7 Design, static and dynamic analysis of automobile chassis

7 Design, static and dynamic analysis of automobile chassis
International Journal of Research in Advanced Engineering and Technology
Online ISSN: 2455-0876
www.engineeringresearchjournal.com
Volume 1; Issue 3; December 2015; Page No. 07-11
Design, static and dynamic analysis of automobile chassis
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G Ganga Rao, 2 M Suresh
M. Tech Student, Department of Mechanical Engineering Chirala Engineering College, Chirala
Assistant Professor, Department of Mechanical Engineering Chirala Engineering College, Chirala
Abstract
In the present scenario, the automotive industry has been one of the rapid growing industries and is facing heavy competition from
the competitors. This necessitates the need to work on various functional aspects of the automobile, starting from chassis design to
aesthetic design. As a part of this, the present work aims to study the static characteristics of automobile chassis.
The chassis acts as a skeleton on which, the engine, wheels, axle assemblies, brakes, suspensions etc. are mounted. The chassis
receives the reaction forces of the wheels during acceleration and braking and also absorbs aerodynamic wind forces and road
shocks through the suspension. So the chassis should be engineered and built to maximize payload capability and to provide
versatility, durability as well as adequate performance.
All real physical structures, when subjected to loads or displacements, behave dynamically. The additional inertia forces,
according to Newton’s second law, are equal to the mass times the acceleration. If the loads or displacements are applied very
slowly, then the inertia forces can be neglected and a static load analysis can be justified, but in reality the loads are dynamic in
nature. Hence, in this work, an effort is made to investigate the static and dynamic response of truck chassis due to road
undulations.
The geometric modeling of the various components of the chassis is carried out in part mode as 3D models using Pro/ENGINEER
2001 software. The section properties, viz, cross-sectional area details of the 3D modeled parts are estimated using the modeling
software. The above properties have been used as input while performing the finite element analysis using ANSYS7.1 software.
The finite element model of the chassis is created using ANSYS 7.1 package. Static analysis is done for vehicle on a plain road and
bump conditions. The model is subjected to static analysis for all the conditions specified. The stress and deflection plots are
determined.
Keywords: Modelling, Static analysis, Dynamic analysis, Chassis
1. Introduction
In this type of chassis construction the frame is the basic unit
to which various components are attached and body is bolted
onto the frame later on.
Functions of the frame:
1. To support the chassis components and the body.
2. To withstand static and dynamic loads without undue
deflection of distortion
Loads on the frame
1. Weight of the vehicle and the passengers, which causes
vertical bending of the side members
2. Vertical loads when the vehicle comes across a bump or
hollow, which results in longitudinal torsion due to one
wheel lifted (or lowered) with other wheels at the usual
road level.
3. Loads due to road camber, side wind, cornering force
while taking a turn, which result in lateral bending side
members.
4. Load due to wheel impact with road obstacles may cause
that particular wheel to remain obstructed while the other
wheels tend to move forward, distorting the frame to
parallelogram shape.
5. Engine torque and braking torque tending to bend the side
members in the vertical plane.
6. Sudden impact loads during a collision, which may result
in a general collapse.
Frame construction
The chassis consists of longitudinal members and the cross
members. The frame is upswept at the rear and front to
accommodate the movement of the axles due to springing. It
also keeps the chassis height low. The frame is narrowed
down at the front either to have a better steering lock, which
gives a smaller turning circle. C are the brackets supporting
the body. Dumb irons to act as bearings for spring shackles.
They also take the bumper brackets. Brackets are meant for
mounting the springs. The extension of the chassis frame
ahead of the front axle is called front overhang, whereas its
extension beyond the rear axle is called rear overhang.
Since the commercial vehicles have to carry large loads,
framed construction is invariably used for these. Because in
these. Because in these vehicles, ground clearance is larger
and sufficient space is otherwise available for steering the
vehicles, the frames for these have only straight members
without taper towards the front or upsweep at the front or rear.
The engine clutch and the transmission are all together to form
one rigid assembly, which is mounted usually on the front end
of the frame. It is supported on the frame at three places by
means of rubber blocks. This helps to isolate the engine from
road shocks and the body from the engine vibrations.
Moreover, this method accommodates any misalignment
between the engine or the transmission relative to the frame or
the body.
7 Various cross sections used for the side members of the
chassis frame. It is seen that the channel section and square
box section have bending stiffness as compare to a solid
square with equal cross section area whose stiffness is taken as
due to this reason, both these sections are used extensively for
side members. Sometimes the box section is formed by
welding a plate to a channel section or by welding twochannel section. That section is sometimes used along with
channel section, for cross members. For heavy-duty
applications, side members may be formed by placing two
channel sections back to back. The side and the cross members
are usually joined by riveting. However, the sub sections are
usually joined by lap welding.
Sub frames
Normally the various components are bolted directly to the
main frame. However, many a time, these components are
mounted on a separate frame called sub frame. The main
frame at three points further supports this sub frame. In this
way the components are
Isolated from the effects of twisting and flexing of the main
frame. The advantages of sub frames are:
The mass of the sub frames alone helps to damp vibrations.
The provision of sub frame simplifies production on the
assembly line and facilitates subsequent overhaul or repair.
1.1 Role of Chassis in Automotives
Every vehicle body consists of two parts; chassis and
bodywork or superstructure. The chassis is the framework of
any vehicle. Its principal function is to safely carry the
maximum load for all designed operating conditions. It must
also absorb engine and driveline torque, endure shock loading
and accommodate twisting on uneven road surfaces. The
chassis receives the reaction forces of the wheels during
acceleration and braking and also absorbs aerodynamic wind
forces and road shocks through the suspension. So the chassis
should be engineered and built to maximize payload capability
and to provide versatility, durability as well as adequate
performance. To achieve a satisfactory performance, the
construction of a heavy vehicle chassis is the result of careful
design and rigorous testing.
It should be noted that this ‘ladder’ type of frame construction
is designed to offer good downward support for the body and
payload and at the same time provide torsional flexibility,
mainly in the region between the gearbox cross member and
the cross member ahead of the rear suspension. This chassis
flexing is necessary because a rigid frame is more likely to fail
than a flexible one that can ‘weave’ when the vehicle is
exposed to arduous conditions. A torsionally flexible frame
also has the advantage of decreasing the suspension loading
when the vehicle is on uneven surfaces.
The chassis which is made of pressed steel members can be
considered structurally as grillages. It acts as a skeleton on
which, the engine, wheels, axle assemblies, brakes,
suspensions etc. are mounted. The frame and cross members
form an important part of the chassis. The frame supports the
cab, engine transmission, axles and various other components.
Cross members are also used for vehicle component mounting,
and protecting the wires and tubing that are routed from one
side of the vehicle to the other. The cross members control
axial rotation and longitudinal motion of the main frame, and
reduce torsion stress transmitted from one rail to the other.
2. Modeling of Chassis
2.1 Geometric Model of Chassis
Geometric modeling of the various components of chassis has
been carried out in part mode as 3-D models using
Pro/ENGINEER 2001. The properties, viz., cross-sectional
area, beam height and area moment of inertia of these 3-D
modeled parts are estimated in Pro/ENGINEER 2001. These
properties have been used as input while performing the finite
element analysis using ANSYS 7.1 software.
Fig 1: Model of chassis assembled in PRO-Engineering
8 2.2 Material
Alloy steel material is generally used for the manufacture of
the chassis and the properties of the material are shown in
Table 1
Heavy-duty chassis are usually manufactured with either
frame rails of steel or aluminum alloy. Each material must be
handled in a specific manner to assure maximum service life.
Many heavy-duty trucks are presently manufactured with
frame rails of mild steel high-strength low-alloy steel or heat
treated steel. Material thickness increases, as the truck’s
intended duty becomes more severe. The depth of the rail also
increases with duty severity. The on-road tractor rails will
usually be less, than the damper rails.
 Steady-state inertial forces (such as gravity or rotational
velocity)
 Imposed (nonzero) displacements
 Temperatures (for thermal strain)
 Fluences (for nuclear swelling)
The deflection and stress pattern in the model of the chassis is
obtained by performing static analysis. The results of model
are shown in Tables.2 and 3 and the general pattern of
deflection, stress and strain are shown in Fig. 2, Fig. 3 and Fig.
4 respectively. The design stress for the alloy steel material of
which the chassis is made is 500 MPa. Based on this, the
factor of safety is estimated as shown in Table 2.
Table 2: Static deflections in chassis
Table 1: Properties of alloy steel
Sl.no.
1
2
3
4
5
Properties
Modulus of rigidity
Poisson’s ratio
Mass density
Yield strength
Element type
Value
210 GPa
0.3
7800 kg / m3
500 MPa
BEAM188
Condition\Model
Vehicle On Plain Road
Front Wheels On Bump
Back Wheels On Bump
Side Wheels On Bump
Diagonal Wheels On Bump
Maximum Deflection (mm)
0.23
4.15
4.1
1.53
1.1
Table 3: Static stresses in chassis
3. Static Analysis
Static analysis is used to determine the displacements,
stresses, strains, and forces in structures or components caused
by loads that do not induce significant inertia and damping
effects. Steady loading and response conditions are assumed;
that is, the loads and the structure's response are assumed to
vary slowly with respect to time. The kinds of loading that can
be applied in a static analysis include:
 Externally applied forces and pressures
Condition\Model
Vehicle On Plain Road
Front Wheels On Bump
Back Wheels On Bump
Side Wheels On Bump
Diagonal Wheels On Bump
Factor of safety
Maximum bending stress (MPa)
6.53
16.6
15.4
10
9
30.12
Fig 2: Maximum strain developed on chassis
Table 4: Results
Maximum Deflection
Maximum Stress
Maximum Strain
Factor Of Safety
0.236MM
16.6MPa
0.22E-6
30.12
4. Dynamic Analysis
4.1 Modal Analysis
Modal analysis is performed on the chassis and the natural
frequencies have been found out, the first five natural
frequencies are listed in Table 5
9 Table 5: Natural Frequencies
Set
1
2
3
4
5
Natural Frequencies (Hz)
52.264
68.680
125.96
151.88
213.67
Fig 5: Fourth Mode Shape
The natural frequencies for model range from 52Hz to 213Hz.
Table 5 indicates that the highest forcing frequency is in the
range 30-40 Hz, whereas the fundamental natural frequency
for models 52 Hz. Hence the fundamental natural frequency is
well above the forcing frequency range, which shows that the
chassis is safe from resonance point of view.
5. Conclusion
 Modelling has been done in solid mode and the section
properties of different models are estimated using
Pro/ENGINEER 2001.
 The deflection and stress pattern in the model of the
chassis is obtained by performing static analysis.
However, include steady inertia loads (such as gravity and
rotational velocity), and time-varying loads are also
included that are approximated as static equivalent loads.
 Maximum Deflection of chassis was found to be .2mm
and maximum stress was found to be 16.6MPa.
 The design stress for the alloy steel material of which the
chassis is made is 500 MPa. Based on this, the factor of
safety is estimated as 30.12.
 We can do case studies by changing the cross section
types of both longitudinal and cross beams of the chassis.
 By altering the locations of the cross members, we can do
a number of case studies.

Material of chassis can be altered
o Alloys of steel for Heavy duty chassis.
o Alloys of Aluminum for light weight chassis.
6. References
1. Theory of Vibrations by Grover.
2. Finite Element analysis, by Krishnamoorthy.
3. Finite and boundary methods in engg by O.P.Guptha
4. Pro/Engineer for Engineers & Designers by Prof. Sham
Tickoo
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