Internal-combustion engines run at high speeds, so a reduction in gearing is necessary to transmit power to the drive wheels, which turn much more slowly.
The gearbox provides a selection of gears for different driving conditions: standing start, climbing a hill, or cruising on level surfaces. The lower the gear, the slower the road wheels turn in relation to the engine speed.
The constant-mesh gearbox
The gearbox is the second stage in the transmission system, after the clutch. It is usually bolted to the rear of the engine, with the clutch between them.
Modern cars with manual transmissions have four or five forward speeds and one reverse, as well as a neutral position.
Syncromesh disengaged
The gear turns freely on a bush, rotated by a meshing gear on the layshaft. The synchromesh unit, splined the the mainshaft, rests near by.
Synchromesh engaged
The fork moves the synchromesh towards the selected gear. Friction surfaces synchronise the shaft speeds, and synchromesh and gear lock together.
Constant-mesh four-speed gearbox
The gears are selected by a system of rods and levers operated by the gear lever. Drive is transmitted through the input shaft to the layshaft and then to the mainshaft, except in direct drive - top gear - when the input shaft and the mainshaft are locked together.
All the gears except those needed for reverse are constantly in mesh. The gears on the output shaft revolve freely around it, while those on the layshaft are fixed. No drive is being transmitted.
First gear
In first gear, the smallest gear on the layshaft (with the fewest teeth) is locked to it, passing drive through the largest gear on the mainshaft, giving high torque and low speed for a standing start.
Second gear
In second gear, the difference in diameter of the gears on the two shafts is reduced, resulting in increased road speed and lower torque increase. The ratio is ideal for climbing very steep hills.
Fourth gear
In fourth gear, the input shaft and mainshaft are locked together, providing 'direct drive': one revolution of the propellor shaft for each revolution of the crankshaft. There is no increase in torque.
Reverse
For reversing, an idler gear is interposed between gears on the two shafts, causing the mainshaft to reverse direction. Reverse gear is usually not synchronised.