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


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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


image


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.


image


Gear ratios 

Neutral


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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


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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


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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


image

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

image

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.



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