Manual transmission

Cherrier two speed gear, circa 1900[1]

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.

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 synchronizer, 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.

Top and side view of a typical manual transmission, in this case, a Ford Toploader, used in vehicles with external floor shifters.

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 synchronizers 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 (dual-clutch automatic 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, has 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 liters .

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 the application.

In 2008, 75.2% of vehicles produced in Western Europe were equipped with manual transmission, versus 16.1% with automatic and 8.7% with other.[2]

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.[3]

The input shaft is connected to the engine and spins at engine speed whenever the clutch is engaged.[4] The countershaft has gears of various sizes, which are permanently meshed with the corresponding gear on the input shaft.[5] 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).[6] 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.[7] 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 main shaft. 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 countershaft) 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 main shaft (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.

• Gear selection in a constant-mesh transmission
• 
  First gear (blue, to back)
• 
  Second gear (blue, to front)
• 
  Third gear (purple, to back)
• 
  Fourth gear (purple, to front)

• 
  Reverse (green engaged)
• 
  Neutral (all disengaged)

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.[8]

The dog clutch is a sliding selector mechanism that 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, the 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.

Synchronizer rings

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. The synchromesh transmission was invented in 1919 by Earl Avery Thompson and first used on production cars by Cadillac in 1928.[9]

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.[10] 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 allow the dog clutch to fall into the 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, bronze or carbon (with the latter usually reserved for high-performance transmissions due to their high cost).[11]

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.

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 the 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 the mesh, 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 synchronizer 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.[12]

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.

An alternative design of transmission that is used in old cars, trucks, and other automotive vehicles is a non-synchronous transmission (also known as a crash gearbox). Non-synchronous transmissions use a sliding mesh design and have the nickname "crash" because the difficulty in changing gears can lead to gear shifts accompanied by crashing/crunching noises.

Starting from a stationary position is a challenge in a manual transmission car, due to the extra force required to accelerate the vehicle up the hill and the potential for the car to roll backward in the time it takes to move the driver's foot from the brake pedal to the accelerator pedal (to increase the engine RPM before letting out the clutch). The traditional method of hill starts in a manual transmission car is to use the parking brake (also called "handbrake" or "e-brake") to hold the vehicle stationary. This means that the driver's right foot is not needed to operate the brake pedal, freeing it up to be used on the throttle pedal instead. Once the required engine RPM is obtained, the driver can release the clutch, also releasing the parking brake as the clutch engages.

A device called the hill-holder was introduced on the 1936 Studebaker. Many modern vehicles use an electronically actuated parking brake, which often includes a hill-holder feature whereby the parking brake is automatically released as the driven wheels start to receive power from the engine.[17]

• Double clutching is sometimes necessary to synchronize the speed of the transmission's input shaft with the road speed of the vehicle.
• Heel-and-toe shifting can be used to match the engine RPM with the road speed when shifting into a lower gear (eg from 3rd to 2nd gear).