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-higher or next-lower 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 locking selected gear pairs to the output shaft inside the transmission. Conversely, most automatic transmissions feature epicyclic (planetary) gearing controlled by brake bands and/or clutch packs to select gear ratio. Automatic transmissions that allow the driver to manually select the current gear are called 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.
The earliest form of the manual transmission is thought to have been invented by Louis-Rene Panhard and Emile Levassor in the late 19th century. This type of transmission offered multiple gear ratios and, in most cases, reverse. The gears were engaged by sliding them (or dog clutches) on their shafts—hence the term "shifting gears," which required a lot of careful timing and throttle manipulation when shifting, so that the gears would be spinning at roughly the same speed when engaged; otherwise, the teeth would refuse to mesh.
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-clutching, 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 not, such transmissions colloquially referred to as "crashboxes." Non-synchronized designs are used for several reasons. The friction material, such as brass, in synchronizers is more prone to wear and breakage than gears, which are forged steel, and the simplicity of the mechanism improves reliability and reduces cost. In addition, the process of shifting a synchromesh transmission is slower than that of shifting a non-synchromesh transmission. For racing of production-based transmissions, sometimes half the teeth (or "dogs") on the synchros are removed to speed the shifting process, at the expense of greater wear.
Heavy duty trucks 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, initially, and then most eventually gravitate to shifting without the 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.
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. This 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, seven 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.)
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.
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.
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.
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":
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.
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 need only be half as long as a traditional countershaft with four gears and two shifters.
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.
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 mid 1980s (last appearing in the 1986 Ford F-150). 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 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.
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.
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.
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:
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.).
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.