Axle configurations - By Bennie Hurter (aka
In this article,
I will discuss the 2 main types of axles in use in 4x4 vehicles
today namely solid axles and independent axles. There are also a
third type in use called portal axles but this configuration is
used seldom, normally only in off-road trucks like the Mercedes
Unimog truck, but I will not discuss them as they are not really
relevant to the target audience of this article.
In all vehicles, the driving wheels are attached to axles that facilitate
power delivery to the wheels. Most vehicles’ axles contain a differential
in the centre of the axle to allow the differences in rotational
speeds between the left and right wheels to be absorbed or cancelled
out. This difference in speeds occur when the vehicle does not move
in a straight line and the wheel inside the turn move slower due
to a shorter path travelled compared to the outer wheel that travels
a longer path.
The Differential is the centre portion of the axle and contains
the following components:
The casing that holds all the pieces intact is called the diff carrier
On imported differentials the triangular piece at the bottom of
the picture including the diff mechanism (carrier, gears, pinion
and bearings) can screw loose separately and this is called the
The input shaft is called the pinion (A)
and the gear at the end is called the pinion gear (B)
The rotation from the prop shaft that is connected to the pinion
rotates the pinion gear which in turn drives the ring gear or crown
Attached to the crown wheel is the spider gears or sometimes called
planetary gears (D)
The spider gears in turn drive the side gears and this is the heart
of the differential system. This setup will allow rotational speed
differences to be absorbed or cancelled out by the spider gears.
If both wheels rotate at the same speed, then the spider gears stand
still and the side gears and crown wheel turn in unison. A differential
lock can be installed into the carrier which will lock the side
gears and crown wheel together and will cause equal power transfer
to both wheels, if one of them were to loose traction.
Read the article on
which explains differentials in more detail.
Check out these cool animations which show a differential while
the vehicle is driving straight and when it is turning
The side gears drive the wheels via the side shafts. There are a
few diff configurations in use.
The first is a solid rear axle also called a rear beam axle and
does not have any rotational joints attached to the side shafts
The side shaft covers or beams (H), the centre portion, also referred
to as the pumpkin, and the wheel hubs (K) are one solid piece.
The next configuration is a solid front axle or front beam axle.
In this case the beams and the pumpkin is still a solid unit but
at the ends there are swivel hubs that allows the steering wheels
in front of the vehicle to be turned left or right.
In order to facilitate this turning movement in the side shaft while
it is rotating, rotational joints (J)
are placed at the ends of the side shafts and connects the stub
axles to the side shafts. These joints can either be universal joints
as used in DANA axles, Birfield joints as used in the Toyota solid
front axles or constant velocity (CV) joints as used in most vehicles
with independent front or rear suspension
The next configuration is a independent rear axle
In this configuration, the hubs and side shafts move independently
from the pumpkin that is fixed to the vehicle, so it remains stationary
while the wheels and side shafts move independently.
To be able to achieve this, two rotational joints (normally CV joints)
are placed at the ends of the side shaft. The inner CV (I) connects
the side shaft to the diff and the Outer CV (J) connects the side
shaft to the sub axle.
In this configuration the side shafts are often exposed.
The next configuration is an independent front axle
The independent front axle works just as the independent rear axle,
except that it also have swivel hubs at the ends like the solid
front axle to facilitate steering of the vehicle.