A Comprehensive Study on Suspension System and Tilting Vehicle

Abstract

Nowadays, many researchers are working on active tilting technology to improve vehicle
cornering. The concept of ‘active tilting technology’ has become quite popular in narrow
tilting road vehicles and modern railway vehicles. In this paper, the development of
active tilting technology has been reviewed. To tilt the vehicle inward during cornering,
tilting actuators are used as an element of the active suspension system. Leading
automotive companies have started to use intelligent suspensions in their high-end
automobiles’ to tilt vehicle. But much more research and developments are required in
design, fabrication and testing the suspension system and many challenges need to be
overcome in this area. This paper high lights different type of suspension systems which
are being widely used. It has been realized that semi-active suspension system is suitable
for road vehicles due to its performance and reliability.

Full text

Australian Journal of Basic and Applied Sciences, 9(30) Special 2015, Pages: 46-53
ISSN:1991-8178
Australian Journal of Basic and Applied Sciences
Journal home page: www.ajbasweb.com
Corresponding Author: S.B.A. Kashem, Faculty of Engineering, Swinburne University of Technology Sarawak, Kuching
93500, Sarawak, Malaysia.
A Comprehensive Study on Suspension System and Tilting Vehicle
1S.B.A. Kashem, 2T. Saravana Kannan, 3T.A. Choudhury, 4M.A. Chowdhury, 5Sajib Roy, 6A.A. Safe, 7M. Ektesabi, 8R. Nagarajah
1,2,4Faculty of Engineering, Swinburne University of Technology Sarawak, Kuching 93500, Sarawak, Malaysia.
3Faculty of Science and Technology, Federation University Australia, Churchill, VIC 3842, Australia.
5Faculty of Engineering , East West University, Dhaka, Bangladesh.
6Faculty of Engineering , Chittagong University of Engineering and Technology, Chittagong, Bangladesh.
7,8Faculty of Science, Swinburne University of Technology, Hawthorn, VIC 3122, Australia.
A RT I CL E I N FO
A B ST RA C T
Article history:
Received 13 November 2013
Accepted 23October 2013
Available online 30November
2011
Keywords:
Vehicle; Semi-active; Suspension;
Damper; Car; Adaptive;
Intelligent; review; comparison.
Nowadays, many researchers are working on active tilting technology to improve vehicle
cornering. The concept of ‘active tilting technology’ has become quite popular in narrow
tilting road vehicles and modern railway vehicles. In this paper, the development of
active tilting technology has been reviewed. To tilt the vehicle inward during cornering,
tilting actuators are used as an element of the active suspension system. Leading
automotive companies have started to use intelligent suspensions in their high-end
automobiles’ to tilt vehicle. But much more research and developments are required in
design, fabrication and testing the suspension system and many challenges need to be
overcome in this area. This paper high lights different type of suspension systems which
are being widely used. It has been realized that semi-active suspension system is suitable
for road vehicles due to its performance and reliability.
© 2015 AENSI Publisher All rights reserved.
To Cite This Article: S.B.A. Kashem, T. Saravana Kannan, T.A. Choudhury, M.A. Chowdhury, Sajib Roy, A.A. Safe, M. Ektesabi, R.
Nagarajah., A Comprehensive Study on Suspension System and Tilting Vehicle. Aust. J. Basic & Appl. Sci., 9(30): 46-53, 2015
INTRODUCTION
Vehicle performance during cornering has been
improved by most of the car manufacturers by using
electronic stability control (ESC). Car manufacturers
use different brand names for ESC, such as, Volvo
named it DSTC (Dynamic Stability and Traction
Control); Mercedes and Holden called it ESP
(Electronic Stability Program); DSC (Dynamic
Stability Control) is the term used by BMW and
Jaguar but despite the term used the processes are
almost the same. To avoid over steering and under
steering during cornering, ESC extends the brake and
different torque on each wheel of the vehicle. But
ESC reduces the longevity of the tire as the tire skids
while random braking. To overcome this problem a
vehicle can be tilted inwards via an active or semi-
active suspension system.
The concept of ‘active tilting technology’ has
become quite popular in narrow tilting road vehicles
and modern railway vehicles. Now in Europe, most
new high-speed trains are fitted with active tilt
control systems and these trains are used as regional
express trains (Goodall, R., 1999; Goodall, R., 1999).
To tilt the train inward during cornering, tilting
actuators are used as an element of the secondary
active suspension system. These actuators are named
as bolsters. In a road vehicle actuators are also used
to affect the vehicle roll angle via an active
suspension system. Since the beginning of the 1950s,
there has been extensive work done in developing the
Narrow Tilting Vehicle by both the automotive
industry and academic researchers (Karnopp, D. and
R. Hibbard, 1992; Hibbard, R. and D. Karnopp,
1996; So, S.G. and D. Karnopp, 1993; Saccon, A.,
2008; Frezza, R. and A. Beghi, 2003).
This particular small and narrow geometric
property of the vehicle poses stability problems when
the vehicle needs to corner or change a lane. There
are also two types of control schemes that have been
used to stabilize the narrow tilting vehicle. These
control schemes are defined as Direct Tilt Control
(DTC) and Steering Tilt Control (STC) systems as
detailed in (Kidane, S., et al., 2006; Piyabongkarn,
D., 2004). A typical passenger vehicle body can be
tilted up to 10° as the maximum suspension travel is
around 0.25 m. Then, the lateral acceleration of the
tilted vehicle caused by gravity can reach a
maximum of about 0.17g (Wang, J. and S. Shen,
2008). Since the lateral acceleration produced by
normal steering manoeuvres is around 0.3–0.5 g, the
active or semi-active suspension systems have the
potential of improving vehicle ride handling
performance. Semi-active or active suspension

47 S.B.A. Kashem et al, 2015
Australian Journal of Basic and Applied Sciences, 9(30) Special 2015, Pages: 46-53
systems can act promptly to tilt the vehicle with the
help of semi-active dampers or actuators. However,
the active suspension systems need to avoid over-
sensitive reaction to driver’s steering commands for
vehicle safety. Recently Bose Corporation presented
the Bose suspension system (Jones, W., 2005) in
which the high-bandwidth linear electromagnetic
dampers improved vehicle cornering. It is able to
counter the body roll of the vehicle by stiffening the
suspension while cornering. Car giant Nissan has
developed a four wheeled ground vehicle named
Land Glider. The vehicle body can lean into a corner
up to 17 degrees for sharper handling considering the
speed, steering angle and yaw rate of the vehicle. In
addition, in the works stated above and other
research, the effect of road bank angle is neither
considered in the control system design nor in the
dynamic model of the tilting standard passenger
vehicles (Bin Abul Kashem, S., 2014; Kashem,
S.B.A., 2012; Gohl, J.B., 2003; Rajamani, R., et al.,
2003; So, S.G. and D. Karnopp, 1997; Sang-Gyun, S.
and D. Karnopp, 1997; Li, Y., 1968; Mourad, L.,
2011; Roqueiro, N., 2011; Amati, N., et al., 2012;
Edelmann, J., 2011; Kashem, S.B.A., 2008). Not
incorporating the road bank angle creates a non-zero
steady state torque requirement. So this phenomena
needs to be addressed while designing the tilt control
and the dynamic model of the full car model. To lean
a vehicle which incorporates the road bank angle, the
response time of the actuator or semi-active damper
plays an important role.
Vehicle suspension system plays an important
role in vehicle tilting. A brief description of different
suspension systems is given in section 2.
Vehicle suspension system:
Fig. 1: Vehicle suspension.
A suspension system is an essential element of a
vehicle to isolate the frame of the vehicle from road
disturbances. Figure 1 shows a car suspension
system. It is required to maintain continuous contact
between a vehicle’s tyres and the road. The most
important element of a suspension system is the
damper. It reduces the consequences of an
unexpected bump on the road by smoothing out the
shock. In most shock absorbers, vibration energy is
converted to heat and dissipates into the
environment. Such as, in the viscous damper, energy
is converted to heat via viscous fluid. In hydraulic
cylinders, the hydraulic fluid is heated up. In air
cylinders, the hot air is emitted into the atmosphere.
But the electromagnetic damper is different; here the
vibration energy is converted into electricity via an
electric motor (induction machine or DC motor or
synchronous machine) and stored in a condenser or
battery for further use (Suda, Y., et al., 2004).
Suspension systems are categorized as passive,
active and semi-active considering their level of
controllability. Although all the types of the
suspension systems have different advantages and
disadvantages, all of them utilize the spring and
damper units.
Passive suspension systems are composed of
conventional springs and oil dampers with constant
damping properties (Figure 2). In this model m1 and
m2 represent the un-sprung mass and sprung mass
respectively, k 1 is the tyre stiffness coefficient or
tyre spring constant, k 2 is the suspension stiffness or
suspension spring constant. c0 and ct are the
suspension damping constant and the tyre damping
constant respectively, Fr is friction of suspension, q,
z1, z2 represents road profile input, displacement of
un-sprung mass and displacement of sprung mass
respectively.
Passive suspension system:
Fig. 2: Passive suspension system.
In most instances, passive suspension systems
are less complex, more reliable and less costly
compared to active or semi-active suspension
systems. The constant damping characteristic is the
main disadvantage of passive suspension systems.
For a passive suspension, the use of soft springs and
moderate to low damping rates is needed but the use
of stiff springs and high damping rates is needed to
reduce the effects of dynamic forces. Designers
utilize soft springs and a damper with low damping
rates for applications that need a smooth and
comfortable ride such as in a luxury automobile.
On the other hand, sports cars incorporate stiff
springs and a damper with high damping rates to gain
greater stability and control at the expense of
comfort. Therefore, the performance in each area is
limited for the two opposing goals (Gillespie, T.,

48 S.B.A. Kashem et al, 2015
Australian Journal of Basic and Applied Sciences, 9(30) Special 2015, Pages: 46-53
2006). There is always a compensation need to be
made between ride comfort and ride handling in the
passive suspension system as spring and damper
characteristics cannot be changed according to the
road profile.
Semi-active suspension System:
Fig. 3: Semi-active suspension system.
The semi-active suspension system was first
proposed by Karnopp et al. in 1973. In this model,
Figure 3 is a semi-active suspension model. Here fd
can generate an active actuating force by an
intelligent controller. Since then, semi-active
suspension systems have continued to acquire
popularity in vehicular suspension system
applications, due to their better performance and
advantageous characteristics over passive suspension
systems. In semi-active suspension systems, the
damping properties of the damper can be changed to
some extent. The adjustable damping characteristics
in semi-active dampers are achieved through a
variety of technologies, such as: Electro-Rheological
(ER) and Magneto-Rheological (MR) fluids,
solenoid-valves and piezoelectric actuators. It has
been widely recognized that a semi-active suspension
system provides better performance than a passive
system. As it is safe, economical and does not need a
large power supply, semi-active suspension has
recently been commercialized for use in high-
performance automobiles (Irmscher, S. and E. Hees,
1966; Konik, D., et al., 1996; Nakayama, T., et al.,
1996; Yi, K. and B.S. Song, 1999;
Sankaranarayanan, V., et al., 2008). However, there
still exist many challenges that have to be overcome
for these technologies to achieve their full potential.
MR degradation with time, sealing problems and
temperature sensitivity are some crucial issues of the
MR dampers that need development.
Active suspension system:
The active suspension system (Figure 4) actuates
the suspension system links by extending or
contracting them through an active power source as
required. Conventionally, automotive suspension
designs have been a compromise between the three
contradictory criteria of road handling, suspension
travel and passengers comfort. In recent years the
use of active suspension systems has allowed car
manufacturers to achieve all three desired criteria
independently.
Fig. 4: Active suspension system.
A similar approach has also been used in train
bogies to improve the curving behaviour of the trains
and decrease the acceleration perceived by
passengers. But this makes the system expensive and
increases the design complexity and energy demands.
From the above discussion, it is apparent that a
semi-active suspension system is more appropriate
for implementing and evaluating the performance of
various control strategies.
Active tilting technology:
The concept of ‘active tilting technology’ has
become quite popular in narrow tilting road vehicles
and modern railway vehicles. Now in Europe, most
new high-speed trains are fitted with active tilt
control systems and these trains are used as regional
express trains. Some of the vehicles use actuator or
active suspension system to tilt the vehicle.The
description of tilting road vehicles technology is
given in the following sections.
Narrow titling road vehicle:
Narrow vehicles are characterized by a high
centre of gravity and relatively narrow track width
compared to the standard production vehicle. These
vehicles would be more efficient and pragmatic
considering parking problems and traffic congestion
in urban areas. They would also reduce energy
consumption. These new cars are small,
approximately half of the width of a conventional car
(less than 2.5m in length, 1m in width and 1.5m in
height). All over the world traffic congestion is a
growing problem. Furthermore, the average number
of occupants including the driver of a single vehicle
in USA is 1.57 persons.

49 S.B.A. Kashem et al, 2015
Australian Journal of Basic and Applied Sciences, 9(30) Special 2015, Pages: 46-53
Fig. 5: Narrow commuter vehicle.
The narrow commuter vehicle shown at Figure 5
can be categorised by two types depending on their
tiling mechanisms. The first one Figure 6(a) uses an
active suspension system to tilt the whole vehicle and
the second one Figure 2-4(b) has an actively
controlled tilting passenger cabin and a non-tilting
chassis frame or rear assembly. An actuator fitted to
the rear assembly controls the tilt action of the
passenger cabin according to the design criteria. The
non-tilting assembly of the vehicle typically consists
of several power train components so therefore it
contributes considerably to the mass and inertia of
the vehicle. Moreover, the non-tilting chassis has to
support the roll torque which has been applied to tilt
the passenger cabin by the actuator. As a result, the
suspension of the vehicle wheel needs to be quite
stiff which may affect the ride comfort. Furthermore,
the energy consumption of this tilting mechanism is
also very high.
This particular small and narrow geometric
property of the vehicle poses stability problems while
cornering or lane change. There are also two types of
control schemes that have been used to stabilize the
narrow tilting vehicle. These control schemes are
defined as Direct Tilt Control (DTC) and Steering
Tilt Control (STC) systems as detailed in (Rajamani,
R., 2006). In the DTC system, the driver steering
input is connected to the front wheel steering
mechanism directly. In a DTC system, dedicated
actuators control the tilt of the vehicle (such as
having an active suspension). In this system, the link
between the wheels and the steering wheel is no
longer mechanical. In an STC system, on the other
hand, STC or steering tilt control, no additional
actuator is used, and the tilt of the vehicle is
controlled by the steering angle input from the driver.
The steering input is used to follow the desired
trajectory as well as stabilize the tilt mode of the
vehicle. This is particularly a steer-by-wire system.
In this system, the driver steering input signal is read
by the controller and the controller determines the tilt
angle. Since the beginning of the 1950s extensive
research has been done on both types of control
systems by the automotive industry and researchers.
(a)
(b)
Fig. 6: (a) Vehicle tilt by suspension, (b) Vehicle tilt
by actuator.
Motorised tilting vehicles have been studied and
developed since the pioneering prototype proposed
by Ernst Neumann in 1945–1950. The Ford Motor
Company developed a two-wheeled lean vehicle in
the middle of the 1950s. It was gyroscopically
stabilised with retractable wheel pods for parking. In
the 1960s, the MIT presented a tilting vehicle which
was equipped with an active roll control. The design
was similar to a motorcycle. At the beginning of the
1970s, General Motors developed a tilting vehicle
called the ‘Lean Machine’. It had a fixed rear frame
and a tilting body module that was controlled by the
rider. The rider had to balance the tilting body using
foot pedals.
More recently, Brink Dynamics developed a
three wheeled car named Carver with a rotating body
and non-tilting rear engine. BMW and the
Universities of Bath and Berlin were presented
Clever in 2003. It consists of a non-tilting two-wheel
rear axle and a single front wheel that tilts with the
main body. The rear body remains in contact with the
ground in the same way as a conventional
automobile rear axle but the main body is connected
to the rear frame by a suspension layout enabling it
to lean like a motorcycle.
The manufacturer Lumeneo presented the Smera
and Piaggio presented MP3. At the Tokyo motor
show 2009 Nissan revealed the Land Glider, which is

50 S.B.A. Kashem et al, 2015
Australian Journal of Basic and Applied Sciences, 9(30) Special 2015, Pages: 46-53
a four wheeled narrow vehicle. Of all the above the
Carver One was sold commercially between 2006 to
mid-2009 and the MP3 has been on the market for
sale since 2006.
From an academic point of view researchers
have done an extensive amount of work on these
cars. D. Karnopp suggested that the narrow tilting
vehicle would have to lean into a corner and also
explained the optimum desired lean angle in his
research. Dean Karnopp and his co-workers have
also carried out a significant amount of research into
dynamic modelling of tilting vehicles. Karnopp and
Hibbard have proposed that a tilt actuator can be
employed to tilt a narrow tilting vehicle to a certain
desired tilt angle with the help of the direct tilt
control strategy. It is apparent that their research lays
down the basic ideas for designing a direct tilt
control system. However in some of their research,
they are unable to take into account the lateral
position acceleration of the vehicle while calculating
the desired tilt angle calculation. This caused the
controller to require a high transient torque.
There are a few publications which have
presented the idea of a virtual driver in a narrow
tilting vehicle. These virtual drivers are able to
follow a path without falling to one side. Saccon et
al. (2008) developed a dynamic inversion of a
simplified motorcycle model. This model is able to
obtain a stabilizing feedback through the standard
Linear Quadratic Regulatory control system. This
model allows the controller to calculate the state and
input trajectories according to a desired output
trajectory of the tilting vehicle. To avoid the direct
deal with the lean instability, Frezza and Beghi
(2003) took the roll angle as control input instead of
the steering angle input from the driver. They have
defined the path tracking as an optimization problem
of the controller design.
Snell (1998) proposed to start the tilting action
with the STC system then to switch to the DTC
system to maintain the tilting position. A three
wheeled prototype of a narrow tilting vehicle was
developed at the University Of Bath, UK. It
employed hydraulic actuators to tilt the cabin with
the help of DTC technology which has a high power
requirement (Poelgeest, A., 2007). Kidane et al.
(2008), applied hybrid control schemes with both
STC and DTC. This work employed a feed forward
plus PID controllers to stabilize the tilt of the vehicle
and a look-ahead error of the trajectory model was
used as the driver model. Chiou proposed a double
loop PID to control and to maintain the tilting
position and the rate of the vehicle (Chiou, J., et al.,
2009).
Defoort (2009) and Nenner et al., (2008) worked
with the trajectory-tracking and robust stabilization
problems of a rider-less bicycle. They developed a
dynamic model that considers geometric-stabilization
mechanisms. They also derived a combined control
system consisting of a second-order sliding mode
controller and disturbance observer. In their research
they adopted a simplified tricycle model as the
dynamic model of a bicycle.
In addition, in the research works stated above
and in other authors’ researches, the effect of road
bank angle is not considered in the control system
design and in the modelling of the dynamic model of
narrow tilting vehicles. The result of not
incorporating road bank angle is a non-zero steady
state torque requirement. It also significantly
increases transient torque requirements. Sang-Gyun
So and D. Karnopp (1993) considered the road bank
angle in their work, but it has no effect on the final
form of the control input. The authors specified that
the lateral acceleration of the vehicle be obtained
from the sensor readings mounted on the vehicle. But
it is evident that the reading of an accelerometer of a
narrow tilting vehicle would be contaminated by the
tilt angle, the road bank angle and the angular
acceleration of the vehicle.
Tilting standard production vehicle:
To improve vehicle performance during
cornering or sudden lane change advance
electromechanical and electronic systems are used,
for example, antilock braking systems, electronic
brake force distribution, active steering and
electronic stability programs. Nowadays, some
researchers have focused on active steering control to
improve vehicle cornering (Marino, R., 2007; Li, B.
and F. Yu, 2009; Du, F., et al., 2010). Recently, a
system was presented by Bose Corporation, namely,
the Bose suspension system. This system consists of
a power amplifier and a linear electromagnetic motor
at each wheel that is controlled by a set of control
algorithms. The high-bandwidth linear
electromagnetic dampers of this system respond
quickly enough to achieve better ride performance.
To date the prototype of the Bose suspension system
is installed in standard production vehicles and able
to achieve superior comfort and control
simultaneously. According to the manufacturer, the
Bose suspension system can counter the body roll of
the vehicle by stiffening the suspension while
cornering. It can also change the ride height
dynamically and is capable of performing the four
quadrant operations and the high bandwidth
operation. But it uses less than one third of the power
of the air conditioning system of a typical vehicle.
However, to date no commercial tests or design
details are available to the world from the Bose
Corporation which would allow an accurate and
unbiased comparison with other competitive
suspension systems.
Vehicle performance during cornering has been
improved by most car manufacturers using electronic
stability control (ESC). Car manufacturers use
different brand names for ESC, such as Volvo call it
DSTC (Dynamic Stability and Traction Control);
Mercedes and Holden call it ESP (Electronic

51 S.B.A. Kashem et al, 2015
Australian Journal of Basic and Applied Sciences, 9(30) Special 2015, Pages: 46-53
Stability Program); DSC (Dynamic Stability Control)
is the term used by BMW and Jaguar but whatever
the term used the processes are almost same. To
avoid over steering and under steering during
cornering, ESC extends the brake and different
torque on each wheel of the vehicle. But ESC
reduces the longevity of the tyre because the tyre
skids during random braking. To overcome this
problem a vehicle can be tilted inwards via an active
or semi-active suspension system.
Car giant Nissan has developed a four wheeled
ground vehicle for the future which is half-scooter
and half-car. The electric-powered Land Glider
shown at Figure 7 is approximately half the width of
a family car and is designed for busy city streets.
Fig. 7: Nissan Land Glider.
It uses a steer-by-wire system to control the
vehicle manoeuvrer and has small motors mounted at
each wheel. A computer in the Land Glider
automatically calculates the amount of lean required
to corner considering the speed, steering angle and
yaw rate of the vehicle. The vehicle body can lean
into a corner up to 17 degrees for sharper handling.
In addition, in the works stated above and other
authors’ researches, the effect of road bank angle is
considered neither in the control system design nor in
the modelling of the dynamic model of the tilting
vehicles.
Conclusion:
For a long time, active and semi-active
suspension systems have been employed as a
practical application for modern control theory. In
this literature review different suspension systems
have been reviewed. It has been explained that the
semi-active suspension system is the most suitable
for road vehicles. A brief literature review on
automotive tilting technology has also been done in
this chapter. This highlights that a direct tilting
method needs to be developed to tilt the standard
passenger vehicle inward during cornering while
considering the road bank angle.
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