1 1. INTRODUCTION This chapter provides the motivation for the research presented in this thesis by describing some of the difficulties inherent in the sport of RC (Remote Control) car racing. Based upon the problems certain goals were established. Next, the objectives and approach to achieve them are explained. The chapter ends with an outline of the thesis. 1.1. Motivation For as long as products have been in development engineers have struggled to trade off between research and development time and quality or performance of the product under development. This is especially true for the RC motorsport industries. Let us take an example from Automotive engineering history to highlight that necessity to continuously improve the efficiency of research and development was due to changing market demands [1]. In the RC motorsport industry this market demand was replaced by the performance of one’ s competition. The RC hobby industry is now riding a wave of enthusiasts and has become a sport and in some formats has specific rules and regulations. The cars itself are a testament to the detailing and over engineering of such small components, that they show some likeness to formula one cars. However the people preparing them are not necessarily well trained and hence they lack the engineering to prepare their cars based upon numbers and stats, instead they choose to buy special parts labelled with superlatives. This thesis hopes to annihilate such misconceptions by offering the novice a chance to learn and use test results to make his
This is for the young enthusiasts who wish to make their final year project on Suspension. This rig was made for RC Cars and costed us around 7000 in total.
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Transcript
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1. INTRODUCTION
This chapter provides the motivation for the research presented in this thesis by
describing some of the difficulties inherent in the sport of RC (Remote Control) car racing.
Based upon the problems certain goals were established. Next, the objectives and approach
to achieve them are explained. The chapter ends with an outline of the thesis.
1.1. Motivation
For as long as products have been in development engineers have struggled to trade off
between research and development time and quality or performance of the product under
development. This is especially true for the RC motorsport industries. Let us take an example
from Automotive engineering history to highlight that necessity to continuously improve the
efficiency of research and development was due to changing market demands [1]. In the RC
motorsport industry this market demand was replaced by the performance of one’s
competition.
The RC hobby industry is now riding a wave of enthusiasts and has become a sport
and in some formats has specific rules and regulations. The cars itself are a testament to the
detailing and over engineering of such small components, that they show some likeness to
formula one cars. However the people preparing them are not necessarily well trained and
hence they lack the engineering to prepare their cars based upon numbers and stats, instead
they choose to buy special parts labelled with superlatives. This thesis hopes to annihilate
such misconceptions by offering the novice a chance to learn and use test results to make his
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car to race specifications without spending. By repeated tests he can find out the accurate
damping, spring stiffness and sprung weight to use. Thus they can self fabricate.
The indoor lab based nature of the product allows the experiments to be documented
properly and also the time taken for successive experiments is lesser. Also the repeatability
and accuracy of the rig are other pillars for this thesis.
Also this thesis is to provide a platform to develop the software and electronic
know how to be able to fabricate a full size test rig for BAJA as well as FSAE team of SRM
University
1.3. Approach
Figure 1.1 The Process flow chart of the thesis.
To achieve the goals the following approach is taken. The state of current full scale
quarter car test rigs is analysed. Then the requirements of the RC car enthusiast was
analysed. Then the rig that is to be built was foreseen to be able to fulfil these rigorous
demands of the customer. Once the conception was complete the CAD modelling and
analysis was started. Once a base frame was finalised, the fabrication process was initiated.
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Later on the electronics and pneumatic systems were prototyped around the completed
mechanical setup and the final prototype was assembled. Rigorous testing was done to
ensure the repeatability and safety of the test bench.
1.4. Outline
The following is a brief outline of the chapters to come. Chapter two provides the
background for such a study which includes complete literary review of the current state of
RC motorsport and the void that this product wishes to bridge. Also the salient features of full
scale quarter car rigs are studied and reviewed. These salient features are to be integrated as
the rig is scaled down. The mechanical design and development aspects are discussed in
chapter 3 with illustrations from CAD modelling and analysis.
In later chapters control aspects are discussed along with the automation and
electronics working and assembly discussed in detail. Once the construction details are
established the theory behind the model are shared and the mathematical models are
presented.
In the final chapter the test results are compared to theoretical models and the
inference is stated. This inference is aimed at establishing the product as an effective tool for
any RC enthusiast.
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2. LITERATURE REVIEW
The literature review aids in providing perspective before pursuing the task at hand.
In the first half of this chapter the problems with existing test rigs are studied. The second
part of this chapter deals with how the electronics are adapted to serve the function of control
as well as interface with the PC.
2.1. Vehicle Test Rigs
It is clear that the main purposes of a shaker rigs regardless of the number of posts is
that they are used to measure noise, vibration and harshness (NVH), to perform durability
tests and to improve handling[1,2,3]. These goals vary depending upon the nature of the
industry they are applicable. The Automotive manufacturing segment would primarily be
interested in the NVH and durability but on some occasions may want to improve their
vehicle’s handling without spending countless hours in the proving grounds. The RC racing
industry is slightly different. They are not interested in the NVH aspects; however durability
and handling are critical.
2.1.1 Complex Shakers
Among the most complex test equipments are complex shakers. Complex shakers can
be of 4-post, 7-post or 8 post variety. In a 4-post shaker each wheel of the car is supported by
a servo actuator. If tyre is not to be tested then the servos are directly coupled to the spindles.
Thus test rig can input various disturbances onto the chassis and the vehicle response can be
measured. The 7-post rig works in a similar manner with the exception of three additional
actuators and in 8-post there are four additional actuators. These extra actuators allow
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increased capability in the form of simulating vehicle response from inputs like acceleration,
braking and aerodynamic loads.
Figure 2.1 Image of Servo Test 7-post Test Rig
Figure 2-1 represents a Servo Test tire-coupled 7-post rig with a Formula 1 race car.
These complex test rigs offer an immense amount of capability, however they are very
expensive to build and maintain. They also present other difficulties. These rigs are very
sophisticated multi-input/multi-output (MIMO) systems which require a high degree of
control knowledge and understanding to use properly. Often, the complex nature of these
multivariable problems requires multi-step iteration to obtain a suitable drive file for
commanding each of the actuators. Once converged data is extracted from tests run on these
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systems, it is often very difficult to interpret and correlate to the real world counterpart. Some
reasons for these issues with more complex test rigs are the lack of literature and other
available documentation [5, 6]. To the authors knowledge only a handful of papers that
discuss multi-post test to any detail exist [2, 4, 7, 8]. It is likely that the lack of available
information is partially due to race teams and automotive companies trying to protect their
competitive advantage.
2.1.2 Current Quarter-Car Rigs
As an answer to the high complexity and expense of these systems, simpler test beds
such as the quarter-car test rig are used. A rig such as this reduces the complexity greatly by
only focusing on one corner or quarter of the vehicle. These may b considered one post or
two-post systems. Often, these systems can be viewed as a single input/single-output (SISO).
This greatly reduces computational time and complexity and often closed form solution may
be reached. This allows for much better understanding of both the problem and results.
Problems with existing quarter car test rigs are mentioned in the following illustration.
Often the suspension components are simplified; hence the suspension is not tested as a single
unit, but in the form of a simple linear spring. The analysis is made simpler at the expense of
correlation of the results with the actual vehicle test data. This point is illustrated in Figure 2.2
which shows a simplified quarter car test rig which has elastomeric mounts in the place of
tyres and air-springs make up for the suspension compliance. The theory is correct but the
execution is wrong.
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Figure 2.2 Simplified Quarter-car Test Rig (VT AVDL)
2.1.3. Functional Requirements
After reviewing the operational functions offered by the current state-of-the-art test rigs,
the following requirements were proposed for a new quarter-car test rig:
• Design for a large range of vehicle corner weights (buggy to truggy).
• Design for sprung mass external forces such as aerodynamic loading and/or weight
transfer
• Design in flexibility to add future functionality such as vehicle roll or rotating and/or
steering the tire.
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These functional requirements are made such that a new state-of-the-art test rig would
be as flexible as possible, allow for more accurate and realistic representation of the test
vehicle, and achieve these goals as inexpensively as possible.
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3. HARDWARE DEVELOPEMENT
This chapter discusses the engineering approach to the design and construction of the
quarter-car test rig used in this research. It begins with a general discussion of the quarter-
car rig’s design. Following, are detailed discussions of how each major component was
developed and or specified. Included is discussion of the implementation of the first
suspension tested with the new rig (Thunder Tiger XXT). The chapter closes by
summarizing the functionality of the new quarter-car rig and presents some future
developments being planned.
3.1. General Description
The Quarter Car is used to represent one corner of the test RC car. The schematic of
the first design is shown in figure 3.1 in this diagram the suspension type shown is Mc
Pherson strut type in the later stages this was changed to double wishbone type due to
availability of parts. However there was no change in the layout and functioning of all other
components.
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Figure 3.1 Schematic of the Quarter-Car test rig.
The above representation shows the critical components of the rig as the sprung mass,
sprung mass adapter plate, tire, tire pan, actuator, load cell and accelerometer. The sprung
mass and adapter plate are constrained to move in a single axis. The actual suspension of the
vehicle is attached to the sprung mass adapter plate via an industrial grade adhesive.
The actuator is fixed to the base plate via a threaded sleeve which in turn is welded
onto the base plate. The actuator is excited by the switching of the pneumatic direction
control solenoid valve or DCV. The solenoid valves are switched via an electrical pulse
width which is decided by the frequency at which the suspension is to be oscillated.
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The amplitude of the oscillation is controlled by the supply pressure which can be
adjusted in two steps via the regulator at the main pressure supply line (Coarse adjustment)
and via flow control valve located just before the DCV. The input vibration is fed into the
suspension system and then the RC vehicle response is noted via an accelerometer located at
the sprung mass plate. The amplitudes of both the wheel and the sprung mass are compared
to understand the suspension efficiency.
Figure 3.2 CAD Model of the test rig.
The above illustration Figure 3.2 shows the initial CAD model done in PRO-E. The
suspension components are shown as in the full assembly. The actuator position and length
were decided after checking the dimensions in this model.
The next illustration Figure 3.3 shows the suspension components much more clearly
as seen the upper and lower control arms as well as the wheel hub and tyre.
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Figure 3.3 The suspension components of and RC car (Thunder Tiger XXT)
3.2. Base Plate and Support Frame
The base plate and frame form the skeleton over which the entire assembly is built
such that it can withstand the vibrations and shocks that will be subjected on it over a period
of time. The base was a 350mm x 400mm x 6mm mild steel plate. The frame consisted of
two long L-sections each 400mm long and 50mm x 50mm cross-section. The L-sections
were placed with their length along the vertical axis and welded on to the base plate.
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3.2.1 Frame Modal Analysis
The vibrations from the actuator could undergo resonance with the frame and base
plate causing damage to rig as well as error in the readings. Before the frame was fabricated,
it was tested in ANSYS to determine the modal frequencies of the frame. The test was
performed on two separate designs namely Design I and Design II. The frame designs are
described below along with their modal analysis results.
Analysis of Frame Design I
Frame Design I consisted of two L-sections welded on to the base plate, the two L-
sections would each have a linear guide way to allow the suspension adapter plate to translate
along the vertical axis. The modal analysis however revealed that the first modal frequency
was quite low. This posed a safety issue, if the frame were to undergo resonance with the
actuators frequency at any point of time the rig would be damaged.
Figure 3.4 Meshing of the frame done in ANSYS
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Figure 3.5 Total displacement of Frame Design I at its modal frequency.
Figure 3.6 First six modal frequencies of frame Design I
119.7
204.66
399.36
766.54
Frame modal analysis of frame I
first modal second modal third modal fourth modal
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ANALYSIS OF FRAME DESIGN II
Figure 3.7 Meshing of Frame II
Figure 3.8 Total displacement of frame II
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Figure 3.9 First six modal frequencies of frame II
Hence it was concluded that the frame design II would be used in the final model.
This design ensured that the rig was dynamically safe. Also this new design minimised the
weight of the rig by 1.445 kg and the centre of mass of the rig as a whole was brought nearer
to the geometric centre.
3.3. Linear Guide
The Figure 3.10 shows the linear guide which is a readymade component available in
the market. It is used in heavy cabinet drawers; it consists of three U-sections separated by
balls which allow each section to slide out one by one. The component was chosen mainly
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192.4212.48
373.98
474.23
Frame modal analysis of frame II
first modal second modal third modal fourth modal
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for its smooth motion as well as the large travel available. The large travel allows a large
range of suspension to be tested on the rig.
3.4. Moving Masses
The moving masses are a critical part of the experiment. The experiment requires a
minimum load of 556gm to be added as minimum sprung mass of the RC car(RC-Radio
Controlled). The RC car weighs a total of 3.125kg the weight distribution was 50-50. The
minimum weight was used to serve as a control setup for the experiment, upon addition of
the remaining 235gm the car would reach its actual weight. For the purpose of fixing and
removing weights an aluminium block was drilled and threaded such that weights could be
added and taken off at will. The minimum weight was secured to the slider directly such
that the load was finally transferred from the slider to the wheel. The weights were secure
and immune to any movement during testing.
3.5. Suspension Adapter Plate
The sprung mass plate is the part to which the suspension components are fastened
securely. This plate has to be light such that it doesn’t disturb the sprung mass. The
material for the sprung mass plate was chosen as Aluminium Composite Polymer (ACP)
the durability combined with the light weight and the excellent machine-ability led to this
choice. The plate itself was 100mm x 100mm x 8mm square piece. The differential of an
RC car was cut and bonded with the ACP using industrial strength Anabond 201. This
ensured the plastics remained together under all loading conditions. The differential
provided the perfect mounting points for the suspension.
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Figure 3.10 Fabricated Test rig
3.6. First Application
The first suspension to be tested on this rig was a Thunder Tiger XXT model left front
suspension assembly; there are three reasons why this particular model was chosen for the
test bench. Firstly, the Thunder Tiger XXT 1:10 scale car is the most common type of RC
car found in India, this fact was supported by SRM WiRL members, many of whom owned
the same model. Secondly, the suspension layout of car had a lot of flexibility and has plenty
of room for adjustment; this would do away with complex movable fixtures for varying
caster, camber toe etc. Thirdly, the easy availability of spares for this model also added in
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the favour of this model; as for all other model cars the spares had to be ordered from
overseas manufactures and the time taken for the shipment would have made it impossible
for completing the project within the deadline.
Figure 3.11 Thunder tiger XXT
3.6.1. Adjustments
The suspension assembly had to be adjusted to ensure the hard point locations were
all consistent with the original RC car. The information for setting up the suspension is
available online at www.thundetiger.com this website provides working ranges for caster,
camber, toe, KPI (King Pin Axis Inclination), Axle loads and cornering weights. The table
below illustrates the ranges defined by the manufacturer and the actual values used in the