Semi-automatic transmission

A semi-automatic transmission (also known as clutchless manual transmission, automated manual transmission, e-gear, shift-tronic, flappy paddle gearbox, or paddle shift gearbox) is a system which uses electronic sensors, processors and actuators to do gear shifts on the command of 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 synchronise the timing and torque required to make gear shifts quick and smooth. The system was designed by European automobile manufacturers to provide a better driving experience, especially in cities where congestion frequently causes stop-and-go traffic patterns.


In standard mass-production automobiles, the gear lever appears similar to manual shifts, except that the gear stick only moves forward and backward to shift into higher and lower gears, instead of the traditional H-pattern. The Bugatti Veyron uses this approach for its 7-speed transmission. In Formula One, the system is adapted to fit onto the steering wheel in the form of two paddles; depressing the right paddle shifts into a higher gear, while depressing the left paddle shifts into a lower one. Numerous road cars have inherited the same mechanism.

Hall effect sensors sense the direction of 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 a 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. 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.

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 to allow a complete clutch operation without the intervention of the driver.

For the needs of parking, reversing and neutralizing the transmission, the driver must engage both paddles at once, after this has been accomplished the car will prompt for one of the three options.

The clutch is really only needed to start the car. For a quicker upshift, the engine power can be cut, and the collar disengaged until the engine drops to the correct speed for the next gear. For the teeth of the collar to slide into the teeth of the rings not only the speed, but also the position must match. This needs sensors to measure not only the speed, but the positions of the teeth, and the throttle may need to opened softer or harder. The even faster shifting techniques like powershifting require a heavier gearbox or clutch or even a twin-clutch gearbox.



According to the Car Crazy episode "Le Mans Museum of the Automobile", the paddle shifter interface could be found as early as in a 1912 Le Mans race car. The system used an inner steering wheel to select a gear level.

In Formula One, the first attempt at clutch-less gear changing was in the early 1970s, with the system being tested by the Lotus team. However, it would be a lot longer until the concept would be brought back to attention; in 1989, John Barnard and Harvey Postlethwaite, then-Ferrari engineers and designers, created a semi-automatic gearbox for use in the Ferrari 640 single-seater. Despite serious problems in testing, the car won its first race at the hands of Nigel Mansell. By 1994, the semi-automatic transmission was dominant in terms of gearbox technology, and the last F1 car fitted with a manual gearbox raced in 1995.

In 2008 the IndyCar Series adopted mandatory paddle shifters for the first time.


Main articles: Vacamatic and Presto-Matic
Historically, the first semi-automatic transmission which was marketed by a major manufacturer was the 1941 M4/Vacamatic Transmission by Chrysler. It was an attempt to compete against rivals' automatic transmissions, though it still had a clutch, primarily to change range. The main difference was the addition of a fluid coupling between engine and clutch, and the shifting mechanism.

In normal driving, the clutch was not used. The transmission itself was a fully synchronised manual type, with four forward gears, one reverse, where the shifting was done 'automatically' by either vacuum cylinders (early, M4, Vacamatic), or hydraulic cylinders (late, M6, Presto-Matic).


Nearly simultaneously, Packard introduced the Electro-Matic clutch, which was a vacuum operated clutch pedal, signaled by the position of the accelerator. Significantly, it came with an 'off' switch, probably due to the fact that the system was somewhat unstable during engine warm-up. Packard's system was used in conjunction with their regular transmission so the H-pattern shifting remained.

Earlier, and by many manufacturers, an arrangement to disengage the clutch during coasting was tried to ease shifting. Called "freewheeling", it was bedeviled by the absence of adequate brakes.


Much later, the Volkswagen Beetle came with an optional "Autostick", which was essentially a clutchless manual with three forward gears.


Mercedes used a system similar to the VW Autostick, called Hydrak. Hydrak had one major flaw- the oil supply for the torque converter was sealed within the converter itself and did not circulate via a pump, and also had no oil cooler. Idling in gear for even short periods would overheat the oil and burn up the seals in the converter, which would then need to be replaced.


Citroën is one manufacturer that committed to semi-automatic transmission. First appearing in 1955 on the Citroën DS, a hydraulic system was used 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 was a clutchless shifting with a single column mounted selector. The Citroën 2CV gained a device named 'Trafficlutch', a centrifugal clutch that enabled clutchless changes in the first two ratios only (for town driving). The DS's semi automatic transmission was nicknamed 'Citro-Matic' in the United States. Later, the manufacturer introduced optional semi automatic transmissions on their medium and large saloon and estate models in the 1970s; the Citroën GS and CX models had the option of 3 speed, semiautomatic transmission marketed as 'C matic'. This was simpler than the previous inasmuch as it used a floor mounted quadrant lever operating a contact breaker and conventional gear selector rods in series, a fluid coupling 'torque converter' and wet plate clutch were cut in and out of phase by an electro valve controlled by the contact breaker. This system was simple in that it dispensed with the former use of hydraulics to operate a clutch AND select the gear ratios. Citroën semi automatic transmission of this era made no use of electronics, the entire gear selecting operation was carried out by simply moving the gear lever from one ratio to the next.


The German automobile manufacturer NSU produced a semi-automatic system for the rotary-engined Ro80 saloon car in the 1960s, similar in concept to Citroën's system except that it used an electric switch on the gear shifter which disengaged the clutch.


Honda marketed both cars and motorcycles with the Hondamatic transmission in the 1970s and early 1980s. This transmission is frequently referred to as the 'Bang-O-Matic' by mechanics. The design is noteworthy because it preserves engine braking by eliminating a sprag between first and second gears.


The 993cc Daihatsu Charade in 1985 at least had the option of a 2-speed semi-automatic transmission, which was similar to a conventional auto with torque converter and planetary gearset but lacked a full valve body for making decisions regarding shifting. This was left entirely to the driver and as a result could be accelerated from rest in top gear if desired, depending entirely on the torque converter action. The standing quarter mile time with two 60kg occupants and using low gear appropriately was 21.0 sec while using top gear only was 21.5 sec.

Other applications

Trucks buses and trains

Semi-automatic transmissions have also made its way into the truck and bus market in the early 2000s. Volvo offers its I-shift on its heavier trucks and buses, 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. These gearboxes have a place in public transport as they have been shown to reduce fuel consumption in some specific cases.

In the UK though, semi-automatic transmission has been very popular on buses for some time, from the 1950s right through to the 1980s, an example being the well known London Routemaster, although the latter could also be driven as a fully automatic in the 3 highest gears. Most heavy-duty bus manufacturers offered this option, using a gearbox from Self-Change 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 transmission, 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, and semi-automatic transmission lost favour. These days, very few buses with semi-automatic transmission remain in service, although many are still on the roads with private owners. Modern types of semi-automatic transmission though is becoming more common, mostly replacing manual gearboxes in coaches and small buses.

The Self-Change Gears semi-automatic 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 engine/semi-automatic gearbox units mounted under the floor. Synchronising controls by control cables connected through the train ensured all the gearboxes under all coaches of the train changed gear together.


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.

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