Southern Illinois University Carbondale
College of Engineering
Saluki Engineering Company
F13-58-MOON
Literature Review
Date Submitted: October 8, 2013
By:
Wei Jun Lim (PM)
Adam Yun Wei Ee
Tengsong Ng
Kelsey Staggs
Anthony Bolton
Kaylyn Shaw
Fall 2013
Table of Contents
Introduction(Wei Jun) ................................................................................................................................... 3
Frame(Wei Jun) ............................................................................................................................................. 4
Suspension(Kelsey) ....................................................................................................................................... 8
Drivetrain(Anthony) .................................................................................................................................... 11
Steering(Tengsong) ..................................................................................................................................... 14
Braking(Tengsong) ...................................................................................................................................... 19
Seating(Adam)............................................................................................................................................. 20
Safety(Adam) .............................................................................................................................................. 22
Telemetry System(Kaylyn) .......................................................................................................................... 24
Conclusion(Wei Jun).................................................................................................................................... 27
Bibliography ................................................................................................................................................ 28
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Introduction
The Great Moonbuggy Race is an annual event sponsored by NASA. It is held at the
Space and Rocket Center in Huntsville, Alabama. The race is held to pay respect to the original
Lunar Roving Vehicles which were used on NASA’s missions to the moon. Engineers of the
original Lunar Roving Vehicle had to create a vehicle which could navigate lunar terrain, light
weight, while still fold to fit on the Lunar Module. These properties are made into rules and
criteria which dictate the challenges faced by participants of the Great Moonbuggy Race.
The Moonbuggy is divided into eight main subsystems; frame, suspension, drivetrain,
steering, brakes, seating, safety, and telemetry system. Each subsystem must be individually
design while being able to be incorporated into the final assembly flawlessly.
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Frame
The frame serves as the main connecting body of the Moonbuggy to all the other systems
on the buggy. For reference, the 2013 Moonbuggy is used as an example.
Figure: 2013 Moonbuggy
There are three distinct parts on the frame of the buggy (see figure 1). The parts in the red boxes
are the mounting parts on the frame, whereas the blue box shows the hinge which allows the
buggy to fold. There are 2 main factors in designing the frame: geometry and materials. The goal
is to create a frame rigid and strong enough to withstand an off-road terrain like the course, while
being as light as possible to increase speed and acceleration.
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Due to the similarities of the Moonbuggy frame to a bicycle frame, research has shown
that there are 4 possible materials; 4130 Steel, Aluminum 6061, Carbon Fiber, Titanium 3/2.5
would be suitable in constructing the frame. The main CTQ’s of a Moonbuggy frame is weight,
strength, elasticity, required tooling, and cost.
Table 1: Material Properties [1]
Material
Density
(lb/in3)
Mod. of
Elasticity
(Mpsi)
Steel 4130
0.283
30
Aluminum 6061
0.098
10
Carbon Fiber
0.05-0.067
~22
Titanium 3/2.5
0.163
15.2-17.4
Yield
Strength
(kpsi)
70-85
35-40
120-175
105
Tooling
Cost ($ per foot of 1”
tubing)
MIG Weld
TIG Weld
Adhesive
TIG Weld
$7.89 (0.083” Wall)
$6.37 (0.125” Wall)
$20.56 (0.060” Wall)
$30.00 (0.065” Wall)
Material selection of the frame will be based on 5 main criteria: density, modulus of
elasticity, yield strength, tooling and cost (see table 1). Density determines the overall weight of
the frame, modulus of elasticity determines the stiffness, and yield strength determines the
amount of load the material can handle safely. Research shows that the material used last year,
4130 steel, also known as chromoly [2], is one of the stiffest material while being the most dense.
It also has considerable yield strength, and is relatively cheap compared to carbon fiber and
titanium. Aluminum is the weakest among the four materials, with the lowest yield strength and
modulus of elasticity. However, its density is only 1/3 of chromoly steel. It is also the least
expensive material to acquire. Carbon fiber is one of the non-metal materials which are
commonly used in bicycle frames. This material is relatively expensive if bought in tube form. It
has the lowest density among all the other material, while having the highest yield strength as
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well as considerable stiffness. However, unlike metals, bonding carbon fiber requires specialized
adhesive. This could pose as a problem as the frame might have multiple angles, which can be
hard to produce using carbon fiber. The final material is titanium 3/2.5 alloy. This alloy is the
strongest metal material among the four, but it is also the most expensive material. Research also
reveals that using TIG welding with this material might compromise its strength due to welding
[3].
Another aspect of the frame is the geometry. The most successful teams always choose
one of two primary designs: single rail and triple rail. The 2012 Moonbuggy utilized a single rail
design [4]. This design is simple, and the folding mechanism is a simple hinge and locking
mechanism thus reducing the overall weight; however, due to the lack of surfaces, mounting of
other components is harder. A single rail might also be less rigid, and more prone to failure due
to the lack of other support structures. A triple rail system can be more complicated; however, it
provides a sufficient amount of surface area to mount other components. A triangular shape can
also allow the buggy to be more rigid; however, triple rail could be significantly heavier due to
the amount of material used.
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Figure 1: 2012 Moonbuggy - Single Rail [4]
Figure 2: 2013 Moonbuggy - Triple Rail [2]
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Suspension
Several types of suspension systems are feasible for the Moonbuggy project. The
suspension system is needed to ensure that the tires stay in contact with the ground as much as
possible, and to give the passengers of the Moonbuggy a safe and comfortable ride. If the tires
lose contact with the ground, the buggy will slow down and passengers may lose control of the
buggy. The same is true for turning.
The first decision to be made is whether a dependent or independent suspension system
would be used. The advantage to a dependent suspension system is that the camber is fixed;
however, this is also a disadvantage, because when turning or going over bumps, the camber
change allows the tires to stay flat to the ground. Independent suspension is the obvious choice to
be implemented, because although more costly and time consuming, the benefits outweigh any
of the difficulties.
The simplest independent suspension system to apply is the MacPherson strut.
Figure 3: MacPherson Strut [5]
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The MacPherson strut is very simple and compact (see figure 4). This is beneficial when it
comes to saving space, which allows for space to put the drivetrain, steering, and frame. The
disadvantages are that it allows for a lot of camber angle change while turning. This means less
contact with the ground during turns which is detrimental for keeping control of the vehicle
along with keeping the maximum amount of force applied through the pedals to the ground. Also,
with this compact design, there are not many changes that can be made to prevent this from
happening. A MacPherson strut is one of the more simple suspension systems that do not prevent
rollover when traveling at high speeds during a turn. Because the strut is attached straight to the
frame, not much wheel travel can be gained and the frame is subjected to a loud vibration [6].
The next choice was a double wishbone suspension system (see figure 5). This uses two
a-arms that provide a cushion for the frame by allowing the wheels to move without the frame
moving. The a-arms are allowed to pivot, transferring the force to the strut rather than the frame.
This gives a smoother ride than the MacPherson strut.
Figure 4: Double Wishbone Suspension [7]
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A double wishbone suspension is very versatile. It can be modified to suit a specific purpose. It
also allows for a large amount of deflection in the wheel travel, which is needed because the
track is very rugged. The disadvantages of two a-arms is that, because there are more parts, the
amount of time and money required to build and adjust everything is much greater. However, if
the upper a-arm is shorter compared to the lower a-arm, the steering for the driver is facilitated,
and the wheels would want to turn back forward after turning. This setup helps with the camber
so that the wheels stay in contact with the ground more [6].
The 2013 Moonbuggy featured a double a-arm suspension system along with a pushrod
and rocker arm [2]. A pushrod and rocker arm suspension are key components in maximizing the
amount of wheel travel out of a shock absorber. Last year the Moonbuggy achieved 7.5 inches of
wheel travel while using a 2.5 inch shock absorber. Figure 6 shows this design. This year the
goal of the suspension system is the same – to gain the most wheel travel as possible. This
challenge can be accomplished by adjusting the geometry of the suspension components,
particularly the rocker arm.
Figure 5: Pushrod Suspension System [4]
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Drivetrain
The drivetrain of the Moonbuggy is the subsystem translates the power provided by the
rider into energy that moves the Moonbuggy. The components include: the pedals, transmission,
differential, drive shafts, axles, wheels, and tires. Of these, the main components would be the
transmission, differential, and drive shafts. All of these parts are a necessity and without them the
buggy would not move.
The transmission is the system which allows the Moonbuggy to run at various gears,
allowing different mechanical advantages throughout the course. During the competition, the
Moonbuggy will traverse many obstacles and rough terrain. Therefore, it is necessary to have
different gears to use for different applications. Pre-2013 Moonbuggies featured gearing hubs
such as the Shimano Nexus hub or the Rohloff Hub [4]. Both of these are internal gear hubs
mainly used for bicycles. However, pre-2013 Moonbuggies gearing usually failed, resulting in
only one useable gear. The 2013 Moonbuggy featured a HammerSchmidt FR two speed
transmission [2]. This transmission uses a planetary gear mechanism and has been proven by
many teams to be reliable. However, the HammerSchmidt transmission is very expensive and
costly. Another alternative to the HammerSchmidt transmission is the Patterson Metropolis
transmission. The Metropolis uses the same design as the HammerSchmidt, but at a lower cost.
Both have a 1.6:1 overdrive ratio, and both can be shifted under any load [8], [9] which means
that the driver does not need to stop, or slow down, their pedaling to shift to a new gear.
Many buggies from last year did not have a multiple gear setup and failed due to not being able
to overcome the obstacles. Consequently, competitors have to dismount the buggy and walk it
over the obstacle, which resulted in a penalty.
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Figure 6: Rohloff Hub
Figure 7: Shimano Nexus Hub
Figure 8: Metopolis Transmission
Figure 9: HammerSchmidt Transmission
One of the most important parts for a successful buggy is the differential, a set of gears
that allows the power being transmitted to be divided so that the wheels can turn at different rates
when turning. This is beneficial because when going into a corner, it is necessary for the outside
wheel to spin faster in order to make up for the greater distance around a corner. Without a
differential, there would be less traction, and increased tire wear. The 2012 Moonbuggy used a
modified golf cart differential in its design, which was a poor choice, as it was very heavy and
was the wrong type of differential to be used in this application.
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The problem with the golf cart differential is that if one wheel is in the air, all the power
will be transmitted to that wheel. Hence, the wheel on the ground will not receive any power,
effectively wasting the riders’ energy while not moving the Moonbuggy forward. The 2013
Moonbuggy featured a freewheel differential design [2]. The freewheel differential design
proved to be effective and much lighter than the golf cart differential. With a freewheel
differential, if one wheel is in the air, power is still transmitted to both wheels. Therefore, even if
one wheel is off the ground the buggy will still be able to move forward.
Figure 10: Components of 2013's Differential
Figure 11: Differential on 2013 Moonbuggy
The driveshaft transfers the power from the differential to the wheels. Because of the
suspension setup of the Moonbuggy, the drive shafts must be telescoping. This means that one
part of the drive shaft must be able to move into the other part of the driveshaft. This part is
important because as the suspension travels, the distance between the wheel and the differential
changes. If the driveshaft was solid and made of a fixed length, it would either limit the
suspension travel or it would break the driveshaft.
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Steering
Steering enables control on the Moonbuggy to travel in the desired direction. A vehicle
without steering will have no control of its direction. Several style of steering has been employed
on previous Moonbuggies, such as the ‘tank style’ steering and the under-seat steering.
The most commonly used steering system is the ‘tank style’ steering. The handlebars of
this kind of steering are installed at the front driver seat and connected to the wheels through tie
rods and linkage, enabling the front driver to push and pull the Moonbuggy’s wheels directing to
desired cornering angle.
The 2013 Moonbuggy has an under-seat steering [2]. In this design, the Moonbuggy
featured its handlebars under the front driver seats which are more spacious and comfortable for
the front driver who is also controlling the steering. With pivotal steering system, drivers can
lean into the turn, causing the frame to pivot resulting in a camber change at wheels. The change
in camber assists the Moonbuggy to turn at a sharper angle.
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Figure 12: 2013 Moonbuggy
Some teams used the standard bicycle steering system, which steers with the handlebars.
In 2000, SIUC Moonbuggy used a joystick steering system. Variation widens when Moonbuggy
is to be viewed as a recumbent bicycle. Over-seat steering, under-seat steering or pivotal-seat
steering could be used as the steering system of the Moonbuggy.
Figure 13: 2000 Moonbuggy - Joystick Steering
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Pivotal-seat steering system is extremely maneuverable. Essentially, the driver uses
his/her bodyweight, leaning left or right to control the travelling direction of the vehicle. A
pivotal-seat steering system also requires extensive practice and training in order to steer the
Moonbuggy successfully. In addition, using pivotal-seat steering system on a Moonbuggy which
has four wheels can be a challenge.
Figure 14: Pivotal-seat Steering
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Another steering system is the over-seat steering system commonly used by bicycles.
This steering system is simple to manhandle and is user-friendly. The mechanism is just a
conventional handlebar stem with steering tube connected. Handlebars are located at the front of
the cyclist to control the steering of the front wheels [10]. This is the most mechanically simple
system with no extra moving parts compared to other systems; however, having the handlebars
in the front of the rider, combined with a highly reclined seat on the Moonbuggy will cause a
restriction of vision. Additionally, this configuration takes up more space. According to the rules
of the competition, a Moonbuggy should be able to fit into a 4-foot cube [11]; this makes space a
very precious resource.
Figure 15: Over-Seat Steering
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An under-seat steering system has its handlebars fixed below the front driver’s seat on its
own pivot, and there is a tie rod running forward to the fork crown [10]. This system makes
maneuvering more comfortable and more space is preserved. Having the handlebar at a lower
point prevents the driver from getting exhausted easily. However, this configuration can be more
difficult to achieve and might need practice to get used to the handling.
Figure 16: Under-seat Steering
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Braking
The forms of braking practical to a Moonbuggy is limited, therefore, there is not much to
choose for a Moonbuggy’s braking system. The two most common braking systems present in
current bicycles are the rim brakes and disc brakes.
Rim brakes work by apply pressure to the rim of the wheel, creating friction force
opposing the angular force of the wheel, resulting in deceleration of vehicle [12]. Compared to
disc brakes, the rim braking system is simple and inexpensive, but works terrible during wet
conditions. For rim brakes to work, a contact point between the brake pad and the rim must also
exist. This can be difficult to achieve because the Moonbuggy wheels are typically mounted on
one side of the wheel, which restricts the mounting capability of the rim brakes.
Disc brakes have an additional disc which is connected to the side of the wheel, in the
case of a bicycle. The disc can be mounted at any point, as long as it is connected to the
drivetrain. Disc brakes clamp onto the disc to create the same friction force as rim brakes [12].
Instead of wearing out an expensive rim, only the disc would have to be replaced. Although disc
brakes cost more, it gives better performance than rim brakes.
Figure 17: Rim Brakes
Figure 18: Disc Brakes
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Seating
Seating is one of the difficult sections of the Moonbuggy to design. It needs to be
comfortable so that any sort of injuries such as muscle cramp or sprains will not occur to the
riders. The seats for the Moonbuggy have been similar since 2010 to 2012, in which they may be
fold before collapsing the Moonbuggy into 4 cubic feet. The seats are installed on the body
frame with a pair of supporting structure designed (see figure 20).
Figure 19: 2013 Moonbuggy Seating
The seating part must be parallel to the driver’s buttock to provide better support. This will
demonstrate a recumbent seating position to increase comfort (see figure 20).
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Figure 20: Recumbent Seating
There are various types of seating position but to enhance comfort. A bucket seat design
provides maximum comfort and one of the best sitting positions, thus, reducing the tiredness
during pedaling. Most race cars use a bucket seat design to increase back support for the driver.
This design accommodates the back bone and helps to relieve fatigue and stress during use.
Figure 22 illustrates a typical bucket seat design.
Figure 21: Bucket Seat
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Safety
Safety has always been paramount in any concepts and items. In automobiles, air bags are
installed for safety purposes. In the case of the Moonbuggy, the potential of overturning exists.
The 2013 Moonbuggy safety was an extended bar that is called the 'keel bars,' specially designed
and attached to the back of the seat [2]. Should the Moonbuggy overturn, the keel bars will
prevent the riders from touching the ground first. Also, the Moonbuggy will not fall on top of the
riders after a flip (see figure 23). The 2013 Moonbuggy’s keel bar is shown in Figure 24.
Figure 22: Moonbuggy rolling Over
Figure 23: 2013 Moonbuggy Keel bar
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The keel bars attached to the backside of the seats used a simple locking mechanism where a
small rod-like was inserted between the keel bars and the backseat bars to hold them in (see
figure 25).
Figure 24: Keel bar locking system
An alternative to this locking mechanism is to modify it into a retractable system. A
retractable keel bar will ease the unfolding procedure of the bars. This modification could save
precious seconds when unfolding the Moonbuggy.
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Telemetry System
“The purpose of a telemetry system is to collect data at a place that is remote or
inconvenient and to relay the data to a point where the data may be evaluated.” Telemetry is
commonly employed in the testing of moving vehicles such as missiles, aircraft, and cars. The
telemetry system is comprised of a specific set of communication systems [13, p. 1]. One
example of such a telemetry system is a wireless telemetry system for a solar car designed by a
senior design team at Georgia Technical University [14].
The telemetry system can be broken down into 7 main subsystems that start with the data
collection subsystem and end with display of the collected data in a meaningful or useful manner
[13, p. 2].
Table 2: Subsystems
Subsystem
1. Data collection
2. Multiplex system
3. Modulator, transmitter,
and antenna
4. Waveform channel
5. Antenna and Receiver
6. Demultiplex system
7. Data processing
Description (if needed)
Grouping
Contains: measurement sensors,
signal conditioners
Types: Time Division (TDM),
Frequency Division (FDM), hybrid
(combination of the two)
Test Vehicle Package
Test Vehicle Package
Test Vehicle Package
“Material” data must be sent
through
Contains: Radio Frequency (RF)
amplifiers, carrier demodulator
Types: TDM, FDM, hybrid
Transmission Medium
Data displayed by means of laptop
or other machine with display
Ground Equipment
Ground Equipment
Ground Equipment
The first three subsystems are a part of the Test Vehicle Package, and at the heart of the
Test Vehicle Package for Georgia Tech’s Solar Car is a Single Board Computer (SBC) which is
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Linux based [13, pp. 2-3], [14, p. 5].This SBC is responsible for the collection of data [14, p. 5].
Additionally, Georgia Tech employs an interface board, which serves as a central termination
point for various sensors and is responsible for scaling signals and prevention of over-current and
over-voltage [14, p. 5]. “The first subsystem, the data collection system, is made up of sensors or
transducers that convert a physical variable into an electrical signal.” This signal is typically
quite small and requires buffering or amplification before transmission to the multiplex system,
the second subsystem [13, p. 3]. Georgia Tech chooses to use Hall-effect type sensors, voltage
sensors, temperature sensors, and a GPS receiver. The second subsystem, the multiplex system,
will receive the output of the data collection system. This subsystem will separate data into
different “bins” for transmission by either frequency division multiplexing (FDM) or time
division multiplexing (TDM). FDM will separate data into different frequency bins, and the
TDM will separate data in the time domain. If the system is comprised of a combination of TDM
and FDM, it will be referred to as a hybrid system [13, p. 3]. For Georgia Tech, the multiplex
system is encapsulated in the SBC and takes data from sensors once per second, storing it in a
USB flash drive [14, p. 13]. “The third subsystem is the modulator, transmitter, and antenna.” In
this stage, the multiplexed data is modulated onto the carrier at the transmitter, which will be
made to drive an antenna. Usually, the system is made to transmit data at 1,435-1,535 MHz or
2,200-2,290 MHz band [14, p. 3]. However, Georgia Tech chooses to transmit over Wi-Fi from a
USB port of the SBC by means of an Asus Wi-Fi dongle mounted to the top-rear of the solar car
[14, p. 13].
The fourth “subsystem” is the medium in which the signal is transmitted. This medium,
for most purposes, is air, which is indeed the medium for Georgia Tech’s Solar Car [13, p. 3],
[14, p. 3].
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The fifth, sixth, and seventh subsystems make up the Ground Equipment grouping [13,
pp. 3-4]. The fifth subsystem is composed of the receiving antenna, radio frequency (RF)
amplifiers, intermediate frequency (IF) amplifiers and a carrier demodulator. The modulated
signal is received by an antenna and absorbed by its receiver, “where the RF signal is amplified
at the RF frequency, then converted into an IF carrier and amplified again.” For Georgia Tech,
the signals are received by a Wi-Fi USB at a laptop computer [14, p. 13]. The data (or
modulation) is extracted from the IF carrier by means of a process referred to as carrier
demodulation or detection [13, p. 5]. Georgia Tech does this on the aforementioned laptop
computer [14, p. 13]. In the sixth subsystem, the demultiplexing system, the data is separated (or
demultiplexed) by FDM or TDM techniques, or both, to direct the data from each individual
sensor to the correct channel [13, p. 5]. Again, Georgia Tech takes care of this on the laptop
computer [14, p. 13]. All programming required for the Ground Equipment is done in the C
language for Georgia Tech [14, p. 13]. Finally, at seventh and final subsystem, the nowseparated data in the channels is taken for display, recording, and processing [13, p. 5]. Georgia
Tech displays data on the same laptop computer [14, p. 7].
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Conclusion
Due to the success of 2013 Moonbuggy’s design, the new Moonbuggy should be
designed based on the same design; however, there is still rooms for improvement such as
reducing the cost, weight, while increasing aspects such as turning radius, suspension travel.
These modifications to the previous design will, hopefully, place this version a contender for
champion in the next Great Moonbuggy Race.
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