| Transmission types |
|---|
|
|
|
|
|
|
|
|
|
|
A manual transmission, also known as a manual gearbox or standard transmission (informally, a manual; standard, straight shift; stick (shift), (US); or straight drive) is a type of transmission used in motor vehicle applications. It generally uses a driver-operated clutch, typically operated by a foot pedal (automobile) or hand lever (motorcycle), for regulating torque transfer from the internal combustion engine to the transmission, and a gear stick, either operated by foot (as on a motorcycle) or by hand (as in an automobile).
A conventional manual transmission is frequently the base equipment in a car; other options include automated transmissions such as an automatic transmission (often a manumatic), a semi-automatic transmission, or a continuously variable transmission (CVT).
Contents |
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. Sequential transmissions are commonly used in auto racing for their ability to make quick shifts.
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 eight gears. Transmission for heavy trucks and other heavy equipment usually have at least 9 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. Some heavy vehicle transmissions have dozens of gears, but many are duplicates, introduced as an accident of combining gear sets, or introduced to simplify shifting. 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 to have been invented by Louis-René Panhard and Emile 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 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. 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 synchronised, 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 (or dogs) on the synchros 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 is not conducive to having the long shift time of a synchronized gearbox. Because of this, it is necessary to synchronize gear speeds by blipping the throttle when shifting into a lower gear on a motorcycle.
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 in to 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.
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. 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),[1] 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 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 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, 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. 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[2] 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 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 change between layshaft and transmission and decrease synchro wear.
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.
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.
It is clear that 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 strange 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-1970s, cars were generally equipped with 3-speed transmissions as standard equipment. 4-speed units began to appear on volume-production models in the 1950s 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.
Today, mass market automotive manual transmissions are nearly all 6-speeds with 5-speed transmissions available in smaller vehicles and entry-level models. 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[3][4] with the first six gear ratios the same as the 6-speed gearbox and the 7th gear being of a higher ratio.
Initially the Tesla Roadster was intended to have a purpose-built two-speed manual transmission[5] but this gearbox proved to be problematic and was later replaced with a fixed-ratio transmission.
In the 1950s, '60s, and '70s, 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.[6]
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.
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 with automatic transmissions 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.
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.
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 rearward.
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. While the layout for gears one through four is nearly universal, the location of reverse is not. 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.
Four-speed transmissions with floor-mounted shifters were sometimes referred to as "four on the floor" during the period when the steering column was the more common shifter location. The latter, often being the standard non-performance transmission, usually had only three forward speeds and was referred to as "three on the tree."
Most 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. Front-wheel drive and rear-engined 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 '50s. 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.[7](Scroll down to "Overdrive In North America".)
Later, European and Japanese models began to have 4-speed column shifters with this shift pattern:
A majority of North American-spec vehicles for USA 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, a 5-speed column shifter has been offered in some vans sold in Asia and Europe, such as Toyota Hiace and Mitsubishi L400.
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.
Newer small cars and MPVs, like the Suzuki MR Wagon, the Fiat Multipla, the Toyota Matrix, the Pontiac Vibe, the Chrysler RT platform cars and the Honda Civic Si EP3 may feature a manual or automatic transmission gear shifter located on the vehicle's instrument panel. 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-lowest or next-highest 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:
6 - 5 - 4 - 3 - 2 N 1
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 riders 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 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 motorcycle 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. 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: BMWs SMG system, for example, can shift from 6th gear directly to 4th gear.
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.
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,[8] 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.[9] 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.[10] 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. 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.[9]
Because manual transmissions are mechanically simpler and have fewer moving parts than automatic transmissions, they require less maintenance and are easier to repair. This is one of the many reasons manual transmissions are still offered today, and why the demand for them still has a place in the market. The price of a new car with a manual transmission will often be lower than the same car with an automatic transmission.
Manual transmissions generally offer 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 makes 7- and 8-speed automatic transmissions. The increased number gears allows for better use of the engine's power band, allowing increased fuel economy, by staying in the most fuel-efficient part of the power band, or higher performance, by staying closer to the engine's peak power.
Because the driver has more direct control over the car with a manual than with an automatic, an experienced, knowledgeable driver who knows the correct procedure for executing a driving maneuver, and wants the vehicle to realize his or her intentions exactly and instantly, can perform actions difficult or impossible with automatic transmissions. When starting forward, for example, the driver can control how much torque goes to the tires, which is useful on slippery surfaces such as ice, snow or mud. This can be done with clutch finesse, or by starting in second gear instead of first. However, some automatic transmissions, particularly those with tiptronic gear selectors, can be placed in second gear for starting on slippery surfaces. An engine coupled with a manual transmission can often be started by the method of push starting. This is particularly useful if the starter is inoperable or defunct, or the battery has drained below operable voltage. Similarly, a vehicle with a manual transmission and no clutch/starter interlock switch can be moved, if necessary, by cranking the starter while in gear. This is useful when the vehicle will not start but must be immediately moved, e.g. off the road in the event of a breakdown, if the vehicle has stalled on a railway crossing, or in extreme off-roading cases such as an engine that has stalled in deep water.
In contrast to most manual gearboxes, most automatic transmissions have a free-wheel-clutch. This means that the engine does not slow down the car when the driver steps off the throttle, also known as engine or compression braking. This leads to more usage of the brakes in cars with automatic transmissions, which can potentially overheat them in hilly or mountainous areas, causing reduced braking ability and the potential for complete failure. However, the automatic gearboxes in commodity Nissans and Hondas disable the free wheel operation completely if the driver has selected a gear position other than "D" - either "1", "2", or "D with overdrive off". This works by blocking the free-wheel sprag using a multi-disk clutch called the "overrun clutch". Similarly, many trucks have a "tow mode" that allows engine braking.
There is a significant learning curve with a manual transmission, and the smoothness and correct timing of gear shifts are wholly dependent on the driver's experience and skill. Because the driver must develop a feel for properly engaging the clutch, especially when starting forward on a steep road or when parking on an incline, an inexperienced driver can easily stall the engine, or cause the car to jerk and bounce abruptly, which is not only uncomfortable, but could potentially be dangerous in an area with little room for error. Additionally, if an inexperienced driver selects the wrong gear by mistake, they can do damage to the engine and/or transmission. Selecting too high a gear can "lug" the motor, lowering the lifespan over time, while selecting too low a gear for the speed of the car can over-rev the engine, causing severe damage very quickly.
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 2nd and 3rd, then back over and into reverse. 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.
Some automatic transmissions can 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 manual transmissions. Even though some automatic transmissions and semi-automatic transmissions can shift faster, many purists still prefer a regular manual transmission.
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 the correct 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.
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 wear out clutches very quickly. 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.
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.
Many types of automobiles are equipped with manual transmissions.
Sports cars are also often equipped with manual transmissions because they offer more direct driver involvement and better performance. 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 and developing countries. 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.[11] 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.[12]
In some places (for example New Zealand (for the second-phase Restricted licence, but not the final Full licence), Belgium, China, Estonia, Dominican Republic, Finland, France, Germany, Ireland, Israel, Jordan, Netherlands, Norway, Poland, Singapore, Slovenia, South Africa, South Korea, Spain, Sri Lanka, Sweden, Turkey, U.A.E and the UK), when a driver takes the licensing road test using an automatic transmission, the resulting license is restricted to the use of automatic transmissions. 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 India, Italy, Pakistan, 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:
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.
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.