Manual Transmission
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,
although some do not. Most automobile manual transmissions allow the driver
to select any gear at any time, but some, such as those commonly mounted on motorcycles
and some types of race cars, only allow the driver to select the next-highest
or next-lowest gear ratio. This second type of transmission is sometimes
called a sequential manual transmission.
Manual transmissions are
characterized by gear ratios that are selectable by engaging pairs of gears inside
the transmission. Conversely, automatic transmissions feature clutch packs
to select gear ratio. Transmissions that employ clutch packs but allow the driver
to manually select the current gear are called semi-automatic transmissions.
Contemporary
automotive manual transmissions are generally available
with four to six forward gears and one reverse gear, although manual transmissions
have been built with as few as 2 and as many as 7 gears. Some manuals are
referred to by the number of forward gears they offer (e.g., 5-speed) as a way
of distinguishing between automatic or other available manual transmissions. In
contrast, a 5-speed automatic transmission is referred to as a 5-speed automatic.
Other
types of transmission in mainstream automotive use are the
automatic transmission, semi-automatic transmission, and the continuously variable
transmission.
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.
Unsynchronized transmission
The earliest automotive transmissions were entirely
mechanical unsynchronized gearing systems. They could be shifted, with multiple
gear ratios available to the operator, and even had reverse. But the gears
were engaged by sliding mechanisms or simple clutches, which required skills of
timing and careful throttle manipulation when shifting, so that the gears would
be spinning at roughly the same speed when engaged; otherwise the teeth would
refuse to mesh.
When upshifting, the speed of the gear driven by the
engine had to drop to match the speed of the next gear; as this happened naturally
when the clutch was depressed, it was just a matter of skill and experience
to hear and feel when the gears managed to mesh. However, when downshifting,
the gear driven by the engine had to be sped up to mesh with the output gear,
requiring engagement of the clutch for the engine to speed up the gears. Double
declutching, that is, shifting once to neutral to speed up the gears and again
to the lower gear, is sometimes needed. In fact, such transmissions are often
easier to shift from without using the clutch at all. The clutch, in these cases,
is only used for starting from a standstill. This procedure is common in racing
vehicles and most production motorcycles.
Even though automotive
transmissions are now almost universally synchronised, heavy trucks and machinery
as well as dedicated racing transmissions are still usually nonsynchromesh
transmissions, known colloquially as "crashboxes", for several reasons. Being
made of brass, synchronizers are prone to wear and breakage more than the actual
gears, which are cast iron, and the rotation of all the sets of gears at once
results in higher frictional losses. In addition, the process of shifting a synchromesh
transmission is slower than that of shifting a nonsynchromesh transmission.
For racing of production based transmissions, sometimes half the dogs on
the synchros are removed to speed the shifting process, at the expense of much
more wear.
Similarly, most modern motorcycles still utilise unsynchronised
transmissions. Synchronisers are generally not necessary or desirable in
motorcycle transmissions. The low gear inertias and higher strengths mean that
'forcing' the gears to alter speed is not damaging, and the selector method on
modern motorcycles (pedal operated) is not conducive to having the long shift
time of a synchronised gearbox. Because of this, it is still necessary to synchronise
gear speeds by 'blipping-the-throttle' when shifting into a lower gear
on a motorcycle.
Synchronized transmission
A modern gearbox
is of the constant mesh type, in which all gears are always in mesh but only
one of these meshed pairs of gears is locked to the shaft on which it is mounted
at any one time, the others being allowed to rotate freely; thus greatly reducing
the skill required to shift gears.
Most modern cars are fitted
with a synchronised gear box, although it is entirely possible to construct a
constant mesh gearbox without synchromesh, as found in motorcycle for example.
In a synchromesh gearbox, the teeth of the gears of all the transmission speeds
are always in mesh and rotating, but the gears are not directly rotationally 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 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. 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 only 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.
The
first synchronized transmission system was introduced by
Cadillac in 1929. The modern cone system was developed by Porsche and introduced
in the 1952 Porsche 356; cone synchronizers were called "Porsche-type" for many
years after this. In the early 1950s only the second-third shift was synchromesh
in most cars, requiring only a single synchro and a simple linkage; drivers'
manuals in cars suggested that if the driver needed to shift from second to
first, it was best to come to a complete stop then shift into first and start
up again. With continuing sophistication of mechanical development, however, fully
synchromesh transmissions with three speeds, then four speeds, five speeds,
six speeds and so on became universal by the 1960s. Reverse gear, however, is
not synchromesh, as there is only one reverse gear in the normal automotive transmission
and changing gears in reverse is not required.
Internals
Shafts
Like
other transmissions, a manual transmission has
several shafts with various gears and other components attached to them. Typically,
there are three shafts: an input shaft, a countershaft and an output shaft.
The countershaft is sometimes called a layshaft.
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. Of course,
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.
Under one
possible design, the transmission's input shaft has just one pinion gear, which
drives the countershaft. Along the countershaft are mounted gears of various
sizes, which rotate when the input shaft rotates. These gears correspond to the
forward speeds and reverse. Each of the forward gears on the countershaft is
permanently meshed with a corresponding gear on the output shaft. However, these
driven gears are not rigidly attached to the output shaft: although the shaft
runs through them, they spin independently of it, which is made possible by bearings
in their hubs. Reverse is typically implemented differently, see the section
on Reverse.
When the transmission is in neutral, and the clutch
is disengaged, the input shaft, clutch disk and countershaft can continue to
rotate under their own inertia. In this state, the engine, the input shaft and
clutch, and the output shaft, all rotate independently.
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.
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. Of course,
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 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 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. There is a
clear description of such a mechanism in the Honda Civic 1996-1998 Service Manual,
which refers to it as a "noise reduction system":
Whenever the
clutch pedal is depressed to shift into reverse, the mainshaft continues to rate
because of its inertia. The resulting speed difference between mainshaft and
reverse idler gear produces gear noise [grinding]. The reverse gear noise reduction
system employs a cam plate which was added to the reverse shift holder. When
shifting into reverse, the 5th/reverse shift piece, connected to the shift lever,
rotates the cam plate. This causes the 5th synchro set to stop the rotating
mainshaft. (13-4)
A reverse gear implemented this way makes a loud whining
sound, which is not heard in the forward gears. The teeth on the forward gears
of consumer automobiles are helically cut. When helical gears rotate, their
teeth slide together, which results in quiet operation. In spite of all forward
gears being always meshed, they do not make a sound that can be easily heard
above the engine noise. By contrast, reverse gears are spur gears, meaning that
they have straight teeth, in order to allow for the sliding engagement of the
idler, which would not be possible with helical gears. The teeth of spur gears
clatter together when the gears spin, generating a characteristic whine.
It
is clear that the spur gear design of reverse gear represents some compromises?less
robust, unsynchronized engagement and loud noise?which are acceptable
due to the small volume of driving that takes place in reverse.
Design
Variations
Gear Variety
Manual transmissions are often
equipped with 4, 5, or 6 forward gears. Nearly all have exactly 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 high-efficiency and
conventional passenger cars.
External Overdrive
On earlier models
with three or four forward speeds, the lack of an overdrive ratio for relaxed
and fuel-efficient highway cruising was often filled by incorporation of a
separate overdrive unit in the rear housing of the transmission, separately actuated
by a knob or button, often incorporated into the gearshift knob.
Shaft
and Gear Configuration
The input shaft need not turn a pinion which
rotates the countershaft. Another possibility is that gears are mounted on
the input shaft itself, meshed with gears on the countershaft, in which case the
countershaft then turns the output shaft. In other words, it's a matter of design
on which shaft the driven and driving gears reside.
The distribution
of the shifters is also a matter of design; it need not be the case that
all of the free-rotating gears with selectors are on one shaft, and the permanently
splined gears on the other. For instance a five speed transmission might have
the first-to-second selectors on the countershaft, but the third-to-fourth
selector and the fifth selector on the mainshaft, which is the configuration in
the 1998 Honda Civic. This means that when the car is stopped and idling in neutral
with the clutch engaged input shaft spinning, the third, fourth and fifth
gear pairs do not rotate.
Clutch
In all vehicles utilizing
a transmission (virtually all modern vehicles), a coupling device is utilized
to be able to separate the engine and transmission when necessary. The clutch
is what accomplishes this in manual transmissions. Without it, the engine and
tires would at all times be inextricably linked, and anytime the vehicle is at
a stop, so would be the engine. In a car the clutch is usually used by a pedal;
on a motorcycle, a lever on the left handlebar serves the purpose.
When
the clutch pedal is fully depressed, the clutch is fully disengaged, and no
torque is transferred from the engine to the transmission, and by extension to
the drive wheels. This allows for the transmission's gears to be independent
of the engine (spinning purely through momentum or, for any engaged gear, the motion
of the vehicle). This allows for shifting without gear grinding.
When
the clutch pedal is fully released, the clutch is fully engaged, and essentially
all of the engine's torque is transferred.
In between these extremes,
the clutch "slips" to varying degrees. Clutch slippage is useful, because the
entire purpose the transmission serves is gear reduction. Because the engine and
tires are designed to be linked in order to drive, one must dictate the speed
of the other. If there was no slippage, the tires would dictate engine speed,
and as such, getting a vehicle to move from rest would be extremely difficult.
This slippage allows for the slow introduction of power, with less resistance
introduced to the engine until enough momentum is built that the engine can operate
normally without output reduction from the clutch.
Note: Automatic transmissions
also utilize a coupling device, however, a clutch is not present.
In these kinds of vehicles, the torque converter is used to separate the engine
and transmission.
Floor-mounted shifter
In most modern cars,
gears are selected through a lever attached to the floor of the automobile?this
selector is often called a gearstick, gear lever, gear selector, or simply shifter.
Moving this lever forward, backward, left, and right allows the driver to
select any given gear. In this configuration, the gear lever must be pushed laterally
before it is pushed longitudinally.
Transmissions equipped with
this shift pattern usually feature a lockout mechanism that requires the driver
to depress a switch or the entire gear lever when entering reverse, so that
he does not accidentally select it when trying to find first gear.
Most
front-engined, rear-wheel drive cars have a transmission that sits between
the driver and the front passenger seat. Floor-mounted shifters are often connected
directly to the transmission. Front-wheel drive and rear-engined cars often
require a mechanical linkage to connect the shifter to the transmission.
A
4-speed floor shifter is sometimes referred to as "Four on the Floor".
Column-mounted
shifter
Some older cars feature a gear lever
which is mounted on the steering column of the car. Many automatic transmissions
still use this placement, but manual column shifters are no longer common.
Column
shifters are mechanically similar to floor shifters, although shifting
occurs in a vertical plane instead of a horizontal one. Column shifters
also generally involve additional linkages to connect the shifter with the transmission.
The
3-speed shift pattern is typical of American cars, trucks,
and vans produced with manual transmissions until the 1950s and 1960s. This
pattern is not "intuitive", as the shifter has to be moved forward (into R)
to make the car go backward (and vice-versa).
First gear in a 3-speed
is often called "low," while third is usually called "high." There is, of course,
no overdrive.
A 3-speed column shifter is sometimes referred to
as "Three on a Tree".
Note that reverse in a car with a column shift
is in nearly the same position as park (P) is on a car with a column-mounted
gear selector with an automatic transmission.
Sequential manual
Some
transmissions do not allow the driver to arbitrarily select any gear. Instead,
the driver may only ever select the next-lowest or next-highest gear ratio.
These transmissions often provide clutch control, but the clutch is only necessary
when selecting first or reverse gear from neutral. Most gear changes can
be performed without the clutch.
Sequential transmissions are generally
controlled by a forward-backward lever, foot pedal, or set of paddles mounted
behind the steering wheel. In some cases, these are connected mechanically
to the transmission. In many modern examples, these controls are attached to sensors
which instruct a transmission computer to perform a shift?many of these systems
can be switched into an automatic mode, where the computer controls the
timing of shifts, much like an automatic transmission.
Semi-manual
Some
very new transmissions (BMW's Sequential Manual Gearbox (SMG) and Audi's
Direct-Shift Gearbox (DSG), for example) are conventional manual transmissions
with a computerized control mechanism. These transmissions feature independently
selectable gears but do not have a clutch pedal. Instead, the transmission
computer controls a servo which disengages the clutch when necessary.
These
transmissions vary from sequential transmissions in that they still allow
nonsequential shifts: BMWs SMG system, for example, can shift from 6th gear
directly to 4th gear when decelerating from high speeds.
Comparison
with automatic transmissions
Manual transmissions are typically compared
to automatic transmissions, as the two represent the majority of options available
to the typical consumer. These comparisons are general guidelines and may not
apply in certain circumstances. Additionally, the recent popularity of semi-manual
and semi-automatic transmissions renders many of these points obsolete.
It should be kept in mind that many some of these points are true of "conventional"
automatic transmissions which shift gears and are coupled to the engine with
a torque converter but are not a true comparison or do not apply to other kinds
of automatic transmissions, like the continuously-variable transmission.
Advantages
Manual
transmissions are typically more efficient than
automatic transmissions. This is because manuals generally involve a clutch instead
of a torque converter, which can cause significant power losses and because
an automatic transmission introduces parasitic losses through the high pressure
hydraulic pumps it requires. This results in both better acceleration and fuel
economy.
Increased fuel economy with a properly operated manual transmission
vehicle versus an equivalent automatic transmission vehicle can range from
5 % to about 15 % depending on driving conditions and style of driving -- extra
urban or urban (highway or city).
It is generally easier to build a
very strong manual transmission than a very strong automatic transmission. Manual
transmissions usually have only one clutch, whereas automatics have many clutch
packs.
Manual transmissions are generally significantly lighter than
torque-converter automatics.
Manual transmissions are typically cheaper to
build than automatic transmissions.
Manual transmissions generally require
less maintenance than automatic transmissions.
Manual transmissions normally
do not require active cooling, because not much power is dissipated as
heat through the transmission.
The heat issue can be important in certain
situations, like climbing long hills in hot weather, particularly if pulling a
load. Unless the automatic's torque converter is locked up (which typically only
happens in an overdrive gear that would not be engaged when going up a hill)
the transmission can overheat. A manual transmission's clutch only generates heat
when it slips, which does not happen unless the driver is riding the clutch
pedal.
A driver has more direct control over the state of the transmission
with a manual than an automatic. This control is important to an experienced,
knowledgeable driver who knows the correct procedure for executing a driving
manoeuver, and wants the machine to obey his or her instructions exactly and instantly.
An example: the driver, anticipating a turn, can downshift to the
appropriate gear while the steering is still straight, and stay in gear through
the turn. This is the correct, safe way to execute a turn.
Another example:
when starting, the driver can control how much torque goes to the tires,
which is useful for starting on slippery surfaces such as ice, snow or mud. This
can be done with clutch finesse, or possibly by starting in second gear instead
of first. The driver of an automatic can only put the car into drive, and play
with the throttle. The torque converter can easily dump too much torque into
the wheels, because when it slips, it acts as an extra low gear, passing through
the engine power, reducing the rotations while multiplying torque.
Yet
another example: passing. When the driver is attempting to pass a slower moving
vehicle by making use of a lane with opposite traffic, he or she can select a
lower gear for more power at exactly the right moment when conditions are right
to begin the manoeuver. Automatics have a delayed reaction time, because the
driver can only indicate his intent by pressing the throttle. The skilled manual
transmission driver has an advantage of superior finesse and confidence in such
situations.
Driving a manual requires more involvement from the driver,
thereby discouraging some dangerous practices. The manual selection of gears
requires the driver to monitor the road and traffic situation, anticipate events
and plan a few steps ahead. If the driver's mind wanders from the driving task,
the machine will soon end up in an incorrect gear, which will be obvious from
excessive or insufficient engine RPM. Related points:
It's much more difficult
for the driver to fidget in a manual transmission car, for instance by
eating, drinking beverages, or talking on a cellular phone without a headset. During
gear shifts, two hands are required. One stays on the wheel, and the other
operates the gear lever. The hand on the wheel is absolutely required during
turns, and tight turns are accompanied by gear changes. If the hand leaves the
wheel, the steering will begin to straighten. In general, the more demanding the
driving situation, the more difficult it is for the manual driver to do anything
but operate the vehicle. The driver of an automatic transmission can engage
in distracting activities in any situation, such as sharp turns through intersections
or stop-and-go traffic.
The driver of a manual transmission car can
develop an accurate intuition for how fast the car is traveling, from the sound
of the motor and the gear selection. It's easier to observe the lower speed
limits like 30 km/h and 50 km/h without glancing at the instrumentation.
Cars
with manual transmissions can often be started when the battery is dead by
pushing the car into motion (or allowing it to roll down a hill) and then engaging
the clutch in third or second gear. This is called a push start.
Manual
transmissions work regardless of the orientation angle of the car with respect
to gravity. Automatic transmissions have a fluid reservoir (pan) at the bottom;
if the car is tilted too much, the fluid pump can be starved, causing a failure
in the hydraulics. This could matter in some extreme off roading circumstances.
Disadvantages
Manual transmissions require more driver
interaction than automatic transmissions.
A driver may inadvertently shift
into the wrong gear with a manual transmission, potentially causing damage to
the engine and transmission as well as compromising safety.
Manual transmissions
are more difficult to learn to drive as one needs to develop a feel for
properly engaging the clutch.
The smooth and quick shifts of an automatic
transmission are not guaranteed when operating a manual transmission.
Manual
transmissions are slightly harder to start when stopped upward on a hill,
but this is overcome with a little experience.
The clutch disc is a wear
item and must be replaced periodically. This is typically a labor intensive
process and can be an expensive service.
Applications and popularity
Many
types of automobiles are equipped with manual transmissions. Small
economy cars predominantly feature manual transmissions because they are relatively
cheap and efficient, although many are optionally equipped with automatics.
Economy cars are also often powered by very small engines, and automatic transmissions
can make them comparatively very slow, while a manual transmission makes
much more efficient use of the power produced.
Sports cars are also
often equipped with manual transmissions because they offer more direct driver
involvement and better performance. Off-road vehicles and trucks often feature
manual transmissions because they allow direct gear selection and are often
more rugged than their automatic counterparts.
Very heavy trucks also
feature manual transmissions because they are efficient and, more importantly,
can withstand the severe stress encountered in hauling heavy loads.
Maintenance
Because
clutches use changes in friction to modulate the transfer
of torque between engine and transmission, they are subject to wear in everyday
use. A very good clutch, when used by an expert driver, can last hundreds
of thousands of kilometres. Weak clutches, downshifting.