In this issue description will be first made to see how the slide-mesh type transmission, of which principle has been explained in the Training Aid before, is shifted from one speed stage to the lower speed stage.
In the slide-mesh type transmission illustrated below, the mainshaft 3rd-speed gear and the countershaft 3rd-speed gear are in mesh, which allows teeth of both gears to run at the same peripheral velocity. The mainshaft 2nd-speed gear has its peripheral velocity larger
than the 3rd-speed gear because the former has larger diameter than the later. While the countershaft 2nd-speed gear has its peripheral velocity smaller than the 3rd-speed gear because the former has smaller diameter than the later. As a result, the countershaft 2nd-speed gear is in rotation at the lower peripheral velocity than the mainshaft 2nd-speed gear.
To shaft the transmission from the 3rd-speed stage to the 2nd-speed stage, first the 3rd-speed gears (on the mainshaft and on the countershaft) are to be disengaged from each other to make the transmission in neutral state, and then the 2nd-speed gears (on the mainshaft and on the counter-shaft) are to be brought in, mesh. During these steps disengagement of the main clutch is required each time of engagement or disengagement of the gears, because it is impossible to engage or disengage the rotating gears.
Even after the disengagement of the main clutch, main-and countershaft remain their rotation due to inertia for a some period of time. During this period, an attempt to engage the 2nd-speed gears (on the mainshaft and on the countershaft) will result in ineffective because the peripheral velocities are different between the two gears.
To ensure smooth shifting down of this transmission the two-step clutching technique is recommended, which requires such additional steps as – engaging the main clutch – depressing the accelerator pedal to speed up the engine – disengaging the main clutch when the peripheral velocity of the countershaft 2nd-speed gear rises up to the same with that of the mainshaft 2nd-speed gear – after the 3rd-speed gears are disengaged and before the 2nd-speed gears are brought in mesh. This technique, however, is somewhat troublesome and requires good skill.