A manual transmission
transfers power from the engine to the differential using a clutch and pairs of
gears that are manually selected using the gear selector and locked to the
output shaft. Most cars today with manual transmissions have four, five or six
different forward gear ratios.
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:
The clutch is
positioned between the engine’s flywheel and the transmission. Normally when
the clutch is engaged, it locks the transmissions input shaft to the flywheel.
When the clutch pedal is depressed, the clutch is disengaged by a thrust
bearing, and no power is transferred. With the clutch disengaged it is possible
to select gears. To start the vehicle moving, the clutch is slowly released,
and slipped slightly. It is important not to ride the clutch once you are
moving because this can lead to clutch
slippage or premature thrust bearing problems. Gears are normally selected
using a shift lever, often mounted on the floor, but sometimes mounted on the
dash or steering column. The gears are usually laid out in an H pattern with
neutral being a space between gears.
Early manual transmissions (and those found in some
trucks, heavy machinery, and racecars) use an unsynchronized design. In an
unsynchronized transmission the gears are selected by sliding them on shafts
until they have engaged the dog clutch. In order for the gears to engage
properly they need to be spinning at the same speed as the output shaft,
otherwise the gears (actually the dog clutches) will grind. To do this you can
either double clutch by essentially using the clutch to shift into neutral, and
then again to shift into the desired gear, or you can rev match by matching the
engine’s RPM with the RPM the engine will be at for the road speed in the
desired gear. Unsynchronized transmissions are generally tougher than
synchronized ones, as the synchros are usually made out of soft brass that wear
much more quickly than the steel gears. Because of this added strength,
unsynchronized transmissions can usually be shifted quickly without using the
clutch, especially when designed with fewer teeth on the dog clutches, as in
some race cars.
In a sequential gear box (usually found in race
cars) there is no neutral between gears, and the gears are selected in
sequential order instead of in an H pattern, often without using the clutch. In
a semi-automatic manual gearbox the actual rev matching, gear shifting and
clutch control is handled by a computer. In these transmissions the clutch is
usually only used to get the vehicle moving. When the driver presses a button
or pulls a lever the computer takes the car out of the current gear, blips the
throttle or waits for the engine RPM to drop to the appropriate level, and then
changes into the requested gear.
Manual transmissions are generally more fuel
efficient than automatic transmissions because there is no fluid coupling or
fluid pump like in a traditional automatic transmission, and no belt to slip
like in a continuously variable transmission. A standard transmission also
allows the driver more control over which gear they are in, potentially
allowing for a lower engine RPM.
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