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 edition of
we'll answer all of these questions and more as we explore the
interior of a manual transmission!
The Goal of a 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
Ideally, the transmission would be so flexible in its
ratios that the engine could always run at its single,
best-performance 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.
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. 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
RPM at Transmission Output Shaft with
Engine at 3,000 rpm
A Very Simple Transmission To understand the
basic idea behind a standard transmission, the diagram below
shows a very simple two-speed transmission in neutral:
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 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
The picture below shows how, when
shifted into first gear, the collar engages the blue gear on
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. Meantime, 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:
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
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
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.
In the next section, we'll take a look at a real
A Real Transmission The following animation
shows you the internal workings of a four-speed transmission
The five-speed manual transmission is fairly
standard on cars today. 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 neutral, first, second
and third gear:
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). Therefore, at all times, the blue reverse gear in
this diagram is turning in a direction opposite to all of the
other blue gears. It would be impossible to throw the
transmission into reverse while the car is moving forward --
the dog teeth would never engage. They will make a lot of
Synchronizers Manual transmissions in modern
passenger cars use synchronizers to eliminate the need
for double-clutching. 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
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.
For more information on transmissions and related topics,
check out the links on the next page.