Manual transmission: Difference between revisions

Revision as of 08:49, 15 June 2020

Amanual transmission, also known as a manual gearbox, a standard transmission, stick shift, or simply stick, clutch, or gearbox, is a type of transmission used in motor vehicle applications. It uses a driver-operated clutch, usually engaged and disengaged by a foot pedal or hand lever, for regulating torque transfer from the engine to the transmission; and a gear selector that can be operated by hands.

A conventional 5- or 6-speed manual transmission is often the standard equipment in a modern base model vehicle, with 5- speed being common on lower end vehicles and commercial vehicles. Higher end vehicles, such as sports cars and luxury cars are often usually equipped with a 6-speed transmission for the base model. Other options include automatic transmissions such as a traditional automatic (hydraulic planetary) transmission (often a manumatic), an automated manual transmission, or a continuously variable transmission (CVT). The number of forward gear ratios is often expressed for automatic transmissions as well (e.g., 9-speed automatic).

Overview

Manual transmissions often feature a driver-operated clutch and a movable gear stick. Most automobile manual transmissions allow the driver to select any forward gear ratio ("gear") at any time, but some, such as those commonly mounted on motorcycles and some types of racing cars, only allow the driver to select the next-higher or next-lower gear. This type of transmission is sometimes called a sequential manual transmission.

In a manual transmission, the flywheel is attached to the engine's crankshaft and spins along with it. The clutch disc is in between the pressure plate and the flywheel, and is held against the flywheel under pressure from the pressure plate. When the engine is running and the clutch is engaged (i.e., clutch pedal up), the flywheel spins the clutch plate and hence the transmission. As the clutch pedal is depressed, the throw out bearing is activated, which causes the pressure plate to stop applying pressure to the clutch disk. This makes the clutch plate stop receiving power from the engine so that the gear can be shifted without damaging the transmission. When the clutch pedal is released, the throw out bearing is deactivated, and the clutch disk is again held against the flywheel, allowing it to start receiving power from the engine.

With manual transmissions, gear ratios can be selected by locking selected gear pairs to the output shaft inside the transmission. Conversely, most automatic transmissions feature epicyclic (planetary) gearing controlled by brake bands and/or clutch packs to select gear ratio. Automatic transmissions that allow the driver to manually select the current gear are called manumatics. A manual-style transmission operated by computer is often called an automated transmission rather than an automatic, even though no distinction between the two terms need be made.

Contemporary automobile manual transmissions typically use four to six forward gear ratios and one reverse gear, although consumer automobile manual transmissions have been built with as few as two and as many as seven gears. Transmissions for heavy trucks and other heavy equipment usually have 8 to 25 gears, so that there are both a wide range of gears and close gear ratios to keep the engine running in the power band. Operating such transmissions often uses the same pattern of shifter movement with a single or multiple switches to engage the next sequence of gears.

History

Unsynchronized transmission

French inventors Louis-Rene Panhard and Emile Levassor are credited with the development of the first modern manual transmission. They demonstrated their three-speed transmission in 1894 and the basic design is still the starting point for most contemporary manual transmissions. This type of transmission offered multiple gear ratios and, in most cases, reverse. The gears were typically engaged by sliding them on their shafts (hence the phrase shifting gears), which required careful timing and throttle manipulation when shifting, so the gears would be spinning at roughly the same speed when engaged; otherwise, the teeth would refuse to mesh. These transmissions are called sliding mesh transmissions or sometimes crash boxes, because of the difficulty in changing gears and the loud grinding sound that often accompanied. Newer manual transmissions on 4+-wheeled vehicles have all gears mesh at all times and are referred to as constant-mesh transmissions, with "synchro-mesh" being a further refinement of the constant mesh principle.

In both types, a particular gear combination can only be engaged when the two parts to engage (either gears or clutches) are at the same speed. To shift to a higher gear, the transmission is put in neutral and the engine allowed to slow down until the transmission parts for the next gear are at a proper speed to engage. The vehicle also slows while in neutral and that slows other transmission parts, so the time in neutral depends on the grade, wind, and other such factors. To shift to a lower gear, the transmission is put in neutral and the throttle is used to speed up the engine and thus the relevant transmission parts, to match speeds for engaging the next lower gear. For both upshifts and downshifts, the clutch is released (engaged) while in neutral. Some drivers use the clutch only for starting from a stop, and shifts are done without the clutch. Other drivers will depress (disengage) the clutch, shift to neutral, then engage the clutch momentarily to force transmission parts to match the engine speed, then depress the clutch again to shift to the next gear, a process called double clutching. Double clutching is easier to get smooth, as speeds that are close but not quite matched need to speed up or slow down only transmission parts, whereas with the clutch engaged to the engine, mismatched speeds are fighting the rotational inertia and power of the engine.

Some automotive manual transmissions had freewheeling capability in the 1930s through 1960s.^{[citation needed]}

Even though automobile and light truck transmissions are now almost universally synchronized, transmissions for heavy trucks and machinery, motorcycles, and for dedicated racing are usually not. Non-synchronized transmission designs are used for several reasons. The friction material, such as brass, in synchronizers is more prone to wear and breakage than gears, which are forged steel, and the simplicity of the mechanism improves reliability and reduces cost. In addition, the process of shifting a non-synchromesh transmission is faster than that of shifting a synchromesh transmission. For racing of production-based transmissions, sometimes half the teeth on the dog clutches are removed to speed the shifting process, at the expense of greater wear.

Heavy duty trucks often use unsynchronized transmissions, though military trucks usually have synchronized transmissions, allowing untrained personnel to operate them in emergencies. In the United States, traffic safety rules refer to non-synchronous transmissions in classes of larger commercial motor vehicles. In Europe, heavy duty trucks use synchronized gearboxes as standard.

Similarly, most modern motorcycles use unsynchronized transmissions: their low gear inertias and higher strengths mean that forcing the gears to alter speed is not damaging, and the pedal operated selector on modern motorcycles, with no neutral position between gears (except, commonly, 1st and 2nd), is not conducive to having the long shift time of a synchronized gearbox.

Synchronized transmission

Most modern manual-transmission vehicles are fitted with synchromesh on the forward gears. The modern cone system was developed by Porsche and introduced in the 1952 Porsche 356; cone synchronisers were called Porsche-type for many years after this. In the early 1950s, only the second-third shift was synchromesh in most vehicles, requiring only a single synchro and a simple linkage; drivers' manuals in vehicles suggested that if the driver needed to shift from second to first, it was best to come to a complete stop then shift into first and start up again. With continuing sophistication of mechanical development, fully synchromesh transmissions with three speeds, then four, and then five, became universal by the 1980s. Many modern manual-transmission vehicles, especially sports cars, now offer six speeds. The 2012 Porsche 911 offers a seven-speed manual transmission, with the seventh gear intended for cruising—the top speed being attained on sixth.

Until the mid-1950s (earlier in Europe and later in the US, on average) vehicles were generally equipped with 3-speed transmissions as standard equipment. 4-speed units began to appear on volume-production models in the 1930s (Europe) and 1950s (USA) and gained popularity in the 1960s; some exotics had 5-speeds. In the 1970s, as fuel prices rose and fuel economy became an important selling feature, 4-speed transmissions with an overdrive 4th gear or 5-speeds were offered in mass market automobiles and even compact pickup trucks, pioneered by Toyota (who advertised the fact by giving each model the suffix SR5 as it acquired the fifth speed). 6-speed transmissions started to emerge in high-performance vehicles in the early 1990s. 7-speed transmissions appeared on extreme high-end supercars, such as the 2005 Bugatti Veyron (semi-automatic manual transmission). In 2012, the Porsche 911 featured a 7-speed manual transmission, becoming the first of its class to support this feature, paving the way for the 2014 Chevrolet Corvette Stingray.

Today, mass-market automotive manual transmissions are nearly all at least 5-speed. 4-speed manual transmissions had fallen into almost total disuse by the end of the 1980s, having gradually become less common on vehicles during the 1980s. By the early 1990s in the USA, they are usually only found on vehicles with engines of around 1–2 litres.^{[citation needed]}

It has been widely anticipated that for electric vehicles (EVs), clutches and multi-speed gearboxes would not be required, as electric motors can drive the vehicle both forward and reverse from zero speed and typically operate over a wider speed range than combustion engines. Elimination of the gearbox represents a significant reduction in powertrain weight and complexity, and also removes a notable source of parasitic losses. The majority of first-generation consumer EVs have therefore been single-speed. However, current trends indicate that multi-speed gearboxes are likely to return for many future EVs. This allows the use of smaller, lower torque motors running at higher speeds to achieve both greater torque at the wheels for low speed tractive effort, and higher top road speed. Modest efficiency gains are also possible by reducing the proportion of the time that the motor(s) operate at very low speeds where efficiency is reduced. The wider speed range of motors means that the number of ratios required is lower than for combustion engine vehicles, with two to four speed designs emerging as the optimum depending on application.

Internals

Shafts

A manual transmission has several shafts with various gears and other components attached to them. Most modern passenger cars use 'constant-mesh' transmissions consisting of three shafts: an input shaft, a countershaft (also called a layshaft) and an output shaft.^[2]

The input shaft is connected to the engine and spins at engine speed whenever the clutch is engaged.^[3] The countershaft has gears of various sizes, which are permanently meshed with the corresponding gear on the input shaft.^[4] The gears on the output shaft are also permanently meshed with a corresponding gear on the countershaft, however the output shaft gears are able to rotate independently of the output shaft itself (through the use of bearings located between the gears and the shaft).^[5] Through the use of collars (operated using the shift rods), the speed of the output shaft becomes temporarily locked to the speed of the selected gear.^[6] Some transmission designs— such as in the Volvo 850 and S70— have two countershafts, both driving an output pinion meshing with the front-wheel-drive transaxle's ring gear. This allows for a narrower transmission, since the length of each countershaft is halved compared with one that contains four gears and two shifters.

The fixed and free gears can be mounted on either the input or output shaft, or both. For example, 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. This means that when the vehicle is stopped and idling in neutral with the clutch engaged and the input shaft spinning, the third-, fourth-, and fifth-gear pairs do not rotate.

When neutral is selected, none of the gears on the output shaft are locked to the shaft, allowing the input and output shafts to rotate independently. For reverse gear, an idler gear is used to reverse the direction in which the output shaft rotates. In many transmissions, the input and output shafts can be directly locked together (bypassing the counter shaft) to create a 1:1 gear ratio which is referred to as direct drive.

In a transmission for longitudinal engined vehicles (e.g. most rear-wheel drive cars), it is common for the input shaft and output shaft to be located on the same axis, since this reduces the torsional forces to which the transmission casing must withstand. The assembly consisting of both the input and output shafts is referred to as the mainshaft (although sometimes this term refers to just the input shaft or output shaft). Independent rotation of the input and output shafts is made possibly by one shaft being located inside the hollow bore of the other shaft, with a bearing located between the two shafts.

In a transmission for transverse engined vehicles (e.g. front-wheel drive cars), there are usually 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. These transmissions also have an integral differential unit, which is connected via a pinion gear at the end of the counter/output shaft.

Dog clutch

In a modern 'constant-mesh' manual transmission, the gear teeth are permanently in contact with each other and dog clutches (sometimes called dog teeth) are used to select the gear ratio for the transmission. When the dog clutches for all gears are disengaged (i.e. when the transmission is in neutral), all of the gears are able to spin freely around the output shaft. When the driver selects a gear, the dog clutch for that gear is engaged (via the gear selector rods), locking the transmission's output shaft to a particular gear set. This means the output shaft rotates at the same speed as the selected gear, thus determining the gear ratio of the transmission.^[7]

The dog clutch is a sliding selector mechanism which sits around the output shaft. It has teeth to fit into the splines on the shaft, forcing that shaft to rotate at the same speed as the gear hub. However, clutch can move back and forth on the shaft, to either engage or disengage the splines. This movement is controlled 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. Unlike some other types of clutches (such as the foot-operated clutch of a manual-transmission car) , a dog clutch provides non-slip coupling and is not suited to intentional slipping.

Synchromesh

In order to provide smooth gearshifts without requiring the driver to manually match the engine revs for each gearshift, most modern passenger car transmissions use 'synchromesh' (also called 'synchronizer rings') on the forward gears. These devices automatically match the speed of the input shaft with that of the gear being selected, thus removing the need for the driver to use techniques such as double clutching. Synchromesh transmission was invented in 1919 by Earl Avery Thompson and first used on production cars by Cadillac in 1928.^[8]

The need for synchromesh in a constant-mesh transmission is because the dog clutches require the input shaft speed to match that of the gear being selected, otherwise the dog teeth will fail to engage and a loud grinding sound will be heard as they clatter together. Therefore, to speed up or slow down the input shaft as required, cone-shaped brass synchronizer rings are attached to each gear. When the driver moves the gearshift lever towards the next gear, these synchronizer rings press on the cone-shaped sleeve on the dog collar so that the friction forces can reduce the difference in rotational speeds.^[9] Once these speeds are equalized, the dog clutch can engage and thus the new gear is now in use. In a modern gearbox, the action of all of these components is so smooth and fast it is hardly noticed. Many transmissions do not include synchromesh on the reverse gear (see Reverse gear section below).

The synchromesh system must also prevent the collar from bridging the locking rings while the speeds are still being synchronized. This is achieved through 'blocker rings' (also called 'baulk rings'). The synchro ring rotates slightly because of the frictional torque from the cone clutch. In this position, the dog clutch is prevented from engaging. Once the speeds are synchronized, friction on the blocker ring is relieved and the blocker ring twists slightly, bringing into alignment certain grooves or notches that allows the dog clutch to fall into engagement.

Common metals for synchronizer rings are brass and steel, and are produced either by forging or sheet metal shaping. The latter involves the stamping the piece out of a sheet metal strip and then machining to obtain the exact shape required. The rings are sometimes coated with anti-wear linings (also called 'friction linings') made from molybdenum, iron, bronzeorcarbon (with the latter usually reserved for high-performance transmissions due to their high cost).^[10]

Mechanical wear of the synchronizer rings and sleeves can cause the synchromesh system to become ineffective over time. These rings and sleeves have to overcome the momentum of the entire input shaft and clutch disk during each gearshift (and also the momentum and power of the engine, if the driver attempts a gearshift without fully disengaging the clutch). Larger differences in speed between the input shaft and the gear require higher friction forces from the synchromesh components, potentially increasing their wear rate.

Reverse gear

Even in modern transmissions where all of the forward gears are in a constant-mesh configuration, often the reverse gear uses the older sliding mesh ('crash box') configuration. This means that moving the gearshift lever into reverse results in gears moving to mesh together. Another unique aspect of reverse gear is that it consists of two gears— an idler gear on the countershaft and another gear on the output shaft— and both of these are directly fixed to the shaft (i.e. they are always rotating at the same speed as the shaft). These gears are usually spur gears with straight-cut teeth which— unlike the helical teeth used for forward gear— results in a whining sound as the vehicle moves in reverse.

When reverse gear is selected, the idler gear is physically moved to mesh with the corresponding gears on the input and output shafts. To avoid grinding as the gears begin to mech, they need to be stationary. Since the input shaft is often still spinning due to momentum (even after the car has stopped), a mechanism is needed to stop the input shaft, such as using the sychronizer rings for 5th gear. However, some vehicles do employ a synchromesh system for the reverse gear, thus preventing possible crunching if reverse gear is selected while the input shaft is still spinning.^[11]

Most transmissions include a lockout mechanism to prevent reverse gear from being accidentally selected while the car is moving forwards. This can take the form of a collar underneath the gear knob which needs to be lifted, or requiring extra force to push the gearshift lever into the plane of reverse gear.

Clutch

Vehicles with a manual transmission use a clutch to decouple the transmission from the engine during gearshifts and when the vehicle is stationary. Without a clutch, the engine would stall any time the vehicle stopped and changing gears would be difficult (deselecting a gear while the transmission requires the driver to adjust the throttle so that the transmission is not under load, and selecting a gear requires the engine RPM to be at the exact speed that matches the road speed for the gear being selected).

Most motor vehicles use a pedal to operate the clutch; except for motorcycles, which usually have a clutch lever on the left handlebar.

Gear stick

In most vehicles with manual transmission, the driver selects gears by manipulating a lever called a gear stick (also called a gearshift, gear leverorshifter). In most automobiles, the gear stick is often located on the floor between the driver and front passenger, however so cars have a gear stick that is mounted to the steering column or centre console.

The movement of the gear stick is transferred (via solid linkages or cables) to the selector forks within the transmission.

Motorcycles typically employ sequential transmissions, although the shift pattern is modified slightly for safety reasons. Gear selection is usually via the left foot pedal with a layout of 1 - N - 2 - 3 - 4 - 5 - 6.

External overdrive

In the 1950s, 1960s, and 1970s, fuel-efficient highway cruising with low engine speed was in some cases enabled on vehicles equipped with 3- or 4-speed transmissions by means of a separate overdrive unit in or behind the rear housing of the transmission. This was actuated either manually while in high gear by throwing a switch or pressing a button on the gearstick knob or on the steering column, or automatically by momentarily lifting the foot from the accelerator with the vehicle travelling above a certain road speed. Automatic overdrives were disengaged by flooring the accelerator, and a lockout control was provided to enable the driver to disable overdrive and operate the transmission as a normal (non-overdrive) transmission.^[12]

The term 'overdrive' is also used to describe a gear with a ratio of less than one (eg if the top gear of a transmission has a ratio of 0.8:1).

Benefits

Fuel economy

The manual transmission couples the engine to the transmission with a rigid clutch instead of the torque converter on an automatic transmission or the v-belt of a continuously variable transmission,^[13] which slip by nature. Manual transmissions also lack the parasitic power consumption of the automatic transmission's hydraulic pump. Also, manual transmissions do not require active cooling and because they are, mechanically, much simpler than automatic transmissions, they generally weigh less than comparable automatics, which can improve economy in stop-and-go traffic.^[14]. Because of this, manual transmissions generally offer better fuel economy than automatic or continuously variable transmissions; however the disparity has been somewhat offset with the introduction of locking torque converters on automatic transmissions.^[14] 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.^[15] The lack of control over downshifting under load in an automatic transmission, coupled with a typical vehicle engine's greater efficiency under higher load, can enable additional fuel gains from a manual transmission by allowing the operator to keep the engine performing under a more efficient load/RPM combination. This is especially true as between manual and automatic versions of older models, as more recent advances including variable valve timing reduce the efficiency disadvantages of automatic transmissions by allowing better performance over a broader RPM range. In recognition of this, many current models (2010 and on) come with manual modes, or overrides on automatic models, although the degree of control varies greatly by the manufacturer. However this gap in economy is being rapidly closed, and many mid- to higher-end model automatic vehicles now get better economy than their standard-spec counterparts.^{[citation needed]} This is due in part to the increasing impact of computers co-ordinating multiple systems, particularly in hybrid models in which the engine and drive motors must be managed, as well as using more gears and more efficient gear ratios or different automatic technology such as CVTs and dual-clutch automatics.

Longevity

Because manual transmissions are mechanically simpler, are more easily manufactured, and have fewer moving parts than automatic transmissions, they require less maintenance and are easier as well as cheaper to repair. Because of their mechanical simplicity, they often last longer than automatic transmissions when used by a skilled driver. Typically, there are no electrical components, pumps and cooling mechanisms in a manual transmission, other than an internal switch to activate reversing lighting. These attributes become extremely vital with a vehicle stuck in mud, snow, etc. The back and forth rocking motion of the vehicle drivers use to dislodge a stuck vehicle can destroy automatic transmissions. Clutches are a wear item that may need to be replaced at some point in the vehicle's lifespan, however the service life of the clutch depends on the operating conditions that it is subjected to.

Cost

The price of a new vehicle with a manual transmission tends to be lower than the same vehicle with an automatic transmission.

Most new vehicles are available with manual or automatic transmissions. There is often a difference in cost between the two. Manual transmissions generally cost less than automatic transmissions. For example, the base price of a Chevrolet Cruze 2LT with a manual transmission is $22,120, while the base price of the automatic is $23,405—a difference of $1,285.

Lubrication

Most manual transmissions rely on splash lubrication although some five speed Rover gearboxes did incorporate an oil pump. The problem with splash lubrication is that it is speed dependent. There are centrifugal effects, hydrodynamic effects and effects from the gears working as pumps. If a gearbox is fitted with Perspex windows and run on a test rig these effects can be observed. As the gearbox is run through its rev range, the oil jets will switch over and move around. Research on the Austin Maxi 1500 gearbox showed that one of the ball races was running dry at 80 miles per hour (130 km/h). The solution was to alter the casting to include a small projection that would intercept the main oil jet that was present at 80 mph and disperse it. This small modification enabled the later Maxi 1750 gearbox to be relatively trouble free. Four speed gearboxes seldom show these problems because at top speed (and maximum power) they are basically a solid shaft and the gears are not transmitting power.^{[citation needed]}

Performance and control

Manual transmissions have generally offered a wider selection of gear ratios. Many vehicles offer a 5-speed or 6-speed manual, whereas the automatic option would typically be a 4-speed. This is generally due to the increased space available inside a manual transmission compared with an automatic, since the latter requires extra components for self-shifting, such as torque converters and pumps. However, automatic transmissions are now adding more speeds as the technology matures. ZF currently manufactures 7- and 8-speed automatic transmissions. ZF is also planning a 9-speed automatic for use in front-wheel drive vehicles.^[16] The increased number of gears allows for better use of the engine's power band, resulting in increased fuel economy by staying in the most fuel-efficient part of the power band, or higher performance, thereby remaining closer to the engine's peak power rating. Even with more forward speeds and the potential of designing more forward gears to offer higher speed and/or torque, the manual transmission remains smaller and much more compact than its larger, automatic cousin, as referenced by the 991 Generation of the Porsche 911 and the C7 Chevrolet Corvette, which offer a 7-speed manual transmission.

Engine braking

In contrast to most manual gearboxes, most automatic transmissions have far less effective engine-braking. This means that the engine does not slow the vehicle as effectively when the automatic transmission driver releases the engine speed control. This leads to more usage of the brakes in vehicles with automatic transmissions, bringing shorter brake life. Brakes are also more likely to overheat in hilly or mountainous areas, causing reduced braking ability, brake fade, and the potential for complete failure with the automatic transmission vehicle.

Push starting

Vehicles with a manual transmission can often be push started when the starter motor is not operational, usually because of a low battery.

When push starting, the energy generated by the wheels moving on the road is transferred to the driveshaft, then the transmission and eventually the crankshaft. When the crankshaft spins as a result of the energy generated by the rolling of the vehicle, the motor is cranked over. This simulates what the starter is intended for and operates in a similar way to crank handles on very old cars from the early 20th century, with the cranking motion being replaced by the pushing of the car.

Drawbacks

Complexity and learning curve

For most people, there is a slight learning curve with a manual transmission, which may be intimidating and unappealing for a novice driver. Because the driver must develop a feel for properly engaging the clutch, an inexperienced driver will often release the clutch pedal abruptly, causing the engine to stall. Most drivers can learn how to drive a vehicle with a manual transmission in as little as an hour, although it may take weeks before it becomes "second nature". Additionally, if an inexperienced driver selects an inappropriate gear by mistake, damage to mechanical components and even loss of control can occur if not rectified quickly. Learning clutch/throttle pedal coordination can be made easier by using the clutch pedal only, on a level surface. This will allow the operator to gauge where the "sweet spot" of clutch engagement is. Correct "release speed" of the clutch pedal (slow for smooth, fast for abrupt) will indicate when and where throttle pedal use should occur.

In many jurisdictions, such as the United Kingdom, a driving licence issued for only vehicles with an automatic transmission is not valid for driving vehicles with a manual transmission, but a licence for manual transmissions covers both.^[17] This is also the case for P1 (provisional-1) car licence holders in New South Wales, Australia, but P2 (provisional-2) licence holders are allowed to drive vehicles with either transmission.^[18]

Shifting speed

Automatic transmissions can typically shift ratios faster than a manual gear change can be accomplished, because of the time required for the average driver to push the clutch pedal to the floor and move the gearstick from one position to another. This is especially true in regards to dual-clutch transmissions, which are specialized computer-controlled automatic transmissions that mechanically operate more like a manual transmission than a traditional automatic one.

Ease of use

Because manual transmissions require the operation of an extra pedal, and keeping the vehicle in the correct gear at all times, they require more concentration, especially in heavy traffic situations. The automatic transmissions, on the other hand, simply require the driver to speed up or slow down as needed, with the vehicle doing the work of choosing an appropriate gear. Manual transmissions also place a greater workload on the driver in heavy traffic situations, when the driver must operate the clutch pedal quite often. Because the clutch pedal can require a substantial amount of force, especially on large trucks, and the long pedal travel compared to the brake or accelerator requires moving the entire leg, not just the foot near the ankle, a manual transmission can cause fatigue, and is more difficult for injured people to drive. Additionally, because automatic transmissions can be driven with only one foot, people with one leg that is missing or impaired can still drive, unlike the manual transmission that requires the use of two feet at once. Likewise, manual transmissions require the driver to remove one hand periodically from the steering wheel while the vehicle is in motion, which can be difficult or impossible to do safely for people with a missing or impaired arm, and requires increased coordination, even for those with full use of both hands.

Stopping on hills

The clutch experiences most of its wear in first gear because moving the vehicle from a standstill involves a great deal of friction at the clutch. When accelerating from a standstill on an incline, this problem is made worse because the amount of work needed to overcome the acceleration of gravity causes the clutch to heat up considerably more. For this reason, stop-and-go driving and hills tend to have an effect on the clutches to a certain degree. Automatic transmissions are better suited for these applications because they have a hydraulic torque converter which is externally cooled, unlike a clutch. Torque converters also do not have a friction material that rubs off over time like a clutch. Some automatics even lock the output shaft so that the vehicle cannot roll backwards when beginning to accelerate up an incline. To reduce wear in these applications, some manual transmissions will have a very low, "granny" gear which provides the leverage to move the vehicle easily at very low speeds. This reduces wear at the clutch because the transmission requires less input torque. However, the issue of handling stops on hills is easy to learn.

Many drivers use the parking brake to prevent the vehicle from rolling backwards when starting to accelerate up a steep hill. This saves precious clutch life. A device called the hill-holder was introduced on the 1936 Studebaker. Some modern manual vehicles such as the Dodge Challenger and most Subaru models have a "hill-start assist"^[19] feature. The vehicle's computer applies just enough brake pressure to prevent the vehicle from rolling backwards. This allows the driver to start normally with no additional effort, even on steep hills.

Starting on a hill with the aid of the parking brake is not always possible since in recent vehicles that feature an electric park brake the parking brake can only be released when the brake pedal is engaged.

Applications and popularity

Sports cars are also often equipped with manual transmissions because they offer more direct driver involvement and better performance, though this is changing as many automakers move to faster dual-clutch transmissions, which are generally shifted with paddles located behind the steering wheel. For example, the 991 Porsche 911 GT3 uses Porsche's PDK. 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 vehicles sold in North America, Australia, and some parts of Asia, although they remain dominant in other parts of Asia, and in Europe, Africa, and Latin America. Nearly all vehicles are available with an automatic transmission option, and family vehicles and large pickup trucks sold in the US are predominantly fitted with automatics. However, in some cases, if a buyer wishes, they can have the vehicle fitted with a manual transmission at the factory. In Europe, most vehicles are sold with manual transmissions. Most luxury vehicles are only available with an automatic transmission. In most cases where both transmissions are available for a given model of vehicle, automatics are an at-cost option, but in some cases the reverse is true. Some vehicles, such as rentals and taxicabs, are nearly universally equipped with automatic transmissions in countries such as the United States, but the opposite is true in Europe.^[20] As of 2008, 75.2% of vehicles made in Western Europe were equipped with manual transmission, versus 16.1% with automatic and 8.7% with other.^[21]

When a driver passes their driving test using a vehicle with an automatic transmission, in some jurisdictions; the resulting driving licence is restricted to the use of vehicles with automatic transmissions only. This is the case in countries such as New Zealand (for the second-phase Restricted license, but not the final Full license), the European Union with the exception of member countries that opt to disallow road tests on automatic vehicles completely, China, Dominican Republic, Israel, Jordan, Norway, Philippines, Russia, Singapore, South Africa, South Korea, Sri Lanka, Switzerland, and the U.A.E. This treatment of the manual transmission skill seems to maintain the widespread use of the manual transmission.^{[citation needed]} As many new drivers worry that their restricted license will become an obstacle for them where most vehicles have manual transmissions, they make the effort to learn with manual transmissions and obtain full licenses. Some other countries (such as Greece, Georgia, India, Pakistan, Portugal, Malaysia, Serbia, Brazil, Ukraine and Denmark) go even further, whereby the license is granted only when a test is passed on a manual transmission. In Denmark and Brazil, drivers are allowed to sit the test in an automatic vehicle if they are disabled, but with such a license they will not be allowed to drive a vehicle with a manual transmission.

Truck transmissions

Some trucks have transmissions that look and behave like ordinary consumer vehicle transmissions—these transmissions are used on lighter trucks, typically have up to 6 gears, and usually have synchromesh.

For trucks needing more gears, the standard "H" pattern can get very complicated, so additional controls are used to select additional gears. The "H" pattern is retained, then an additional control selects among alternatives. In older trucks, the control is often a separate lever mounted on the floor or more recently a pneumatic switch mounted on the "H" lever; in newer trucks the control is often an electrical switch mounted on the "H" lever. Multi-control transmissions are built in much higher power ratings, but rarely use synchromesh.

There are several common alternatives for the shifting pattern. Usual types are:

Although there are many gear positions, shifting through gears usually follows a regular pattern. For example, a series of upshifts might use "move to splitter direct; move to splitter overdrive; move shift lever to No. 2 and move splitter to underdrive; move splitter to direct; move splitter to overdrive; move shift lever to No. 3 and move splitter to underdrive"; and so on. In older trucks using floor-mounted levers, a bigger problem is common gear shifts require the drivers to move their hands between shift levers in a single shift, and without synchromesh, shifts must be carefully timed or the transmission will not engage. For this reason, some splitter transmissions have an additional "under under" range, so when the splitter is already in "under" it can be quickly downshifted again, without the delay of a double shift.

Today's truck transmissions are most commonly "range-splitter". The most common 13-speed has a standard H pattern, and the pattern from left upper corner is as follows: R, down to L, over and up to 1, down to 2, up and over to 3, down to 4. The "butterfly" range lever in the center front of the knob is flipped up to high range while in 4th, then shifted back to 1. The 1 through 4 positions of the knob are repeated. Also, each can be split using the thumb-actuated under-overdrive lever on the left side of the knob while in high range. The "thumb" lever is not available in low range, except in 18 speeds; 1 through 4 in low range can be split using the thumb lever and L can be split with the "Butterfly" lever. L cannot be split using the thumb lever in either the 13- or 18-speed. The 9-speed transmission is basically a 13-speed without the under-overdrive thumb lever.

Truck transmissions use many physical layouts. For example, the output of an N-speed transmission may drive an M-speed secondary transmission, giving a total of N*M gear combinations; for example a 4-speed main box and 3-speed splitter gives 12 ratios. Transmissions may be in separate cases with a shaft in between; in separate cases bolted together; or all in one case, using the same lubricating oil. The second transmission is often called a "Brownie" or "Brownie box" after a popular brand. With a third transmission, gears are multiplied yet again, giving greater range or closer spacing. Some trucks thus have dozens of gear positions, although most are duplicates. Sometimes a secondary transmission is integrated with the differential in the rear axle, called a "two-speed rear end". Two-speed differentials are always splitters. In newer transmissions, there may be two countershafts, so each main shaft gear can be driven from one or the other countershaft; this allows construction with short and robust countershafts, while still allowing many gear combinations inside a single gear case.

Heavy-duty transmissions are almost always non-synchromesh. One argument is synchromesh adds weight that could be payload, is one more thing to fail, and drivers spend thousands of hours driving so can take the time to learn to drive efficiently with a non-synchromesh transmission. Heavy-duty trucks driven frequently in city traffic, such as cement mixers, need to be shifted very often and in stop-and-go traffic. Since few heavy-duty transmissions have synchromesh, automatic transmissions are commonly used instead, despite their increased weight, cost, and loss of efficiency.

Heavy trucks are usually powered with diesel engines. Diesel truck engines from the 1970s and earlier tend to have a narrow power band, so need many close-spaced gears. Starting with the 1968 Maxidyne, diesel truck engines have increasingly used turbochargers and electronic controls that widen the power band, allowing fewer and fewer gear ratios. A transmission with fewer ratios is lighter and may be more efficient because there are fewer transmissions in series. Fewer shifts also makes the truck more drivable. As of 2005, fleet operators often use 9, 10, 13 or 18-speed transmissions, but automated manual and semi-automatic transmissions are becoming more common on heavy vehicles, as they can improve efficiency and drivability, reduce the barrier to entry for new drivers, and may improve safety by allowing the driver to concentrate on road conditions. ^{[citation needed]}

Float shifting

Float shifting or floating gears is changing gears without depressing the clutch, usually on a non-synchronized transmission. Since the clutch is not used, it is easy to mismatch speeds of gears, and the driver can quickly cause major (and expensive) damage to the gears and the transmission. Float shifting is often done on large trucks with standard (non-synchronized) gearboxes.

Crash gearbox

Acrash gearbox, also known as a crash box, is a transmission type used in old cars, trucks, and other automotive vehicles. It is more properly called a "sliding mesh" gearbox and has the nickname "crash" because it is difficult to change gears, so gear changes are often accompanied by loud noises. The etymology of "crash" is probably "clash".

In a sliding-mesh gearbox, individual gears are mounted so they always engage the shaft, but gears on one shaft can be moved axially. To engage a particular pair of gears, one gear is slid axially until it fully engages a gear on the other shaft. If the gear shafts are spinning so the two gears have the same surface speed, the gears are relatively easy to engage. However, if speeds are mismatched, the gears tend to "bounce" off each other at first contact and resist engagement. Thus, gear engagement relies on the driver carefully matching speeds, typically through practice and intuition.

In contrast, newer "constant mesh" transmissions use gears that are held axially, but gears on one shaft spin freely on the shaft. Gear pairs in the transmission are always in mesh, though at most one is engaged at any time. Each free-spinning gear has a dog clutch which is engaged by an axial sliding collar that transfers power to the shaft. The dog clutch may be plain, also called "non-synchromesh", or may use an additional synchromesh mechanism that helps get the parts moving at the same speed to assist engagement. Many constant mesh transmissions use a sliding-mesh gear for reverse, but since reverse is only engaged from near a stop, it is still easy to engage.

A constant-mesh transmission offers several advantages over a sliding-mesh design. First, the dog clutch is designed for the task, rather than asking the gear to do dual duty of power transmission and sliding engagement. Second, any shock from engagement of the dog clutch is spread out among many teeth rather than just one or two teeth on the gears themselves. This greatly reduces the resulting stress when gears are engaged. Third, the dog clutch is typically smaller in diameter than the gear it controls, so absolute speeds of the engaging parts are lower, aiding engagement. Thus, while a non-synchromesh transmission still relies on the operator to match speeds, gears are easier to engage. Fourth, a constant-mesh transmission can easily use helical gears which are smoother, quieter, and can carry more torque for a given size of gear. Fifth, a constant-mesh transmission can use synchromesh for easier shifting; while many heavy vehicle transmissions do not use it, most medium- and light-duty automotive transmissions do.

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. Other factors that can lead to excessive clutch wear include increased urban driving (since the clutch is used more for a given distance travelled) and excessively tall gearing (which requires more slippage of the clutch to start from a standing stop).

Manual transmissions are lubricated with gear oil or engine oil in some vehicles, which must be changed periodically in some vehicles, although not as frequently as the fluid in an automatic transmission. Some manufacturers specify that changing the gear oil is never necessary except after transmission work or to rectify a leak. However, manual transmission oil is usually not filtered and accumulates metal particles from normal wear. Also, manual transmission oil has additives (such as friction modifiers to improve synchro function) which lose effectiveness with time. For these reasons, it is best to periodically change transmission oil, even on vehicles which specify a ‘lifetime fill’.

Gear oil has a characteristic aroma because it contains added sulfur-bearing anti-wear compounds. These compounds are used to reduce the high sliding friction by the helical gear cut of the teeth (this cut eliminates the characteristic whine of straight cut spur gears). On motorcycles with "wet" clutches (clutch is bathed in engine oil), there is usually nothing separating the lower part of the engine from the transmission, so the same oil lubricates both the engine and transmission. The original Mini placed the gearbox in the oil sump below the engine, thus using the same oil for both. The clutch was however a fairly conventional dry plate clutch.

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