synchromesh n : an automotive system for shifting gears in which the gears revolve at the same speed and so shift smoothly

English

Noun

1. System of synchronized transmission found in modern gearboxes to make gear changing easier and smoother.

A manual transmission (also known as a stick shift or just 'stick', '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 forward gear at any time, but some, such as those commonly mounted on motorcycles and some types of racing cars, only allow the driver to select the next-highest or next-lowest gear ratio. This second type of transmission is sometimes called a sequential manual transmission.

Manual transmissions are characterized by gear ratios that are selectable by engaging pairs of gears inside the transmission. Conversely, most 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. Tractor units have at least 10 gears and as many as 24. Some manuals are referred to by the number of forward gears they offer (e.g., 5-speed) as a way of distinguishing between automatic or other available manual transmissions. Similarly, a 5-speed automatic transmission is referred to as a 5-speed automatic.

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

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

Unsynchronized transmission

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

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

Even though automotive transmissions are now almost universally synchronised, heavy 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.

Heavy duty trucks utilize unsynchronized transmissions in the interest of saving weight. Military edition trucks, which do not have to obey weight laws, usually have a synchronized transmission. Highway use heavy-duty trucks in the United States are limited to 80,000 pounds GVWR, and the lighter the curb weight for the truck, the more cargo can be carried, and with a synchronizer adding weight to a truck that could otherwise be used to carry cargo, most drivers are simply taught how to double clutch.

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

Synchronized transmission

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

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

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

Internals

Shafts

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

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

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

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

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

Dog clutch

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

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

Synchromesh

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

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

Reverse

The previous discussion 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. However, many modern transmissions now include a reverse gear synchronizer and helical gearing.

Design variations

Gear variety

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

External overdrive

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

Shaft and gear configuration

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

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

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

Clutch

In all vehicles using a transmission (virtually all modern vehicles), a coupling device is used to separate the engine and transmission when necessary. The clutch accomplishes this in manual transmissions. Without it, the engine and tires would at all times be inextricably linked, and any time the vehicle stopped the engine would perforce stall. Without the clutch, changing gears would be very difficult, even with the vehicle moving already: deselecting a gear while the transmission is under load requires considerable force, and 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 uncoupled state it is possible to select gears or to stop the car without stopping the engine.
• When the clutch pedal is fully released, the clutch is fully engaged, and practically all of the engine's torque is transferred. In this coupled state, the clutch does not slip, but rather acts as rigid coupling, and power is transmitted to the wheels with minimal practical waste heat.
• Between these extremes of engagement and disengagement the clutch slips to varying degrees. When the clutch slips it still transmits torque despite the difference in speeds between the engine crankshaft and the transmission input. Because this torque is transmitted by means of friction rather than direct mechanical contact, considerable power is wasted as heat (which is dissipated by the clutch). Properly applied, slip allows the vehicle to be started from a standstill, and when it is already moving, allows the engine rotation to gradually adjust to a newly selected gear ratio.
• Learning to use the clutch efficiently requires the development of muscle memory and a level of coordination analogous to that required to learn a musical instrument or to play a sport.
• A rider of a highly-tuned motocross or off-road motorcycle may "hit" or "fan" the clutch when exiting corners to assist the engine in revving to the point where it delivers the most power.
• Note: Automatic transmissions also use a coupling device; however, a clutch is not present. In these kinds of vehicles, the torque converter is used to separate the engine and transmission.

Gear selection

Floor-mounted shifter

In most modern passenger cars, gears are selected through a lever attached to the floor of the automobile—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, 1960s-1980s Ford Europe models, and current VW/Audi models), lower left (Fj Cruiser, Ferrari), the lower right (Jeep CJ7, Datsun models, and Honda Civic), or upper right (Corvette), so caution is always warranted in gear selection. The shift pattern for a specific transmission is usually molded on the gear knob.

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

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

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

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

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, Opels, most Volkswagens (though some have reverse towards second gear,) older Volvo 240s and some Renault models (12, 9, 19, 5, Mégane, Twingo 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.

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 '50s. 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 North 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.

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:

            6          5 ┘       4 ┘     3 ┘   2 ┘ N 1

The pedal goes one step - both up and down - from the center, before it reaches its limit and has to be allowed to move back to the center position. Thus, changing multiple gears in one direction is accomplished by repeatedly pumping the pedal, either up or down. Although neutral is listed as being between first and second gears for this type of transmission, it "feels" more like first and second gear are just "further away" from each other than any other two sequential gears. Because this can lead to difficulty in finding neutral for inexperienced riders most motorcycles have a neutral indicator light on the instrument panel to help 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.

The shift pattern for most underbone motorcycles with automatic centrifugal clutch is also modified for 2 key reasons - to enable the less-experienced riders to shift the gears without problems of "finding" the neutral gear, and also due to more force needed to "lift" the gearshift lever (because gearshift pedal of an underbone motorcycle also operates the clutch). The gearshift lever of an underbone motorcycle has two ends, therefore the rider clicks down the front end with the left toe all the way to the top gear and clicks down the rear end with the heel all the way down to neutral. Some underbone models such as Honda Wave have "rotary" shift pattern, which means that the rider can shift directly to neutral from the top gear, but this is only possible when the motorcycle is stationary for safety reasons. Some models also have gear position indicators for all gear positions at the instrument panel.

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 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.

Advantages

• Manual transmissions typically offer better fuel economy compared to automatics. 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.
  • 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.
  • Geartrain efficiency. Automatics may require power to be transmitted through multiple planetary gearsets before attaining the desired gear ratio.
• Manual transmissions are still more efficient than belt-driven continuously-variable transmissions.
• Manual transmissions are generally significantly lighter than torque-converter automatics. A manual transmission's clutch only generates heat when it slips, which does not happen unless the clutch is very worn or 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 realize 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, provided they have a quick reaction time to driver input.
  • 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. 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.
• 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.
• 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), "crash starting" (in New Zealand), "roll starting" (in Australia) or "bump starting" (in the UK). However, this practice is strongly discouraged by most manufacturers, citing possible damage to emissions control devices such as the catalytic converter.
• 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.
• 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, but some manufacturers have begun to add a clutch start override switch so that this advantage may still be enjoyed when necessary.

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 do not allow the driver to have both hands on the steering wheel at all times.
• Manual transmissions often require the driver to place their full and continuous attention on operating the vehicle, preventing them from multitasking. This can also be seen as an advantage, as listed above, as it can prevent the driver from potential distractions like mobile phone or radio use.
• Inexperienced drivers may place more of their attention on shifting the gears of the transmission, 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 or transmission. It may also result in loss of control due to a sudden change in the vehicle's speed.
• Manual transmissions require a learning curve as one must develop a feel for properly engaging the clutch.
• While it can easily be overcome with experience, manual transmission vehicles require good accelerator pedal application and clutch control when starting the car from a standstill. Excessive RPMs may cause the car to redline, exacerbating engine wear, whereas insufficient RPMs upon clutch release causes the engine to stall due to the lack of momentum required to sustain engine operation.
• The smooth and timely shifts of an automatic transmission are not guaranteed when operating a manual transmission; such changes are dependent on the driver's experience and timing.
• Manual transmissions burden the driver in heavy traffic situations since the driver is often operating the clutch pedal. In comparison, automatic transmissions merely require moving the foot from the accelerator pedal to the brake pedal, and vice versa.
• 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 a rest when positioned upward on a hill as it requires coordination of the accelerator, the clutch pedals, and the handbrake. This can be easily feasible, however, with experience.
• The clutch disc wears, and therefore must be replaced periodically. While this is typically a labor intensive process that can be an expensive service, it should not prove more expensive than periodic service to an automatic gearbox in the long run.

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.

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, when a driver takes the licensing road test using an automatic transmission, the resulting license is restricted to the use of automatic transmissions (or clutchless manual vehicles in the case of Australia). This treatment of the manual transmission skill seems to maintain the widespread use of the manual transmission, as many new drivers worry that their restricted license will become an obstacle for them where most cars have manual transmissions, so they make the effort to learn with manual transmissions and obtain full licenses. Some other countries (such as Pakistan) go even further, whereby the license is granted only when a test is passed on a manual transmission.

Truck transmissions

Very heavy trucks also feature manual transmissions because they are efficient and, more importantly, can withstand the severe stress encountered in hauling heavy loads. Some trucks have transmissions that look and behave as ordinary car transmissions - these transmissions are used on smaller trucks and typically have up to 6 gears. Then there are range, splitter and range-splitter transmissions for larger trucks which need more gears:

Range transmissions works like ordinary car transmission as well but have a high-low selector dividing the transmission in a higher and a lower range. The selector is used to switch from high and low mode using the fingers while the palm rests on the gear lever. This is usually done at 40km/h (25 mph). Lets say it's a 8-speed range transmission. It means that it has an H shift pattern with four gears when the selector is in low mode. Now to access the fifth gear, depress clutch, flip up the selector and move the gear to the gear lever to the "first gear" and release clutch. When the selector in this example is in high mode first gear becomes fifth, second gear becomes sixth and so on, as they share positions. Keep in mind that the reverse also become a high gear when the selector is in high mode. It is advised to never reverse in high gear. Always make sure the selector is in low mode while taking off, otherwise the engine will stall. Splitter transmissions works in a similar way and have a high-low selector. Although the selector does not devide the transmission in one upper and lower range. But instead split the gears in two. Which means first and second gear is in the same position. To shift up from first to second, let your palm rest on the gear lever, flip the selector with your thumb, depress the clutch, release it and the truck is now in the second gear. Just like range transmission the reverse also has two gears, one high and one low. Make sure the selector is in low mode before reversing. Range-Splitter transmissions is a combination of these two mentioned above. These transmissions allow even more gears and can be seen as trickier to drive. To separate the range and split. The range selector is placed in front of the gear lever. The splitter selector is placed on the side of the gear lever and is operated by thumb.

Most companies with fleets of large trucks use 10 speed non-synchronized manual transmissions, because their shift patterns are simpler to learn than the super-10 and 13 speed transmissions. The shift pattern for a standard 10-speed transmission on a truck, such as the Eaton-Fuller RoadRanger 10 speed, is the same as a 5 speed standard in a passenger car. There is a high-low selector switch on the gear shift itself, and after going through 5th gear, which is approximately though it may be geared differently, the switch is flipped up, and the shifter moved back to the first gear position, which is now 6th gear. This causes Reverse to have a HIGH reverse also, which will allow the vehicle to move at speeds up to in reverse, and is definitely not recommended.

To start a large truck moving from a standstill, with the engine started and the transmission in neutral, the clutch pedal must be pressed all the way to the floor to engage the clutch brake. The service brakes should also be applied. Once the clutch brake is engaged, the shifter is moved to the low/1st gear, and the clutch and brakes can be released. It is VERY important to note that a heavy-duty truck engine is capable of producing over of torque and can destroy a clutch fairly easily, so the throttle should not be touched until the clutch pedal is completely released. Giving the vehicle throttle while the clutch is not fully engaged will do nothing more than make the clutch slip, and won't actually help move the vehicle at all, while at the same time overheating the clutch.

However, Automated Manual Transmissions (AMTs) 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.).

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 (or miles). Weak clutches, abrupt downshifting, inexperienced drivers, and aggressive driving can lead to more frequent repair or replacement.

Manual transmissions are lubricated with gear oil or engine oil in some cars, which must be changed periodically in some cars, although not as frequently as the automatic transmission fluid in a vehicle so equipped. (Some manufacturers specify that changing the gear oil is never necessary except after transmission work or to rectify a leak.)

Gear oil has a characteristic aroma due to the addition of 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). 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.

See also

• Automatic transmission
• Non-synchronous transmission
• Transmission (mechanics)
• Toploader Transmission
• Freewheel
• Gear ratio
• Borg-Warner T-56

References

External links

• www.standardshift.com Manual transmission community site

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