It's called bump shift because rather than shifting smoothly down through the gears as you roll to a stop, one of the lower gears engages harshly. Rather than a normal downshift, it almost feels like someone has lightly bumped the back of your car.
The gear ratio
of a gear train, also known as its speed ratio
, is the ratio of the angular velocity of the input gear to the angular velocity of the output gear. The gear ratio can be calculated directly from the numbers of teeth on the gears in the gear train. The torque ratio of the gear train, also known as its mechanical advantage, is determined by the gear ratio. The speed ratio and mechanical advantage are defined so they yield the same number in an ideal linkage.
In a gear train, the input gear
, or drive gear
, transmits power to the output gear
, also known as the driven gear
. The input gear, which is usually connected to a power source, such as a motor or engine, transmits power through any other gears that may be in the gear train to the output gear.
The simplest gear train is a pair of meshing gears in which the input gear drives the output gear. The gear teeth are designed so that the pitch circles of the two gears roll on each other without slipping. The velocity, represented by v
, of the points of contact of the two pitch circles are the same. If the input gear GA
has the radius rA
and angular velocity
, and meshes with output gear GB
of radius rB
and angular velocity
The number of teeth on a gear is proportional to the radius of its pitch circle, which means that the ratios of the gears' angular velocities, radii, and number of teeth are equal. Where NA
is the number of teeth on the input gear and NB
is the number of teeth on the output gear, the following equation is formed:
This shows that a simple gear train with two gears has the gear ratio R
This equation shows that if the number of teeth on the output gear GB
is larger than the number of teeth on the input gear GA
, then the input gear GA
must rotate faster than the output gear GB
The teeth of a gear are distributed on the circumference of the pitch circle so that the thickness of each tooth t
and the space between two teeth are the same. The pitch p
of a gear, which is the distance between the equivalent points on two teeth, is equal to twice the thickness of a tooth,
The pitch of a gear GA
can be computed from the number of teeth NA
and the radius rA
of its pitch circle
In order to mesh smoothly two gears GA
must have the same sized teeth and therefore they must have the same pitch p
, which means
This equation shows that the ratio of the circumference, the diameters and the radii of two meshing gears is equal to the ratio of their number of teeth,
The speed ratio of two gears rolling without slipping on their pitch circles is given by,
In other words, the gear ratio, or speed ratio, is inversely proportional to ratio of the radii of the pitch circles and the number of teeth of the two gears.
A gear train can be analyzed using the principle of virtual work to show that its torque ratio, which is the ratio of its output torque to its input torque, is equal to the gear ratio, or speed ratio, of the gear train.
This means that the input torque TA
applied to the input gear GA
and the output torque TB" on the output gear
GB are related by the ratio
is the gear ratio of the gear train.
The torque ratio of a gear train is also known as its mechanical advantage, thus
In a sequence of gears chained together, the ratio depends only on the number of teeth on the first and last gear. The intermediate gears, regardless of their size, do not alter the overall gear ratio of the chain. However, the addition of each intermediate gear reverses the direction of rotation of the final gear.
An intermediate gear which does not drive a shaft to perform any work is called an idler gear. Sometimes, a single idler gear is used to reverse the direction, in which case it may be referred to as a reverse idler
. For instance, the typical automobile manual transmission engages reverse gear by means of inserting a reverse idler between two gears.
Idler gears can also transmit rotation among distant shafts in situations where it would be impractical to simply make the distant gears larger to bring them together. Not only do larger gears occupy more space, the mass and rotational inertia (moment of inertia) of a gear is proportional to the square of its radius. Instead of idler gears, a toothed belt or chain can be used to transmit torque over distance.
If a simple gear train has three gears, such that the input gear GA
meshes with an intermediate gear GI
which in turn meshes with the output gear GB
, then the pitch circle of the intermediate gear rolls without slipping on both the pitch circles of the input and output gears. This yields the two relations
The speed ratio of this gear train is obtained by multiplying these two equations to obtain
Notice that this gear ratio is exactly the same as for the case when the gears GA
engaged directly. The intermediate gear provides spacing but does not affect the gear ratio. For this reason it is called an idler
gear. The same gear ratio is obtained for a sequence of idler gears and hence an idler gear is used to provide the same direction to rotate the driver and driven gear, if the driver gear moves in clockwise direction, then the driven gear also moves in the clockwise direction with the help of the idler gear.
Assuming that in the photo the smallest gear is connected to the motor, it is the driver gear. The somewhat larger gear on the upper left is called an idler gear. It is not connected directly to either the motor or the output shaft and only transmits power between the input and output gears. There is a third gear in the upper-right corner of the photo. Assuming that that gear is connected to the machine's output shaft, it is the output or driven gear.
The input gear in this gear train has 13 teeth and the idler gear has 21 teeth. Considering only these gears, the gear ratio between the idler and the input gear can be calculated as if the idler gear was the output gear. Therefore, the gear ratio is driven/driver = 21/13 = ~1.62 or 1.62:1.
This ratio means that the driver gear must make 1.62 revolutions to turn the driven gear once. It also means that for every one revolution of the driver, the driven gear has made 1/1.62, or 0.62, revolutions. Essentially, the larger gear turns more slowly.
The third gear in the picture has 42 teeth. The gear ratio between the idler and third gear is thus 42/21, or 2:1, and hence the final gear ratio is 1.62x2=~3.23. For every 3.23 revolutions of the smallest gear, the largest gear turns one revolution, or for every one revolution of the smallest gear, the largest gear turns 0.31 (1/3.23) revolution, a total reduction of about 1:3.23 (Gear Reduction Ratio (GRR) = 1/Gear Ratio (GR)).
Since the idler gear contacts directly both the smaller and the larger gear, it can be removed from the calculation, also giving a ratio of 42/13 = ~3.23.
Belts can have teeth in them also and be coupled to gear-like pulleys. Special gears called sprockets can be coupled together with chains, as on bicycles and some motorcycles. Again, exact accounting of teeth and revolutions can be applied with these machines.
For example, a belt with teeth, called the timing belt, is used in some internal combustion engines to synchronize the movement of the camshaft with that of the crankshaft, so that the valves open and close at the top of each cylinder at exactly the right time relative to the movement of each piston. A chain, called a timing chain, is used on some automobiles for this purpose, while in others, the camshaft and crankshaft are coupled directly together through meshed gears. Regardless of which form of drive is employed, the crankshaft to camshaft gear ratio is always 2:1 on four-stroke engines, which means that for every two revolutions of the crankshaft the camshaft will rotate once.
Automobile drivetrains generally have two or more major areas where gearing is used. Gearing is employed in the transmission, which contains a number of different sets of gears that can be changed to allow a wide range of vehicle speeds, and also in the differential, which contains the final drive to provide further speed reduction at the wheels. In addition, the differential contains further gearing that splits torque equally between the two wheels while permitting them to have different speeds when travelling in a curved path. The transmission and final drive might be separate and connected by a driveshaft, or they might be combined into one unit called a transaxle. The gear ratios in transmission and final drive are important because different gear ratios will change the characteristics of a vehicle's performance.]
A 2004 Chevrolet Corvette C5 Z06 with a six-speed manual transmission has the following gear ratios in the transmission:
In 1st gear, the engine makes 2.97 revolutions for every revolution of the transmission’s output. In 4th gear, the gear ratio of 1:1 means that the engine and the transmission's output rotate at the same speed. 5th and 6th gears are known as overdrive gears, in which the output of the transmission is revolving faster than the engine's output.
The Corvette above has a differential ratio of 3.42:1, meaning that for every 3.42 revolutions of the transmission’s output, the wheels make one revolution. The differential ratio multiplies with the transmission ratio, so in 1st gear, the engine makes 10.16 revolutions for every revolution of the wheels.
The car’s tires can almost be thought of as a third type of gearing. This car is equipped with 295/35-18 tires, which have a circumference of 82.1 inches. This means that for every complete revolution of the wheel, the car travels 82.1 inches (209 cm). If the Corvette had larger tires, it would travel farther with each revolution of the wheel, which would be like a higher gear. If the car had smaller tires, it would be like a lower gear.
With the gear ratios of the transmission and differential, and the size of the tires, it becomes possible to calculate the speed of the car for a particular gear at a particular engine RPM.
For example, it is possible to determine the distance the car will travel for one revolution of the engine by dividing the circumference of the tire by the combined gear ratio of the transmission and differential.
It is also possible to determine a car's speed from the engine speed by multiplying the circumference of the tire by the engine speed and dividing by the combined gear ratio.
A close-ratio transmission is a transmission in which there is a relatively little difference between the gear ratios of the gears. For example, a transmission with an engine shaft to drive shaft ratio of 4:1 in first gear and 2:1 in second gear would be considered wide-ratio when compared to another transmission with a ratio of 4:1 in first and 3:1 in second. This is because the close-ratio transmission has less of a progression between gears. For the wide-ratio transmission, the first gear ratio is 4:1 or 4, and in second gear it is 2:1 or 2, so the progression is equal to 4/2 = 2 (or 200%). For the close-ratio transmission, first gear has a 4:1 ratio or 4, and second gear has a ratio of 3:1 or 3, so the progression between gears is 4/3, or 133%. Since 133% is less than 200%, the transmission with the smaller progression between gears is considered close-ratio. However, the difference between a close-ratio and wide-ratio transmission is subjective and relative.
Close-ratio transmissions are generally offered in sports cars, sport bikes, and especially in race vehicles, where the engine is tuned for maximum power in a narrow range of operating speeds, and the driver or rider can be expected to shift often to keep the engine in its power band.
Factory 4-speed or 5-speed transmission ratios generally have a greater difference between gear ratios and tend to be effective for ordinary driving and moderate performance use. Wider gaps between ratios allow a higher 1st gear ratio for better manners in traffic, but cause engine speed to decrease more when shifting. Narrowing the gaps will increase acceleration at speed, and potentially improve top speed under certain conditions, but acceleration from a stopped position and operation in daily driving will suffer.
is the torque multiplication difference between 1st and 4th gears; wider-ratio gear-sets have more, typically between 2.8 and 3.2. This is the single most important determinant of low-speed acceleration from stopped.
is the reduction or decay in the percentage drop in engine speed in the next gear, for example after shifting from 1st to 2nd gear. Most transmissions have some degree of progression in that the RPM drop on the 1-2 shift is larger than the RPM drop on the 2-3 shift, which is in turn larger than the RPM drop on the 3-4 shift. The progression may not be linear (continuously reduced) or done in proportionate stages for various reasons, including a special need for a gear to reach a specific speed or RPM for passing, racing and so on, or simply economic necessity that the parts were available.
Range and progression are not mutually exclusive, but each limits the number of options for the other. A wide range, which gives a strong torque multiplication in 1st gear for excellent manners in low-speed traffic, especially with a smaller motor, heavy vehicle, or numerically low axle ratio such as 2.50, means that the progression percentages must be high. The amount of engine speed, and therefore power, lost on each up-shift is geater than would be the case in a transmission with less range, but less power in 1st gear. A numerically low 1st gear, such as 2:1, reduces available torque in 1st gear, but allows more choices of progression.
There is no optimal choice of transmission gear ratios or a final drive ratio for best performance at all speeds, as gear ratios are compromises, and not necessarily better than the original ratios for certain purposes.
In modern usage, a torque converter
is generally a type of fluid coupling (but also being able to multiply torque) that is used to transfer rotating power from a prime mover, such as an internal combustion engine or electric motor, to a rotating driven load. The torque converter normally takes the place of a mechanical clutch in a vehicle with an automatic transmission, allowing the load to be separated from the power source. It is usually located between the engine's flexplate and the transmission.
The key characteristic of a torque converter is its ability to multiply torque when there is a substantial difference between input and output rotational speed, thus providing the equivalent of a reduction gear. Some of these devices are also equipped with a temporary locking mechanism which rigidly binds the engine to the transmission when their speeds are nearly equal, to avoid slippage and a resulting loss of efficiency.
By far the most common form of torque converter in automobile transmissions is the device described here. However, in the 1920s there was also the pendulum-based Constantinesco torque converter. There are also mechanical designs for continuously variable transmissions and these also have the ability to multiply torque, e.g. the Variomatic with expanding pulleys and a belt drive.
A fluid coupling is a two element drive that is incapable of multiplying torque, while a torque converter has at least one extra element—the stator—which alters the drive's characteristics during periods of high slippage, producing an increase in output torque.
In a torque converter there are at least three rotating elements: the impeller, which is mechanically driven by the prime mover; the turbine, which drives the load; and the stator, which is interposed between the impeller and turbine so that it can alter oil flow returning from the turbine to the impeller. The classic torque converter design dictates that the stator be prevented from rotating under any condition, hence the term stator
. In practice, however, the stator is mounted on an overrunning clutch, which prevents the stator from counter-rotating with respect to the prime mover but allows forward rotation.
Modifications to the basic three element design have been periodically incorporated, especially in applications where higher than normal torque multiplication is required. Most commonly, these have taken the form of multiple turbines and stators, each set being designed to produce differing amounts of torque multiplication. For example, the Buick Dynaflow automatic transmission was a non-shifting design and, under normal conditions, relied solely upon the converter to multiply torque. The Dynaflow used a five element converter to produce the wide range of torque multiplication needed to propel a heavy vehicle.
Although not strictly a part of classic torque converter design, many automotive converters include a lock-up clutch to improve cruising power transmission efficiency and reduce heat. The application of the clutch locks the turbine to the impeller, causing all power transmission to be mechanical, thus eliminating losses associated with fluid drive.
A torque converter has three stages of operation:
The key to the torque converter's ability to multiply torque lies in the stator. In the classic fluid coupling design, periods of high slippage cause the fluid flow returning from the turbine to the impeller to oppose the direction of impeller rotation, leading to a significant loss of efficiency and the generation of considerable waste heat. Under the same condition in a torque converter, the returning fluid will be redirected by the stator so that it aids the rotation of the impeller, instead of impeding it. The result is that much of the energy in the returning fluid is recovered and added to the energy being applied to the impeller by the prime mover. This action causes a substantial increase in the mass of fluid being directed to the turbine, producing an increase in output torque. Since the returning fluid is initially traveling in a direction opposite to impeller rotation, the stator will likewise attempt to counter-rotate as it forces the fluid to change direction, an effect that is prevented by the one-way stator clutch.
Unlike the radially straight blades used in a plain fluid coupling, a torque converter's turbine and stator use angled and curved blades. The blade shape of the stator is what alters the path of the fluid, forcing it to coincide with the impeller rotation. The matching curve of the turbine blades helps to correctly direct the returning fluid to the stator so the latter can do its job. The shape of the blades is important as minor variations can result in significant changes to the converter's performance.
During the stall and acceleration phases, in which torque multiplication occurs, the stator remains stationary due to the action of its one-way clutch. However, as the torque converter approaches the coupling phase, the energy and volume of the fluid returning from the turbine will gradually decrease, causing pressure on the stator to likewise decrease. Once in the coupling phase, the returning fluid will reverse direction and now rotate in the direction of the impeller and turbine, an effect which will attempt to forward-rotate the stator. At this point, the stator clutch will release and the impeller, turbine and stator will all (more or less) turn as a unit.
Unavoidably, some of the fluid's kinetic energy will be lost due to friction and turbulence, causing the converter to generate waste heat (dissipated in many applications by water cooling). This effect, often referred to as pumping loss, will be most pronounced at or near stall conditions. In modern designs, the blade geometry minimizes oil velocity at low impeller speeds, which allows the turbine to be stalled for long periods with little danger of overheating.
A torque converter cannot achieve 100 percent coupling efficiency. The classic three element torque converter has an efficiency curve that resembles ∩: zero efficiency at stall, generally increasing efficiency during the acceleration phase and low efficiency in the coupling phase. The loss of efficiency as the converter enters the coupling phase is a result of the turbulence and fluid flow interference generated by the stator, and as previously mentioned, is commonly overcome by mounting the stator on a one-way clutch.
Even with the benefit of the one-way stator clutch, a converter cannot achieve the same level of efficiency in the coupling phase as an equivalently sized fluid coupling. Some loss is due to the presence of the stator (even though rotating as part of the assembly), as it always generates some power-absorbing turbulence. Most of the loss, however, is caused by the curved and angled turbine blades, which do not absorb kinetic energy from the fluid mass as well as radially straight blades. Since the turbine blade geometry is a crucial factor in the converter's ability to multiply torque, trade-offs between torque multiplication and coupling efficiency are inevitable. In automotive applications, where steady improvements in fuel economy have been mandated by market forces and government edict, the nearly universal use of a lock-up clutch has helped to eliminate the converter from the efficiency equation during cruising operation.
The maximum amount of torque multiplication produced by a converter is highly dependent on the size and geometry of the turbine and stator blades, and is generated only when the converter is at or near the stall phase of operation. Typical stall torque multiplication ratios range from 1.8:1 to 2.5:1 for most automotive applications (although multi-element designs as used in the Buick Dynaflow and Chevrolet Turboglide could produce more). Specialized converters designed for industrial, rail, or heavy marine power transmission systems are capable of as much as 5.0:1 multiplication. Generally speaking, there is a trade-off between maximum torque multiplication and efficiency—high stall ratio converters tend to be relatively inefficient below the coupling speed, whereas low stall ratio converters tend to provide less possible torque multiplication.
While torque multiplication increases the torque delivered to the turbine output shaft, it also increases the slippage within the converter, raising the temperature of the fluid and reducing overall efficiency. For this reason, the characteristics of the torque converter must be carefully matched to the torque curve of the power source and the intended application. Changing the blade geometry of the stator and/or turbine will change the torque-stall characteristics, as well as the overall efficiency of the unit. For example, drag racing automatic transmissions often use converters modified to produce high stall speeds to improve off-the-line torque, and to get into the power band of the engine more quickly. Highway vehicles generally use lower stall torque converters to limit heat production, and provide a more firm feeling to the vehicle's characteristics.
A design feature once found in some General Motors automatic transmissions was the variable-pitch stator, in which the blades' angle of attack could be varied in response to changes in engine speed and load. The effect of this was to vary the amount of torque multiplication produced by the converter. At the normal angle of attack, the stator caused the converter to produce a moderate amount of multiplication but with a higher level of efficiency. If the driver abruptly opened the throttle, a valve would switch the stator pitch to a different angle of attack, increasing torque multiplication at the expense of efficiency.
Some torque converters use multiple stators and/or multiple turbines to provide a wider range of torque multiplication. Such multiple-element converters are more common in industrial environments than in automotive transmissions, but automotive applications such as Buick's Triple Turbine Dynaflow and Chevrolet's Turboglide also existed. The Buick Dyna flow utilized the torque-multiplying characteristics of its planetary gear set in conjunction with the torque converter for low gear and bypassed the first turbine, using only the second turbine as vehicle speed increased. The unavoidable trade-off with this arrangement was low efficiency and eventually these transmissions were discontinued in favor of the more efficient three speed units with a conventional three element torque converter.
As described above, impelling losses within the torque converter reduce efficiency and generate waste heat. In modern automotive applications, this problem is commonly avoided by use of a lock-up clutch that physically links the impeller and turbine, effectively changing the converter into a purely mechanical coupling. The result is no slippage, and virtually no power loss.
The first automotive application of the lock-up principle was Packard's Ultramatic transmission, introduced in 1949, which locked up the converter at cruising speeds, unlocking when the throttle was floored for quick acceleration or as the vehicle slowed down. This feature was also present in some Borg-Warner transmissions produced during the 1950s. It fell out of favor in subsequent years due to its extra complexity and cost. In the late 1970s lock-up clutches started to reappear in response to demands for improved fuel economy, and are now nearly universal in automotive applications.
As with a basic fluid coupling the theoretical torque capacity of a converter is proportional to
is the mass density of the fluid (kg/m³),
is the impeller speed (rpm), and
is the diameter(m). In practice, the maximum torque capacity is limited by the mechanical characteristics of the materials used in the converter's components, as well as the ability of the converter to dissipate heat (often through water cooling). As an aid to strength, reliability and economy of production, most automotive converter housings are of welded construction. Industrial units are usually assembled with bolted housings, a design feature that eases the process of inspection and repair, but adds to the cost of producing the converter.
In high performance, racing and heavy duty commercial converters, the pump and turbine may be further strengthened by a process called furnace brazing, in which molten brass is drawn into seams and joints to produce a stronger bond between the blades, hubs and annular ring(s). Because the furnace brazing process creates a small radius at the point where a blade meets with a hub or annular ring, a theoretical decrease in turbulence will occur, resulting in a corresponding increase in efficiency.
Overloading a converter can result in several failure modes, some of them potentially dangerous in nature:
An overdrive (OD) is a mechanism that allows an automobile to cruise at sustained speed with reduced engine RPM, leading to better fuel economy, lower noise, and lower wear.
Use of the term is confused, as it is applied to several different, but related, meanings. The most fundamental meaning is that of an overall gear ratio between engine and wheels, such that the car is now over-geared and can no longer reach its potential top speed, i.e. the car could travel faster if it were in a lower gear, with the engine turning more quickly. In some cases this implies a second definition is also true, that the gearbox output driveshaft is rotating faster than the original engine RPM. This later definition was common in the past, but many cars no longer have a driveshaft and it is not technically correct.
In the era of front-engine, rear-wheel drive layouts, the device for achieving an overdrive transmission was usually a small separate gearbox, attached to the rear of the main gearbox and controlled by its own shift lever or electrical actuation button. These were often an optional extra on some models of the same car. As popular cars became faster relative to legal limits and fuel costs became more important, particularly after the 1973 oil crisis, the use of 5-speed gearboxes became more common in mass-market cars, with the 5th gear being an overdrive, eliminating the need for a separate gearbox.
With the popularity of front wheel drive cars, the separate gearbox and final drive have merged into a single transaxle. However the fundamental meaning, that of an overall ratio higher than the ratio for maximum speed, still applies. Although the deliberate labelling of an overdrive is now rare, the underlying feature is now found across all cars.
The power needed to propel a car at any given set of conditions and speed is straightforward to calculate, based primarily on the total weight and the vehicle's speed. These produce two primary forces slowing the car, rolling resistance and air drag. The former varies roughly with the speed of the vehicle, while the latter varies with the square of the speed. Calculating these from first principles is generally difficult due to a variety of real-world factors, so this is often measured directly in wind tunnels and similar systems.
The power produced by an engine increases with the engine's speed to a maximum, then falls away. This is known as the point of maximum power. Given a curve describing the overall drag on the vehicle, it is simple to find the speed at which the total drag forces are the same as the maximum power of the engine. This defines the maximum speed the vehicle is able to reach. The rotational speed of the wheels for that given forward speed is simple to calculate, it is simply the tire diameter times the RPM. As the tire RPM at maximum speed is not the same as the engine RPM at that power, a transmission is used with a gear ratio to convert one to the other.
At even slightly lower speeds than maximum, the total drag on the vehicle is considerably less, and the engine needs to deliver this greatly reduced amount of power. In this case the RPM of the engine has changed significantly while the RPM of the wheels has changed very little. Clearly this condition calls for a different gear ratio. If one is not supplied, the engine is forced to run at a higher RPM than optimal. As the engine requires more power to overcome internal friction at higher RPM, this means more gas is used simply to keep the engine running at this speed. Every cycle of the engine leads to wear, so keeping the engine at higher RPM is also bad for engine life. Additionally, the sound of an engine is strongly related to the RPM, so running at lower RPM is generally quieter.
If one runs the same RPM transmission exercise outlined above for maximum speed, but instead sets the "maximum speed" to that of highway cruising, the output is a higher gear ratio that provides ideal gas milage. In an era when cars were not able to travel very fast, the maximum power point might be near enough to the desired speed that additional gears were not needed. But as more powerful cars appeared, especially during the 1960s, this disparity between the maximum power point and desired speed grew considerably. This meant that cars were often operating far from their most efficient point. As the desire for better fuel economy grew, especially after the 1973 oil crisis, the need for a "cruising gear" became more pressing.
The obvious solution to this problem would be to add more gears to the transmission. Indeed, in modern vehicles this is common. However, due to historical particularities, this was not always practical.
In the conventional rear-wheel drive layout, the transmission system normally contained two sections, the "gearbox" mounted next to the engine, and the "final drive" normally mounted in the transaxle at the rear of the car. The reason for this separation of duties between the front and back of the car was to allow the drive shaft to run at higher RPM. As power is the product of RPM and torque, running the shaft at higher RPM allowed more power to be transferred at lower torque. Doing so reduced the torque the driveshaft had to carry, and thus the strength and weight it required.
Although the designer was theoretically free to choose any ratio for the gearbox and final drive, there is one additional consideration which meant that the top gear of most gearboxes was 1:1 or "direct drive". This is chosen for efficiency, as it does not require any gears to transmit power and so reduces the power lost by them. This was particularly important in the early days of cars, as their straight-cut gears were poorly finished, noisy and inefficient. The final drive then took this output and adjusted it in a fixed-ratio transmission arrangement that was much simpler to build. Final drive ratios of 4:1 were common, meaning that the wheels would turn four times as slow as the engine if they were directly connected.
In an era when different models of car with different wheel sizes could be accommodated by simply changing the final drive ratio, it made sense for all transmissions to use direct drive as the highest gear. As noted earlier, however, this would cause the engine to operate at too high an RPM for efficient cruising. Although adding the cruising gear to the main gearbox was possible, it was generally simpler to add a separate two-gear overdrive system to the existing gearbox. This not only meant that it could be tuned for different vehicles, but had the additional advantage that it could be offered as an option, which was easy to add.
With the use of front-wheel drive layouts, the gearbox and final drive are combined into a single transaxle. There is no longer a drive shaft between them and so the notion of "direct drive" is simply inapplicable. Although "overdrive" is still referred to, this is now mostly a marketing term to refer to any extra-high ratio for efficient cruising, whether it is achieved through the gearbox ratios, or by an unusually high final drive.
Generally speaking, overdrive is the highest gear in the transmission. Overdrive allows the engine to operate at a lower RPM for a given road speed. This allows the vehicle to achieve better fuel efficiency, and often quieter operation on the highway. When it is switched on, an automatic transmission can shift into overdrive mode after a certain speed is reached (usually 70+ km/h [40-45 mph or more] depending on the load). When it is off, the automatic transmission shifting is limited to the lower gears. For an automatic transmission, it is almost always best to select overdrive and allow the transmission to control engagement of the overdrive. (It may be necessary to switch it off if the vehicle is being operated in a mountainous area or when towing a trailer.) With a manual transmission, overdrive should usually be selected when the average speed is above 70 km/h (40-45 mph).
The automatic transmission automatically shifts from OD to direct drive when more load is present. When less load is present, it shifts back to OD. Under certain conditions, for example driving uphill, or towing a trailer, the transmission may "hunt" between OD and the next highest gear, shifting back and forth. In this case, switching it off can help the transmission to "decide". It may also be advantageous to switch it off if engine braking is desired, for example when driving downhill. The vehicle's owner's manual will often contain information and suitable procedures regarding such situations, for each given vehicle.
Virtually all vehicles (cars and trucks) have overdrive today whether manual transmission or automatic. In the automotive aftermarket you can also retrofit overdrive to existing early transmissions. Overdrive was widely used in European automobiles with manual transmission in the 60s and 70s to improve mileage and sport driving as a bolt-on option but it became increasingly more common for later transmissions to have this gear built in. If a vehicle is equipped with a bolt-on overdrive (e.g.: GKN or Gear Vendors) as opposed to a having overdrive built in one will typically have the option to use the overdrive in more gears than just the top gear. In this case gear changing is still possible in all gears, even with overdrive disconnected. Overdrive simply adds effective ranges to the gears, thus overdrive third and fourth become in effect "third-and-a-half" and a fifth gear. In practice this gives the driver more ratios which are closer together providing greater flexibility particularly in performance cars.
The overdrive consists of an electrically or hydraulically operated epicyclic gear train bolted behind the transmission unit. It can either couple the input driveshaft directly to the output shaft (or propeller shaft) (1:1), or increase the output speed so that it turns faster than the input shaft (1:1 + n). Thus the output shaft may be "overdriven" relative to the input shaft. In newer transmissions, the overdrive speed(s) are typically as a result of combinations of planetary/epicyclic gearsets which are integrated in the transmission. In these cases, there is no separately identifiable "overdrive" unit. In older vehicles, it is sometimes actuated by a knob or button, often incorporated into the gearshift knob, and does not require operation of the clutch. Newer vehicles have electronic overdrive in which the computer automatically adjusts to the conditions of power need and load.
The vast majority of overdrives in European cars were manufactured by an English company called Laycock de Normanville (later GKN Laycock), at its Little London Road site in Sheffield, which is now demolished and remanufactured in the UK by an ex-Laycock de Normanville employee trading as Overdrive Spares. The system was devised by a Briton, named Captain Edgar J de Normanville (1884–1968), through a chance meeting with a Laycock Products Engineer. De Normanville overdrives were found in vehicles manufactured by Ford, British Leyland, Jaguar, Rootes Group and Volvo to name a few. Another British company, the former aircraft builder Fairey, built a successful all-mechanical unit for the Land Rover, which is still in production in America today.
The first production vehicle to feature the Laycock system was the 1948 Standard Vanguard Saloon. The first unit to be created was the A-type overdrive, which was fitted to many sports cars during the 1950s, and into the late 1960s. Several famous marques used A-type overdrives, including Jaguar, Aston Martin, Ferrari, Austin-Healey, Jensen, Bristol, AC, Armstrong Siddeley and Triumph's TR sports car range, until the end of TR5 & TR250 production in 1969.
In 1959, the Laycock Engineering Company introduced the D-type overdrive, which was fitted to a variety of motor cars including Volvo 120 and 1800s, Sunbeam Alpines and Rapiers, Triumph Spitfires, and also 1962-1967 MGBs (those with 3-synchro transmissions).
From 1967 the LH-type overdrive was introduced, and this featured in a variety of models, including 1968-1980 MGBs, the MGC, the Ford Zephyr, early Reliant Scimitars, TVRs, and Gilberns.
The J-type overdrive was introduced in the late 1960s, and was adapted to fit Volvo, Triumph, Vauxhall/Opel, American Motors and Chrysler motorcars, and Ford Transit vans.
The P-type overdrive marked the last updates and included both a Gear Vendors U.S. version and a Volvo version. The Volvo version kept the same package size as the J-type but with the updated 18 element freewheel and stronger splines through the planet carrier. The Gear Vendors U.S. version uses a larger 1.375 outer diameter output shaft for higher capacity and a longer rear case.
Over a period of 40 years, Laycock Engineering manufactured over three and a half million overdrive Units, and over one million of these were fitted to Volvo motorcars.
In 2008 the U.S. company Gear Vendors, Inc. of El Cajon, California purchased all the overdrive assets of GKN to continue production of the U.S. version and all spares for J and P types worldwide.
The system features an oil pressure operated device attached to the back of the standard gearbox operating on the gearbox output shaft. Through a system of oil pressure, solenoids and pistons, the overdrive would drop the revs on whatever gears it was used on by 22% (.778). For instance, the overdrive system applied to a Triumph TR5 operates on 2nd, 3rd and top gear. When engaged, the overdrive would drop the revs from 3000 by 666 RPM, or from 3500 the drop would be 777 RPM to 2723 net. The advantages this reduced rpm had on fuel consumption was most often quite near 22% decrease during highway driving.
In the days before automatic transmissions were common, especially in the 1950s, many rear-wheel drive American cars were available with an overdrive option. Borg Warner provided the box that was factory-installed between the transmission and a foreshortened driveshaft. Since the overdrive function, if enabled, could be shifted by simply easing up on the accelerator without depressing the clutch pedal, the action was much like a semi-automatic. Also, an electrically operated solenoid would deactivate the unit via a switch under the accelerator pedal providing the equivalent of the kickdown of the automatic. A knob connected to a bowden cable, similar to some emergency brake applications, was also provided to lock out the unit mechanically. Using overdrive with the main 3-speed transmission in 2nd gear was similar in ratio to 3rd gear, and with the main transmission in 3rd, the overall ratio was fractional (i.e., "true overdrive").
Such add-on overdrive boxes were available from the 1930s to the 1970s for cars and light trucks.
Today, most gas and diesel cars and trucks come with an overdrive transmission because of the benefit to fuel economy.][ Overdrive is included in both automatic and manual transmissions as an extra gear (or two in some cases).
Since 1979, add-on overdrives are provided by Gear Vendors, Inc., El Cajon CA. for most U.S. made vehicles from the 1920s through to current (2010) cars and light trucks.][
When using overdrive gearing, the car's engine speed drops, reducing wear and normally saving fuel. Since 1981 U.S. corporate average fuel economy (CAFE) legislation, virtually all domestic vehicles have included overdrive to save fuel. One should refer to the car's owner's manual for the proper speed to run at overdrive. All engines have a range of peak efficiency and it is possible for the use of overdrive to keep the engine out of this range for all or part of the time of its use if used at inappropriate speeds, thus cutting into any fuel savings from the lower engine speed.
Overall drivetrain reduction comes down to three basic factors: transmission gearing (including overdrive), differential gearing (in the axle), and tire size. The rotation speed problem comes into effect when the differential gearing is a high ratio and an overdrive is used to compensate. This may create unpleasant vibrations at high speeds and possible destruction of the driveshaft due to the centripetal forces or uneven balance.
The driveshaft is usually a hollow metal tube that requires balancing to reduce vibration and contains no internal bracing.
The higher speeds on the driveshaft and related parts can cause heat and wear problems if an overdrive and high differential gearing (or even very small tires) are combined, and create unnecessary friction. This is especially important because the differential gears are bathed in heavy oil and seldom provided with any cooling besides air blowing over the housing.
The impetus is to minimize overdrive use and provide a higher ratio first gear, which means more gears between the first and the last to keep the engine at its most efficient speed. This is part of the reason that modern automobiles tend to have larger numbers of gears in their transmissions. It is also why more than one overdrive gear is seldom seen in a vehicle except in special circumstances i.e. where high (numerical) differential gear is required to get the vehicle moving as in trucks or performance cars though double overdrive transmissions are common in other vehicles, often with a small number on the axle gear reduction, but usually only engage at speeds exceeding 100 kilometres per hour (60 mph).
A gear stick is the device is used to change gear; in a manual transmission vehicle this will normally be done whilst depressing the clutch pedal with the left foot to disengage the engine from the drivetrain and wheels. Automatic transmission vehicles, robotised manuals, and those with continuously variable transmission gearboxes, do not require a clutch pedal.
Gear sticks are most commonly found between the front seats of the vehicle, either on the center console (sometimes even quite far up on the dashboard), the transmission tunnel, or directly on the floor. Some vehicles have a column shift where the lever is mounted on the steering column—this arrangement was once almost standard practice in the United States until relatively recently, which had the added benefit of allowing for a full width bench-type front seat (though some models with bucket seating as an option include it). It has since fallen out of favor, although it can still be found widely on US-market pick-up trucks, vans, emergency vehicles (both law enforcement and EMS - the column shifter is retained where a floor shifter is unfeasible due to mounting the mobile data terminal and 2-way radio), and "full-size" US sedans such as the Ford Crown Victoria. A dashboard mounted shift was common on certain French models such as the Citroen 2CV and Renault 4. Both the Bentley Mark VI and the Riley Pathfinder had their gear lever to the right of the right-hand drive driver's seat, alongside the driver's door, where it was not unknown for British cars to also have their handbrake. (Left-hand drive models received a column shift.)
In some modern sports cars, the gear lever has been replaced entirely by "paddles", which are a pair of levers, usually operating electrical switches (rather than a mechanical connection to the gearbox), mounted on either side of the steering column, where one increments the gears up, and the other down. Formula 1 cars used to hide the gear stick behind the steering wheel within the nose bodywork before the modern practice of mounting the "paddles" on the (removable) steering wheel itself.
A knob, variously called gear knob, shift knob, gear shift knob, or stick shift knob forms the handle for the gear stick. Typically the gear knob includes a diagram of the shift pattern of the gear selection system; i.e. the positions to which the gear stick should be moved when selecting a gear. In some older manual transmission vehicles it may incorporate a switch to engage an overdrive; in some automatic transmission vehicles it may incorporate a switch to engage a special mode such as a sports mode. Both of the above-mentioned switches may also be found on the console or on steering column stalks instead. Manual shifters on the steering column, if having only three forward speeds, are typically called a "three on the tree." The lowest of these gears, if set at a much lower ratio than standard 1st-gear ratio, is often called a "granny gear."
Many automatic transmission vehicles have extra controls on the gear stick, or very close by, which modify the choices made by the transmission system depending on engine and road speed; for example, "sports" or "economy" modes which will broadly speaking allow, respectively, for higher and lower revolutions per gear, before changing up.
Some specialist vehicles have controls for other functions on the gear stick. The Land Rover Freelander introduced a button for that company's Hill Descent Control system feature, which uses the brakes to simulate the function of a low-ratio gearbox in steep descents.
In some traditional four-wheel drive "off-road" vehicles there can be a second gear lever which engages a low-ratio gearbox, used on tough terrain. Further, similar-looking levers may switch between two- and four-wheel drive, or engage differential locks; these are not "gear levers," however.
The shift pattern refers to the layout of the gears. In a typical manual transmission car, first gear is located to the left, and forwards. In many trucks and some sports cars it is instead in a "dog leg" position, to the left and rearwards. There is usually a spring-loading to return the stick to the central position. Reverse gear is commonly positioned in the best choice of location to avoid accidental engagement.
A typical manual transmission vehicle, with (for example) five forward gears, will thus have seven possible positions: the five forwards gears, reverse gear, and a central "neutral" position. Some vehicles have a special button to prevent accidental engagement of reverse. Others require that the lever be lifted, pressed down, or moved with extra force to engage reverse. In transmissions with reverse directly below fifth, there may be a mechanical lock-out preventing selection of reverse other than from neutral, thus preventing a driver used to a six-speed transmission from engaging reverse while trying to select sixth.
This shift pattern is another five-speed shift pattern which can be found in Saabs, BMWs, some Audis, Eagle, Volvos, Volkswagens, Škodas, Opels, Hyundais, most Renaults, some diesel Fords, and more.
This gear pattern can also be found on some heavy vehicles in which first gear is an extra-low ratio for use in extreme standing-start conditions, and would see little use in normal driving.
Automatic transmissions traditionally have had a straight pattern beginning at the most forward position with "park" (which locks up the transmission), and running through reverse, neutral, drive (all gears available), and then the lower gears (often three or two more positions, each locking out a successive upper gear), with the rear-most position allowing first gear only. More modern automatic transmissions have employed a "J-gate" (pioneered by Jaguar) where some gears are on the left-hand "arm", some on the right, and there is a sideways movement at the rear of the pattern.
Some modern gearboxes such as the Alfa Romeo Sportronic have a traditional automatic shift pattern to the right, with a special position to the left in which movement of the stick forwards and backwards increments the gears up and down respectively. This can be useful in snow or dirt conditions, where it may be necessary to start from second gear.
A shift knob also known as a gear knob, gear shift knob and stick shift knob is the physical interface between the manual transmission stick shift and the drivers hand. Made of many materials from simple plastics through to platinum it comes in many shapes sizes and weights. Generally spherical in shape the OEM versions tend towards the conservative and the automotive aftermarket versions can be found to be of very original design.
The shift knob's principal function is the ergonomical interface between driver and the manual transmission stick shift. The stick shift, as the name implies, is often just a machined or cast aluminium or steel rod with or without threading the shift knob is fitted on its end.
In recent years, manufacturers have increased the variety of shifts knobs available to the consumer from inexpensive plastics to diamond studded white gold.
'Big Daddy' Roth 'bloodshot eyeball' shift knob, a 1960s craze.
Stylized shift knobs can be found on many types of vehicles. This aluminum, hand made, skull shift knob is mounted on a vintage Harley Davidson Knucklehead's "jockey shifter".
A weighted shift knob is a performance driven aftermarket modification which entirely replaces the OEM shift knob and sits atop the aftermarket short shifter or OEM manual shift stick. Initially designed to be used in tandem with a short shifter, it is increasingly being purchased for stock stick shifts. The weight generally varies between 400 and 600 grams or more, depending on the material used. The principle of the weighted shift knob is to make the stick shifter top heavy, thus increasing the available leverage in order to decrease time between shifts. Weighted gear knobs are offered for sale by a variety of North American based manufacturers in many shapes and finishes, though they are significantly more expensive than the commonly available aftermarket shift knob.
A speed bump, speed hump or ramp is a traffic calming feature of road design used to slow traffic, or reduce through traffic, via vertical deflection. They are also known as a sleeping policeman in British English and Caribbean English and a judder bar in New Zealand English. A speed bump is a bump in a roadway with heights typically ranging between 3 and 4 inches (7.6 and 10 cm). The depth of speed bumps is typically less than or near to 1 foot (30 cm); contrasting with the wider speed humps which are typically 10 to 14 feet (3.0 to 4.3 m) in depth.
Speed bumps can be made of recycled plastic, metal, asphalt or rubber. Speed bumps of various sizes can be placed on a road, from using two four foot or six foot devices on it with a space on either side for drainage, and not designed for avoiding the bump on one side of the car. It may also be connected across the entire road surface.
The use of speed bumps is widespread around the world, and they are most commonly found where vehicle speeds are statutorily mandated to be low, usually 40 km/h (25 mph), or 8 to 16 km/h (5 to 10 mph) in car parks.][ Although speed bumps are very effective in keeping vehicle speed down, their use is sometimes controversial as they can cause noise and possibly vehicle damage if taken at too great a speed.][ Poorly designed speed bumps often found in private car parks (too tall, too sharp an angle for the expected speed) can be hard to negotiate in vehicles with low ground clearance, such as sports cars, even at very slow speeds.][ Speed bumps can also pose serious hazards to motorcyclists and bicyclists if not easily noticed, though in some cases a small cut in the bump allows those vehicles to pass through without impedance. Speed bumps cost between $50–200 though they do have to be replaced after wear.
On June 7, 1906, The New York Times reported on an early implementation of what might be considered speed bumps in the U.S. town of Chatham, New Jersey, which planned to raise its crosswalks five inches above the road level: "This scheme of stopping automobile speeding has been discussed by different municipalities, but Chatham is the first place to put it in practice". The average automobile's top speed at the time was around 30 mph.
Arthur Holly Compton was a physicist and winner of the Nobel Prize in physics in 1927 for his discoveries resulting in major changes in electromagnetic theory. He is commonly known for his work on the Compton Effect with X-rays. He also invented what he called "traffic control bumps," the basic design for the speed hump, in 1953. Compton began designs on the speed bump after noticing the speed at which motorists passed Brookings Hall at Washington University in St. Louis, Missouri, where he was chancellor.
The British Transport and Road Research Laboratory published a comprehensive report in 1973 examining vehicle behavior for a large variety of different bump geometries. At the time speed humps were not permitted on public roads but had been installed on private roads.
According to a publication by the Institute of Transportation Engineers, the first speed bump in Europe was built in 1970 in the city of Delft in the Netherlands.
In the United Kingdom, vertical deflection in highways for the purpose of traffic calming typically takes one of the following forms:
The Department for Transport defines the regulations for the design and use of road humps.
After complaints from Derby residents, 146 speed bumps were removed from streets at a cost of £460,000. Similar incidents have been reported elsewhere in the UK.
UK news sources reported a cyclist being killed in a crash while attempting to avoid a speed bump.
Speed bumps in some areas have been removed after protests by local residents. Such protests cite the lack of any consultation as one factor.
Local authorities have cited disadvantages to speed bumps:
Other sources argue that speed bumps:
In 2003, the chairman of the London Ambulance Service, Sigurd Reinton was reported as claiming that delay due to speed bumps was responsible for up to 500 avoidable deaths from cardiac arrest each year. He later denied the statement.
In Sweden, an evaluation of spinal stress in bus drivers against ISO 2631-5 required on health grounds:
A potential may exist for liability or at least a law suit when a driver damages his car by going too fast over speed bumps.
Dynamic speed bumps differ from conventional speed bumps in that they only activate if a vehicle is traveling above a certain speed. Vehicles traveling below this speed will not experience the discomfort of a conventional speed bump. Dynamic speed bumps may allow the passage of emergency vehicles at higher speeds.
In one design, a rubber housing is fitted with a pressure valve which determines the speed of a vehicle. If the vehicle is traveling below the set speed the valve opens allowing the bump to deflate as the vehicle drives over it, but remains closed if the vehicle is traveling too fast. The valve can also be set to allow heavy vehicles, such as fire trucks, ambulances, and buses to cross at higher speeds.
A double clutch (also called a double declutch) is a method of shifting gears primarily used for vehicles with an unsynchronized manual transmission, such as commercial trucks and specialty vehicles. A double clutch is not necessary in a vehicle that has a synchronized manual transmission. Opposed to operating a single clutch by holding the clutch in once and shifting directly to another gear, a double clutch is operated by first shifting the transmission into neutral before shifting to the next desired gear. The clutch is held with each change.
The double clutching technique involves the following steps:
In a gearbox with neutral between each gear, a typical shift actually involves two gear changes, once into neutral, and again into the next gear. During any shift, disconnecting drive components via a clutch properly unloads the engine and transmission of undue pressure applied by the opposing components. Fully utilizing the clutch for each shift out of, and then into each gear is double (de)clutching. Due to the absence of a neutral spacing, double clutching is ill-advised for sequential gear changes, as in a fully sequential gearbox such as a typical motorcycle.
Before the introduction of transmission synchronizers (in the 1920s), double clutching was a technique required to prevent damage to an automobile's gear system (except for cars like the Model T with a planetary gearbox). Due to the difficulty and most often unnecessary redundancy involved in learning the technique, coupled with the advent of synchronized gearing systems, it has largely fallen into disuse. However, drivers of large trucks often use the double clutching technique when unable to keep the transmission unloaded during shifting, as large vehicles are usually equipped with older, simpler and more durable unsynchronized gearboxes.
The purpose of the double-clutch technique is to aid in matching the rotational speed of the input shaft being driven by the engine to the rotational speed of the gear the driver wishes to select (directly connected to rotating wheels). When the speeds are matched, the gear will engage smoothly and no clutch is required. If the speeds are not matched, the dog teeth on the collar will "clash" or grate as they attempt to fit into the holes on the desired gear. A modern synchromesh gearbox accomplishes this synchronization more efficiently. However, when the engine speed is significantly different from the transmission speed, the desired gear can often not be engaged even in a fully synchronized gearbox. An example is trying to shift into a gear while travelling outside the gear's speed or directional range, such as accidentally into 1st from near the top of 2nd, or intentionally from reverse to a forward gear whilst still moving at speed.
Double clutching, although time consuming, eases gear selection when an extended delay or variance exists between engine and transmission speeds.
Although double clutching is a testing requirement when obtaining a commercial driver's license, most experienced truckers learn to shift gears without using the clutch. This is known as float gears, which thus eliminates the clutch except during starting and stopping. Skip shifting is when a gear is left out, usually on an upshift, for example shifting 2-4-6 while accelerating with the help of gravity down a hill. This technique saves unnecessary shifting work and saves fuel.
Conversely, in order to shift down, engine RPM must be increased while the gearbox is in neutral and the clutch is engaged. This requires the driver to slow the vehicle sufficiently, shift into neutral, apply throttle to bring the RPM up to a suitable speed, and finally shift into gear. This operation can be very difficult to master, as it requires the driver to gauge the speed of the vehicle and throttle to the intended gear accurately; vehicle weight and road gradient are important factors as they influence the vehicle's acceleration or deceleration during the shift. Double clutching is when the clutch pedal is depressed while shifting to neutral to match engine speed to the intended gear and vehicle speed, and again depressed for shifting into gear.
Sometimes, truck drivers use the engine brake to help match the engine speed to the gear. The most common situation is with a loaded vehicle which has no split gears or half gears in the lower range, from gears 1-4. In this case, it is especially difficult and sometimes impossible to get from 1 to 2, and sometimes even from 2-3 while starting on a hill. The problem is that by the time the engine speed has dropped sufficiently to enable a shift into the higher gear, the vehicle will have slowed down too much or possibly even stopped, making the shift impossible. The engine brakes, which on some models can be set to different intensities (retarding variable numbers of engine cylinders) enable a shift by dropping the engine speed quickly enough to catch the higher gear before the vehicle has decelerated too much. This technique, sometimes called "jake shifting", requires high skill and much practice shifting without the clutch, and is usually not recommended among truck drivers because mistakes can cause damage to the transmission.
A related downshifting technique is called heel-and-toe, in which the brake and accelerator pedals are pressed simultaneously. Classically, the brake is pressed with the ball of the right foot and the accelerator pedal is controlled by the right heel, while the clutch pedal is pressed by the left foot. However, variants are possible, with the brake and accelerator pressed by sides of the right foot.
Proper heel-and-toe technique aids both slowing the vehicle while at the same time accelerating the engine for a matched downshift. Note that neglecting to rev-match in any downshifting scenario can be extremely dangerous, especially in low-traction conditions. Heel-and-Toe may be used with any type of gearbox when simultaneous braking and downshifting is necessary. Though difficult, mastering the heel-and-toe technique in conjunction with necessary clutching is essential for high performance driving (e.g., rally racing) to stay in the optimal gear regardless of the simultaneous braking, accelerating, and clutching required for shifts. This allows the engine to stay in the RPM "powerband" and allows one to drive as fast as possible. Left foot braking while accelerating the engine with the right foot to accommodate downshifting in a clutchless situation accomplishes the same feat.
The purpose of the heel-toe-double-clutch is to downshift into the correct gear, and thus optimal engine RPM, for exiting the corner while placing the least wear and tear on the entire drivetrain. Note that racers will sometimes skip gears during downshifts depending on the vehicle speed. With double-clutching there is no need to shift through every gear when significant velocity has been lost.
Electrohydraulic manual transmission is a type of semi-automatic transmission system, which uses an automated clutch unlike conventional manual transmissions where the driver operates the clutch. The clutch is controlled by electronic computers and hydraulics. To change gears, the driver selects the desired gear with the transmission shift lever, and the system automatically operates the clutch and throttle to match revs and engage the clutch again. Also, many such transmissions operate in sequential mode where the driver can only upshift or downshift by one gear at a time.
Depending on the implementation, some computer-controlled electrohydraulic manual transmissions will automatically shift gears at the right points (like an automatic transmission), while others require the driver to manually select the gear even when the engine is at the redline. Despite superficial similarity, clutchless manual transmission differ significantly in internal operation and driver's 'feel' from manumatics, the latter of which is an automatic transmission (automatics use a torque converter instead of clutch to manage the link between the engine and the transmission) with ability to signal shifts manually.
In 1984, Isuzu introduced the "NAVi5", a semi-automatic gearbox with electronically controlled hydraulics, for their domestic-market Aska. Initially available with an automatic mode only, the later incarnations added a manual mode. It was operated with an H-pattern shift lever, not a sequential lever or paddles popular in today’s cars.
The most famous application of a sequential transmission on road-cars would be their use in some Ferraris since the late-nineties, beginning with the F355 F1. Their system, the most current version of which is called "F1-Superfast," with shift times of 60 ms is designed to serve as a link to their Formula One 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 in 1999 was the first sequential transmission in a mainstream car, derived from the Ferrari system.][
BMW offered a system simply called "sequential manual gearbox" (SMG) on the E36 M3, and later "SMG-II" on the E46 M3. The BMW SMG transmission has both automatic and manual shift modes. Inside the different modes there are different programmes, with six settings to control the upshift/downshift speed for manual operation, and five settings for automatic mode.
Later, the 3rd generation Toyota MR2 used Toyota's version, known as the "Sequential Manual Transmission" (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.][
Finally, Volkswagen Group (parent owner of Lamborghini) introduced a sequential transmission to the Lamborghini Gallardo (E gear), and then adding it to the Audi R8 (R tronic).
BMW has since switched over to a Getrag dual clutch transmission in the latest M3, and Ferrari as well in 2009 with the California and 458 Italia.
Not to be confused with Downshifting, the social trend.
Downshift is the name of three fictional characters in the Transformers universes. Bump
A gear or cogwheel is a rotating machine part having cut teeth, or cogs, which mesh with another toothed part in order to transmit torque, in most cases with teeth on the one gear of identical shape, and often also with that shape (or at least width) on the other gear. Two or more gears working in tandem are called a transmission and can produce a mechanical advantage through a gear ratio and thus may be considered a simple machine. Geared devices can change the speed, torque, and direction of a power source. The most common situation is for a gear to mesh with another gear; however, a gear can also mesh with a non-rotating toothed part, called a rack, thereby producing translation instead of rotation.
The gears in a transmission are analogous to the wheels in a crossed belt pulley system. An advantage of gears is that the teeth of a gear prevent slipping. Physics
Mechanical engineering is a discipline of engineering that applies the principles of engineering, physics and materials science for analysis, design, manufacturing, and maintenance of mechanical systems. It is the branch of engineering that involves the production and usage of heat and mechanical power for the design, production, and operation of machines and tools. It is one of the oldest and broadest engineering disciplines.
The engineering field requires an understanding of core concepts including mechanics, kinematics, thermodynamics, materials science, structural analysis, and electricity. Mechanical engineers use these core principles along with tools like computer-aided engineering, and product lifecycle management to design and analyze manufacturing plants, industrial equipment and machinery, heating and cooling systems, transport systems, aircraft, watercraft, robotics, medical devices, weapons, and others. Mechanics