A '''manual transmission''' (also known as a '''stick shift''' or just 'stick', 'straight drive', or '''standard transmission''') is a type of transmission used in driver-operated clutch and a movable gear selector. Most automobile manual transmissions allow the driver to select any forward gear at any time, but some, such as those commonly mounted on motorcycles and some types of racing cars, only allow the driver to select the next-highest or next-lowest gear ratio. This second type of transmission is sometimes called a sequential manual transmission.

Manual transmissions are characterized by gear ratios that are selectable by engaging pairs of gears inside the transmission. Conversely, most epicyclic (planetary) gearing controlled by brake bands and/or clutch packs to select gear ratio. Automatic transmissions that allow the driver to manually select the current gear are called semi-automatic transmissions.

Contemporary automotive manual transmissions are generally available with four to six forward gears and one reverse gear, although manual transmissions have been built with as few as two and as many as eight gears. Tractor units have at least 10 gears and as many as 24.  Some manuals are referred to by the number of forward gears they offer (e.g., 5-speed) as a way of distinguishing between automatic or other available manual transmissions. Similarly, a 5-speed automatic transmission is referred to as a ''5-speed automatic''.

Other types of transmission in mainstream automotive use are the automatic transmission, semi-automatic transmission, and the continuously variable transmission.

Manual transmissions come in two basic types: simple non-synchronous systems, where gears are spinning freely and their relative speeds must be synchronized by the operator to avoid noisy and damaging "clashing" and "grinding" when trying to mesh the rotating teeth; and synchronized systems, which eliminate this necessity while changing gears.

==Unsynchronized transmission==
The earliest vehicle transmissions could be shifted, with multiple gear ratios available to the operator, and even had reverse. However, the gears were engaged by sliding mechanisms or simple clutches, which required a lot of careful timing and throttle manipulation when shifting, so that the gears would be spinning at roughly the same speed when engaged; otherwise, the teeth would refuse to mesh.

When upshifting, the speed of the gear driven by the engine had to drop to match the speed of the next gear; as this happened naturally when the clutch was depressed or disengaged, it was just a matter of skill and experience to hear and feel when the gears managed to mesh. However, when downshifting, the gear driven by the engine had to be sped up to mesh with the output gear, requiring letting the clutch up (engagement) for the engine to speed up the gears. Double clutching, that is, shifting once to neutral to speed up the gears and again to the lower gear, is sometimes needed. In fact, such transmissions are often easier to shift without using the clutch at all. When using this method, the driver has to time the shift with relative precision to avoid grinding the gears. The clutch, in these cases, is only used for starting from a standstill. This procedure is common in racing vehicles and most production motorcycles.

Even though automotive transmissions are now almost universally synchronised, heavy racing transmissions are usually non-synchromesh transmissions, known colloquially as "crashboxes," for several reasons. The friction material, such as dogs") on the synchros are removed to speed the shifting process, at the expense of greater wear.

Heavy duty trucks utilize unsynchronized transmissions in the interest of saving weight. Military edition trucks, which do not have to obey weight laws, usually have a synchronized transmission. Highway use heavy-duty trucks in the United States are limited to 80,000 pounds GVWR, and the lighter the curb weight for the truck, the more cargo can be carried, and with a synchronizer adding weight to a truck that could otherwise be used to carry cargo, most drivers are simply taught how to double clutch, initially, and then most eventually gravitate to shifting without the clutch.

Similarly, most modern motorcycles still utilize unsynchronized transmissions as synchronizers are generally not necessary or desirable. Their low gear inertias and higher strengths mean that 'forcing' the gears to alter speed is not damaging, and the selector method on modern motorcycles (pedal operated) is not conducive to having the long shift time of a synchronized gearbox. Because of this, it is still necessary to synchronize gear speeds by "blipping" the throttle when shifting into a lower gear on a motorcycle.

==Synchronized transmission==
thumb|right|400px|Top and side view of a typical manual transmission, in this case a Ford [[Toploader Transmission|"Toploader", used in cars with external floor shifters.]]
Modern gearboxes are '''constant mesh''', i.e. all gears are always in mesh.  Only one of these meshed pairs of gears is locked to the shaft on which it is mounted at any one time, while the others are allowed to rotate freely.  Thus, it greatly reduces the skill required to shift gears. 

Most modern cars are fitted with a synchronized gear box, although it is entirely possible to construct a constant mesh gearbox without a synchromesh, as found in a motorcycle, for example. In a constant mesh gearbox, the transmission gears are always in mesh and rotating, but the gears are not rigidly connected to the shafts on which they rotate. Instead, the gears can freely rotate or be locked to the shaft on which they are carried. The locking mechanism for any individual gear consists of a collar (or "dog collar") on the shaft which is able to slide sideways so that teeth (or "dogs") on its inner surface bridge two circular rings with teeth on their outer circumference: one attached to the gear, one to the shaft (one collar typically serves for two gears; sliding in one direction selects one transmission speed, in the other direction selects the other). When the rings are bridged by the collar, that particular gear is rotationally locked to the shaft and determines the output speed of the transmission. In a synchromesh gearbox, to correctly match the speed of the gear to that of the shaft as the gear is engaged, the collar initially applies a force to a cone-shaped brass clutch which is attached to the gear, which brings the speeds to match prior to the collar locking into place. The collar is prevented from bridging the locking rings when the speeds are mismatched by synchro rings (also called blocker rings or balk rings, the latter being spelled "baulk" in the UK). The gearshift lever manipulates the collars using a set of linkages, so arranged so that one collar may be permitted to lock only one gear at any one time; when "shifting gears," the locking collar from one gear is disengaged and that of another engaged. In a modern gearbox, the action of all of these components is so smooth and fast it is hardly noticed.

The modern cone system was developed by Porsche and introduced in the 1952 Porsche 356;  cone synchronizers were called "Porsche-type" for many years after this. In the early 1950s only the second-third shift was synchromesh in most cars, requiring only a single synchro and a simple linkage; drivers' manuals in cars suggested that if the driver needed to shift from second to first, it was best to come to a complete stop then shift into first and start up again. With continuing sophistication of mechanical development, however, fully synchromesh transmissions with three speeds, then four speeds, five speeds, six speeds and so on became universal by the 1960s. Reverse gear, however, is usually not synchromesh, as there is only one reverse gear in the normal automotive transmission and changing gears in reverse is not required. (The obvious exception to this is in cars made by Lamborghini, almost all of whose models have synchromesh on reverse gear - presumably because the designers were thinking of drivers engaging reverse while still moving forward.)

==Internals==
===Shafts===
Like other transmissions, a manual transmission has several shafts with various gears and other components attached to them. Typically, a rear-wheel-drive transmission has three shafts: an input shaft, a ''countershaft'' and an output shaft. The countershaft is sometimes called a ''layshaft''.

In a rear-wheel-drive transmission, the input and output shaft lie along the same line, and may in fact be combined into a single shaft within the transmission. This single shaft is called a ''mainshaft''. The input and output ends of this combined shaft rotate independently, at different speeds, which is possible because one piece slides into a hollow bore in the other piece, where it is supported by a bearing. Sometimes the term ''mainshaft'' refers to just the input shaft or just the output shaft, rather than the entire assembly.

In some transmissions, it's possible for the input and output components of the mainshaft to be locked together to create a 1:1 gear ratio, causing the power flow to bypass the countershaft. The mainshaft then behaves like a single, solid shaft, a situation referred to as ''direct drive''.

Even in transmissions that do not feature direct drive, it's an advantage for the input and output to lie along the same line, because this reduces the amount of torsion that the transmission case has to bear.

Under one possible design, the transmission's input shaft has just one pinion gear, which drives the countershaft.  Along the countershaft are mounted gears of various sizes, which rotate when the input shaft rotates. These gears correspond to the forward speeds and reverse. Each of the forward gears on the countershaft is permanently meshed with a corresponding gear on the output shaft. However, these driven gears are not rigidly attached to the output shaft: although the shaft runs through them, they spin independently of it, which is made possible by Reverse.

Most front-wheel-drive transmissions for transverse engine mounting are designed differently. For one thing, they have an integral final drive and differential. For another, they usually have only two shafts; input and countershaft, sometimes called input and output. The input shaft runs the whole length of the gearbox, and there is no separate input pinion. At the end of the second (counter/output) shaft is a pinion gear that mates with the ring gear on the differential.

Front-wheel and rear-wheel-drive transmissions operate similarly. When the transmission is in neutral, and the clutch is disengaged, the input shaft, clutch disk and countershaft can continue to rotate under their own inertia. In this state, the engine, the input shaft and clutch, and the output shaft all rotate independently.

===Dog clutch===
The gear selector does not engage or disengage the actual gear teeth which are permanently meshed. Rather, the action of the gear selector is to lock one of the freely spinning gears to the shaft that runs through its hub. The shaft then spins together with that gear. The output shaft's speed relative to the countershaft is determined by the ratio of the two gears: the one permanently attached to the countershaft, and that gear's mate which is now locked to the output shaft.

Locking the output shaft with a gear is achieved by means of a dog clutch selector. The dog clutch is a sliding selector mechanism which is splined to the output shaft, meaning that its hub has teeth that fit into slots (splines) on the shaft, forcing it to rotate with that shaft. However, the splines allow the selector to move back and forth on the shaft, which happens when it is pushed by a selector fork that is linked to the gear lever. The fork does not rotate, so it is attached to a collar bearing on the selector. The selector is typically symmetric: it slides between two gears and has a synchromesh and teeth on each side in order to lock either gear to the shaft.

===Synchromesh===
If the teeth, the so-called dog teeth, make contact with the gear, but the two parts are spinning at different speeds, the teeth will fail to engage and a loud grinding sound will be heard as they clatter together.  For this reason, a modern dog clutch in an automobile has a synchronizer mechanism or ''synchromesh'', where before the teeth can engage, a cone clutch is engaged which brings the selector and gear to the same speed.  Moreover, until synchronization occurs, the teeth are prevented from making contact, because further motion of the selector is prevented by a blocker (or "baulk") ring.  When synchronization occurs, friction on the blocker ring is relieved and it twists slightly, bringing into alignment certain grooves and notches that allow further passage of the selector which brings the teeth together. Of course, the exact design of the synchronizer varies from manufacturer to manufacturer.

The synchronizer<ref></ref> has to change the momentum of the entire input shaft and clutch disk. Additionally, it can be abused by exposure to the momentum and power of the engine itself, which is what happens when attempts are made to select a gear without fully disengaging the clutch. This causes extra wear on the rings and sleeves, reducing their service life. When an experimenting driver tries to "match the revs" on a synchronized transmission and force it into gear without using the clutch, the synchronizer will make up for any discrepancy in RPM.  The success in engaging the gear without clutching can deceive the driver into thinking that the RPM of the layshaft and transmission were actually exactly matched.  Nevertheless, approximate "rev-matching" ''with clutching'' can decrease the general delta between layshaft and transmission and decrease synchro wear.

===Reverse===
The previous discussion normally applies only to the forward gears. The implementation of the reverse gear is usually different, implemented in the following way to reduce the cost of the transmission. Reverse is also a pair of gears: one gear on the countershaft and one on the output shaft. However, whereas all the forward gears are always meshed together, there is a gap between the reverse gears. Moreover, they are both attached to their shafts: neither one rotates freely about the shaft. What happens when reverse is selected is that a small gear, called an ''idler gear'' or ''reverse idler'', is slid between them. The idler has teeth which mesh with both gears, and thus it couples these gears together and reverses the direction of rotation without changing the gear ratio.

Thus, in other words, when reverse gear is selected, in fact it '''is''' actual gear teeth that are being meshed, with no aid from a synchronization mechanism. For this reason, the output shaft must not be rotating when reverse is selected: the car must be stopped.  In order that reverse can be selected without grinding even if the input shaft is spinning inertially, there may be a mechanism to stop the input shaft from spinning.  The driver brings the vehicle to a stop, and selects reverse. As that selection is made, some mechanism in the transmission stops the input shaft. Both gears are stopped and the idler can be inserted between them. There is a clear description of such a mechanism in the Honda Civic 1996-1998 Service Manual, which refers to it as a "noise reduction system":

:Whenever the clutch pedal is depressed to shift into reverse, the mainshaft continues to rotate because of its inertia. The resulting speed difference between mainshaft and reverse idler gear produces gear noise [grinding]. The reverse gear noise reduction system employs a cam plate which was added to the reverse shift holder. When shifting into reverse, the 5th/reverse shift piece, connected to the shift lever, rotates the cam plate. This causes the 5th synchro set to stop the rotating mainshaft. (13-4)

A reverse gear implemented this way makes a loud whining sound, which is not heard in the forward gears. The teeth on the forward gears of consumer automobiles are helically cut. When helical gears rotate, their teeth slide together, which results in quiet operation. In spite of all forward gears being always meshed, they do not make a sound that can be easily heard above the engine noise. By contrast, reverse gears are spur gears, meaning that they have straight teeth, in order to allow for the sliding engagement of the idler, which would not be possible with helical gears. The teeth of spur gears clatter together when the gears spin, generating a characteristic whine.

It is clear that the spur gear design of reverse gear represents some compromises&mdash;less robust, unsynchronized engagement and loud noise&mdash;which are acceptable due to the relatively small amount of driving that takes place in reverse. However, many modern transmissions now include a reverse gear synchronizer and helical gearing.

===Design variations===
====Gear variety====
Manual transmissions are often equipped with 4, 5, or 6 forward gears.  Nearly all have one reverse gear. In three or four speed transmissions, in most cases, the topmost gear is "direct", ''i.e.'' a 1:1 ratio. For five speed or higher transmissions, the highest gear is usually an overdrive gear, with a ratio of less than 1:1. Older cars were generally equipped with 3-speed transmissions, or 4-speed transmissions for high performance models and 5-speeds for the most sophisticated of automobiles; in the 1970s, 5-speed transmissions began to appear in low priced mass market automobiles and even compact pickup trucks, pioneered by Toyota (who advertised the fact by giving each model the suffix ''SR5'' as it acquired the fifth speed). Today, mass market automotive manual transmissions are essentially all 5-speeds, with 6-speed transmissions beginning to emerge in high performance vehicles in the early 1990s, and recently beginning to be offered on some high-efficiency and conventional passenger cars.  A very small number of 7-speed 'manual derived' transmissions are offered on high-end performance cars, such as the Bugatti Veyron 16.4, or the BMW M5. Both of these cars feature a "Paddle Shifter".

====External overdrive====
On earlier models with three or four forward speeds, the lack of an overdrive ratio for relaxed and fuel-efficient highway cruising was often filled by incorporating a separate overdrive unit in the rear housing of the transmission. This unit was separately actuated by a knob or button, often incorporated into the gearshift knob.

====Shaft and gear configuration====
The input shaft need not turn a pinion which rotates the countershaft. Another possibility is that gears are mounted on the input shaft itself, meshed with gears on the countershaft, in which case the countershaft then turns the output shaft. In other words, it's a matter of design on which shaft the driven and driving gears reside. 

The distribution of the shifters is also a matter of design; it need not be the case that all of the free-rotating gears with selectors are on one shaft, and the permanently splined gears on the other. For instance a five speed transmission might have the first-to-second selectors on the countershaft, but the third-to-fourth selector and the fifth selector on the mainshaft, which is the configuration in the 1998 Honda Civic. This means that when the car is stopped and idling in neutral with the clutch engaged input shaft spinning, the third, fourth and fifth gear pairs do not rotate. 

In some transmission designs (Volvo 850 and V/S70 series, for example) there are actually two countershafts, both driving an output pinion meshing with the front-wheel-drive transaxle's ring gear. This allows the transmission designer to make the transmission narrower, since each countershaft need only be half as long as a traditional countershaft with four gears and two shifters.

==Clutch==

In all vehicles using a transmission (virtually all modern vehicles), a coupling device is used to separate the engine and transmission when necessary. The pedal; on a motorcycle, a lever on the left handlebar serves the purpose.
thumb|250px|Pedal setup on a 2007 [[Subaru Legacy. From left to right, foot rest, clutch, brake, and accelerator.]]
* When the clutch pedal is fully depressed, the clutch is fully disengaged, and no torque is transferred from the engine to the transmission (and by extension to the drive wheels). In this uncoupled state it is possible to select gears or to stop the car without stopping the engine.
* When the clutch pedal is fully released, the clutch is fully engaged, and practically all of the engine's torque is transferred. In this coupled state, the clutch does not slip, but rather acts as rigid coupling, and power is transmitted to the wheels with minimal practical waste heat. 
* Between these extremes of engagement and disengagement the clutch slips to varying degrees. When the clutch slips it still transmits torque despite the difference in speeds between the engine crankshaft and the transmission input. Because this torque is transmitted by means of friction rather than direct mechanical contact, considerable power is wasted as heat (which is dissipated by the clutch). Properly applied, slip allows the vehicle to be started from a standstill, and when it is already moving, allows the engine rotation to gradually adjust to a newly selected gear ratio.
* Learning to use the clutch efficiently requires the development of muscle memory and a level of coordination analogous to that required to learn a musical instrument or to play a sport.
* A rider of a highly-tuned motocross or off-road motorcycle may "hit" or "fan" the clutch when exiting corners to assist the engine in revving to the point where it delivers the most power.
*Note: Automatic transmissions also use a coupling device; however, a clutch is not present. In these kinds of vehicles, the torque converter is used to separate the engine and transmission.

==Gear selection==
===Floor-mounted shifter===

In most modern passenger cars, gears are selected through a lever attached to the floor of the automobile&mdash;this selector is often called a gearstick, gear lever, gear selector, or simply 'shifter'.  Moving this lever forward, backward, left, and right allows the driver to select any given gear.  In this configuration, the gear lever must be pushed laterally before it is pushed longitudinally.

thumb|250px|5 speed shift stick of a 1999 [[Mazda Protege.]]

A sample layout of a ''four-speed'' transmission is shown below.  '''N''' marks ''neutral'', or the position where no gears are engaged.  In reality, the entire horizontal line is a ''neutral'' position, although the shifter is usually equipped with springs so that it will return to the N position if not left in another gear.  The '''R''' denotes reverse, which is technically a fifth gear on this transmission.

Image:Manual Layout4d.PNG

This layout is called the '''shift pattern'''. Because of the shift quadrants, the basic arrangement is often called an ''H-pattern''. While the layout for gears one through four is nearly universal, the location of reverse is not. Reverse can be found outside of the quadrant at the upper left (late 1960s GM models and AMC models, 1960s-1980s Ford Europe models, and current VW/Audi models), lower left (Fj Cruiser, Corvette), so caution is always warranted in gear selection. The shift pattern for a specific transmission is usually molded on the gear knob.

The image below shows the most common five-speed layout found in the USA and the UK.

Image:Manual Layout.PNG

This layout is reasonably intuitive because it starts at the upper left and works top to bottom, left to right, with reverse far away and toward the rear of the car. There is usually a mechanism that only allows selection of reverse from the neutral position, so reverse will be less likely to be accidentally chosen when downshifting from 5th to 4th (or by someone used to a 6-speed transmission and trying to shift from 5th to the non-existent 6th).

Image:Manual Dogleg.png

This five-speed layout, found on many race cars and some older model passenger cars, is commonly referred to as a "dog-leg first" or "racing" pattern, because of the "up and over" 1-2 shift. Its use is common in race cars and sports cars, but is diminishing as six speed and sequential gearboxes are becoming more common. Having 1st gear across the dog leg is beneficial as first gear is traditionally only used for getting the car moving and hence it allows 2nd and 3rd gear to be in the same vertical plane, which makes downshifting into 2nd gear easier. As most of the gearboxes are non-syncromesh there is no appreciable delay when upshifting from 1st through the dog leg into 2nd.

This gear pattern can also be found on some heavy vehicles where 1st gear is a crawler gear and would see little normal use.

Another five-speed shift pattern (common on many European cars) is this:

'''Механическая (ручная) коробка переключения передач''' автомобиля.

Устанавливается на автомобиль для изменения передаточного числа двигателя внутреннего сгорания.
Механическая коробка передач имеет валы, шестерни и фрикционное сцепление, управляемое водителем, а также рычаг переключения передач.
Характеризуется минимальными потерями при передаче крутящего момента двигателя, из-за чего часто устанавлявается на спортивные или околоспортивные автомобили.
Недостаток МКПП — необходимость водителю самостоятельно переключать передачи, отвлекаясь от вождения и уставая при длительном передвижении, особенно в городской черте. На время переключения передач происходит разрыв потока мощности — то есть двигатель временно отключается от трансмиссии при помощи сцепления.

Автомобили оснащенные МКПП преобладают в Европе, СНГ, а также странах с низким доходом населения.

В США МКПП практически полностью вытеснена АКПП и CVT.