Manual transmission

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Transmission types
Manual Automatic Tiptronic Semi-automatic Twin-clutch gearbox Saxomat Continuously-variable Multitronic Derailleur gears Hub gears

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. 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 racing cars, only allow the driver to select the next-highest or next-lowest gear ratio. This second type of transmission is sometimes called a sequential (manual) transmission.

Manual transmissions are characterized by gear ratios that are selectable by engaging pairs of gears inside the transmission. Conversely, 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 semi-automatic transmissions.

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

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

Manual transmissions come in two basic types: simple 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.

Contents 1 Unsynchronized transmission 2 Synchronized transmission 3 Internals 3.1 Shafts 3.2 Dog clutch 3.3 Synchromesh 3.4 Reverse 3.5 Design Variations 3.5.1 Gear Variety 3.5.2 External Overdrive 3.5.3 Shaft and Gear Configuration 4 Clutch 5 Gear selection 5.1 Floor-mounted shifter 5.2 Column-mounted shifter 5.3 Sequential manual 5.4 Semi-manual 6 Comparison with automatic transmissions 6.1 Advantages 6.2 Disadvantages 7 Applications and popularity 8 Maintenance 9 See also 10 References 11 External links

[edit] Unsynchronized transmission

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. However, the gears were engaged by sliding mechanisms or simple clutches, which required careful timing and throttle manipulation when shifting, so that the gears would be spinning at roughly the same speed when engaged; otherwise, the teeth would refuse to mesh.

When upshifting, the speed of the gear driven by the engine had to drop to match the speed of the next gear; as this happened naturally when the clutch was depressed or disengaged, it was just a matter of skill and experience to hear and feel when the gears managed to mesh. However, when downshifting, the gear driven by the engine had to be sped up to mesh with the output gear, requiring letting the clutch up (engagement) for the engine to speed up the gears. Double 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 usually non-synchromesh transmissions, known colloquially as "crashboxes", 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.

Similarly, most modern motorcycles still utilise unsynchronised transmissions as synchronisers are generally not necessary or desirable. Their low gear inertias and higher strengths mean that 'forcing' the gears to alter speed is not damaging, and the selector method on modern motorcycles (pedal operated) is not conducive to having the long shift time of a 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.

[edit] Synchronized transmission

Modern gearboxes are constant mesh. I.e. all gears are always in mesh. Only one of these meshed pairs of gears is locked to the shaft on which it is mounted at any one time, while the others are allowed to rotate freely. Thus, it greatly reduces the skill required to shift gears.

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

The first synchronized transmission system was introduced by Cadillac in 1929^{[citation needed]}. 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.

[edit] Internals

[edit] Shafts

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

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

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

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

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

[edit] Dog clutch

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

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

[edit] Synchromesh

If the teeth, the so-called dog teeth, make contact with the gear, but the two parts are spinning at different speeds, the teeth will fail to engage and a loud grinding sound will be heard as they clatter together. For this reason, a modern dog clutch in an automobile has a synchronizer mechanism or synchromesh. Thanks to this mechanism, before the teeth can engage, a frictional contact is made between "synchro cones" 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 (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 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.

[edit] Reverse

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 relatively small amount of driving that takes place in reverse.

[edit] Design Variations

[edit] Gear Variety

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

[edit] External Overdrive

On earlier models with three or four forward speeds, the lack of an overdrive ratio for relaxed and fuel-efficient highway cruising was often filled by 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.

[edit] Shaft and Gear Configuration

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

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

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

[edit] Clutch

In all vehicles using a transmission (virtually all modern vehicles), a coupling device is used to separate the engine and transmission when necessary. The clutch accomplishes this in manual transmissions. Without it, the engine and tires would at all times be inextricably linked, and anytime the vehicle was at a stop, so would 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 undesirable 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.
• A rider of a highly-tuned motocross or off-road motorcycle may "hit" or "fan" the clutch when exiting corners to assist the engine in revving to point where it makes the best power.
• Note: Automatic transmissions also use a coupling device, however, a clutch is not present. In these kinds of vehicles, the torque converter is used to separate the engine and transmission.

[edit] Gear selection

[edit] Floor-mounted shifter

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 molded on the gear knob.

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

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 and race cars, is commonly referred to as a "dog-leg first" or "racing" pattern, because of the "up and over" 1-2 shift. Its use was common on race cars and sports cars in the days before six and seven speed transmissions. By putting 4th and 5th on the same vertical plane, shifting at high speed becomes easier and faster. Putting first gear on a dogleg in a race or sports car is only a minor inconvenience since, theoretically, those cars would rarely use their first gears.

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

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

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. This style of pattern (including depressing the gear lever) is common on BMWs, most Volkswagens (though some have reverse towards second gear,) older Volvo 240s and some Renault models (12, 9, 19, Mégane and Clio).

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. 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 (in left-hand drive cars) 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.

Historically, 4-speed floor shifters were sometimes referred to as "Four on the Floor", when steering column mounted shifters were more common.

Image:Manual Layout10.png

[edit] Column-mounted shifter

Some cars have a gear lever mounted on the steering column of the car. It was common in the past but is no longer common today. However, many automatic transmissions still use this placement.

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. Also, the pattern is not "intuitive", as the shifter has to be moved backward and upward into R to make the car go backward.

A 3-speed column shifter, nicknamed "Three on the Tree" (alternatively, "Three in the Tree"), began appearing in America in the late 1930s and became common during the 1940s and 50's. Its layout is as shown below:

First gear in a 3-speed is often called "low," while third is usually called "high." There is, of course, no overdrive. Later European and Japanese models began to have 4-speed column shifter and some of these made their way to the USA. Its layout is shown here:

However, the column manual shifter disappeared in America by the late 1970s. But in the rest of the world, the column mounted shifter continued to be made, and was in fact common in some places. For example, all Toyota Crown and Nissan Cedric taxis in Hong Kong had the 4-speed column shift until 1999 when automatic began to be offered. Since the late 1980s or early 1990s, 5-speed column shifter has been made in some vans sold in Asia and Europe, such as Toyota Hiace and Mitsubishi L400.

[edit] Sequential manual

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:

         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 finding the neutral gear. 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.

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.

[edit] Semi-manual

Some new transmissions (Fiat'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 6^th gear directly to 4^th gear when decelerating from high speeds.

[edit] 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.

[edit] Advantages

• Manual transmissions typically offer better fuel economy than automatics.^[1] 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.^[2]
  • 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.^{[citation needed]}
  • Geartrain efficiency. Automatics may require power to be transmitted through multiple planetary gearsets before attaining the desired gear ratio. In comparison, manual transmissions usually transmit power through one or two gearsets at most.^{[citation needed]}
• Manual transmissions are still more efficient than belt-driven continuously-variable transmissions.^[3][4]
• Manual transmissions are generally significantly lighter than torque-converter automatics.^[1]
• Manual transmissions are typically cheaper to build than automatic transmissions.^{[citation needed]}
• Manual transmissions generally require less maintenance than automatic transmissions.^{[citation needed]}
• Manual transmissions normally do not require active cooling, because not much power is dissipated as heat through the transmission.^[4]
  • 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.^[5] 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 maneuver, 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.^{[citation needed]} Some cars, such as the Saab NG900 Automatic transmission, have a special mode for low traction situations.
  • 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 maneuver. 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 or their U.S. and Imperial counterparts, 20 mph and 30 mph—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 downhill, and then engaging the clutch in third or second gear. This is commonly known as a "push start", "popping the clutch" (in the USA) or Bump starting, which in the UK describes the action of suddenly releasing the clutch pedal after putting it in gear.
• 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.^{[citation needed]}
• 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). It is also easier to put a car with a manual transmission into neutral, even when the transmission has suffered damage from an accident or malfunction. Many modern vehicles will not allow the starter to be run without the clutch fully depressed, negating this advantage.

[edit] Disadvantages

Many of the disadvantages of a manual transmission involve the driver interaction with the vehicle. While most of these can be overcome with practice and experience, they should be considered:

• Manual transmissions often require the driver to place their full and continuous attention on the road, which may be seen as a disadvantage. Some consider this an advantage, as it prevents the driver from other potential distractions like cell phone or radio use.
• Inexperienced drivers may place more of their attention on shifting the gears, potentially distracting them from the road surroundings.
• A driver may inadvertently shift into the wrong gear with a manual transmission, potentially causing damage to the engine and transmission, or the vehicle's body and its surroundings^{[verification needed]} if the intended gear was reverse. However this can be offset with a lockout on the reverse such as found on many European cars.
• Manual transmissions require a learning curve as one must 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; such changes are dependent on the driver.
• For a person with physical impairment, an automatic transmission might be the only available shifting option. The comparable systems for hand operated clutch and brakes for a manual transmission equipped car are usable only by people with just lower body handicap. Retrofit of such a system also requires extensive modifications to the car.
• Vehicles with manual transmissions are more difficult to start from rest when stopped upward on a hill because the clutch must be depressed and the gas applied very quickly once the brake is removed to prevent slipping backward. However, this can be overcome with experience and/ or the use of the handbrake.
• The clutch disc is a wear item and must be replaced periodically. While this is typically a labor intensive process that can be an expensive service, it shouldn't prove more expensive than periodic service to an automatic transmission in the long run^{[citation needed]}.

[edit] 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 or may be 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. However, Automated Manual Transmissions (AMT's) and semi-automatic transmissions are becoming more common on heavy vehicles, particularly in the European market. Mercedes-Benz is one of the manufacturers leading the introduction of AMT and semi-automatic gearboxes. This has been closely followed by other leading truck manufacturers, such as MAN, Scania, Volvo, and DAF. The use of fully automatic gearboxes is more common on buses, with Voith and Allison being the leading manufacturers of heavy automatic gearboxes, the use of this type of transmission is also common in specialist vehicles, such as fire appliances and municipal vehicles (road-sweepers, refuse collection vehicles, etc.).

Conversely, manual transmissions are no longer popular in many classes of cars sold in North America and Japan, 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 (except Japan) 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, Ireland, Switzerland, Germany, The Netherlands, Sweden, Japan, Belgium, Australia, Finland and Singapore, when a driver takes the licencing road test using an automatic transmission, the resulting licence 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 licence will become an obstacle for them where most cars have manual transmissions, so they make the effort to learn with manual transmissions and obtain full licences.

[edit] Maintenance

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 (100,000 kilometres is equal to 62,137 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, 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. 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). 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 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.

[edit] See also

• Borg-Warner T-56
• Toploader Transmission

[edit] References

1. ^ ^{a b} 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."
2. ^ For more information, see the BSFC page:

   A reciprocating engine achieves maximum efficiency at torque-peak speed and wide-open throttle.

3. ^ 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)

4. ^ ^{a b} 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.

5. ^ Car Repair and Maintenance on Yahoo

   Extended discusson about automatic transmission overheating issues.

[edit] External links

• How Manual Transmissions Work on HowStuffWorks
• Standardshift.com: A Forum for Standardshift Enthusiasts and New-comers to the Wonderful Word of Stick