A manual transmission (also known as a stick shift, straight drive, or standard transmission) is a type of transmission used in automotive applications. Manual transmissions often feature a driver-operated clutch and a movable gear selector, although some do not. Most automobile manual transmissions allow the driver to select any gear at any time, but some, such as those commonly mounted on motorcycles and some types of race cars, only allow the driver to select the next-highest or next-lowest gear ratio. This second type of transmission is sometimes called a sequential manual transmission.
Manual transmissions are characterized by gear ratios that are selectable by engaging pairs of gears inside the transmission. Conversely, automatic transmissions feature clutch packs to select gear ratio. Transmissions that employ clutch packs but allow the driver to manually select the current gear are called semi-automatic transmissions.
Contemporary automotive manual transmissions are generally available with four to six forward gears and one reverse gear, although manual transmissions have been built with as few as 2 and as many as 7 gears. 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. In contrast, a 5-speed automatic transmission is referred to as a 5-speed automatic.
Manual transmissions come in two basic types: simple unsynchronized systems, where gears are spinning freely and their relative speeds must be synchronized by the operator to avoid noisy and damaging "clashing" and "grinding" when trying to mesh the rotating teeth; and synchronized systems, which eliminate this necessity while changing gears.
- 1 Unsynchronized transmission
- 2 Synchronized transmission
- 3 Internals
- 4 Clutch
- 5 Gear selection
- 6 Comparison with automatic transmissions
- 7 Applications and popularity
- 8 Maintenance
- 9 See also
The earliest automotive transmissions were entirely mechanical unsynchronized gearing systems. They could be shifted, with multiple gear ratios available to the operator, and even had reverse. But the gears were engaged by sliding mechanisms or simple clutches, which required skills of timing and careful throttle manipulation when shifting, so that the gears would be spinning at roughly the same speed when engaged; otherwise the teeth would refuse to mesh.
When upshifting, the speed of the gear driven by the engine had to drop to match the speed of the next gear; as this happened naturally when the clutch was depressed, it was just a matter of skill and experience to hear and feel when the gears managed to mesh. However, when downshifting, the gear driven by the engine had to be sped up to mesh with the output gear, requiring engagement of the clutch for the engine to speed up the gears. Double declutching, that is, shifting once to neutral to speed up the gears and again to the lower gear, is sometimes needed. In fact, such transmissions are often easier to shift from without using the clutch at all. The clutch, in these cases, is only used for starting from a standstill. This procedure is common in racing vehicles and most production motorcycles.
Even though automotive transmissions are now almost universally synchronised, heavy trucks and machinery as well as dedicated racing transmissions are still usually nonsynchromesh transmissions, known colloquially as "crashboxes", for several reasons. Being made of brass, synchronizers are prone to wear and breakage more than the actual gears, which are cast iron, and the rotation of all the sets of gears at once results in higher frictional losses. In addition, the process of shifting a synchromesh transmission is slower than that of shifting a nonsynchromesh transmission. For racing of production based transmissions, sometimes half the dogs on the synchros are removed to speed the shifting process, at the expense of much more wear.
Similarly, most modern motorcycles still utilise unsynchronised transmissions. Synchronisers are generally not necessary or desirable in motorcycle transmissions. The low gear inertias and higher strengths mean that 'forcing' the gears to alter speed is not damaging, and the selector method on modern motorcycles (pedal operated) is not conducive to having the long shift time of a synchronised gearbox. Because of this, it is still necessary to synchronise gear speeds by 'blipping-the-throttle' when shifting into a lower gear on a motorcycle.
A modern gearbox is of the constant mesh type, in which all gears are always in mesh but only one of these meshed pairs of gears is locked to the shaft on which it is mounted at any one time, the others being allowed to rotate freely; thus greatly reducing the skill required to shift gears.
Most modern cars are fitted with a synchronised gear box, although it is entirely possible to construct a constant mesh gearbox without synchromesh, as found in motorcycle for example. In a constant mesh gearbox, the gears of the different transmission speeds are always in mesh and rotating, but the gears are not directly rotationally connected to the shafts on which they rotate. Instead, the gears can freely rotate or be locked to the shaft on which they are carried. The locking mechanism for any individual gear consists of a collar on the shaft which is able to slide sideways so that teeth or "dogs" on its inner surface bridge two circular rings with teeth on their outer circumference; one attached to the gear, one to the shaft. (One collar typically serves for two gears; sliding in one direction selects one transmission speed, in the other direction selects the other) When the rings are bridged by the collar, that particular gear is rotationally locked to the shaft and determines the output speed of the transmission. In a synchromesh gearbox, to correctly match the speed of the gear to that of the shaft as the gear is engaged, the collar initially applies a force to a cone-shaped brass clutch which is attached to the gear, which brings the speeds to match prior to the collar locking into place. The collar is prevented from bridging the locking rings when the speeds are mismatched by synchro rings (also called blocker rings or balk rings, the latter being spelled "baulk" in the UK). The gearshift lever manipulates the collars using a set of linkages, so arranged so that only one collar may be permitted to lock only one gear at any one time; when "shifting gears", the locking collar from one gear is disengaged and that of another engaged. In a modern gearbox, the action of all of these components is so smooth and fast it is hardly noticed.
The first synchronized transmission system was introduced by Cadillac in 1929. The modern cone system was developed by Porsche and introduced in the 1952 Porsche 356; cone synchronizers were called "Porsche-type" for many years after this. In the early 1950s only the second-third shift was synchromesh in most cars, requiring only a single synchro and a simple linkage; drivers' manuals in cars suggested that if the driver needed to shift from second to first, it was best to come to a complete stop then shift into first and start up again. With continuing sophistication of mechanical development, however, fully synchromesh transmissions with three speeds, then four speeds, five speeds, six speeds and so on became universal by the 1960s. Reverse gear, however, is usually not synchromesh, as there is only one reverse gear in the normal automotive transmission and changing gears in reverse is not required.
Like other transmissions, a manual transmission has several shafts with various gears and other components attached to them. Typically, there are three shafts: an input shaft, a countershaft and an output shaft. The countershaft is sometimes called a layshaft.
The input and output shaft lie along the same line, and may in fact be combined into a single shaft within the transmission. This single shaft is called a mainshaft. The input and output ends of this combined shaft rotate independently, at different speeds, which is possible because one piece slides into a hollow bore in the other piece, where it is supported by a bearing. Sometimes the term mainshaft refers to just the input shaft or just the output shaft, rather than the entire assembly.
In some transmissions, it's possible for the input and output components of the mainshaft to be locked together to create a 1:1 gear ratio, causing the power flow to bypass the countershaft. The mainshaft then behaves like a single, solid shaft, a situation referred to as direct drive.
Even in transmissions that do not feature direct drive, it's an advantage for the input and output to lie along the same line, because this reduces the amount of torsion that the transmission case has to bear.
Under one possible design, the transmission's input shaft has just one pinion gear, which drives the countershaft. Along the countershaft are mounted gears of various sizes, which rotate when the input shaft rotates. These gears correspond to the forward speeds and reverse. Each of the forward gears on the countershaft is permanently meshed with a corresponding gear on the output shaft. However, these driven gears are not rigidly attached to the output shaft: although the shaft runs through them, they spin independently of it, which is made possible by bearings in their hubs. Reverse is typically implemented differently, see the section on Reverse.
When the transmission is in neutral, and the clutch is disengaged, the input shaft, clutch disk and countershaft can continue to rotate under their own inertia. In this state, the engine, the input shaft and clutch, and the output shaft, all rotate independently.
The gear selector does not engage or disengage the actual gear teeth which are permanently meshed. Rather, the action of the gear selector is to lock one of the freely spinning gears to the shaft that runs through its hub. The shaft then spins together with that gear. The output shaft's speed relative to the countershaft is determined by the ratio of the two gears: the one permanently attached to the countershaft, and that gear's mate which is now locked to the output shaft.
Locking the output shaft with a gear is achieved by means of a dog clutch selector. The dog clutch is a sliding selector mechanism which is splined to the output shaft, meaning that its hub has teeth that fit into slots (splines) on the shaft, forcing it to rotate with that shaft. However, the splines allow the selector to move back and forth on the shaft, which happens when it is pushed by a selector fork that is linked to the gear lever. The fork does not rotate, so it is attached to a collar bearing on the selector. The selector is typically symmetric: it slides between two gears and has a synchromesh and teeth on each side in order to lock either gear to the shaft.
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. Thanks to this mechanism, before the teeth can engage, a frictional contact is made which brings the selector and gear to two parts to rotate at the same speed. Moreover, until synchronization occurs, the teeth are prevented from making contact, because further motion of the selector is prevented by a blocker 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 has to change the momentum of the entire input shaft and clutch disk. Additionally, it can be abused by exposure to the momentum and power of the engine itself, which is what happens when attempts are made to select a gear without fully disengaging the clutch. This causes extra wear on the rings and sleeves, reducing their service life. When an experimenting driver tries to "match the revs" on a synchronized transmission and force it into gear without using the clutch, it is actually the synchronizer that makes up for any discrepancy in RPM, deceiving the driver into an exaggerated sense of how much human skill was involved.
The previous discussion applies to the forward gears. The implementation of the reverse gear is usually different, implemented in the following way to reduce the cost of the transmission. Reverse is also a pair of gears: one gear on the countershaft and one on the output shaft. However, whereas all the forward gears are always meshed together, there is a gap between the reverse gears. Moreover, they are both attached to their shafts: neither one rotates freely about the shaft. What happens when reverse is selected is that a small gear, called an idler gear or reverse idler, is slid between them. The idler has teeth which mesh with both gears, and thus it couples these gears together and reverses the direction of rotation without changing the gear ratio.
Thus, in other words, when reverse gear is selected, in fact it is actual gear teeth that are being meshed, with no aid from a synchronization mechanism. For this reason, the output shaft must not be rotating when reverse is selected: the car must be stopped. In order that reverse can be selected without grinding even if the input shaft is spinning inertially, there may be a mechanism to stop the input shaft from spinning. The driver brings the vehicle to a stop, and selects reverse. As that selection is made, some mechanism in the transmission stops the input shaft. Both gears are stopped and the idler can be inserted between them. There is a clear description of such a mechanism in the Honda Civic 1996-1998 Service Manual, which refers to it as a "noise reduction system":
- Whenever the clutch pedal is depressed to shift into reverse, the mainshaft continues to rotate because of its inertia. The resulting speed difference between mainshaft and reverse idler gear produces gear noise [grinding]. The reverse gear noise reduction system employs a cam plate which was added to the reverse shift holder. When shifting into reverse, the 5th/reverse shift piece, connected to the shift lever, rotates the cam plate. This causes the 5th synchro set to stop the rotating mainshaft. (13-4)
A reverse gear implemented this way makes a loud whining sound, which is not heard in the forward gears. The teeth on the forward gears of consumer automobiles are helically cut. When helical gears rotate, their teeth slide together, which results in quiet operation. In spite of all forward gears being always meshed, they do not make a sound that can be easily heard above the engine noise. By contrast, reverse gears are spur gears, meaning that they have straight teeth, in order to allow for the sliding engagement of the idler, which would not be possible with helical gears. The teeth of spur gears clatter together when the gears spin, generating a characteristic whine.
It is clear that the spur gear design of reverse gear represents some compromises—less robust, unsynchronized engagement and loud noise—which are acceptable due to the small volume of driving that takes place in reverse.
Manual transmissions are often equipped with 4, 5, or 6 forward gears. Nearly all have exactly one reverse gear. In three or four speed transmissions, in most cases, the topmost gear is "direct", i.e. a 1:1 ratio. For five speed or higher transmissions, the highest gear is usually an overdrive gear, with a ratio of less than 1:1. Older cars were generally equipped with 3-speed transmissions, or 4-speed transmissions for high performance models and 5-speeds for the most sophisticated of automobiles; in the 1970s, 5-speed transmissions began to appear in low priced mass market automobiles and even compact pickup trucks, pioneered by Toyota (who advertised the fact by giving each model the suffix SR5 as it acquired the fifth speed). Today, mass market automotive manual transmissions are essentially all 5-speeds, with 6-speed transmissions beginning to emerge in high performance vehicles in the early 1990s, and recently beginning to be offered on some high-efficiency and conventional passenger cars.
On earlier models with three or four forward speeds, the lack of an overdrive ratio for relaxed and fuel-efficient highway cruising was often filled by incorporation of a separate overdrive unit in the rear housing of the transmission, separately actuated by a knob or button, often incorporated into the gearshift knob.
Shaft and Gear Configuration
The input shaft need not turn a pinion which rotates the countershaft. Another possibility is that gears are mounted on the input shaft itself, meshed with gears on the countershaft, in which case the countershaft then turns the output shaft. In other words, it's a matter of design on which shaft the driven and driving gears reside.
The distribution of the shifters is also a matter of design; it need not be the case that all of the free-rotating gears with selectors are on one shaft, and the permanently splined gears on the other. For instance a five speed transmission might have the first-to-second selectors on the countershaft, but the third-to-fourth selector and the fifth selector on the mainshaft, which is the configuration in the 1998 Honda Civic. This means that when the car is stopped and idling in neutral with the clutch engaged input shaft spinning, the third, fourth and fifth gear pairs do not rotate.
In all vehicles using a transmission (virtually all modern vehicles), a coupling device is used to be able to separate the engine and transmission when necessary. The clutch is what accomplishes this in manual transmissions. Without it, the engine and tires would at all times be inextricably linked, and anytime the vehicle is at a stop, so would be the engine. Moreover, 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 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. 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 state, it's possible to select gears or stop the car.
- When the clutch pedal is fully released, the clutch is fully engaged, and essentially all of the engine's torque is transferred. In this state, the clutch does not slip, but rather behaves like a rigid coupling. Power is transmitted to the wheels with minimal loss.
- In between these extremes, the clutch slips to varying degrees. When the clutch slips, it transmits torque, in spite of the difference in speeds between the engine crankshaft and the transmission input. Because the torque is transmitted by means of friction, a lot of power is wasted as heat, which must be dissipated by the clutch. Slip allows the vehicle to be started from a standstill, and when it is already moving, slip allows the engine rotation to gradually adjust to a newly selected gear ratio, resulting in a smooth, jolt-free gear change.
- Because of the heat that a slipping clutch generates, slip cannot be maintained for a long time. Moreover, because energy is wasted, it would be undesireable to do so. Skilled drivers rarely allow a clutch to slip for more than about one second. Making effective use of clutch slip requires the development of feeling through practice, similar to learning to play a musical instrument or to play a sport.
- Note: Automatic transmissions also use a coupling device, however, a clutch is not present. In these kinds of vehicles, the torque converter is used to separate the engine and transmission.
In most modern cars, gears are selected through a lever attached to the floor of the automobile—this selector is often called a gearstick, gear lever, gear selector, or simply shifter. Moving this lever forward, backward, left, and right allows the driver to select any given gear. In this configuration, the gear lever must be pushed laterally before it is pushed longitudinally.
A sample layout of a four-speed transmission is shown below. N marks neutral, or the position where no gears are engaged. In reality, the entire horizontal line is a neutral position, although the shifter is usually equipped with springs so that it will return to the N position if not left in another gear. The R denotes reverse, which is technically a fifth gear on this transmission.
This layout is called the shift pattern. Because of the shift quadrants, the basic arrangement is often called an H-pattern. While the layout for gears one through four is nearly universal, the location of reverse is not. Reverse can be found outside of the quadrant at the upper left (late 1960s GM models and AMC models), lower left (Toyota Land Cruiser FJ), or the lower right (Jeep CJ7, Datsun models, and Honda Civic), so caution is always warranted in gear selection. The shift pattern for a specific transmission is usually printed on the shifter knob.
The image below shows the most common five-speed layout found in the United States.
This layout is reasonably intuitive because it starts at the upper left and works top to bottom, left to right, with reverse far away and toward the rear of the car. There is usually a mechanism that only allows selection of reverse from the neutral position, so reverse will be less likely to be accidentally chosen when downshifting from 5th to 4th (or by someone used to a 6-speed transmission and trying to shift from 5th to the non-existent 6th).
This five-speed layout, found on a number of older models including Lamborghinis, is commonly referred to as a "dog-leg" pattern, because of the "up and over" 1-2 shift. Its use, especially on sports cars, has since been deprecated because the long, offset 1-2 shift can have a notable effect on a car's acceleration, especially from 0-60 mph.
Another five-speed shift pattern (common on many European cars) is this:
Transmissions equipped with this shift pattern usually feature a lockout mechanism that requires the driver to depress a switch or the entire gear lever when entering reverse, so that reverse is not accidentally selected when trying to find first gear.
A typical pattern for the more modern six-speed transmission is shown here
A six-speed manual transmission (seven speeds with reverse) is widely considered to be the largest number of gears that can be contained within a variation of the "H" shift pattern. It is for this reason that BMW, after succumbing to American market pressures for a conventional manual transmission in their M5 and M6 models, was forced to switch from a seven-speed sequential gearbox to a six-speed conventional manual. Note that: reverse is placed outside of the "H", with a canted shift leg. This is to prevent the shift lever from intruding too far into the driver's footwell when reverse is selected. This is the most common layout for a six-speed manual transmission.
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.
A 4-speed floor shifter is sometimes referred to as "Four on the Floor".
Some older cars feature a gear lever which is mounted on the steering column of the car. Many automatic transmissions still use this placement, but manual column shifters are no longer common.
Column shifters are mechanically similar to floor shifters, although shifting occurs in a vertical plane instead of a horizontal one. Column shifters also generally involve additional linkages to connect the shifter with the transmission.
The 3-speed shift pattern is typical of American cars, trucks, and vans produced with manual transmissions until the 1950s and 1960s. This pattern is not "intuitive", as the shifter has to be moved forward (into R) to make the car go backward (and vice-versa).
First gear in a 3-speed is often called "low," while third is usually called "high." There is, of course, no overdrive.
A 3-speed column shifter is sometimes referred to as "Three on a Tree".
Note that reverse in a car with a column shift is in nearly the same position as park (P) is on a car with a column-mounted gear selector with an automatic transmission.
Some automakers, including Mercedes-Benz and Toyota, have made 4- and even 5-speed column-mounted shifters (the Toyota Hiace van had a "five on the tree" shifter well into the 1990s).
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. These transmissions often provide clutch control, but the clutch is only necessary when selecting first or reverse gear from neutral. Most gear changes can be performed without the clutch.
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:
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 into 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. For inexperienced riders, this can lead to difficulty in finding 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. You will not accidentally shift into neutral. 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.
Some very new transmissions (BMW's Sequential Manual Gearbox (SMG) and Audi's Direct-Shift Gearbox (DSG), 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 when decelerating from high speeds.
Comparison with automatic transmissions
Manual transmissions are typically compared to automatic transmissions, as the two represent the majority of options available to the typical consumer. These comparisons are general guidelines and may not apply in certain circumstances. Additionally, the recent popularity of semi-manual and semi-automatic transmissions renders many of these points obsolete. It should be kept in mind that some of these points are true of "conventional" automatic transmissions which shift gears and are coupled to the engine with a torque converter but are not a true comparison or do not apply to other kinds of automatic transmissions, like the continuously-variable transmission.
- Manual transmissions typically offer better fuel economy than automatics.<ref name="fueleconomy">The United States Department of Energy website is dedicated to providing information about the fuel consumption of many makes and models of vehicles, with separate entries for the manual and automatic transmission variants of a model, if they exist. The site's Transmission Technologies page states that "Manual transmissions are lighter than conventional automatic transmissions and suffer fewer energy losses." </ref> 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 -- extra urban or urban (highway or city). There are several reasons for this:
- Mechanical efficiency. The manual transmission couples the engine to the transmission with a rigid clutch instead of a torque converter that introduces significant power losses. The automatic transmission also suffers parasitic losses by driving the high pressure hydraulic pumps required for its operation.
- Driver control. Certain fuel-saving modes of operation simply do not occur in an automatic transmission vehicle, but are accessible to the manual transmission driver. For example, the manual-transmission vehicle can be accelerated gently, yet with a fully open throttle (accelerator pedal to the floor), by means of shifting early to a higher gear, keeping the engine RPM in a low power band. By contrast, in an automatic transmission, the throttle position serves as the indicator of how fast the driver wishes to accelerate. If the accelerator pedal is floored, the transmission will shift to a lower gear, resulting in high engine RPM and aggressive acceleration. The thermodynamically efficient combination of open throttle and low RPMs is unavailable to the automatic transmission driver. Fuel-efficient acceleration is important to achieving fuel economy in stop-and-go city driving.<ref>For more information, see the BSFC page:
- A reciprocating engine achieves maximum efficiency at torque-peak speed and wide-open throttle.</ref>
- Fuel cut-off. The torque converter of the automatic transmission is designed for transmitting power from the engine to the wheels. Its ability to transmit power in the reverse direction is limited. During deceleration, if the torque converter's rotation drops beneath its stall speed, the momentum of the car can no longer turn the engine, requiring the engine to be idled. By contrast, a manual transmission, with the clutch engaged, can use the car's momentum to keep the engine turning, in principle, all the way down to zero RPM. This means that there are better opportunities, in a manual car, for the electronic control unit (ECU) to impose deceleration fuel cut-off (DFCO), a fuel-saving mode whereby the fuel injectors are turned off if the throttle is closed (foot off the accelerator pedal) and the engine is being driven by the momentum of the vehicle. Automatics further reduce opportunities for DFCO by shifting to a higher gear when the accelerator pedal is released, causing the RPM to drop.||}}
- Manual transmissions are still more efficient than belt-driven continuously-variable transmissions.<ref>An Investigation into The Loss Mechanisms associated with a Pushing Metal V-Belt Continuously Variable Transmission, Sam Akehurst, 2001, Ph. D Thesis, University of Bath.
- Despite these theoretical predictions to date reduced fuel consumptions and emissions have not been realised by production cars fitted with CVTs. Rather fuel economy figures compared to equivalent fixed ratio vehicles have been at best equal and in most cases considerably lower. (p. 1-2)</ref><ref name=
efficiency>An Overview of Current Automatic, Manual and Continuously Variable Transmission Efficiencies and Their Projected Future Improvements, Kluger and Log, SAE 1999-01-1259
- This publication assigns 94% efficiency to current 5 speed manual transmissions, 70-80% efficiency (city-highway) to a current four-speed automatics, and predicts 88% efficiency for future continuously-variable designs.</ref>
- It is generally easier to build a very strong manual transmission than a very strong automatic transmission. Manual transmissions usually have only one clutch, whereas automatics have many clutch packs.||}}
- Manual transmissions are generally significantly lighter than torque-converter automatics.<ref name="fueleconomy"/>
- Manual transmissions are typically cheaper to build than automatic transmissions.||}}
- Manual transmissions generally require less maintenance than automatic transmissions.||}}
- Manual transmissions normally do not require active cooling, because not much power is dissipated as heat through the transmission.<ref name="efficiency"/>
- The heat issue can be important in certain situations, like climbing long hills in hot weather, particularly if pulling a load. Unless the automatic's torque converter is locked up (which typically only happens in an overdrive gear that would not be engaged when going up a hill) the transmission can overheat.<ref>Car Repair and Maintenance on Yahoo
- Extended discusson about automatic transmission overheating issues.</ref> A manual transmission's clutch only generates heat when it slips, which does not happen unless the driver is riding the clutch pedal.
- A driver has more direct control over the state of the transmission with a manual than an automatic. This control is important to an experienced, knowledgeable driver who knows the correct procedure for executing a driving manoeuver, and wants the machine to realise his or her intentions exactly and instantly. Manual transmissions are particularly advantageous for performance driving or driving on steep and winding roads. Note that this advantage applies equally to manual-automatic transmissions, such as tiptronic.
- An example: the driver, anticipating a turn, can downshift to the appropriate gear while the steering is still straight, and stay in gear through the turn. This is the correct, safe way to execute a turn. An unanticipated change of gear during a sharp turn can cause skidding if the road is slippery.
- Another example: when starting, the driver can control how much torque goes to the tires, which is useful for starting on slippery surfaces such as ice, snow or mud. This can be done with clutch finesse, or possibly by starting in second gear instead of first. The driver of an automatic can only put the car into drive, and play with the throttle. The torque converter can easily dump too much torque into the wheels, because when it slips, it acts as an extra low gear, passing through the engine power, reducing the rotations while multiplying torque. An automatic equipped with ESC, however, does not have this disadvantage.||}}
- Yet another example: passing. When the driver is attempting to pass a slower moving vehicle by making use of a lane with opposite traffic, he or she can select a lower gear for more power at exactly the right moment when conditions are right to begin the manoeuver. Automatics have a delayed reaction time, because the driver can only indicate his intent by pressing the throttle. The skilled manual transmission driver has an advantage of superior finesse and confidence in such situations.||}}
- Driving a manual requires more involvement from the driver, thereby discouraging some dangerous practices. The manual selection of gears requires the driver to monitor the road and traffic situation, anticipate events and plan a few steps ahead. If the driver's mind wanders from the driving task, the machine will soon end up in an incorrect gear, which will be obvious from excessive or insufficient engine RPM. Related points:
- It's much more difficult for the driver to fidget in a manual transmission car, for instance by eating, drinking beverages, or talking on a cellular phone without a headset. During gear shifts, two hands are required. One stays on the wheel, and the other operates the gear lever. The hand on the wheel is absolutely required during turns, and tight turns are accompanied by gear changes. If the hand leaves the wheel, the steering will begin to straighten. In general, the more demanding the driving situation, the more difficult it is for the manual driver to do anything but operate the vehicle. The driver of an automatic transmission can engage in distracting activities in any situation, such as sharp turns through intersections or stop-and-go traffic.
- The driver of a manual transmission car can develop an accurate intuition for how fast the car is traveling, from the sound of the motor and the gear selection. It's easier to observe the lower speed limits like 30 km/h and 50 km/h without glancing at the instrumentation.
- Cars with manual transmissions can often be started when the battery is dead by pushing the car into motion (or allowing it to roll down a hill) and then engaging the clutch in third or second gear. This is called a push start or commonly, "popping the clutch."
- Manual transmissions work regardless of the orientation angle of the car with respect to gravity. Automatic transmissions have a fluid reservoir (pan) at the bottom; if the car is tilted too much, the fluid pump can be starved, causing a failure in the hydraulics. This could matter in some extreme off roading circumstances.||}}
- It is sometimes possible to move a vehicle with a manual transmission just by putting it in gear and cranking the starter. This is useful in an emergency situation where the vehicle will not start, but must be immediately moved (from an intersection or railroad crossing, for example).
- Manual transmissions require more driver interaction than automatic transmissions. Whether this is a disadvantage is debatable since many consider interaction with the car a good thing. It's much more difficult for the driver to fidget in a manual transmission car, for instance by eating, drinking beverages, or talking on a cellular phone without a headset. During gear shifts, two hands are required. One stays on the wheel, and the other operates the gear lever. The hand on the wheel is absolutely required during turns, and tight turns are accompanied by gear changes. If the hand leaves the wheel, the steering will begin to straighten. In general, the more demanding the driving situation, the more difficult it is for the manual driver to do anything but operate the vehicle. The driver of an automatic transmission can engage in distracting activities in any situation, such as sharp turns through intersections or stop-and-go traffic.
- A driver may inadvertently shift into the wrong gear with a manual transmission, potentially causing damage to the engine and transmission as well as compromising safety.
- Manual transmissions are more difficult to learn to drive as one needs to develop a feel for properly engaging the clutch.
- The smooth and quick shifts of an automatic transmission are not guaranteed when operating a manual transmission.
- Manual transmissions are slightly harder to start when stopped upward on a hill, but this is overcome with a little experience.
- The clutch disc is a wear item and must be replaced periodically. This is typically a labor intensive process and can be an expensive service.
Applications and popularity
Many types of automobiles are equipped with manual transmissions. Small economy cars predominantly feature manual transmissions because they are relatively cheap and efficient, although many are optionally equipped with automatics. Economy cars are also often powered by very small engines, and automatic transmissions can make them comparatively very slow, while a manual transmission makes much more efficient use of the power produced.
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.
Very heavy trucks also feature manual transmissions because they are efficient and, more importantly, can withstand the severe stress encountered in hauling heavy loads.
Conversely, manual transmissions are no longer popular in many classes of cars sold in North America, although they remain dominant in Europe. 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. In Europe and Asia most cars are sold with manual transmissions. Most luxury cars are only available with an automatic transmission. In situations where automatics and manual transmissions are sold side-by-side, the manual transmission is the base equipment, and the automatic is optional—although the automatic is sometimes available at no extra cost. 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.
In some countries, such as the United Kingdom, Germany and Japan, when a driver takes the licensing road test using an automatic transmission, the resulting license is restricted to the use automatic transmissions. Consequently, people who wish to obtain an unrestricted license take extra lessons to learn manual. This formal treatment of the manual transmission skill seems to maintain the widespread use of the manual transmission, rather than to diminish it. Some new drivers worry that their restricted driver's license will become an obstacle for them in a culture where many cars have manual transmissions, so they take the extra lessons to obtain a full license. However, in countries where manual transmission is dominant, almost everyone learns to drive using a manual transmission. By means of this exposure, many new drivers become manual transmission drivers.
City streets are typically arranged according to a pattern that resembles a spider web, rather than a grid: streets which are approximately circular (such as the Ringstraße around Vienna) concentrically encircle the city core, intersected by radial streets that emanate from the centre. One can drive to a desired location in the city by following one of these rings, and then turn into the correct cross street. The traffic lights along the rings can be timed such that the traffic rarely has stop. In fact, it's possible to encircle the city several times without ever stopping at a red light, or having to yield right-of-way to another traffic flow.
All of these features of these traffic systems promote traffic flow, and reduce wear on all those millions of clutches, as well as millions of drivers' nerves.
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. Weak clutches, downshifting, inexperienced drivers, and aggressive driving can lead to more frequent repair or replacement.
Manual transmissions are lubricated with gear oil, 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 molybdenum disulfide compounds, to lubricate the large degree of sliding friction seen by the teeth due to their helical cut, which in turn is done to eliminate the characteristic whine of straight cut gears. Some manufacturers, however, such as Honda, do not use this additive in their gear lube, specifying regular motor oil until recently, and now their own brand of gear lube which seems to be an enhanced version of motor oil. On motorcyles 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.