What is a Manual Gearbox / 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. 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 are characterized
by gear ratios that are selectable by engaging pairs of gears inside the transmission.
Conversely, automatic transmissions feature epicyclic (planetary) gearing controlled
by brake bands and/or clutch packs to select gear ratio.
Automatic transmissions that
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 two and as many as eight gears.
Semi-trailers have at least 13 gears and as many as 24. 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. Similarly, 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.
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. However, the gears were engaged by sliding mechanisms or simple
clutches, which required careful timing and 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 or disengaged, 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
letting the clutch up (engagement) 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 usually non-synchromesh transmissions, known colloquially
as "crashboxes", for several reasons. The friction material, such as brass,
in synchronizers is more prone to wear and breakage than gears, which are forged
steel, and the simplicity of the mechanism improves reliability and reduces cost.
In addition, the process of shifting a synchromesh transmission is slower than that
of shifting a non-synchromesh transmission. For racing of production-based transmissions,
sometimes half the teeth (or "dogs") on the synchros are removed to speed
the shifting process, at the expense of greater wear.
Similarly, most modern motorcycles
still utilize unsynchronised transmissions as synchronisers are generally not necessary
or desirable. Their 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.
Top and side view of a typical manual transmission, in this case a
used in cars with external floor shifters. 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
The first synchronized transmission
system was introduced by Cadillac in 1929. The modern cone system was developed
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 usually not synchromesh, as there is only
one reverse gear in the normal automotive transmission and changing gears in reverse
is not required.
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 high-efficiency 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".
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 incorporating
a separate overdrive unit in the rear housing of the transmission. This unit was
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.
In some transmission designs (Volvo
850 and V/S70 series, for example) there are actually two countershafts, both driving
an output pinion meshing with the front-wheel-drive transaxle's ring gear. This
allows the transmission designer to make the transmission narrower, since each countershaft
must be only half as long as a traditional countershaft with four gears and two