Semi-automatic transmission

A semi-automatic transmission (also known as an automated manual[1][2][3], clutchless manual[4], auto-manual[5], semi-manual[6], is an automobile transmission that combines mechanisms of a manual and automatic transmission.

Semi-automatic transmission refers to a conventional manual transmission with an automatic clutch. It requires full driver-control of the manual gear ratio selection, and the driver must manually shift through all the gears. They facilitate gear shifts for the driver by operating the clutch system automatically, while still requiring the driver to manually shift gears. This system is commonly found on older vehicles, such as automobiles and buses, without a clutch pedal, and on some motorcycles, minus the hand clutch lever.[7][8]

Comparison to modern transmissionsEdit

Semi-automatic transmissions are conventional manual transmissions, usually operated with an automatic clutch or another kind of partially automatic transmission mechanism. However, they require full control of the manual gear selection by the driver. The driver must manually operate and shift through the gear ratios via the H-pattern shifter. An example of this transmission type in automobiles is the VW Autostick semi-automatic transmission. The semi-automatic transmission does not have an automatic mode, unlike the more modern automated manual transmissions, which are essentially conventional manual transmissions containing both manual and automatic shifting modes.

Modern automated manual transmissions are a type of Automatic transmission which have a fully-automatic mode, where the driver does not need to change gears at all, operating in the same manner as a conventional torque converter automatic transmission, by allowing the transmission's computer to automatically shift gear if, for example, the driver were redlining the engine. The AMT can be engaged in a manual mode wherein one can up-shift or down-shift using the console-mounted shifter selector or the paddle shifters just behind the steering wheel, without the need of a clutch pedal. The ability to shift gears manually, often via paddle shifters, can also be found on other automatic transmissions (manumatics such as Tiptronic) and continuous variable transmissions (CVTs) (such as Lineartronic). Automated manual transmission is a modern type of Automatic transmission. It consists of a conventional manual transmission with an electronically-actuated hydraulic clutch and computerized gear shift control, and the driver can usually override the computer control with a clutchless "manual" mode.[9] It has a lower cost than any other types of automatic transmissions[10] An automated manual transmission can simply and best be described as a standard manual transmission, with an automated clutch, and automated clutch and gear shift control. It is a type of automatic transmission.[5]

Despite superficial similarity to other automated transmissions, modern automated manual and older semi-automatic transmissions differ significantly in internal operation and driver's "feel" from manumatic and CVTs. A manumatic, like a standard automatic transmission, uses a torque converter instead of clutch to manage the link between the transmission and the engine, while a CVT uses a belt instead of a fixed number of gears. An AMT utilizes a clutch, and offers a more direct connection between the engine and wheels than a manumatic and this responsiveness is preferred in high-performance driving applications, while a manumatic is better for street use because its fluid coupling makes it easier for the transmission to consistently perform smooth shifts,[11][12] and CVTs are generally found in gasoline-electric hybrid engine applications.

Typically automated manual transmissions are more expensive than manumatics and CVTs, for instance BMW's 7-speed Dual Clutch Transmission is a CAD 3900 upgrade from the standard 6-speed manual, while the 6-speed Steptronic Automatic was only a CAD 1600 option in 2007.[13] In a given market, very few models have two choices of automated transmissions; for instance the BMW 545i (E60) and BMW 645Ci/650i (E63/64) (standard 6-speed manual) had as an option a 6-speed automatic "Steptronic" transmission or a 7-speed Getrag SMG III single-clutch automated manual transmission until after the 2008 model year, when the SMG III was dropped.[14] Many sport luxury manufacturers such as BMW offer the manumatic transmissions for their mainstream lineup (such as the BMW 328i and BMW 535i) and the automated manual gearbox for their high-performance models (the BMW M3 and BMW M5).[13]

Modern clutch-based automatic transmission can be derived from a conventional hydraulic automatic; for instance, Mercedes-Benz's AMG-Speedshift MCT automatic transmission is based on the 7G-Tronic manumatic, however, the torque converter has been replaced with a wet, multi-plate launch clutch.[15] Chevrolet's Torque-Drive was based on GM's conventional Powerglide, but lacked the vacuum modulator that controls automatic gear shifts. Other automated manual transmissions have their roots in a conventional manual; the SMG II drivelogic (found in the BMW M3 (E46) is a Getrag 6-speed manual transmission, but with an electrohydraulically actuated clutch pedal, similar to a Formula One style transmission. Alfa Romeo "Selespeed" and Ferrari "F1" transmissions worked in the same way.[16][17][18] The most common type of automated manual transmission in recent years has been the dual clutch type, since single-clutch types such as the SMG III have been criticized for their general lack of smoothness in everyday driving (although being responsive at the track).[19]


VW Autostick semi-automatic transmission diagram
How a typical semi-automatic transmission operates

Semi-automatics facilitate gear shifts by dispensing the need to depress a clutch pedal at the same time as changing gears. Depending on the mechanical build and design, they can use electronic sensors, hydraulics, pneumatics, processors, and actuators to execute gear shifts when requested by the driver. This removes the need for a clutch pedal which the driver otherwise needs to depress before making a gear change since the clutch itself is actuated by electronic equipment which can synchronize the timing and torque required to make quick, smooth gear shifts. The system was designed by automobile manufacturers to provide a better driving experience through fast overtaking maneuvers on highways. Motorcycles with this system use a conventional sequential foot-shift lever (like on a manual motorcycle), but use a centrifugal (automatic) clutch system, removing the need for a hand-clutch lever, and manual clutch actuation.[20]

The operation of semi-automatic transmissions has evolved over time. Many different times of clutch actuation systems have been used, from electro-hydraulic, pneumatic, and electromechanical clutches, while other manufactures have used alternate methods of actuation, like vacuum-operated or electromagnetic clutches. automated manuals have evolved as vehicle manufacturers experimented with different systems. The clutches are usually connected to an electronic control system, which uses various electrical sensors and actuators to detect when the driver touches the gearshift and requests a shift. The gearshift will usually be connected electronically to the clutch, and the clutch will disengage once the driver moves the gearshift. In one example, Ferrari offered their Mondial model with a clutchless manual, which Ferrari called the Valeo transmission. In this system, the gearshift of a conventional manual transmission was retained; and moving the shifter automatically engaged the electro-mechanical clutch. Saab's Sensonic transmission worked in a similar fashion. Most semi-automatic transmissions work in a similar fashion, once the driver moved the shift lever to switch gears, the clutch would disengage, and re-engage once the gear was selected.

Hall effect sensors sense the direction of the requested shift, and this input, together with a sensor in the gear box which senses the current speed and gear selected, feeds into a central processing unit. This unit then determines the optimal timing and torque required for smooth clutch engagement, based on input from these two sensors as well as other factors, such as engine rotation, the Electronic Stability Control, air conditioner and dashboard instruments.

The central processing unit powers a hydro-mechanical unit to either engage or disengage the clutch, which is kept in close synchronization with the gear-shifting action the driver has started. In some cases, the hydro-mechanical unit contains a servomotor coupled to a gear arrangement for a linear actuator, which uses brake fluid from the braking system to impel a hydraulic cylinder to move the main clutch actuator. In other cases, the clutch actuator may be completely electric. The actuators and sensors which control the clutch are usually connected to an electronic servomechanism, operated via the transmission control unit (TCU).

The power of the system lies in the fact that electronic equipment can react much faster and more precisely than a human and takes advantage of the precision of electronic signals and hydraulic actuators to complete the clutch operation faster, and without the manual intervention of the driver.

Usage in passenger carsEdit

1900s to 1920sEdit

Bollée Type F Torpedo with gear shift ring located inside the steering wheel

In 1901, Amédée Bollée developed a method of shifting gears which did not require the use of a clutch and was activated by a ring mounted within the steering wheel.[21] One car using this system was the 1912 Bollée Type F Torpedo.

1930s to 1940sEdit

Prior to the arrival of the first mass-produced hydraulic automatic transmission in 1940 (the General Motors Hydra-Matic), several American manufacturers offered various devices to reduce the amount of clutch or gear shifter usage required. These devices were intended to reduce the difficulty of operating the unsynchronised manual transmissions ("crash gearboxes") that were commonly used, especially in stop-start driving.

An early step towards automated transmissions was the 1933-1935 REO Self-Shifter,[22] which automatically shifted between two forward gears in the "Forward" mode (or between two shorter gear ratios in the "Emergency low" mode). Standing starts required the driver to use the clutch pedal. The Self-Shifter first appeared in May 1933 and was offered as standard on the Royale and as an option on the Reo Flying Cloud S-4.[23]

In 1937, the Oldsmobile 4-speed Automatic Safety Transmission was introduced on the Oldsmobile Six and Oldsmobile Eight models.[22] It used a planetary gearset with a clutch pedal for starting from standstill and switching between the "Low" and "High" ranges.[24][25][26] The Automatic Safety Transmission was replaced by the fully automatic Hydra-Matic for the 1940 model year.

The 1938-1939 Buick Special was available with the Self-shifter 4-speed semi-automatic transmission.[27][citation needed] transmission. This transmission uses a manual clutch for starting from standstill, and an automated clutch for gear changes.

The 1941 Chrysler M4 Vacamatic transmission used a two-speed manual transmission with integral underdrive unit, a traditional manual clutch and a fluid coupling between the engine and the clutch.[28][29][30] The two-speed transmission was used as "High" and "Low" ranges, and the clutch was needed when the driver wanted to switch between ranges. For normal driving, the driver would press the clutch, select High range and then release the clutch. One the accelerator was pressed, the fluid coupling would engage and the car would begin moving forward, with the underdrive unit engaged to provide a lower gear ratio. At between 15–20 mph (24–32 km/h), the driver would lift off the accelerator and the underdrive unit would disengage. The Vacmatic was replaced by the similar M6 Presto-Matic transmission for the 1946 model year.

Similar designs were used for the 1941-1950 Hudson Drive-Master[31][32] and the ill-fated 1942 Lincoln Liquimatic.[33][34] Both of these used a 3-speed transmission with automated shifting between 2nd and 3rd gears, instead of the Vacamatic's underdrive unit.

The Packard Electro-Matic — introduced in the 1941 Packard Clipper and Packard 180 — is an early a clutchless manual transmission that uses a traditional friction clutch with automatic operation. In the case of the Electro-Matic, the clutch was vacuum-operated and controlled by the position of the accelerator.

1950s to 1960sEdit

VW Automatic Stickshift diagram

The Automotive Products Manumatic system, available on the 1953 Ford Anglia 100E was a vacuum-powered automatic clutch system that was actuated by a switch that was triggered whenever the gear level was moved. The system could control the throttle cable (to keep the engine at the required RPM for the gear change) and could vary the rate of clutch engagement.[35] The successive Newtondrive system, available on the 1957-1958 Ford Anglia, also had a provision for choke control.

The Citroën DS, introduced in 1955, used a hydraulic system to select gears and operate the conventional clutch using hydraulic servos. There was also a speed controller and idle speed step-up device, all hydraulically operated. This allowed clutchless shifting with a single selector mounted behind the steering wheel. This system was nicknamed 'Citro-Matic' in the U.S.

The Chevrolet Torque Drive transmission, introduced on the 1968 Chevrolet Nova and Camaro, is one of few examples where a semi-automatic transmission was based on a hydraulic automatic transmission (rather than a manual transmission). The Torque Drive was essentially a 2-speed Powerglide transmission without the vacuum modulator, requiring the driver to manually shift gears between Low and High. The quadrant indicator on Torque Drive cars was, Park R N Hi 1st. The driver would start the car in "1st," then move the lever to "Hi" when desired. The torque drive was discontinued at the end of 1971 and replaced by a traditional hydraulic automatic transmission. Other examples of semi-automatic transmissions based on hydraulic automatics are the Ford Semi-Automatic Transmission 3-speed transmission used in the 1970-1971 Ford Maverick (Americas), early versions of Honda's 1976-1988 Hondamatic 2-speed and 3-speed transmissions, and the Diahatsu Diamatic 2-speed transmission used in the 1985-1991 Daihatsu Charade.

Other examplesEdit

Illustation of the Saab "Sensonic" transmission system
Years Name Notes
1953-1954 Plymouth Hy-Drive Torque convertor added to a 3-speed manual transmission, so it could be driven solely in top gear (to avoid using the manual clutch).
1956-1963 Renault Ferlec Automatic clutch (electromagnetic). Used in the Renault Dauphine.[36][37]
1957-1961 Mercedes-Benz Hydrak Automatic clutch (vacuum-powered), plus a torque convertor for standing starts.[38]
1959-???? Citroën Trafficlutch Automatic clutch (centrifugal). Used in the Citroën 2CV.
1966-???? Simca automatic clutch Automatic clutch plus a torque convertor. Used in the Simca 1000.[39][40][41][42]
1967-1977 NSU automatic clutch Automatic clutch (vacuum-powered) plus a torque convertor. Used in the NSU Ro 80.
1968-1979 Volkswagen Autostick Automatic clutch (electro-pneumatic) plus a torque convertor. Used in the Volkswagen Beetle and Volkswagen Karmann Ghia.[43]
1976-1980 Citroën C-matic Automated clutch plus a torque convertor. Used in the Citroën GS and Citroën CX.
1988-1993 Ferrari Valeo Automatic clutch (electro-mechanical). Used in the Ferrari Mondial.
1995-1996 Saab Sensonic Automatic clutch. Used in the Saab 900 NG.

Usage in motor racingEdit

The first Formula One car to use a semi-automatic transmission was the 1989 Ferrari 640.[21][44] This system used electro-hydraulic actuators for controlling the clutch and shifting, and was operated by paddle-shifters mounted behind the steering wheel. Another paddle on the steering wheel controlled the clutch, which was only needed when starting from a standstill. The car won its debut race at the Brazillian Grand Prix, however for much of the season suffered from reliability problems.[45] Other teams began switching to similar semi-automatic transmissions, and the 1991 Williams FW14 was the first to use a sequential drum-rotation mechanism (similar to the system used in motorcycle transmissions) to produce a more compact design that required only one actuator to rotate the drum and move the gear selector forks. A further development was made possible by the introduction of electronic throttle control soon after, which made it possible for the car to automatically rev-match during downshifts.[46] By 1993, most teams were using semi-automatic transmissions. The last F1 car fitted with a conventional manual gearbox, the Forti FG01, raced in 1995.[47]

Following concerns about the potential for Formula One cars to shift gears automatically without any driver input (i.e. a fully automatic transmission), mandatory software was introduced[when?] that ensured that gear changes only occurred when instructed by the driver. Buttons on the steering wheel to skip directly to a particular gear (instead of stepping through the gears using the paddles) are also permitted.

Other applicationsEdit

Trucks and busesEdit

Automated manual transmissions have also made their way into the truck and bus market in the early 2000s. Volvo offers its I-Shift on its heavier trucks and buses as well UD Trucks with ESCOT, while ZF markets its ASTronic system for trucks, buses and coaches. In North America, Eaton offers the "AutoShift" system which is an add-on to traditional non-synchromesh manual transmissions for heavy trucks.[48] These gearboxes have a place in public transport as they have been shown to reduce fuel consumption in some specific cases.

Bristol/Daimler/Leyland busesEdit

The British employed pneumatic valve bodies to regulate gear shifting by charging pistons with compressed air within the gearbox. These pneumatic pistons or gear-levers are activated by a series of valve bodies and controlled by electronic actuators linked to the gear shifter. As each gear cycle is energized, air valves open and close to engage the corresponding gear-lever. Compressed air is drawn from the braking system and in the event of loss of pressure, the transmission will remain in the last gear selected or if in neutral, will not shift into gear.

In the UK though, semi-automatic transmissions have been very popular on buses for some time, from the 1950s right through to the 1980s, an example being the well known London AEC Routemaster, although the latter could also be driven as a full automatic in the three highest gears. Most heavy-duty bus manufacturers offered this option, using a gearbox from Self-Changing Gears Ltd of Coventry, and on urban single- and double-deck buses it was the norm by the 1970s. This coincided with the development of city buses with engines and transmissions at the rear rather than the front, which was beyond the capability of a manual gearchange/clutch linkage from the driver's position. Leyland manufactured many buses with semi-automatic transmissions, including its Leopard and Tiger coaches. Fully automatic transmission became popular with increasing numbers of continental buses being bought in the UK, and more and more British manufacturers began offering automatic options, mostly using imported gearboxes (such as those made by Voith and ZF), and semi-automatic transmissions lost favor. These days, very few buses with semi-automatic transmissions remain in service, although many are still on the roads with private owners. Modern types of manumatic and automated manual transmissions, though, are becoming more common, mostly replacing manual gearboxes in coaches.



The Self-Changing Gears automated gearbox was also fitted to the several thousand diesel railcars built for the British railway system in the late 1950s-early 1960s, which lasted in service until the 1990s-2000s. Their whole engine-transmission system was based on that from the main bus manufacturers of the period such as Leyland and AEC. Gear selection was by the train driver with a hand-held lever as the train accelerated. Such trains were formed of a number of such railcars coupled together and each power car had two engines/automated gearbox units mounted under the floor. Synchronizing controls by control cables connected through the train ensured all the gearboxes under all coaches of the train changed gear together.


The semi-automatic transmission was also used on many small diesels shunting locomotives, such as British Rail Class 03 and British Rail Class 04. A widely used type was the Wilson-Drewry epicyclic gearbox.

Another early semi-automatic transmission was the Sinclair S.S.S. (synchro-self-shifting) Powerflow gearbox. which was applied to Huwood-Hudswell diesel mines locomotives and on the British Rail Class D2/7 and British Rail Class D2/12.[49] This gearbox was of the layshaft type with constant-mesh gears and dog clutch engagement. A special feature was that the drive was maintained during upward gear changes. It was also applied to some road vehicles.[50]


On high-performance sports bikes, this system is sometimes called trigger-shift, so-called because the rider shifts gears with a handlebar-mounted trigger or paddle, to switch up and down gears. On dirt bikes and some other motorcycles, this may sometimes be referred to as an auto-clutch transmission, since the driver is still required to shift gears manually with the foot-lever, but the clutch system is controlled automatically.

The first motorcycles with semi-automatic gearboxes were developed and patented in the mid 1960s by the Czechoslovakian Jawa motorcycle company.[51]

In 1965, Honda copied the Jawa semi-automatic clutch for the Honda Cub step-through motorcycle series and so Jawa sued them in court for patent infringement. Honda settled out of court and Jawa granted them indefinite license with royalties charged on each Honda motorcycle that used the Jawa system.[51]

The first large-capacity motorcycle with a semi-automatic gearbox was the 1975 948 cc (57.9 cu in) Moto Guzzi Convert (the name was chosen to denote the torque converter which was at the heart of this 110 mph motorcycle)[citation needed] a fairly heavy motorcycle which despite its weight still handled, and cornered remarkably well, which was thanks to the race-bred frame from the Moto Guzzi Le Mans.[citation needed]

Honda also had a range of bikes fitted with a gearbox that mated a torque converter and a two speed gearbox. This style of gearbox still required the rider to manually select neutral and either of the two gears using the foot gear lever.[52] These models were the CB750A, CB400A, CM400A and CM450A. These bikes were badged and marketed as Hondamatics.

In addition to the Hondamatic system noted above, Yamaha Motor Company introduced a semi-automatic transmission on its 2007 model year FJR1300 sport-touring motorcycle in 2006. Notably, this system can be shifted either with the lever in the traditional position near the left foot, or with a switch accessible to the left hand where the clutch lever would go on traditional motorcycles.

Honda began production of the VFR1200F on 8 September 2009,[53] which includes an optional dual clutch transmission, the first to be fitted to a motorcycle.

The BRP Can-Am Spyder Roadster is available with a clutchless semi-automatic transmission (the SE5 or SE6, with five or six speeds, depending on the model).

Small capacity underbone or "step-thru" types of motorcycles, such as the Honda Super Cub, Suzuki FR50 and FR80 and Yamaha Townmate, use a semi-automatic gearbox with a "heel and toe" foot change in the standard motorcycle position but without the need for conventional clutch operation.

Many modern motorcycles feature a device called quick-shifter for up-shifts without pressing the clutch lever or closing (rolling-off) the throttle. The special sensor recognizes pressure on the gear shift rod and quick-shifter sends a signal to the ECU to either stop fuelling for a short time (milliseconds) or suppress the spark at the plug, which unloads the gearbox and allows a gear change. The idea came from racing where it helps to minimize the time when the motorcycle is not at full power. An alternative device for down-shifts is called auto-blipper and is less widespread. It artificially "blips" the throttle to match engine speed to speed of the rear wheel to avoid sudden spikes in torque transfer.[citation needed]


Honda released automated electric shift ATVs starting in the model year 1998 with the TRX450FE aka Foreman 450ES ESP (Electric Shift Program). Shifting is accomplished by pressing either one of the gear selector arrows on the left handlebar control. The currently selected gear is indicated by a digital display. The primary components of the shifting mechanisms were the same on both the manual and electric shift models, but the major difference was the deletion of the shift pedal and the addition of an internal electric shift servo which actuated the components (clutch assembly, shift drum, etc.)in one motion instead of the traditional foot lever. In the event of a malfunction, a supplied override lever can be placed on a shaft protruding from the crankcase in the traditional spot where the pedal would have been. This electric shift technology was later applied to their complete line of ATVs.


See alsoEdit


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