A manual transmission (also known as a stick shift or standard transmission) is a type of transmission used in automotive applications. Manual transmissions often feature a driver-operated clutch and a movable gear selector, although some do not. Most automobile manual transmissions allow the driver to select any gear at any time, but some, such as those commonly mounted on motorcycles and some types of race cars, only allow the driver to select the next-highest or next-lowest gear ratio. This second type of transmission is sometimes called a sequential manual transmission.
Manual transmissions are characterized by gear ratios which are selectable by engaging pairs of gears inside the transmission. Conversely, automatic transmissions feature clutch packs to select gear ratio. Transmissions which employ clutch packs but allow the driver to manually select the current gear are called semi-automatic transmissions.
Contemporary automotive manual transmissions are generally available with between 4 and 6 forward gears and one reverse gear, although manual transmissions have been built with as few as 2 and as many as 7 gears. Some manuals are referred to by the number of gears they offer (e.g., 5-speed) as a way of distinguishing between automatic or other available manual transmissions. In contrast, a 5-speed automatic transmission is referred to as a 5-speed automatic.
Other types of transmission in mainstream automotive use are the automatic transmission, semi-automatic transmission, and the continuously variable transmission.
Like other transmissions, a manual transmission features both input and output shafts. Pairs of gears are attached to these shafts that, when selected, will cause the output shaft to rotate at a given ratio of the input shaft speed. When a driver selects a gear, he is simply selecting a pair of these gears to be used; mechanical connections translate the driver's selection into an appropriate connection of gears and prevent more than one set of gears being engaged at any given time (as that would cause the transmission to lock).
Manual transmissions are often equipped with 4, 5, or 6 forward gears. Nearly all have exactly one reverse gear. In most cases, the topmost gear is an overdrive gear; the ratio directly below this is usually 1:1. Older cars are generally equipped with 3-speed or 4-speed transmissions, and 5-speed transmissions became popular in the 1980s and remain so today. 6-speed transmissions emerged in the early 1990s and have generally been reserved for high-performance vehicles, although they have recently been offered on high-efficiency and conventional passenger cars.
In most vehicles equipped with manual transmissions, the engine and transmission are separated by a clutch, controlled by a pedal on the left hand side of the driver's footwell (usually operated with the left foot) that modulates the transfer of power between the two subsystems.
- When the clutch pedal is fully depressed, the clutch is fully disengaged, and no torque is transferred from the engine to the transmission, and by extension to the drive wheels.
- When the clutch pedal is fully released, the clutch is fully engaged, and essentially all of the engine's torque is transferred.
- In between these extremes, the clutch "slips" to varying degrees.
In most modern cars, gears are selected through a lever attached to the floor of the automobile—this selector is ofter called a gearstick, gear lever, gear selector, or simply shifter. Moving this lever forward, backward, left, and right allows the driver to select any given gear. In this configuration, the gear lever must be pushed laterally before it is pushed longitudinally.
A common layout for a 5-speed transmission is shown below. N marks neutral, or the position where no gears are engaged. In reality, the entire horizontal line is a neutral position, although the shifter is usually equipped with springs so that it will return to the N position if not left in another gear. The R denotes reverse, which is technically a sixth gear on this transmission.
1 3 5
| | | ^
----N---- | Forward
| | | |
2 4 R
This layout is called the shift pattern. The shift pattern for a specific transmission is usually printed on the shifter knob.
Another common five-speed shift pattern is:
R 1 3 5
| | | | ^
------N------ | Forward
| | |
Transmissions equipped with this shift pattern usually feature a lockout mechanism that requires the driver to depress a switch or the entire gear lever when entering reverse, so that he does not accidentally select it when trying to find first gear.
Most front-engined, rear-wheel drive cars have a transmission that sits between the driver and the front passenger seat. Floor-mounted shifters are often connected directly to the transmission. Front-wheel drive and rear-engined cars often require a mechanical linkage to connect the shifter to the transmission.
Some older cars feature a gear lever which is mounted on the steering column of the car. Many automatic transmissions still use this placement, but manual column shifters are no longer common.
Column shifters are mechanically similar to floor shifters, although shifting occurs in a vertical plane instead of a horizontal one. Column shifters also generally involve additional linkages to connect the shifter with the transmission.
R 2 ^
| | | Up
| | -> Forward
The 3-speed shift pattern is typical of American cars, trucks, and vans produced with manual transmissions during the 1950s and 1960s.
First gear in a 3-speed is often called "low," while third is usually called "high." There is, of course, no overdrive.
Note that reverse in a car with a column shift is in nearly the same position as park (P) is on a car with a column-mounted gear selector with an automatic transmission.
Some transmissions do not allow the driver to arbitrarily select any gear. Instead, the driver may only ever select the next-lowest or next-highest gear ratio. These transmissions often provide clutch control, but the clutch is only necessary when selecting first or reverse gear from neutral. Most gear changes can be performed without the clutch.
Sequential transmissions are generally controlled by a forward-backward lever, foot pedal, or set of paddles mounted behind the steering wheel. In some cases, these are connected mechanically to the transmission. In many modern examples, these controls are attached to sensors which instruct a transmission computer to perform a shift—many of these systems can be switched into an automatic mode, where the computer controls the timing of shifts, much like an automatic transmission.
Motorcycles typically employ sequential transmissions, although the shift pattern is modified slightly for safety reasons.
Some very new transmissions (BMW's Sequential Manual Gearbox (SMG) and Audi's Direct-Shift Gearbox (DSG), for example) are conventional manual transmissions with a computerized control mechanism. These transmissions feature independently selectable gears but do not have a clutch pedal. Instead, the transmission computer controls a servo which disengages the clutch when necessary.
These transmissions vary from sequential transmissions in that they still allow nonsequential shifts: BMWs SMG system, for example, can shift from 6th gear directly to 4th gear when decelerating from high speeds.
Comparison with automatic transmissions
Manual transmissions are typically compared to automatic transmissions, as the two represent the majority of options available to the typical consumer. These comparisons are general guidelines and may not apply in certain circumstances. Additionally, the recent popularity of semi-manual and semi-automatic transmissions renders many of these points obsolete.
- Manual transmissions are typically more efficient than automatic transmissions. This is because manuals generally involve a clutch instead of a torque converter, which can cause significant power losses. This results in both better acceleration and fuel economy.
- It is generally easier to build very strong manual transmissions than automatic transmissions. Manual transmissions usually have only one clutch, whereas automatics have many clutch packs.
- Manual transmissions normally do not require active cooling, because not much power is dissipated as heat through the transmission.
- A driver has more direct control over the state of the transmission with a manual than an automatic.
- Manual transmissions are typically cheaper to build than automatic transmissions.
- Manual transmissions generally require less maintenance than automatic transmissions.
- Manual transmissions require more driver interaction than automatic transmissions.
- A driver may inadvertently shift into the wrong gear with a manual transmission, potentially causing damage to the engine and transmission as well as compromising safety.
- Manual transmissions are more difficult to learn to drive as one needs to develop a feel for properly engaging the clutch.
- The smooth and quick shifts of an automatic transmission are not guaranteed when operating a manual transmission.
- Manual transmissions require more controls than automatic transmissions. This is an issue in cramped cockpits, cars where a floor-shifter is inconvenient, or in vehicles equipped for disabled drivers.
- Manual transmissions make it especially challenging to start when stopped upward on a hill.
Applications and popularity
Many types of automobiles are equipped with manual transmissions. Small economy cars predominantly feature manual transmissions because they are relatively cheap and efficient, although many are optionally equipped with automatics. Economy cars are also often powered by very small engines, and automatic transmissions can make them comparatively very slow.
Sports cars are also often equipped with manual transmissions because they offer more direct driver involvement and better performance. Off-road vehicles and trucks often feature manual transmissions because they allow direct gear selection and are often more rugged than their automatic counterparts.
Very heavy trucks also feature manual transmissions because they are efficient and, more importantly, can withstand the severe loads encountered in hauling heavy loads.
Conversely, manual transmissions are no longer popular in many classes of cars sold in North America. Nearly all cars are available with an automatic transmission option, and family cars and large trucks are sold predominantly with automatics. Most luxury cars are unavailable with a manual transmission. In situations where automatics and manual transmissions are sold side-by-side, the manual transmission is the base equipment, and the automatic is optional—although the automatic is sometimes available at no extra cost.
Some cars, such as rental cars and taxis, are nearly universally equipped with automatic transmissions in the US.
- See Manual transmission driving technique.
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, whereas weak clutches or inexperienced drivers can lead to more frequent repair or replacement.
Manual transmissions are lubricated with gear oil, which must be changed periodically, although not as frequently as the automatic transmission fluid in a vehicle so equipped.