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International Journal of Current Engineering and Technology E-ISSN 2277 – 4106, P-ISSN 2347 – 5161
©2018 INPRESSCO®, All Rights Reserved Available at http://inpressco.com/category/ijcet
Research Article
361| International Journal of Current Engineering and Technology, Vol.8, No.2 (March/April 2018)
Four-Wheel Drive System: Architecture, Basic Vehicle Dynamics and
Traction
Bir Armaan Singh Gill, Mayank Sehdev and Hardeep Singh*
Department of Mechanical Enginering, School of Engineering, GD Goenka University, India
Received 12 Feb 2018, Accepted 15 April 2018, Available online 19 April 2018, Vol.8, No.2 (March/April 2018)
Abstract
Technology has been developing at a very tremendous pace to improve the safety and comfort of the people and when
it comes to luxury and comfort, automobiles section is one of the common topic of interest, the most trending among
this section is the “Four Wheel Drive (4WD) system” which has become more advanced and sophisticated with time. It
has become an essential and appealing part of automobiles in today's era. This system has made the vehicles more
secure, both off-road and on-road, enhanced traction of the wheels and power delivery of the engine. With the
manipulation in the fundamental structure of a 4WD, various other 4WD options have become available, suiting
different types of lifestyle and environment. This paper attempts to explain the different types of 4WD system, their
basic structure, basic vehicle dynamics affecting the traction control system and provide a conclusion on various
philosophies that the 4WD system adhere.
Keywords: Four Wheel Drive, Two Wheel Drive, All Wheel Drive, Traction.
Introduction
1
The transmission of energy with least losses has
always been a major interest and a problem to
overcome and hence with the series of advancements
we now have the following transmission system
described below:
2WD: In 2WD(Two Wheel Drive Mode), typically
used in sedans and hatchbacks, only one axle(out of the
two) is powered by the vehicle, that is the torque is
only split between the two wheels. This can be either
used as a RWD (Rear Wheel Drive) or a FWD (Front
Wheel Drive) based on the position of the engine,
vehicle dynamics, fuel efficiency and weight
distribution of the automobile. The torque split ratio
between the two axles is 0:100 as only one axle is
powered and the other axle is a 'dead axle'.
Part Time 4WD: Part Time 4WD is basically a 2WD
but is equipped with a "Transfer Case". On standard
conditions the vehicle runs on two wheels but also has
an option to engage the front two wheels by locking the
hubs on the front wheels. The transfer Case has a
second gear lever and locking hubs on the wheels
attached to the front axle. The power is delivered to the
front wheels through the differential present at the
front axle. There is absence of centre differential which
means that both the axles rotate at same speed making
4WD unsuitable for hard surfaces like bitumen which
*Correspoding author’ ORCID ID: 0000-0003-0495-407
DOI: https://doi.org/10.14741/ijcet/v.8.2.28
can lead to "transmission wind-up". The ratio of the
torque to each wheel depends upon the traction of the
wheel with the surface it's being driven on as well as
the road conditions when the 4WD is engaged. The
vehicle also has an option to select "High-Range 4WD"
which lets the vehicle to power all four wheels with
same speed ratio as that of 2WD and "Low-Range
4WD" which decreases the speed ratio leading to the
increase in RPMs(Rotation Per Minute- of the
combustion cycle in the Internal Combustion(IC)
engine) generating more torque in less speed
(http://www.outbackcrossing.com.au/FourWheelDriv
e/Different_Types_of_4WD.shtml).
Full Time 4WD: A Full Time 4WD is the system in
which both the axles are powering the vehicle at all
times. There is no selection of 2WD available in such
automobiles. A Full Time 4WD is equipped with a
centre differential (inter-axial differential) which
allows the all the four wheels to rotate at different
speeds, avoiding the transmission wind-
up.(http://www.outbackcrossing.com.au/FourWheelD
rive/Different_Types_of_4WD.shtml) The inter-axial
differentials allows slippage between the two axles
whereas the differentials preset at either end allow the
slippage between each wheel. The slippage occurs
when the driving driveshaft becomes faster than the
driven driveshaft (http://www.awdwiki.com/en/all
wheel drive explained/#selectable_all_wheel_drive) If
the centre differential is "locked" the transmission will
work as a Part Time 4WD.
Bir Armaan Singh Gill et al Four-Wheel Drive System: Architecture, Basic Vehicle Dynamics and Traction
362| International Journal of Current Engineering and Technology, Vol.8, No.2 (March/April 2018)
All Wheel Drive (AWD): AWD systems are often
confused with other 4WD systems but have a
completely different proposition .An AWD system also
has an inter-axial differential but does not have the
ability to lock the inter-axial differential, restricting it
to perform like a Part Time 4WD system. An AWD
system can be made to operate in various ways
depending upon the preference on the
manufacturer.(http://www.outbackcrossing.com.au/F
ourWheelDrive/Different_Types_of_4WD.shtml) In an
AWD system, with the presence of 'On-Demand Mode'
the vehicle can operate in 2WD mode and engage the
AWD automatically when there is wheel slippage and
as soon as it is eliminated, it goes back to the 2WD
mode. This is done by using a multi-plate hydraulic
clutch, viscous coupling or any other device which can
engage the other driveshaft (http://www.
outbackcrossing.com.au/FourWheelDrive/Different_Ty
pes_of_4WD.html)(http://www.fourwheeler.com/how
-to/68958-how-transfer-cases-work)
4x4 Transmissions in automobiles are being used
for various purposes since early 1900s. This system
has now evolved to become more sophisticated from
basic mechanical devices with the integration of
technology over the years. 4x4 transmissions are
mostly available in heavy duty vehicles such as pickup
trucks and SUVs which are often used off-road and on
rugged terrains. In a nutshell, the four-wheel
drive(4WD) helps in providing better mobility of the
vehicle that is, providing traction to all the four wheels
of the vehicle. With the incorporation of technology
over the years, the basic 4WD system which was once
cumbersome to use and highly required driver
capabilities, has now become more user-friendly. It has
enhanced the safety of the vehicles on-road, provided
more traction control and increased the practicality of
an automobile. The typical 4WD system has given birth
to other advanced drive systems such as AWD (All
Wheel Drive) which follow the same basic principles of
a 4WD but differ in their mechanisms and provide
more driving options. The advancement of the basic
4WD has now also helped the automakers to
incorporate the 4WD as an additional feature in other
smaller transmissions such as FWD(Front Wheel
Drive) and RWD(Rear Wheel Drive), used in vehicles
which are often made fuel efficient.(Mohan, S, 2000)
The various options available in the 4WD systems have
different placement of the various components which
make up the architecture of a 4WD vehicle leading to
difference in their traction control system and vehicle
dynamics. This paper reflects over the differences and
comparisons of the mechanisms of these advanced
4WD options available across multiple range of
vehicles and explains the usage of these systems on
different terrains.
Structure
The structures of typical AWD, 4WD transmissions are
shown in figure1. The mechanical functioning in the
engine is modified by the transmission and is
distributed to the wheels of the vehicle by the transfer
case through the drive shafts and the axles (Mohan, S,
2000).
1) Internal Combustion engine, 2) Clutch / Torque converter, 3)
Gearbox, 4) Rear Differential, 5) Rear Propeller (longitudinal) Shaft,
6) Transfer Case [with central differential and gear reductor
(optional)], 7) Front Propeller (longitudinal) shaft, 8) Front
Differential, 9) Coupling Device (viscous, electromagnetic)
Figure 1: The structures of typical AWD, 4WD
transmissions
Engine-An engine is the powerhouse of the vehicle.
Vehicles usually have engines which are fuel
injected(IC-Internal Combustion) and are controlled by
an ECU(Electronic Control Unit).(Mohan, S, 2000) The
Spark-Ignition unit, which gives fast response times
and allows power control in small ranges is
responsible for the combustion of the fuel to generate
torque.(Mohan, S, 2000) This torque is transferred
from engine to the transmission through the crankshaft
via the flywheel to the input shaft of the transmission
using clutch or hydraulics which is then divided
between the wheels.
Transmission-The transmission(gearbox) delivers
the torque and speed conversions generated by the
engine to the wheels using gear ratios according to the
variation of speed. The input shaft of the transmission
is connected to the crankshaft of the engine of the
vehicle and the output shaft(drive shaft) of the
transmission is connected the axles, which in turn
power the wheels through the differential(s). A
transmission consists of some key elements such as
gear, gear ratio, clutch, transmission, shift lever and the
H pattern. A vehicle can either have an automatic
transmission or a manual transmission but other
transmission such as CVT(Continuously Variable
Transmission) are also gaining popularity.(Staff, 2017)
In manual, the driver has the full control. By engaging
the clutch which disengages (disconnects the engine
from the input shaft of the transmission) the engine
from the transmission, the driver can select the
required gear in 2WD and also engage the part time
modes of 4WD via the levers on the transfer case. In
automatic transmission the shifting of gears is handled
by the fluid dynamics and the hydraulics. In automatic
transmissions, the integration of traction control
system to control the torque to the wheels is fairly easy
Bir Armaan Singh Gill et al Four-Wheel Drive System: Architecture, Basic Vehicle Dynamics and Traction
363| International Journal of Current Engineering and Technology, Vol.8, No.2 (March/April 2018)
(Staff, 2017) In a CVT, rather than using the gears, a
belt between the two pulleys is used. One of the belt is
driven by the shaft of the engine while the other is
driven by the differential unit and drive axles. Both of
the pulleys are split up so the halves of these pulleys
can come closer and farther apart. With the lowering
and the high movement of the belt, the gear ratios can
be changed between the driving and the driven shaft
(Chaikin Don, 2015)
A 'Transaxle' operates in a same manner as that of
the transmission. In a typical transmission the
transmission is connected via long driveshaft to the
rear axle. Whereas, in a transaxle, the output shaft
drives a larger gear with is coupled with the
differential's ring gear directly. The differential axle
which is mounted on the rear axle in a RWD vehicle, is
located in the transaxle housing. It is mounted parallel
to the transmission. Thereafter, the differential
distributes the power to the front two wheels via
halfshafts (Chaikin Don, 2015).
For 4WD systems, the transmission system gets a
combination of differentials and transfer case
depending on the type of 4WD.
The schematic illustration in figure 2 shows the
basics of the transmission:
Figure 2: The basics of the transmission
(http://www.svseeker.com/wp/sv-seeker-
2/sailboat/propulsion/engines-and-more)
Axle: The axle is a rod which connects to the wheel(s)
of an automobile. The axles houses the differentials and
drive shafts to each wheel. The output shaft from the
transfer case powers the input gears of the axle
differential. Typically, the axle is connected to the
chassis of the automobiles via the suspension system
and bears a significant weight of the automobile on
itself. The axles allows the wheel to rotate at different
speeds when the vehicle is in turning motion by
distributing the torque between the wheels through
axle differentials. The axles can typically be classified
as of two types: live axles and dead axles. The live axles
power the vehicle whereas the dead axle freely rotates.
The dead axle can also be engaged and function as a
live axle when the vehicle switches to 4WD mode
(Mohan, S, 2000).
Transfer Case: A transfer case is an additional
gear box used for the 4WD architecture. The work of
the transfer case is to split the engine torque coming
through the transmission and delivers the torque to
the output shafts which connect to the driveshafts of
both the front and back axles
(http://www.fourwheeler.com/how-to/68958-how-
transfer-cases-work). The torque ratio of the front axle
(%F) to the rear axle(%R) is known as the torque split
ratio(F:R) of the transfer case (Mohan, S, 2000). The
typical transfer cases can have multiple modes of
operation. The switching among different modes can
be either electric or manual. The transfer case can also
have an additional option of 'low range' which helps to
provide greater gear reduction to provide extreme
torque support to wheels on challenging terrains. A
transfer case can fundamentally be classified in two
types for a 4WD vehicle:
Part Time Mode-In part time mode, both the front
and rear axles are coupled in the transfer case. Since
there is no inter-axial differential in this system, any
speed differentiation in the axles can cause
transmission 'wind-up'. Hence, the vehicle normally
operates in 2WD mode and the 4WD should be
engaged on loose surfaces where the wind-up in
unlikely. In part time, options based on gear ratios such
as 4WD High(Four Wheel mode for high speeds and
torque) and4WD Low(Four Wheel mode for lower
speeds and extreme torque demands) can also be
present.(Mohan, S, 2000)
Full Time Mode-In full time mode both the axles
are powered at all times. The inter-axial differential
allows the axles to rotate at different speeds avoiding
transmission 'wind-up'. If the inter-axial differential is
locked, the vehicle can perform as a part time 4WD.
The general torque split between the axles is 50:50
which can be varied with the introduction of different
types of differentials. A vehicle that permanently
operated in Full Time Mode is often termed as AWD
(All Wheel Drive). But, the AWD lacks the ability to lock
the inter-axial differential disabling its ability to act as
a part time mode limiting their off-road ability on
challenging terrains.(Mohan, S, 2000)(http://
www.fourwheeler.com/ how-to/68958-how-transfer-
cases-work)
With developments in technology, "One-Demand
Mode" has also been introduced into 4WD. In this
mode, the vehicle mostly operates in 2WD. The torque
to the secondary axle is transferred "on demand" by
the vehicle, by the modulation of transfer clutch from
'Open' to a rigidly coupled state and also avoiding the
driveline wind-up. The modulation done in the torque
can be done by electronic or hydraulic control systems
based on wheel slippage.(Mohan, S, 2000) The
schematic illustration in figure 3 shows the basics of
the transfer case system.
Differential: A differential is a mechanical device
which balances the torque between the wheels. The
inter-axial differential (Center Differential) is
positioned in the transfer case or between the two
output shafts. The axial differentials are present on the
axles of the vehicle, between the axle driveshafts
connected to the wheel. The input rotation
(transmission output) in the differential gets
distributed to the two outputs (axle driveshafts). In
4WD vehicles, the front wheels rotate more swiftly as
Bir Armaan Singh Gill et al Four-Wheel Drive System: Architecture, Basic Vehicle Dynamics and Traction
364| International Journal of Current Engineering and Technology, Vol.8, No.2 (March/April 2018)
compared to the rear wheels. Hence, a differential
provides the axles to have the ability of turning at
different speeds (Mohan, S, 2000).
Depending on the type and modulation based on
specific preferences of a 4WD vehicle, the number, type
and the placement of differential(s) also varies. A Part
Time 4WD has one differential on each axle whereas in
Full Time 4WD and AWD there is a center differential
along with the axle differentials.=
(http://www.4wdonline.com/A/Diff.locks.html)
A standard differential, often termed as an 'Open'
differential is suitable when the wheels are on a
surface with high traction. If any one of the wheels
loses traction, then all the torque of the axle attached
to the particular wheel(s) will be directed to the free
tire(wheel which has lost the traction) due to the
phenomena of "Path of least resistance". Hence, to
avoid the direction of the power to the free wheel and
rather direct it to the wheel with traction, 'differential
locks' are introduced. These differential locks play a
vital role in 4WD vehicles. There are various types of
differential locks present in the market such as:
Limited slip differentials, Torsen Differentials, Detroit
Differentials, Selectable Lockers, Spools
(http://www.crawlpedia.com/locker_comparison.htm)
Basic vehicle dynamics and traction
According to inertia, a body in rest or motion, will
always try to maintain the status it is in. In order to
bring change to the status, external forces have to
applied to the body. There are various forces which act
on the body regardless of its state of motion such as
gravitational force. According to figure, in a vehicle,
apart from the gravitation force acting on it due to its
weight, there are various different types of forces
which act on it. These forces can be distributed along
the longitudinal axis (Example: rolling friction) or
laterally (Example: steering force) as illustrated in
figure 4 (Reif, 2014).
Figure 4: Distribution of forces on a vehicle (Reif,
2014)
In order for the vehicle to move, the engine torque
must overcome all the forces which resist the motion
such as the friction between the tires and the road. The
forces which act on the tires make the vehicle change
its direction or move in a specific way. The forces can
be the circumferential force, vertical force (normal
force) or the lateral force. The circumferential force is
produced by the transmission(torque) or the
brakes(frictional force) which make the vehicle
increase or decrease their speed. The vertical forces
are the forces between the tires and the road which
acts at all the times irrespective of the state(motion or
rest) of the vehicle. The part of the tire in contact with
the road gets deformed due to the vertical force. As the
tire turns, this vertical force does not stay balanced so
the deformability also changes and a trapezoidal
pressure-distribution pattern is produced. The lateral
force acts upon the wheels when the vehicle is getting
steered or there is a crosswind affecting the
aerodynamics of the vehicle (Reif, 2014).
The coefficient of friction (µ) and the contact load
(between surface and object) of the tires limit the
maximum friction force that can be exerted at the tire
patch. Apart from the longitudinal and lateral
components of force, wheel slippage is also a factor
upon which the traction and steering forces depend.
The torque applied to the wheel is directly
proportional to the wheel slippage. If the torque is
increased, the wheel slippage will also increase to
generate traction. Once the applied torque will exceed
the maximum traction available at the tire patch, the
wheel will obtain a quick-escape slip condition (Mohan,
S, 2000)
In the conditions where the surfaces have low
coefficient of friction, wheel slippage should be
avoided. On surfaces covered with snow, the wheel
tries to push the upper layer of the snow which makes
the bottom layers to be more compact making the
surface with even less µ which further reduces the
traction. On surfaces like that of sand and mud, the
wheel buries itself into the surface due to the wheel
spinning action which then requires more effort by the
vehicle to get out (Mohan, S, 2000).
Yaw movements (causing the rear end to swing
around) occur on the launch of the vehicle whether the
wheels are in straightened or angular direction. During
the cornering/turning of the vehicle, the yaw
movement is very dependent upon the steering
geometry (figure 5) and the speed of the vehicle.
During slow turning, the steering angles are
significantly high and there is less slide-slip of the tires.
If there is improper alignment between the steering
geometry and the driveline of the vehicle, it can make
the tires follow a path different than that of the vehicle.
This can subsequently lead to the driveline and
suspension wind up and the release of the formed
energy through tire slippage in the surface can lead to
the uneven linear and yaw motion. This can also be
experienced by the driver through the steering
feedback as the steering may stiffen up (Mohan, S,
2000).
Bir Armaan Singh Gill et al Four-Wheel Drive System: Architecture, Basic Vehicle Dynamics and Traction
365| International Journal of Current Engineering and Technology, Vol.8, No.2 (March/April 2018)
Figure 5: (Mohan, S, 2000)
Lateral forces experienced on the tires become
significant at higher cornering speeds. To counter
these forces, the wheels need to push the vehicle into
the desired path which is done with the help of
steering forces. The load and the coefficient of the
friction are the limiting factors for the total force of
friction at the tire. Acceleration and braking can reduce
the lateral steering forces at the tires. Having an
unbalanced steering force and lateral forces can cause
the vehicle to under steer or over steer.(Mohan, S,
2000) A vehicle under steers when the lateral
acceleration increases, due to which the front axle
lateral slip angle increases more than that of the rear
axle lateral slip angle. When the rear axle lateral slip
angle is more than that of the front axle lateral slip
angle, the vehicle over steers. When the centrifugal
force acting on the vehicle becomes more than the
lateral forces acting on the wheel, the vehicle shifts
from the desired course. The vehicle will under steer if
it slips at the front wheel and over steer if it slips at the
wheel axles (Reif, 2014).
Figure 6: (Reif, 2014)
The directional stability of the vehicle is important to
move the vehicle in intended direction. Traction
Control System (TCS) is a sort of dynamic security
control framework, which may enhance a driver's
capacity to control a vehicle under unfavourable
conditions. For instance, a wet or frozen street and
control the slip proportions of driving wheels work in
the ideal traction execution conditions in the order to
augment the traction power between the vehicle's tires
and the street. At exhibit, rationale edge technique in
view of tests is broadly utilized as a part of TCS (Liu Z,
2011).
Traction control system is not the newly developed
technology and also there has been always the need of
such system in hilly areas. The root of the system lies in
1950’s where the system was originally developed for
airplanes but further have been optimized by the
automotive engineers to be used in cars to enhance the
safety on roads. The system was first introduced by
Mercedes Benz in 1987, further BMW came into the
privileged once and later it was implemented in Supra
(Liu Z, 2011).
The system is completely based on the spinning of
the wheels. Once it senses the significant fasting of one
wheel over the other it invokes ABS(Anti-Braking
System consisting of ECU-Electronic Control Unit) and
hence brakes create friction to the wheel spinning with
lessened traction, as a outcome of this mechanism the
power is transmitted to differential. Hence to be
equipped with this technology, each wheel should be
equipped with speed sensors and the ECU should be
designed accordingly (Liu Z, 2011).
Conclusion
The infusion of technology into the fundamental
architecture of 4x4 Transmissions has helped to make
the vehicles more secured, automated and more power
efficient. While there will always be a segment in the
market which would prefer the simple transfer case
and Part Time 4x4 mode which are less electronically
complex, the future will bring the advancements which
will be more technologically dependent benefitting the
more tech savvy segment. Introduction of the traction
control systems have made the vehicles more capable
in their performance on both off-road and on-road
surfaces by keeping the driver's safety as a priority. As
it is still a newly developed technology having a
complex functionality, the technology incorporated
into the advanced 4x4 transmissions such as AWD and
Full Time 4WD still has a long way to go. These lesser
technologically complex system such as the Part Time
4WD still overshadow the present advance systems in
performance in many harsh environmental conditions.
As these advanced systems are more electronically
dependent today, failure of a single component such as
speed sensors can lead to multiple failures in the
system whereas the fundamental mechanical devices
are more rugged and durable. These electronic
components in an advanced technological system can
Bir Armaan Singh Gill et al Four-Wheel Drive System: Architecture, Basic Vehicle Dynamics and Traction
366| International Journal of Current Engineering and Technology, Vol.8, No.2 (March/April 2018)
be adversely affected by the harsh environmental
conditions. Future promises the advanced systems to
be more secure and efficient, making the technology
incorporated in these systems to be more lighter,
transparent, efficient and safe. Therefore, the
fundamental mechanical devices should be considered
as a more promising option in the present time.
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