A manual transmission, also known as a manual gearbox, stick shift, n-speed manual (where n is the number of gears), standard, M/T or in colloquial U.S. English a stick (for vehicles with hand-lever shifters), is a type of transmission used in motor vehicle applications. It uses a driver-operated clutch engaged and disengaged by a foot pedal (automobile) or hand lever (motorcycle), for regulating torque transfer from the engine to the transmission; and a gear selector operated by hand (automobile) or by foot (motorcycle).
A conventional, 5 or 6-speed manual transmission is often the standard equipment in a base-model car; other options include automated transmissions such as an automatic transmission (often a manumatic), a semi-automatic transmission, or a continuously variable transmission (CVT). The number of gears is often expressed for automatic transmissions as well (e.g., 3-speed automatic), but without qualifier it usually implies a manual transmission.
- 1 Overview
- 2 Unsynchronized transmission
- 3 Synchronized transmission
- 4 Internals
- 5 Clutch
- 6 Gear shift types
- 7 Benefits
- 8 Drawbacks
- 9 Applications and popularity
- 10 Truck transmissions
- 11 Crash gearbox
- 12 Maintenance
- 13 See also
- 14 References
Manual transmissions often feature a driver-operated clutch and a movable gear stick. Most automobile manual transmissions allow the driver to select any forward gear ratio ("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-higher or next-lower gear. This type of transmission is sometimes called a sequential manual transmission.
In a manual transmission, the flywheel is attached to the engine's crankshaft and spins along with it. The clutch disk is in between the pressure plate and the flywheel, and is held against the flywheel under pressure from the pressure plate. When the engine is running and the clutch is engaged (i.e., clutch pedal up), the flywheel spins the clutch plate and hence the transmission. As the clutch pedal is depressed, the throw out bearing is activated, which causes the pressure plate to stop applying pressure to the clutch disk. This makes the clutch plate stop receiving power from the engine, so that the gear can be shifted without damaging the transmission. When the clutch pedal is released, the throw out bearing is deactivated, and the clutch disk is again held against the flywheel, allowing it to start receiving power from the engine.
Manual transmissions are characterized by gear ratios that are selectable by locking selected gear pairs to the output shaft inside the transmission. Conversely, most automatic transmissions feature 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 Manumatics. A manual-style transmission operated by computer is often called an automated transmission rather than an automatic.
Contemporary automobile manual transmissions typically use four to six forward gears and one reverse gear, although automobile manual transmissions have been built with as few as two and as many as seven gears. Transmissions for heavy trucks and other heavy equipment usually have at least 8 to 25 gears so the transmission can offer both a wide range of gears and close gear ratios to keep the engine running in the power band. Operating aforementioned transmissions often use the same pattern of shifter movement with a single or multiple switches to engage the next sequence of gear selection. 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."
The earliest form of a manual transmission is thought[by whom?] to have been invented by Louis-René Panhard and Émile Levassor in the late 19th century. This type of transmission offered multiple gear ratios and, in most cases, reverse. The gears were typically engaged by sliding them on their shafts (hence the phrase shifting gears), which required careful timing and throttle manipulation when shifting, so the gears would be spinning at roughly the same speed when engaged; otherwise, the teeth would refuse to mesh. These transmissions are called sliding mesh transmissions or sometimes crash boxes, because of the difficulty in changing gears and the loud grinding sound that often accompanied. Newer manual transmissions on cars have all gears mesh at all times and are referred to as constant-mesh transmissions, with "synchro-mesh" being a further refinement of the constant mesh principle.
In both types, a particular gear combination can only be engaged when the two parts to engage (either gears or clutches) are at the same speed. To shift to a higher gear, the transmission is put in neutral and the engine allowed to slow down until the transmission parts for the next gear are at a proper speed to engage. The vehicle also slows while in neutral and that slows other transmission parts, so the time in neutral depends on the grade, wind, and other such factors. To shift to a lower gear, the transmission is put in neutral and the throttle is used to speed up the engine and thus the relevant transmission parts, to match speeds for engaging the next lower gear. For both upshifts and downshifts, the clutch is released (engaged) while in neutral. Some drivers use the clutch only for starting from a stop, and shifts are done without the clutch. Other drivers will depress (disengage) the clutch, shift to neutral, then engage the clutch momentarily to force transmission parts to match the engine speed, then depress the clutch again to shift to the next gear, a process called double clutching. Double clutching is easier to get smooth, as speeds that are close but not quite matched need to speed up or slow down only transmission parts, whereas with the clutch engaged to the engine, mismatched speeds are fighting the rotational inertia and power of the engine.
Even though automobile and light truck transmissions are now almost universally synchronized, transmissions for heavy trucks and machinery, motorcycles, and for dedicated racing are usually not. Non-synchronized transmission designs are used for several reasons. The friction material, such as brass, in synchronizers is more prone to wear and breakage than gears, which are forged steel, and the simplicity of the mechanism improves reliability and reduces cost. In addition, the process of shifting a synchromesh transmission is slower than that of shifting a non-synchromesh transmission. For racing of production-based transmissions, sometimes half the teeth on the dog clutches are removed to speed the shifting process, at the expense of greater wear.
Heavy duty trucks often use unsynchronized transmissions, though military trucks usually have synchronized transmissions, allowing untrained personnel to operate them in emergencies. In the United States, traffic safety rules refer to non-synchronous transmissions in classes of larger commercial motor vehicles. In Europe, heavy duty trucks use synchronized gearboxes as standard.
Similarly, most modern motorcycles use unsynchronized transmissions: their low gear inertias and higher strengths mean that forcing the gears to alter speed is not damaging, and the pedal operated selector on modern motorcycles, with no neutral position between gears (except, commonly, 1st and 2nd), is not conducive to having the long shift time of a synchronized gearbox. On bikes with a 1-N-2(-3-4...) transmission, it is necessary either to stop, slow down, or synchronize gear speeds by blipping the throttle when shifting from 2nd into 1st.
Most modern manual-transmission vehicles are fitted with a synchronized gear box. Transmission gears are always in mesh and rotating, but gears on one shaft can freely rotate or be locked to the shaft. The locking mechanism for a 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. 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. 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 before that of another is engaged. One collar often serves for two gears; sliding in one direction selects one transmission speed, in the other direction selects another.
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 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 baulk rings, the latter being spelled balk in the U.S.). The synchro ring rotates slightly due to the frictional torque from the cone clutch. In this position, the dog clutch is prevented from engaging. The brass clutch ring gradually causes parts to spin at the same speed. When they do spin the same speed, there is no more torque from the cone clutch and the dog clutch is allowed to fall into engagement. 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 synchronisers 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, fully synchromesh transmissions with three speeds, then four, and then five, became universal by the 1980s. Many modern manual transmission cars, especially sports cars, now offer six speeds. The 2012 Porsche 911 offers a seven-speed manual transmission, with the seventh gear intended for cruising- top speed being attained on sixth.
Reverse gear is usually not synchromesh, as there is only one reverse gear in the normal automotive transmission and changing gears into reverse while moving is not required - and often highly undesirable, particularly at high forward speed. Additionally, the usual method of providing reverse, with an idler gear sliding into place to bridge what would otherwise be two mismatched forward gears, is necessarily similar to the operation of a crash box. Among the cars that have synchromesh in reverse are the 1995-2000 Ford Contour and Mercury Mystique, '00-'05 Chevrolet Cavalier, Mercedes 190 2.3-16, the V6 equipped Alfa Romeo GTV/Spider (916), certain Chrysler, Jeep, and GM products which use the New Venture NV3500 and NV3550 units, the European Ford Sierra and Granada/Scorpio equipped with the MT75 gearbox, the Volvo 850, and almost all Lamborghinis, Hondas and BMWs.
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 many transmissions the input and output components of the mainshaft can 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 bearings in their hubs. Reverse is typically implemented differently; see the section on 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 put 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.
Among many different types of clutches, a dog clutch provides non-slip coupling of two rotating members. It is not at all suited to intentional slipping, in contrast with the foot-operated friction clutch of a manual-transmission car.
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 that shaft to rotate with it. 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.
If the 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, which consists of a cone clutch and blocking ring. Before the teeth can engage, the cone clutch engages first, which brings the selector and gear to the same speed using friction. 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 or notches that allow further passage of the selector which brings the teeth together. The exact design of the synchronizer varies among manufacturers.
The synchronizer has to overcome the momentum of the entire input shaft and clutch disk when it is changing shaft rpm to match the new gear ratio. It can be abused by exposure to the momentum and power of the engine, 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 difference in rotational speed between the layshaft and transmission gear shaft, therefore decreasing synchro wear.
Synchronizing rings are made of metal and can be provided with anti-wear coatings called a friction lining. Common metals for synchronizer rings are brass and steel. The linings typically consist of molybdenum, iron, bronze or carbon. The synchronizing rings are produced either by massive forming (common forging) or sheet metal shaping. The latter involves the stamping of the blank out of a sheet metal strip and the subsequent machining with follow-on composite tools or transfer tools. A friction lining usually consists of thermally splashed molybdenum. Alternatively, iron or bronze sinter friction layers can be used. Carbon-coated synchronizer rings are particularly wear resistant and offer very good friction behavior. Due to their higher price, these are reserved for high-performance transmissions.
Transmissions with brass synchronizer components are generally not suitable for use with GL-5 specification oil unless specifically stated by the manufacturer as the extreme pressure (EP) additives in the oil are corrosive to brass and bronze components at high temperatures and decrease the synchronizer effectiveness at low temperatures. The additives in GL-5 oil also cause physical damage to brass synchronizers as the EP additives bond more strongly to the brass than the brass does to itself, causing a small layer of brass to be worn off with every gear change. Instead, oil which meets only the GL-4 specification should be used whenever possible.
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. When reverse is selected 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.
In other words, when reverse gear is selected, it is in fact 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 normally heard in the forward gears. The teeth on the forward gears of most consumer automobiles are helically cut. When helical gears rotate, there is constant contact between gears, 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, most reverse gears are spur gears, meaning that they have straight teeth, in order to allow for the sliding engagement of the idler, which is difficult with helical gears. The teeth of spur gears clatter together when the gears spin, generating a characteristic whine.
Attempting to select reverse while the vehicle is moving forward causes severe gear wear (except in transmissions with synchromesh on the reverse gear). However, most manual transmissions have a gate that locks out reverse directly from 5th gear to help prevent this. In order to engage reverse from 5th, the shift lever has to be moved to the center position between 3rd and 4th, then back over and into reverse. Another widespread solution places reverse to the left of 1st gear, instead of behind the 5th (where you might expect to find a 6th gear). Similarly, many newer six-speed manual transmissions have a collar under the shift knob which must be lifted to engage reverse to also help prevent this.
The spur gear design of reverse gear represents some compromises (less robust, unsynchronized engagement and loud noise) which are acceptable due to the relatively small amount of driving that takes place in reverse. The gearbox of the classic SAAB 900 is a notable example of a gearbox with a helical reverse gear engaged in the same unsynchronized manner as the spur gears described above. Its design allows reverse to share cogs with first gear, and is exceptionally quiet, but results in difficult engagement and unreliable operation. However, many modern transmissions now include a reverse gear synchronizer and helical gearing.
Until the mid-1950s (earlier in Europe and later in the US, on average) cars were generally equipped with 3-speed transmissions as standard equipment. 4-speed units began to appear on volume-production models in the 1930s (Europe) and 1950s (USA) and gained popularity in the 1960s; some exotics had 5-speeds. In the 1970s, as fuel prices rose and fuel economy became an important selling feature, 4-speed transmissions with an overdrive 4th gear or 5-speeds were offered in 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). 6-speed transmissions started to emerge in high-performance vehicles in the early 1990s. 7-speed transmissions appeared on extreme high-end supercars, such as the 2005 Bugatti Veyron (semi-automatic manual transmission). In 2012, the Porsche 911 featured a 7-speed manual transmission, becoming the first of its class to support this feature, paving the way for the 2014 Chevrolet Corvette Stingray.
Today, mass market automotive manual transmissions are nearly all at least 5-speed. 4-speed manual transmission had fallen into almost total disuse by the end of the 1980s, having gradually become less common on cars during the 1980s. By the early 1990s, it was normally only found on cars with engines of around 1.0 litres. Recently Porsche announced the next-generation 911 will be available with a 7-speed manual transmission, the first of its kind for a normal automobile with the first six gear ratios the same as the 6-speed gearbox and the 7th gear being of a higher ratio. In 2013, the new 2014 Chevrolet Corvette Stingray will also feature a 7-speed manual transmission.
It has been widely anticipated that for electric vehicles (EVs), clutches and multi-speed gearboxes would not be required, as electric motors can drive the vehicle both forward and reverse from zero speed and typically operate over a wider speed range than combustion engines. Elimination of the gearbox represents a significant reduction in powertrain weight and complexity, and also removes a notable source of parasitic losses. The majority of first-generation consumer EVs have therefore been single-speed. However, current trends indicate that multi-speed gearboxes are likely to return for many future EVs. This allows the use of smaller, lower torque motors running at higher speeds to achieve both greater torque at the wheels for low speed tractive effort, and higher top road speed. Modest efficiency gains are also possible by reducing the proportion of the time that the motor(s) operate at very low speeds where efficiency is reduced. The wider speed range of motors means that the number of ratios required is lower than for combustion engine vehicles, with two to four speed designs emerging as the optimum depending on application.
The slowest gears (designated '1' or low gear) in most automotive applications allow for three to four engine rotations for each output revolution (3:1). High, or "top", gear in many earlier three or four speed manual transmissions locks the output shaft to spin at the same speed as the engine (1:1). Five and six speed gearboxes are almost always 'overdrive' in top gear with the engine turning less than a full turn for each revolution of the output shaft, 0.8:1 for example (however, the final drive, or differential, always has further reduction gearing).
In the 1950s, 1960s, and 1970s, fuel-efficient highway cruising with low engine speed was in some cases enabled on cars equipped with 3- or 4-speed transmissions by means of a separate overdrive unit in or behind the rear housing of the transmission. This was actuated either manually while in high gear by throwing a switch or pressing a button on the gearstick knob or on the steering column, or automatically by momentarily lifting the foot from the accelerator with the car travelling above a certain road speed. Automatic overdrives were disengaged by flooring the accelerator, and a lockout control was provided to enable the driver to disable overdrive and operate the transmission as a normal (non-overdrive) transmission.
Shaft and gear configuration
On a conventional rear-drive transmission, there are three basic shafts; the input, the output, and the countershaft. The input and output together are called the mainshaft, since they are joined inside the transmission so they appear to be a single shaft, although they rotate totally independently of each other. The input length of this shaft is much shorter than the output shaft. Parallel to the mainshaft is the countershaft. There are a number of gears fixed along the countershaft, and matching gears along the output shaft, although these are not fixed, and rotate independently of the output shaft. There are sliding dog collars, or dog clutches, between the gears on the output shaft, and to engage a gear to the shaft, the collar slides into the space between the shaft and the inside space of the gear, thus rotating the shaft as well. One collar is usually mounted between two gears, and slides both ways to engage one or the other gears, so on a four-speed there would be two collars. A front-drive transmission is basically the same, but may be simplified. There often are two shafts, the input and the output, but depending on the direction of rotation of the engine, three may be required. Rather than the input shaft driving the countershaft with a pinion gear, the input shaft takes over the countershaft's job, and the output shaft runs parallel to it. The gears are positioned and engaged just as they are on the countershaft and output shaft of a rear-drive. This merely eliminates one major component, the pinion gear. Part of the reason that the input and output are in-line on a rear drive unit is to relieve torsional stress on the transmission and mountings, but this isn't an issue in a front-drive as the gearbox is integrated into the transaxle.
The basic process is not universal. The fixed and free gears can be mounted on either the input or output shaft, or both.
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 and the 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.
Some automotive manual transmissions had freewheeling capability in the 1930s through 1960s.
In all vehicles using a transmission (virtually all modern vehicles), a coupling device is used to separate the engine and transmission when necessary. This is because an internal-combustion engine must continue to run when in use, although a few modern cars shut off the engine at a stoplight. The clutch accomplishes this in manual transmissions. Without it, the engine and tires would at all times be inextricably linked, and any time the vehicle stopped, the engine would stall. Without the clutch, changing gears would be very difficult, even with the vehicle moving already: deselecting a gear while the transmission is under load requires considerable force (and risks significant damage). As well, selecting a gear requires the revolution speed of the engine to be held at a very precise value which depends on the vehicle speed and desired gear – the speeds inside the transmission have to match. In a car, the clutch is usually operated by a pedal; on a motorcycle, a lever on the left handlebar serves the purpose.
- 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 all of the engine's torque is transferred. In this coupled state, the clutch does not slip, but rather acts as rigid coupling to transmit power to the gearbox.
- Between these extremes of engagement and disengagement the clutch slips to varying degrees. When slipping 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.
- 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.
The clutch is typically disengaged by a thrust bearing that makes contact with pressure petals on the clutch ring plate and pushes them inward to release the clutch pad friction. Normally the bearing remains retracted away from the petals and does not spin. However, the bearing can be "burned out" and damaged by using the clutch pedal as a foot rest, which causes the bearing to spin continuously from touching the clutch plates.
Float shifting or floating gears is changing gears without depressing the clutch, usually on a non-synchronized transmission. Since the clutch is not used, it is easy to mismatch speeds of gears, and the driver can quickly cause major (and expensive) damage to the gears and the transmission. Float shifting is often done on large trucks with standard (non-synchronized) gearboxes.
Gear shift types
In most vehicles with manual transmission, gears are selected by manipulating a lever called a gear stick, shift stick, gearshift, gear lever, gear selector, or shifter connected to the transmission via linkage or cables and mounted on the floor, dashboard, or steering column. Moving the lever forward, backward, left, and right into specific positions selects particular gears.
A sample layout of a four-speed transmission is shown below. N marks neutral, the position wherein no gears are engaged and the engine is decoupled from the vehicle's drive wheels. The entire horizontal line is a neutral position, though the shifter is usually spring-loaded so it will return to the centre of the N position if not moved to another gear. The R marks reverse, the gear position used for moving the vehicle backward.
This layout is called the shift pattern. Because of the shift quadrants, the basic arrangement is often called an H-pattern. The shift pattern is usually molded or printed on or near the gear knob.
Typically, first gear is engaged at the top left position with second below, third up to the right with fourth, below, and so on. The only other pattern used in production vehicle manual transmissions is known as a Dog-leg gearbox pattern. This pattern locates first at bottom left position, second up and to the right with third below, fourth up and to the right, and so on. This pattern is found primarily in race and race inspired vehicles. Placing the selection position for second gear above the position for third gear is desirable in racing as more frequent shifting occurs from second to third than from first to second.
Independent of the shift pattern, the location of the reverse gear may vary. Depending on the particular transmission design, reverse may be located at the upper left extent of the shift pattern, at the lower left, at the lower right, or at the upper right. There is often a mechanism that allows selection of reverse only from the neutral position, or a reverse lockout that must be released by depressing the spring-loaded gear knob or lifting a spring-loaded collar on the shift stick, to reduce the likelihood of the driver inadvertently selecting reverse.
"Three on the Tree" vs "Four on the Floor"
During the period when U.S. cars usually had only three forward speeds and the steering column was the most common shifter location, this layout was sometimes called "three on the tree." In contrast European cars and performance cars mostly used a four-speed transmission with floor-mounted shifters. This layout was then referred to as "four on the floor".
Most FR(front-engined, rear-wheel drive) cars have a transmission that sits between the driver and the front passenger seat. Floor-mounted shifters are often connected directly to the transmission. FF(front-engined, front-wheel drive) and RR(rear-engined, rear-wheel drive) cars often require a mechanical linkage to connect the shifter to the transmission.
Some cars have a gear lever mounted on the steering column of the car. A 3-speed column shifter, which came to be popularly known as a "Three on the Tree", began appearing in America in the late 1930s and became common during the 1940s and 1950s. If a U.S. vehicle was equipped with overdrive, it was very likely to be a Borg-Warner type, operated by briefly backing off the gas when above 28 mph to enable, and momentarily flooring the gas pedal to return to normal gear. The control simply disables overdrive for such situations as parking on a hill or preventing unwanted shifting into overdrive.
Later,[vague] European and Japanese models began to have 4-speed column shifters with this shift pattern:
A majority of American-spec vehicles sold in the U.S. and Canada had a 3-speed column-mounted shifter - the first generation Chevrolet/GMC vans of 1964-70 vintage had an ultra-rare 4-speed column shifter. The column-mounted manual shifter disappeared in North America by the mid-1980s, last appearing in the 1987 Chevrolet pickup truck. Outside North America, the column-mounted shifter remained in production. All Toyota Crown and Nissan Cedric taxis in Hong Kong had the 4-speed column shift until 1999 when automatic transmissions were first offered. Since the late 1980s or early 1990s,[vague] a 5-speed column shifter has been offered in some vans sold in Asia and Europe, such as Toyota Hiace, Mitsubishi L400 and the first-gen Fiat Ducato.
Column shifters are mechanically similar to floor shifters, although shifting occurs in a vertical plane instead of a horizontal one. Because the shifter is further away from the transmission, and the movements at the shifter and at the transmission are in different planes, column shifters require more complicated linkage than floor shifters. Advantages of a column shifter are the ability to switch between the two most commonly used gears—second and third—without letting go of the steering wheel, and the lack of interference with passenger seating space in vehicles equipped with a bench seat.
Some smaller cars in the 1950s and 1960s, such as Citroën 2CV, Renault 4L and early Renault 5 feature a shifter in the dash panel. This was cheaper to manufacture than a column shifter and more practical, as the gearbox was mounted in front of the engine. The linkage for the shifter could then be positioned on top of the engine. The disadvantage is that shifting is less comfortable and usually slower to operate.
Newer small cars and MPVs, like the Suzuki MR Wagon, the Fiat Multipla, the Toyota Matrix, the Pontiac Vibe, the Chrysler RT platform cars, the Honda Element, the Honda Civic, and the Honda Avancier may feature a manual or automatic transmission gear shifter located on the vehicle's instrument panel, similar to the mid-1950s Chryslers and Powerglide Corvairs. Console-mounted shifters are similar to floor-mounted gear shifters in that most of the ones used in modern cars operate on a horizontal plane and can be mounted to the vehicle's transmission in much the same way a floor-mounted shifter can. However, because of the location of the gear shifter in comparison to the locations of the column shifter and the floor shifter, as well as the positioning of the shifter to the rest of the controls on the panel often require that the gearshift be mounted in a space that does not feature a lot of controls integral to the vehicle's operation or frequently used controls, such as those for the car stereo or car air conditioning, to help prevent accidental activation or driver confusion, especially in right-hand drive cars.
More and more small cars and vans from manufacturers such as Suzuki, Honda, and Volkswagen are featuring console shifters in that they free up space on the floor for other car features such as storage compartments without requiring that the gear shift be mounted on the steering column. Also, the basic location of the gear shift in comparison to the column shifter makes console shifters easier to operate than column shifters.
Some transmissions do not allow the driver to arbitrarily select any gear. Instead, the driver may only ever select the next-lower or next-higher gear ratio. Sequential transmissions often incorporate a synchro-less dog-clutch engagement mechanism (instead of the synchromesh dog clutch common on H-pattern automotive transmissions), in which case the clutch is only necessary when selecting first or reverse gear from neutral, and most gear changes can be performed without the clutch. However, sequential shifting and synchro-less engagement are not inherently linked, though they often occur together due to the environment(s) in which these transmissions are used, such as racing cars and motorcycles.
Sequential transmissions are generally controlled by a forward-backward lever, foot pedal, or set of paddles mounted behind the steering wheel. In some cases, these are connected mechanically to the transmission. In many modern examples, these controls are attached to sensors which instruct a transmission computer to perform a shift—many of these systems can be switched into an automatic mode, where the computer controls the timing of shifts, much like an automatic transmission.
Motorcycles typically employ sequential transmissions, although the shift pattern is modified slightly for safety reasons. In a motorcycle the gears are usually shifted with the left foot pedal, the layout being this:
1 - N - 2 - 3 - 4 - 5 (- 6)
The pedal goes one step–both up and down–from the center, before it reaches its limit and has to be allowed to move back to the center position. Thus, changing multiple gears in one direction is accomplished by repeatedly pumping the pedal, either up or down. Although neutral is listed as being between first and second gears for this type of transmission, it "feels" more like first and second gear are just "further away" from each other than any other two sequential gears. Because this can lead to difficulty in finding neutral for inexperienced riders most motorcycles have a neutral indicator light on the instrument panel to help find neutral. The reason neutral does not actually have its own spot in the sequence is to make it quicker to shift from first to second when moving. Neutral can be accidentally shifted into, though most high end, newer model motorcycles have means of avoiding this. The reason for having neutral between the first and second gears instead of at the bottom is that when stopped, the rider can just click down repeatedly and know that they will end up in first and not neutral. This allows a rider to quickly move his bike from a standstill in an emergency situation. This may also help on a steep hill on which high torque is required. It could be disadvantageous or even dangerous to attempt to be in first without realizing it, then try for a lower gear, only to get neutral.
On motorcycles used on race tracks, the shifting pattern is often reversed, that is, the rider clicks down to upshift. This usage pattern increases the ground clearance by placing the rider's foot above the shift lever when the rider is most likely to need it, namely when leaning over and exiting a tight turn.
The shift pattern for most underbone or miniature motorcycles with an automatic centrifugal clutch is also modified for two key reasons - to enable the less-experienced riders to shift the gears without problems of "finding" neutral, and also due to the greater force needed to "lift" the gearshift lever (because the gearshift pedal of an underbone motorcycle also operates the clutch). The gearshift lever of an underbone has two ends. The rider clicks down the front end with the left toe all the way to the top gear and clicks down the rear end with the heel all the way down to neutral, while miniatures still retain a single-end lever, with the rider clicks down to upshift and lift the lever up to downshift (or vice versa). Some underbone models such as the Honda Wave have a "rotary" shift pattern, which means that the rider can shift directly to neutral from the top gear, but for safety reasons this is only possible when the motorcycle is stationary. Some models also have gear position indicators for all gear positions at the instrument panel.
Some new transmissions (Alfa Romeo's Selespeed gearbox and BMW's Sequential Manual Gearbox (SMG) for example) are conventional manual transmissions with a computerized control mechanism. These transmissions feature independently selectable gears but do not have a clutch pedal. Instead, the transmission computer controls a servo which disengages the clutch when necessary.
These transmissions vary from sequential transmissions in that they still allow nonsequential shifts: the SMG system formerly used by BMW, for example, could shift from 6th gear directly to 4th gear.
An early version of this type of transmission was the Autostick, which was used in the Volkswagen Beetle and Karmann Ghia from 1967 to 1976, where the clutch was disengaged by servo when the driver pushed downward slightly on the gear shift lever. This was a 3-speed unit.
In the case of the early second generation Saab 900, a 'Sensonic' option was available where gears were shifted with a conventional shifter, but the clutch is controlled by a computer.
See semi-automatic transmission for more examples.
A short shifter, also known as a short throw shifter, is the result of an automotive aftermarket modification of the manual transmissions' stick shift either by modification of the existing stick shift or, alternately, by the replacement of the entire part.
The purpose of the modification is to mechanically reduce time between the changing of gears while accelerating or decelerating, thus improving the automobiles' performance. The modification of the existing stick shift, also known as a manual gear stick, can take two forms: either the physical shortening of the existing stick shift, known in the industry as 'chopping', or bending. By reducing the length of the stick shift, the distance it must travel to change gears is effectively reduced, thus reducing the time spent shifting. At the same time, the amount of force required to shift increases due to a shorter lever.
Some major car manufacturers such as Subaru, Mazda and Porsche offer short shifters as stock modifications such as in the Subaru Impreza WRX, Subaru Impreza WRX STI, Subaru BRZ, Mazda Miata, and as an option such as in the Porsche 911.
|This section requires expansion. (July 2014)|
In Japan, finger shift is used on buses. フィンガーシフト
Its system is made in Robert Bosch GmbH.
The manual transmission couples the engine to the transmission with a rigid clutch instead of the torque converter on an automatic transmission or the v-belt of a continuously variable transmission, which slip by nature. Manual transmissions also lack the parasitic power consumption of the automatic transmission's hydraulic pump. Because of this, manual transmissions generally offer better fuel economy than automatic or continuously variable transmissions; however the disparity has been somewhat offset with the introduction of locking torque converters on automatic transmissions. Increased fuel economy with a properly operated manual transmission vehicle versus an equivalent automatic transmission vehicle can range from 5% to about 15% depending on driving conditions and style of driving. The lack of control over downshifting under load in an automatic transmission, coupled with a typical vehicle engine's greater efficiency under higher load, can enable additional fuel gains from a manual transmission by allowing the operator to keep the engine performing under a more efficient load/RPM combination. This is especially true for older models, as advances like variable valve timing allow better performance over a broader RPM range. In recognition of this, many current models (2010 and on) come with manual modes, or overrides on automatic models, although the degree of control varies greatly by the manufacturer. Also, manual transmissions do not require active cooling and because they are, mechanically, much simpler than automatic transmissions, they generally weigh less than comparable automatics, which can improve economy in stop-and-go traffic. However this gap in economy is being rapidly closed, and many mid to higher end model automatic cars now get better economy than their standard spec counterparts. This is in part due to the increasing impact of computers co-ordinating multiple systems, particularly in hybrid models in which the engine and drive motors must be managed.
Longevity and cost
Because manual transmissions are mechanically simpler, are more easily manufactured, and have fewer moving parts than automatic transmissions, they require less maintenance and are easier as well as cheaper to repair. Due to their mechanical simplicity, they often last longer than automatic transmissions when used by a skilled driver. Typically, there are no electrical components, pumps and cooling mechanisms in a manual transmission, other than an internal switch to activate reversing lighting. These attributes become extremely vital with a vehicle stuck in mud, snow, etc. The back and forth rocking motion of the vehicle drivers use to dislodge a stuck vehicle can destroy automatic transmissions. Clutches are a wear item which may need to be replaced at some point in the vehicle's lifespan, however the service life of the clutch depends on the operating conditions that it is subjected to.
The price of a new car with a manual transmission will commonly be lower than the same car with an automatic transmission.
Most new cars are available in manual or automatic transmission versions. There is often a difference in cost between the two. Manual transmissions generally cost less than automatic transmissions. For example, the base price of a Chevrolet Cruze 2LT with a manual transmission is $22,120, while the base price of the automatic is $23,405 - a difference of $1,285.
Most manual transmissions rely on splash lubrication although some five speed Rover gearboxes did incorporate an oil pump. The problem with splash lubrication is that it is speed dependent. There are centrifugal effects, hydrodynamic effects and effects from the gears working as pumps. If a gearbox is fitted with Perspex windows and run on a test rig these effects can be observed. As the gearbox is run through its rev range, the oil jets will switch over and move around. Research on the Austin Maxi 1500 gearbox showed that one of the ball races was running dry at 80 miles per hour (130 km/h). The solution was to alter the casting to include a small projection that would intercept the main oil jet that was present at 80 mph and disperse it. This small modification enabled the later Maxi 1750 gearbox to be relatively trouble free. Four speed gearboxes seldom show these problems because at top speed (and maximum power) they are basically a solid shaft and the gears are not transmitting power.
Performance and control
Manual transmissions have generally offered a wider selection of gear ratios. Many vehicles offer a 5-speed or 6-speed manual, whereas the automatic option would typically be a 4-speed. This is generally due to the increased space available inside a manual transmission compared with an automatic, since the latter requires extra components for self-shifting, such as torque converters and pumps. However, automatic transmissions are now adding more speeds as the technology matures. ZF currently manufactures 7- and 8-speed automatic transmissions. ZF is also planning a 9-speed automatic for use in front-wheel drive vehicles. The increased number of gears allows for better use of the engine's power band, resulting in increased fuel economy by staying in the most fuel-efficient part of the power band, or higher performance, thereby remaining closer to the engine's peak power rating. Even with more forward speeds and the potential of designing more forward gears to offer higher speed and/or torque, the manual transmission remains smaller and much more compact than its larger, automatic cousin, as referenced by the 991 Generation of the Porsche 911 and the 2014 Chevy Corvette, which has a 7-speed manual transmission.
In contrast to most manual gearboxes, most automatic transmissions have far less effective engine braking. This means that the engine does not slow the car as effectively when the automatic transmission driver releases the engine speed control. This leads to more usage of the brakes in cars with automatic transmissions, bringing shorter brake life. Brakes are also more likely to overheat in hilly or mountainous areas, causing reduced braking ability brake fade and the potential for complete failure with the automatic transmission vehicle.
Complexity and learning curve
For most people, there is a slight learning curve with a manual transmission, which may be intimidating and unappealing for an inexperienced driver. Because the driver must develop a feel for properly engaging the clutch, an inexperienced driver will often stall the engine. Most drivers can learn how to drive a car with a manual transmission in as little as an hour, although it may take weeks before it becomes "second nature." Additionally, if an inexperienced driver selects an inappropriate gear by mistake, damage to mechanical components and even loss of control may occur. Learning clutch/gas pedal coordination can be made easier by using the clutch pedal only, on a level surface. This will allow the operator to gauge where the "sweet spot" of clutch engagement is. Correct "release speed" of the clutch pedal (slow for smooth, fast for abrupt) will indicate when and where gas pedal use should occur.
In many jurisdictions, a driving license issued for cars with an automatic transmission is not valid for driving vehicles with a manual transmission, but a license for manual transmissions covers both.
Automatic transmissions can typically shift ratios faster than a manual gear change can be accomplished, due to the time required for the average driver to push the clutch pedal to the floor and move the gearstick from one position to another. This is especially true in regards to dual-clutch transmissions, which are specialized computer-controlled automatic transmissions that mechanically operate more like a manual transmission than a traditional automatic one.
Ease of use
Because manual transmissions require the operation of an extra pedal, and keeping the car in the correct gear at all times, they require a bit more concentration, especially in heavy traffic situations. The automatic transmissions, on the other hand, simply require the driver to speed up or slow down as needed, with the car doing the work of choosing an appropriate gear. Manual transmissions also place a greater workload on the driver in heavy traffic situations, when the driver must operate the clutch pedal quite often. Because the clutch pedal can require a substantial amount of force, especially on large trucks, and the long pedal travel compared to the brake or accelerator requires moving the entire leg, not just the foot near the ankle, a manual transmission can cause fatigue, and is more difficult for weak or injured people to drive. Additionally, because automatic transmissions can be driven with only one foot, people with one leg that is missing or impaired can still drive, unlike the manual transmission that requires the use of two feet at once. Likewise, manual transmissions require the driver to remove one hand periodically from the steering wheel while the vehicle is in motion, which can be difficult or impossible to do safely for people with a missing or impaired arm, and requires increased coordination, even for those with full use of both hands.
Stopping on hills
The clutch experiences most of its wear in first gear because moving the car from a standstill involves a great deal of friction at the clutch. When accelerating from a standstill on an incline, this problem is made worse because the amount of work needed to overcome the acceleration of gravity causes the clutch to heat up considerably more. For this reason, stop-and-go driving and hills tend to have an effect on the clutches to a certain degree. Automatic transmissions are better suited for these applications because they have a hydraulic torque converter which is externally cooled, unlike a clutch. Torque converters also do not have a friction material that rubs off over time like a clutch. Some automatics even lock the output shaft so that the car cannot roll backwards when beginning to accelerate up an incline. To reduce wear in these applications, some manual transmissions will have a very low, "granny" gear which provides the leverage to move the vehicle easily at very low speeds. This reduces wear at the clutch because the transmission requires less input torque. However, the issue of handling stops on hills is easy to learn.
Many drivers use the parking brake to prevent the car from rolling backward when starting to accelerate up a steep hill. This saves precious clutch life. Some modern cars such as the Dodge Challenger and most Subaru models have a "Hill-Start Assist" feature. The vehicle's computer applies just enough brake pressure to prevent the car from rolling backwards. This allows the driver to start normally with no additional effort, even on steep hills.
Starting on a hill with the aid of the parking brake is not always possible since in recent cars that feature an Electric Park Brake the parking brake can only be released when the brake pedal is engaged.
Applications and popularity
Sports cars are also often equipped with manual transmissions because they offer more direct driver involvement and better performance, though this is changing as many automakers move to faster dual-clutch transmissions, which are generally shifted with paddles located behind the steering wheel. For example, the 991 Porsche 911 GT3 uses Porsche's PDK. Off-road vehicles and trucks often feature manual transmissions because they allow direct gear selection and are often more rugged than their automatic counterparts.
Conversely, manual transmissions are no longer popular in many classes of cars sold in North America, Australia and some parts of Asia, although they remain dominant in Europe, Asia, Africa and Latin America. Nearly all cars are available with an automatic transmission option, and family cars and large trucks sold in the US are predominantly fitted with automatics, however in some cases if a buyer wishes he/she can have the car fitted with a manual transmission at the factory. In Europe most cars are sold with manual transmissions. Most luxury cars are only available with an automatic transmission. In most cases where both transmissions are available for a given car, automatics are an at cost option, but in some cases the reverse is true. Some cars, such as rental cars and taxis, are nearly universally equipped with automatic transmissions in countries such as the US, but the opposite is true in Europe. As of 2008, 75.2% of vehicles made in Western Europe were equipped with manual transmission, versus 16.1% with automatic and 8.7% with other.
When a driver takes the licensing road test using an automatic transmission in some places, the resulting license is restricted to the use of automatic transmissions. This is the case in countries such as New Zealand (for the second-phase Restricted license, but not the final Full license), the European Union with the exception of member countries that opt to disallow road tests on automatic vehicles completely, China, Dominican Republic, Israel, Jordan, Norway, Philippines, Singapore, South Africa, South Korea, Sri Lanka, Switzerland, and the U.A.E. This treatment of the manual transmission skill seems to maintain the widespread use of the manual transmission. As many new drivers worry that their restricted license will become an obstacle for them where most cars have manual transmissions, they make the effort to learn with manual transmissions and obtain full licenses. Some other countries (such as Turkey, Greece, Georgia, India, Pakistan, Portugal, Malaysia, Serbia, Brazil, Russia, Ukraine and Denmark) go even further, whereby the license is granted only when a test is passed on a manual transmission. In Denmark and Brazil drivers are allowed to take the test on an automatic if they are handicapped, but with such a license they will not be allowed to drive a car with a manual transmission.
Some trucks have transmissions that look and behave like ordinary car transmissions - these transmissions are used on lighter trucks, typically have up to 6 gears, and usually have synchromesh.
For trucks needing more gears, the standard "H" pattern can get very complicated, so additional controls are used to select additional gears. The "H" pattern is retained, then an additional control selects among alternatives. In older trucks, the control is often a separate lever mounted on the floor or more recently a pneumatic switch mounted on the "H" lever; in newer trucks the control is often an electrical switch mounted on the "H" lever. Multi-control transmissions are built in much higher power ratings, but rarely use synchromesh.
There are several common alternatives for the shifting pattern. Usual types are:
- Range transmissions use an "H" pattern through a narrow range of gears, then a "range" control shifts the "H" pattern between high and low ranges. For example, an 8-speed range transmission has an H shift pattern with four gears. The first through fourth gears are accessed when low range is selected. To access the fifth through eighth gears, the range selector is moved to high range, and the gear lever again shifted through the first through fourth gear positions. In high range, the first gear position becomes fifth, the second gear position becomes sixth, and so on.
- Splitter transmissions use an "H" pattern with a wide range of gears, and the other selector splits each sequential gear position in two: First gear is in first position/low split, second gear is in first position/high split, third gear is in second position/low split, fourth gear is in second position/high split, and so on.
- Range-Splitter transmissions combine range-splitting and gear-splitting. This allows even more gear ratios. Both a range selector and a splitter selector are provided.
Although there are many gear positions, shifting through gears usually follows a regular pattern. For example, a series of upshifts might use "move to splitter direct; move to splitter overdrive; move shift lever to No. 2 and move splitter to underdrive; move splitter to direct; move splitter to overdrive; move shift lever to No. 3 and move splitter to underdrive"; and so on. In older trucks using floor-mounted levers, a bigger problem is common gear shifts require the drivers to move their hands between shift levers in a single shift, and without synchromesh, shifts must be carefully timed or the transmission will not engage. For this reason, some splitter transmissions have an additional "under under" range, so when the splitter is already in "under" it can be quickly downshifted again, without the delay of a double shift.
Today's truck transmissions are most commonly "range-splitter". The most common 13-speed has a standard H pattern, and the pattern from left upper corner is as follows: R, down to L, over and up to 1, down to 2, up and over to 3, down to 4. The "butterfly" range lever in the center front of the knob is flipped up to high range while in 4th, then shifted back to 1. The 1 through 4 positions of the knob are repeated. Also, each can be split using the thumb-actuated under-overdrive lever on the left side of the knob while in high range. The "thumb" lever is not available in low range, except in 18 speeds; 1 through 4 in low range can be split using the thumb lever and L can be split with the "Butterfly" lever. L cannot be split using the thumb lever in either the 13- or 18-speed. The 9-speed transmission is basically a 13-speed without the under-overdrive thumb lever.
Truck transmissions use many physical layouts. For example, the output of an N-speed transmission may drive an M-speed secondary transmission, giving a total of N*M gear combinations; for example a 4-speed main box and 3-speed splitter gives 12 ratios. Transmissions may be in separate cases with a shaft in between; in separate cases bolted together; or all in one case, using the same lubricating oil. The second transmission is often called a "Brownie" or "Brownie box" after a popular brand. With a third transmission, gears are multiplied yet again, giving greater range or closer spacing. Some trucks thus have dozens of gear positions, although most are duplicates. Sometimes a secondary transmission is integrated with the differential in the rear axle, called a "two-speed rear end." Two-speed differentials are always splitters. In newer transmissions, there may be two countershafts, so each main shaft gear can be driven from one or the other countershaft; this allows construction with short and robust countershafts, while still allowing many gear combinations inside a single gear case.
Heavy-duty transmissions are almost always non-synchromesh. One argument is synchromesh adds weight that could be payload, is one more thing to fail, and drivers spend thousands of hours driving so can take the time to learn to drive efficiently with a non-synchromesh transmission. Heavy-duty trucks driven frequently in city traffic, such as cement mixers, need to be shifted very often and in stop-and-go traffic. Since few heavy-duty transmissions have synchromesh, automatic transmissions are commonly used instead, despite their increased weight, cost, and loss of efficiency.
Heavy trucks are usually powered with diesel engines. Diesel truck engines from the 1970s and earlier tend to have a narrow power band, so need many close-spaced gears. Starting with the 1968 Maxidyne, diesel truck engines have increasingly used turbochargers and electronic controls that widen the power band, allowing fewer and fewer gear ratios. A transmission with fewer ratios is lighter and may be more efficient due to fewer transmissions in series. Fewer shifts also makes the truck more drivable. As of 2005, fleet operators often use 9,10,13 or 18-speed transmissions, but automated manual and semi-automatic transmissions are becoming more common on heavy vehicles, as they can improve efficiency and drivability, reduce the barrier to entry for new drivers, and may improve safety by allowing the driver to concentrate on road conditions.
A crash gearbox, also known as a crash box, is a transmission type used in old cars, trucks, and other automotive vehicles. It is more properly called a "sliding mesh" gearbox and has the nickname "crash" because it is difficult to change gears, so gear changes are often accompanied by loud noises. The etymology of "crash" is probably "clash".
In a sliding-mesh gearbox, individual gears are mounted so they always engage the shaft, but gears on one shaft can be moved axially. To engage a particular pair of gears, one gear is slid axially until it fully engages a gear on the other shaft. If the gear shafts are spinning so the two gears have the same surface velocity, the gears are relatively easy to engage. However, if velocities are mismatched, the gears tend to "bounce" off each other at first contact and resist engagement. Thus, gear engagement relies on the driver carefully matching speeds, typically through practice and intuition.
In contrast, newer "constant mesh" transmissions use gears that are held axially, but gears on one shaft spin freely on the shaft. Gear pairs in the transmission are always in mesh, though at most one is engaged at any time. Each free-spinning gear has a dog clutch which is engaged by an axial sliding collar that transfers power to the shaft. The dog clutch may be plain, also called "non-synchromesh", or may use an additional synchromesh mechanism that helps get the parts moving at the same speed to assist engagement. Many constant mesh transmissions use a sliding-mesh gear for reverse, but since reverse is only engaged from near a stop, it is still easy to engage.
A constant-mesh transmission offers several advantages over a sliding-mesh design. First, the dog clutch is designed for the task, rather than asking the gear to do dual duty of power transmission and sliding engagement. Second, the dog clutch is typically smaller in diameter than the gear it controls, so absolute speeds of the engaging parts are lower, aiding engagement. Thus, while a non-synchromesh transmission still relies on the operator to match speeds, gears are easier to engage. Third, a constant-mesh transmission can easily use helical gears which are smoother, quieter, and can carry more torque for a given size of gear. Fourth, a constant-mesh transmission can use synchromesh for easier shifting; while many heavy vehicle transmissions do not use it, most medium- and light-duty automotive transmissions do.
Because clutches use changes in friction to modulate the transfer of torque between engine and transmission, they are subject to wear in everyday use. A very good clutch, when used by an expert driver, can last hundreds of thousands of kilometres (or miles). Weak clutches, abrupt downshifting, inexperienced drivers, and aggressive driving can lead to more frequent repair or replacement.
Manual transmissions are lubricated with gear oil or engine oil in some cars, which must be changed periodically in some cars, although not as frequently as the automatic transmission fluid in a vehicle so equipped. (Some manufacturers specify that changing the gear oil is never necessary except after transmission work or to rectify a leak.)
Gear oil has a characteristic aroma due to the addition of sulfur-bearing anti-wear compounds. These compounds are used to reduce the high sliding friction by the helical gear cut of the teeth (this cut eliminates the characteristic whine of straight cut spur gears). On motorcycles with "wet" clutches (clutch is bathed in engine oil), there is usually nothing separating the lower part of the engine from the transmission, so the same oil lubricates both the engine and transmission. The original Mini placed the gearbox in the oil sump below the engine, thus using the same oil for both. The clutch was however a fairly conventional dry plate clutch.
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