In the constant mesh box, for the smooth engagement of the dog clutchs it is necessary that the speed of main shaft gear & the sliding ged must be equal. Therefor to obtain lower gear, the speed of the clutch shaft, lay shaft & main shaft must be increased. This is done by double declutching. The procedure for double declutching is as given below:
The clutch is disengaged & the gear is brought to neutral. Then the clutch is engaged & accelerator pedal pressed to increase the speed of the main shaft gears. After this the clutch is again disengaged & the gear moved to the required lower gear & the clutch is again engaged . As the clutch is disengaged twice in this process, it is called double declutching.
For changing to higher gear , however, reverse effect is desired i.e the driver has to wait with the gear in neutral till the main shaft speed is decreased sufficiently for a smooth engagement of gear.

• In a constant mesh gear box, for smooth engagement of dog clutches, the speed of the main shaft gear and the dog clutch needs to be equal.
• Therefore to shift from lower gear (i.e. 2nd) to higher gear (i.e. 3rd), the speed of the clutch shaft and the lay shaft must be decreased.
• This is achieved by double declutching.
• First, the clutch is disengaged and brought to neutral.
• Then the clutch is engaged and the driver has to wait till the speed of the engine drops sufficiently for a smooth engagement. The driver can also apply brake gently to speed up the process.
• After this, clutch is pressed again and the gear is moved to the higher gear (3rd gear).
• As the speed of the clutch shaft is now same as the dog clutch, the dog clutch is engaged smoothly.
• In this way, we can shift from 2nd gear to 3rd gear in a constant mesh gear box.