case showing the knuckle pin roller and the quadrant.

Inside the inner and outer covers
Inside the inner case and the outer cover with the gearbox in 4th gear.

2nd gear lay & selector forks
2nd gear lay with its selector fork.

inside the gearbox

This picture shows the inside of the gearbox with both 1st gears and 2nd gear main removed. The quadrant is in the upper right and fits behind the cam plate. The gear in the lower left is 2nd lay and the gear above it is 3rd main. You can see that both 2nd lay and 3rd main have selector forks attached.

cam plate

The cam plate has five indices or notches on its side, one for each gear and one for neutral. You can see the notch for neutral is smaller than the gear notches and is designed this way so you can shift from 1st to 2nd without stopping in neutral. You can also see why you might have trouble finding neutral sometimes. Each notch in the cam plate is to locate the proper position of the cam plate and corresponding gear positioning on their shafts. Each notch is located by the spring loaded cam plunger. You can see the plunger in the above picture with the cam plate in 4th gear.

The gears:

In this section I will explain how the power is transmitted through the gear train from the turning of the main shaft by the engine to the turning of the sleeve gear (4th Main) which in turn, turns the rear wheel. The following pictures show the position of the gear positions inside the gearbox for each gear. In a Commando box, the gears ascend (starting from the kickstart side) in order from 1st gear to 4th gear. In all the following pictures of the gear train, the 1st gears are on the right and the 4th gears on the left. Also the main shaft is on the top and the lay shaft is on the bottom.

1st gear
1st Gear.

The 1st gear power transmission is:

  • The main shaft turns the 1st gear main.
  • The 1st gear main turns the 1st gear lay.
  • The 1st gear lay turns the 2nd gear lay. The 2nd gear is moving over to the 1st gear and dogged to it.
  • The 2nd gear lay turns the lay shaft which turns the 4th gear lay.
  • The 4th gear lay turns the 4th gear main.
You will notice that the only gears dogged together are 1st and 2nd lay.

Neutral
Neutral.

There is no power transmitted in this configuration. You will notice that their are no gears dogged together.

2nd gear
2nd Gear.

The 2nd gear power transmission is:

  • The main shaft turns the 3rd gear main.
  • The 3rd gear main turns the 2nd gear lay. The 3rd gear is moving over to the 2nd gear and dogged to it.
  • The 2nd gear main turns the 2nd gear lay.
  • The 2nd gear lay turns the lay shaft which turns the 4th gear lay.
  • The 4th gear lay turns the 4th gear main.
You will notice that the only gears dogged together are 3rd and 2nd main.

3rd gear
3rd Gear.

The 3nd gear power transmission is:

  • The main shaft turns the 3rd gear main.
  • The 3rd gear main turns the 3rd gear lay.
  • The 3rd gear lay turns the 2nd gear lay. The 3rd gear main has been moved away from the 2nd gear main and the 2nd gear lay has been moved over to the 3nd gear lay and dogged to it.
  • The 2nd gear lay turns the lay shaft which turns the 4th gear lay.
  • The 4th gear lay turns the 4th gear main.
You will notice that the only gears dogged together are 3rd and 2nd lay.

4th gear
4th Gear.

The 4th gear power transmission is:

  • The main shaft turns the 3rd gear main.
  • The 3rd gear main turns the 4th gear main. The 2nd gear lay has been moved away from the 3rd gear lay and the 3nd gear main has been moved over to the 4th gear main and dogged to it.
You will notice that the only gears dogged together are 3rd and 4th main.


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