Difference between revisions of "Sequential manual transmission"
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Latest revision as of 22:27, 3 July 2009
A sequential manual transmission (or sequential manual gearbox) is a type of manual transmission used on motorcycles and high-performance cars or auto racing, where gears are selected in order, and direct access to specific gears is not possible.
With traditional manual transmissions, the driver can move from any gear, to any gear, by moving the shifter to the appropriate position. This type of transmission is often referred to as a H-pattern because of the path that the shift lever takes as it selects the various gears. A clutch must be disengaged before the new gear is selected, to disengage the running engine from the transmission, thus stopping all torque transfer. In auto racing, this process is slow and prone to human error; hence the development of the sequential transmission. A true sequential transmission will very often use dog clutch engagement rather than the more usual syncromesh as fitted to a normal H-pattern road car gearbox. Engagement using dogs only requires a very brief interruption of engine torque to complete a shift into any adjacent gear. This allows shifting between gears without the use of the clutch. The clutch would normally be used only for standing starts.
Sequential manual transmissions works by providing the driver with the ability to select the gear directly before or after the gear currently engaged. Usually the shift lever is pulled back to select the adjacent higher gear and pushed forwards to select the adjacent lower gear. On a true sequential gearbox, the shift lever operates a ratchet mechanism that converts the fore & aft motion of the shift lever into a rotary motion. This rotary action turns a selector drum (sometimes called a barrel) which has 3 or 4 tracks machined around its circumference. Running in the tracks are the selector forks, either directly or via selector rods. These tracks deviate around the circumference and as the drum rotates, the selector forks running in the tracks are moved to select the required gear. Only a true sequential transmission has a shift mechanism that operates in this way.
Sequential gearboxes are also used in nearly all modern motorcycles as it is too cumbersome to have a conventional H-pattern shifter and would take up too much space in the confines of a motorcycle frame. Having control over the gear shifter with the rider's left foot frees the his hands to operate both the clutch and brake without letting go of the handle bars.
Sequential manual transmissions are true manual transmissions, and should not be confused with automatic transmissions that provide some degree of user shifting input. One commercial example of this type of automatic transmission is the Tiptronic transmission. User shifting through buttons or lever does not necessarily mean that the transmission is a manual transmission.
Beyond the ease of use from a driver's standpoint, an additional benefit of sequential manual gearboxes is that use of the clutch via foot pedal or hand control can be minimized or completely obviated, with the clutch only used for starting from a complete stop. Formula One cars of the 1990s made the most high-profile debut of this technology in motor sports, and enhanced variations on this theme are still in use in many forms of road racing and drag racing today. The simple push-pull action of the shift mechanism also lends itself to semi-automatic control using either hydraulic or pneumatic actuators — a system often referred to as paddle-shift. Instead of a manual gear lever, the driver is provided with (usually) a pair of flipper paddles on the steering wheel, rally cars often utilize just a double-acting single paddle. Pulling on the right-hand paddle makes an up-shift and pulling on the left-hand paddle makes a down-shift. The paddle-shift system will use a sophisticated electronic control unit to provide the necessary intelligence to operate the shift mechanism. This type of paddle-shift system fitted to race and rally cars should not be confused with most of the current crop of so-called paddle-shift systems fitted to some high-end road cars. More often than not, these systems are nothing more than conventional automatic transmissions which allow driver input to select the gears.
Use in road cars
The most famous application of a sequential transmission on road-cars would be their use in some Ferraris since the mid-nineties. Their system, the most current version of which is called "F1-Superfast," is designed to serve as a link to their Formula 1 efforts. This technology has also trickled down to the cars of their sister company, Maserati where it is known as "Cambiocorsa". Alfa Romeo's Selespeed was the first sequential transmission in a "normal" car. This transmission was derived from the Ferrari system. The Direct Shift Gearbox (DSG) found in some Volkswagens and Audis is a sequential manual transmission of sorts. This transmission is actually 2 separate gearboxes each with its own clutch. One gearbox contains the ratios for 1st, 3rd & 5th, while the other has 2nd, 4th & 6th. Intelligent predictive electronic control pre-selects the next gear and when the driver requests the shift the drive is passed from one gearbox to the other by use of the two clutches. Volkswagen (parent owner of Lamborghini) has also recently introduced sequential transmission to the Lamborghini Gallardo. BMW also has a system simply called "SMG" (an abbreviation for sequential manual gearbox), but it is not a true sequential, just a modified H-pattern gearbox which uses separate electronically controlled actuators on each of the 3 selector rails. One should not confuse the Porsche Tiptronic transmission with sequential transmissions either. Tiptronic is an automatic transmission which allows the driver to select gears. The 3rd generation Toyota MR2 uses Toyota's version, known as the Sequential Manual Transmission or SMT. Although it does not perform as well as the European-designed transmissions, Toyota's is the cheapest system to manufacture, and the MR2 is the least expensive car to possess a true sequential gearbox. The majority of so-called sequential transmissions fitted to road cars offer little or no performance benefit because, in many cases, they are either conventional automatics with a torque converter or they retain syncro engagement if they are genuine manuals. The shift is invariably slow compared to a dog clutch gearbox and the electronic logic control will often override a shift request from the driver. This is often highly frustrating to the enthusiastic driver.
While a sequential manual transmission can (depending on the model) offer faster shift speeds and thus faster race times, many road-car-driving enthusiasts prefer a standard manual transmission, which—with a clutch pedal and the ability to skip gears—allows more driver input and a more traditional driving experience.