How manual gearboxes work

Constant-mesh four-speed gearbox The input shaft takes the drivefrom the engine (when theclutch is engaged) into thegearbox. The selector rods and forks move thesynchromesh units back and forth,according to which gear is selected.Some gearboxes have only oneselector rod, others three; but theprinciple is the same. A spring-loaded ball at thebase of the gear lever alignswith a recess in the selectorrod holding the lever inposition until the driverchanges gears. When the reverse idler gear isselected, it is interposed inthe gear train, reversing thenormal direction of themainshaft. The layshaft transmits drivefrom the input shaft to themainshaft when a constantlymeshed gear is locked to themainshaft by means of asplined mesh. The mainshaft, or outputshaft, transmits drive fromthe layshaft to the propellorshaft, when a gear isselected.

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.

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

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

Synchromesh engaged

The fork moves the synchromesh towards the selected gear. Friction surfaces synchronise the shaft speeds, and synchromesh and gear lock together.

The gear lever, operated by the driver, is connected to a series of selector rods in the top or side of the gearbox. The selector rods lie parallel with shafts carrying the gears.

The most popular design is the constant-mesh gearbox. It has three shafts: the input shaft, the layshaft and the mainshaft, which run in bearings in the gearbox casing.

There is also a shaft on which the reverse-gear idler pinion rotates.

The engine drives the input shaft, which drives the layshaft. The layshaft rotates the gears on the mainshaft, but these rotate freely until they are locked by means of the synchromesh device, which is splined to the shaft.

It is the synchromesh device which is actually operated by the driver, through a selector rod with a fork on it which moves the synchromesh to engage the gear.

The baulk ring, a delaying device in the synchromesh, is the final refinement in the modern gearbox. It prevents engagement of a gear until the shaft speeds are synchronised.

On some cars an additional gear, called overdrive, is fitted. It is higher than top gear and so gives economic driving at cruising speeds.

Synchronising the gears

The synchromesh device is a ring with teeth on the inside that is mounted on a toothed hub which is splined to the shaft.

When the driver selects a gear, matching cone-shaped friction surfaces on the hub and the gear transmit drive, from the turning gear through the hub to the shaft, synchronising the speeds of the two shafts.

With further movement of the gear lever, the ring moves along the hub for a short distance, until its teeth mesh with bevelled dog teeth on the side of the gear, so that splined hub and gear are locked together.

Modern designs also include a baulk ring, interposed between the friction surfaces. The baulk ring also has dog teeth; it is made of softer metal and is a looser fit on the shaft than the hub.

The baulk ring must be located precisely on the side of the hub, by means of lugs or 'fingers', before its teeth will line up with those on the ring.

In the time it takes to locate itself, the speeds of the shafts have been synchronised, so that the driver cannot make any teeth clash, and the synchromesh is said to be 'unbeatable'.

Most modern cars have synchromesh on all forward gears, but on earlier cars it is not provided on first gear.

We also have this article in Spanish and French

The ultimate video course

We take this car to pieces and then build it again, explaining how every single part works.

By the time you finish watching this, you'll understand everything inside a car.

Watch us take a Mazda MX5 Miatta to pieces, and then build it back together again into a modern working car.

  • Every part explained in detail.
  • We've created the most detailed 3D model ever produced so we can show you everything working.
  • Over 20 hours of footage — see the contents.
  • Preorder and download the How a Car Works PDF for free.
  • Support the video by preordering and we'll put your name in the credits.

This will be the most in-depth course on car mechanics ever produced. Pre-order your copy now and save 75%.

Preorder for $20
Normal price $80. Pre-release in May.

Read more essential guides

How automatic gearboxes work

Most modern automatic gearboxes have a set of gears called a planetary or epicyclic gear train. ...

How automatic gearboxes work

Most modern automatic gearboxes have a set of gears called a planetary or epicyclic gear train. ...

How the transmission works

Driving through a propeller shaft In a front-engined rear-wheel-drive car, power is transmitted ...

Diagnosing faults in automatic transmission

Although they are considerably more complicated than manual gearboxes, with control and operating...

Driven wheel hubs

The hubs of the driven wheels often look similar to those of undriven ones, but there are some d...