Manual Transmissions
How Manual Transmissions Work
Source: http://auto.howstuffworks.com/transmission.htm
(viewed 2008-04-06)
If you drive a stick-shift car, then you may have several questions floating
in your head:

How does the funny "H" pattern that I am moving this shift knob
through have any relation to the gears inside the transmission? What
is moving inside the transmission when I move the shifter?

When I mess up and hear that horrible grinding sound, what is
actually grinding?

What would happen if I were to accidentally shift into reverse while I
am speeding down the freeway? Would the entire transmission
explode?
In this article, we'll answer all of these questions and more as we explore
the interior of a manual transmission.
Photo courtesy DaimlerChrysler
Mercedes-Benz C-class, 6-speed
manual transmission
Cars need transmissions because of the physics of the gasoline engine. First, any engine has a redline -- a
maximum rpm value above which the engine cannot go without exploding. Second, if you have read How
Horsepower Works, then you know that engines have narrow rpm ranges where horsepower and torque are
at their maximum. For example, an engine might produce its maximum horsepower at 5,500 rpm. The
transmission allows the gear ratio between the engine and the drive wheels to change as the car speeds up
and slows down. You shift gears so the engine can stay below the redline and near the rpm band of its best
performance.
Photo courtesy DaimlerChrysler
Mercedes-Benz Actros, manual transmission
Ideally, the transmission would be so flexible in its ratios that the engine could always run at its single, bestperformance rpm value. That is the idea behind the continuously variable transmission (CVT).
A CVT has a nearly infinite range of gear ratios. In the past, CVTs could not compete with four-speed and
five-speed transmissions in terms of cost, size and reliability, so you didn't see them in production
automobiles. These days, improvements in design have made CVTs more common. The Toyota Prius is a
hybrid car that uses a CVT.
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The transmission is connected to the engine through the clutch. The input shaft of the transmission therefore
turns at the same rpm as the engine.
Photo courtesy DaimlerChrysler
Mercedes-Benz C-class sport coupe, six-speed manual
transmission, graphic illustration
A five-speed transmission applies one of five different gear ratios to the input shaft to produce a different rpm
value at the output shaft. Here are some typical gear ratios:
Gear
Ratio
RPM at Transmission Output Shaft
with Engine at 3,000 rpm
1st
2.315:1
1,295
2nd
1.568:1
1,913
3rd
1.195:1
2,510
4th
1.000:1
3,000
5th
0.915:1
3,278
A Very Simple Transmission
To understand the basic idea behind a standard transmission, the diagram below shows a very simple twospeed transmission in neutral:
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Let's look at each of the parts in this diagram to understand how they fit together:

The green shaft comes from the engine through the clutch. The green shaft and green gear are
connected as a single unit. (The clutch is a device that lets you connect and disconnect the engine and
the transmission. When you push in the clutch pedal, the engine and the transmission are disconnected
so the engine can run even if the car is standing still. When you release the clutch pedal, the engine and
the green shaft are directly connected to one another. The green shaft and gear turn at the same rpm
as the engine.)

The red shaft and gears are called the layshaft. These are also connected as a single piece, so all of
the gears on the layshaft and the layshaft itself spin as one unit. The green shaft and the red shaft are
directly connected through their meshed gears so that if the green shaft is spinning, so is the red shaft.
In this way, the layshaft receives its power directly from the engine whenever the clutch is engaged.

The yellow shaft is a splined shaft that connects directly to the drive shaft through the differential to the
drive wheels of the car. If the wheels are spinning, the yellow shaft is spinning.

The blue gears ride on bearings, so they spin on the yellow shaft. If the engine is off but the car is
coasting, the yellow shaft can turn inside the blue gears while the blue gears and the layshaft are
motionless.

The purpose of the collar is to connect one of the two blue gears to the yellow drive shaft. The collar is
connected, through the splines, directly to the yellow shaft and spins with the yellow shaft. However, the
collar can slide left or right along the yellow shaft to engage either of the blue gears. Teeth on the collar,
called dog teeth, fit into holes on the sides of the blue gears to engage them.
Now, let's see what happens when you shift into first gear.
First Gear
The picture below shows how, when shifted into first gear, the collar engages the blue gear on the right:
In this picture, the green shaft from the engine turns the layshaft, which turns the blue gear on the right. This
gear transmits its energy through the collar to drive the yellow drive shaft. Meanwhile, the blue gear on the
left is turning, but it is freewheeling on its bearing so it has no effect on the yellow shaft.
When the collar is between the two gears (as shown in the first figure), the transmission is in neutral. Both of
the blue gears freewheel on the yellow shaft at the different rates controlled by their ratios to the layshaft.
From this discussion, you can answer several questions:
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Manual Transmissions

When you make a mistake while shifting and hear a horrible grinding sound, you are not hearing the
sound of gear teeth mis-meshing. As you can see in these diagrams, all gear teeth are all fully meshed
at all times. The grinding is the sound of the dog teeth trying unsuccessfully to engage the holes in the
side of a blue gear.

The transmission shown here does not have "synchros" (discussed later in the article), so if you were
using this transmission you would have to double-clutch it. Double-clutching was common in older cars
and is still common in some modern race cars. In double-clutching, you first push the clutch pedal in
once to disengage the engine from the transmission. This takes the pressure off the dog teeth so you
can move the collar into neutral. Then you release the clutch pedal and rev the engine to the "right
speed." The right speed is the rpm value at which the engine should be running in the next gear. The
idea is to get the blue gear of the next gear and the collar rotating at the same speed so that the dog
teeth can engage. Then you push the clutch pedal in again and lock the collar into the new gear. At
every gear change you have to press and release the clutch twice, hence the name "double-clutching."

You can also see how a small linear motion in the gear shift knob allows you to change gears. The gear
shift knob moves a rod connected to the fork. The fork slides the collar on the yellow shaft to engage
one of two gears.
A Real Transmission
The five-speed manual transmission is fairly standard on cars today. Internally, it looks something like this:
There are three forks controlled by three rods that are engaged by the shift lever. Looking at the shift rods
from the top, they look like this in reverse, first and second gear:
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Keep in mind that the shift lever has a rotation point in the middle. When you push the knob forward to
engage first gear, you are actually pulling the rod and fork for first gear back.
You can see that as you move the shifter left and right you are engaging different forks (and therefore
different collars). Moving the knob forward and backward moves the collar to engage one of the gears.
Reverse gear is handled by a small idler gear (purple). At all times, the blue reverse gear in this diagram is
turning in a direction opposite to all of the other blue gears. Therefore, it would be impossible to throw the
transmission into reverse while the car is moving forward -- the dog teeth would never engage. However,
they will make a lot of noise!
Synchronizers
Manual transmissions in modern passenger cars use synchronizers to eliminate the need for doubleclutching. A synchro's purpose is to allow the collar and the gear to make frictional contact before the dog
teeth make contact. This lets the collar and the gear synchronize their speeds before the teeth need to
engage, like this:
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The cone on the blue gear fits into the cone-shaped area in the collar, and friction between the cone and the
collar synchronize the collar and the gear. The outer portion of the collar then slides so that the dog teeth
can engage the gear.
Every manufacturer implements transmissions and synchros in different ways, but this is the general idea.
www.howstuffworks.com
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Manual Transmissions
How Manual Transmissions Work
Source: http://en.wikipedia.org/wiki/Manual_transmission (viewed 2008-04-05)
A manual transmission (also known as a stick shift, straight drive, 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. 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 racing 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 come in two basic types: simple unsynchronized systems, where gears are spinning
freely and their relative speeds must be synchronized by the operator to avoid noisy and damaging
"clashing" and "grinding" when trying to mesh the rotating teeth; and synchronized systems, which eliminate
this necessity while changing gears.
Synchronized transmission
Modern gearboxes are constant mesh, i.e. all gears are always in mesh. Only one of these meshed pairs of
gears is locked to the shaft on which it is mounted at any one time, while the others are allowed to rotate
freely. Thus, it greatly reduces the skill required to shift gears.
Most modern cars are fitted with a synchronized gear box, although it is entirely possible to construct a
constant mesh gearbox without a synchromesh, as found in a motorcycle, for example. In a constant mesh
gearbox, the transmission gears are always in mesh and rotating, but the gears are not rigidly connected to
the shafts on which they rotate. Instead, the gears can freely rotate or be locked to the shaft on which they
are carried. The locking mechanism for any individual gear consists of a collar (or "dog collar") on the shaft
which is able to slide sideways so that teeth (or "dogs") on its inner surface bridge two circular rings with
teeth on their outer circumference: one attached to the gear, one to the shaft. (One collar typically serves for
two gears; sliding in one direction selects one transmission speed, in the other direction selects the other.)
When the rings are bridged by the collar, that particular gear is rotationally locked to the shaft and
determines the output speed of the transmission. In a synchromesh gearbox, to correctly match the speed of
the gear to that of the shaft as the gear is engaged, the collar initially applies a force to a cone-shaped brass
clutch which is attached to the gear, which brings the speeds to match prior to the collar locking into place.
The collar is prevented from bridging the locking rings when the speeds are mismatched by synchro rings
(also called blocker rings or balk rings, the latter being spelled "baulk" in the UK). The gearshift lever
manipulates the collars using a set of linkages, so arranged so that one collar may be permitted to lock only
one gear at any one time; when "shifting gears", the locking collar from one gear is disengaged and that of
another engaged. In a modern gearbox, the action of all of these components is so smooth and fast it is
hardly noticed.
Shafts
Like other transmissions, a manual transmission has several shafts with various gears and other
components attached to them. Typically, a rear-wheel-drive transmission has three shafts: an input shaft, a
countershaft and an output shaft. The countershaft is sometimes called a layshaft.
In a rear-wheel-drive transmission, the input and output shaft lie along the same line, and may in fact be
combined into a single shaft within the transmission. This single shaft is called a mainshaft. The input and
output ends of this combined shaft rotate independently, at different speeds, which is possible because one
piece slides into a hollow bore in the other piece, where it is supported by a bearing. Sometimes the term
mainshaft refers to just the input shaft or just the output shaft, rather than the entire assembly.
In some transmissions, it's possible for the input and output components of the mainshaft to be locked
together to create a 1:1 gear ratio, causing the power flow to bypass the countershaft. The mainshaft then
behaves like a single, solid shaft, a situation referred to as direct drive.
Even in transmissions that do not feature direct drive, it's an advantage for the input and output to lie along
the same line, because this reduces the amount of torsion that the transmission case has to bear.
Dog clutch
The gear selector does not engage or disengage the actual gear teeth which are permanently meshed.
Rather, the action of the gear selector is to lock one of the freely spinning gears to the shaft that runs
through its hub. The shaft then spins together with that gear. The output shaft's speed relative to the
countershaft is determined by the ratio of the two gears: the one permanently attached to the countershaft,
and that gear's mate which is now locked to the output shaft.
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Locking the output shaft with a gear is achieved by means of a dog clutch selector. The dog clutch is a
sliding selector mechanism which is splined to the output shaft, meaning that its hub has teeth that fit into
slots (splines) on the shaft, forcing it to rotate with that shaft. However, the splines allow the selector to move
back and forth on the shaft, which happens when it is pushed 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.
Synchromesh
If the teeth, the so-called dog teeth, make contact with the gear, but the two parts are spinning at different
speeds, the teeth will fail to engage and a loud grinding sound will be heard as they clatter together. For this
reason, a modern dog clutch in an automobile has a synchronizer mechanism or synchromesh. Thanks to
this mechanism, before the teeth can engage, a frictional contact is made between "synchro cones" which
brings the selector and gear to two parts to rotate at the same speed. Moreover, until synchronization occurs,
the teeth are prevented from making contact, because further motion of the selector is prevented by a
blocker (or "baulk") ring. When synchronization occurs, friction on the blocker ring is relieved and it twists
slightly, bringing into alignment certain grooves and notches that allow further passage of the selector which
brings the teeth together. Of course, the exact design of the synchronizer varies from manufacturer to
manufacturer.
The synchronizer has to change the momentum of the entire input shaft and clutch disk. Additionally, it can
be abused by exposure to the momentum and power of the engine itself, which is what happens when
attempts are made to select a gear without fully disengaging the clutch. This causes extra wear on the rings
and sleeves, reducing their service life. When an experimenting driver tries to "match the revs" on a
synchronized transmission and force it into gear without using the clutch, it is actually the synchronizer that
makes up for any discrepancy in RPM, deceiving the driver into an exaggerated sense of how much human
skill was involved.
Reverse
The previous discussion applies to the forward gears. The implementation of the reverse gear is usually
different, implemented in the following way to reduce the cost of the transmission. Reverse is also a pair of
gears: one gear on the countershaft and one on the output shaft. However, whereas all the forward gears
are always meshed together, there is a gap between the reverse gears. Moreover, they are both attached to
their shafts: neither one rotates freely about the shaft. What happens when reverse is selected is that a small
gear, called an idler gear or reverse idler, is slid between them. The idler has teeth which mesh with both
gears, and thus it couples these gears together and reverses the direction of rotation without changing the
gear ratio.
Thus, in other words, when reverse gear is selected, in fact it is actual gear teeth that are being meshed, with
no aid from a synchronization mechanism. For this reason, the output shaft must not be rotating when
reverse is selected: the car must be stopped. In order that reverse can be selected without grinding even if
the input shaft is spinning inertially, there may be a mechanism to stop the input shaft from spinning. The
driver brings the vehicle to a stop, and selects reverse. As that selection is made, some mechanism in the
transmission stops the input shaft. Both gears are stopped and the idler can be inserted between them.
Gear Variety
Manual transmissions are often equipped with 4, 5, or 6 forward gears. Nearly all have one reverse gear. In
three or four speed transmissions, in most cases, the topmost gear is "direct", i.e. a 1:1 ratio. For five speed
or higher transmissions, the highest gear is usually an overdrive gear, with a ratio of less than 1:1. Older cars
were generally equipped with 3-speed transmissions, or 4-speed transmissions for high performance models
and 5-speeds for the most sophisticated of automobiles; in the 1970s, 5-speed transmissions began to
appear in low priced 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). Today, mass market
automotive manual transmissions are essentially all 5-speeds, with 6-speed transmissions beginning to
emerge in high performance vehicles in the early 1990s, and recently beginning to be offered on some highefficiency and conventional passenger cars. A very small number of 7-speed 'manual derived' transmissions
are offered on extremely high-end performance cars (supercars), such as the Bugatti Veyron 16.4, or the
BMW M5. Both of these cars feature a "Paddle Shifter".
Clutch
In all vehicles using a transmission (virtually all modern vehicles), a coupling device is used to separate the
engine and transmission when necessary. The clutch accomplishes this in manual transmissions. Without it,
the engine and tires would at all times be inextricably linked, and anytime the vehicle was at a stop, so would
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the engine. Moreover, without the clutch, changing gears would be very difficult, even with the vehicle
moving already: deselecting a gear while the transmission is under load requires considerable force, and
selecting a gear requires the revolution speed of the engine to be held at a very precise value which
depends on the vehicle speed and desired gear. In a car the clutch is usually operated by a pedal; on a
motorcycle, a lever on the left handlebar serves the purpose.
http://a332.g.akamai.net/f/332/936/12h/www.edmunds.com/media/ownership/tech_center/transmission_basi
cs/mustangt5.500.jpg
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CVT Enters the Mainstream
http://www.edmunds.com/ownership/techcenter/articles/45104/article.html (viewed 2008-04-04)
Our staff, as well as other automotive enthusiasts, has noticed an ever-increasing level of
interest about a new/old technology called CVT (continuously variable transmission) and
its burgeoning rise in the automotive world.
We say "new/old" because CVT has actually been around since 1886, but has only
recently begun to find its way into production automobiles.
CVT's promise, both as a boon to fuel economy and as a low-cost alternative to
conventional transmissions, has prompted us to revisit the topic.
Since our article CVT For You and Me appeared, a number of automakers have brought
new CVT-equipped vehicles to market, and more are on the way. We felt now was a good
time to take a closer look at this innovative — and time-tested — technology.
As we mentioned above, CVT has been around for more than a hundred years. However,
until recently, it was reserved for industrial applications — running lathes or light-duty drill
presses, for instance. With the introduction of improved materials, such as high-density
rubber belts, advanced hydraulics and, more recently, high-speed sensors and
microprocessors, the stage was set for CVT's rise in the automobile.
CVT's design advantages lie not only in its efficiency but in its simplicity. It consists of very
few components. A continuously variable transmission typically includes the following
major component groups:

a high-power/density rubber belt

a hydraulically operated driving pulley

a mechanical torque-sensing driving pulley

microprocessors and sensors
That's it.
Because of this simplicity in design, CVT offers some advantages over traditional transmissions, although it
also has certain drawbacks. For instance, its belt-driven orientation limits its application; until recently, cars
with engines larger than 1.2 liters were considered incompatible with CVT. More and more, however, CVTs
are becoming available that can handle more powerful engines, such as the V6 power plants found in some
Nissan and Audi vehicles.
Other disadvantages include its larger size and weight.
Still, in the right situation, CVT's advantages outweigh its disadvantages. Less complexity and moving parts
theoretically mean fewer things to go wrong and maintain.
The first U.S.-sold production automobile in the world to offer a CVT transmission was the Subaru Justy GL,
from 1989 through 1993. The engine in that car was 1.2 liters.
How it Works
Although there are different variations on the CVT theme, most passenger cars use a similar setup.
Essentially, a CVT transmission operates by varying the working diameters of the two main pulleys in the
transmission.
The pulleys have V-shaped grooves in which the connecting belt rides. One side of the pulley is fixed; the
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other side is moveable, actuated by a hydraulic cylinder. When actuated, the cylinder can increase or reduce
the amount of space between the two sides of the pulley. This allows the belt to ride lower or higher along
the walls of the pulley, depending on driving conditions, thereby changing the gear ratio. If you think about it,
the action is similar to the way a mountain bike shifts gears, by "derailing" the chain from one sprocket to the
next — except that, in the case of CVT, this action is infinitely variable, with no "steps" between.
The "stepless" nature of its design is CVT's biggest draw for automotive engineers. Because of this, a CVT
can work to keep the engine in its optimum power range, thereby increasing efficiency and gas mileage. A
CVT can convert every point on the engine's operating curve to a corresponding point on its own operating
curve.
With these advantages, it's easy to understand why manufacturers of high-mileage vehicles often
incorporate CVT technology into their drivetrains.
Look for more CVTs in the coming years as the battle for improved gas mileage accelerates and
technological advances further widen their functionality.
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