SOUTHERN ILLINOIS UNIVERSITY CARBONDALE
COLLEGE OF ENGINEERING
DEPARTMENT OF MECHANICAL ENGINEERING
ME 495A – SENIOR ENGINEERING DESIGN
SAE SALUKI BAJA FRAME DESIGN
F13-BAJA-60
LITERATURE REVIEW
DATE SUBMITTED: 10/8/2013
SUBMITTED TO: SALUKI ENGINEERING COMPANY
TEAM MEMBERS:
Austin Lewandowski [AL]
Preston Mathis [PM]
Keegan Lohman [KL]
Kyle Koester [KK]
Steven Baldwin [SB]
Thang Quang Tran [TT]
FALL 2013
Table of Contents
Introduction [AL] [KL] ________________________________1
Frame Design [KL] [SB] [TT] ___________________________1
A.
B.
C.
D.
E.
Frame Design Overview [TT] [SB] [KL]___________________________2
General Baja Car Specification [TT]______________________________3
Problem Statement of Frame Design [TT] _________________________4
Solution [TT] [SB] ____________________________________________4
Triangulation [TT] [SB]________________________________________4
Material Selection [PM]________________________________5
A.
Material Comparison [PM] ________________________________6
Finite Element Analysis [AL] [KK]_______________________6
A.
B.
C.
D.
E.
Strategy of FEA [KK] [AL] ________________________________6
Calculating Forces [KK] __________________________________7
Force Application Points [KK] _____________________________7
The FEA Standard [KK] [AL]______________________________7
Analyzing FEA Results [KK] [AL] __________________________8
Conclusion___________________________________________9
References__________________________________________10
Introduction [AL] [KL]
The Society of Automotive Engineers (SAE) has many guidelines in the development of
an SAE Baja off-road vehicle. Teams must design a vehicle capable of conquering nearly any
terrain while still maintaining a high level of safety. This task provides several complications
while designing and building a vehicle that could be marketed to the general public as a
recreational vehicle in the amount of 4000 units per year [1]. The ultimate goal of this project is
to build a high performance, light weight, durable vehicle. This goal can be accomplished
through an analysis of the following areas: frame design, Finite Element Analysis (FEA), and
material selection. These areas will be researched in depth so that flaws present in current
methodology can be identified and improved upon.
Frame Design [KL] [SB] [TT]
The frame of any vehicle in its most basic form is an interior skeleton. This skeleton must
be strong enough to protect the driver from any potential harm. A 2014 Baja SAE rule states that
there must be a lateral spacing of 6-inch clearance around the driver’s helmet and a 3 inch
clearance around the driver’s shoulders, torso, hips, thighs, knees, arms, elbows, and hands.
Another rule is that for the elements of the roll cage which consist of primary members of the
Rear Roll Hoop (RRH), Roll Hoop Overhead Members (RHO), Front Bracing Members (FBM),
Lateral Cross Member (LC), and Front Lateral Cross Member (FLC) made of tubular steel. This
tubular steel must have a “bending stiffness and bending strength exceeding that of circular steel
tubing with an outside diameter of 1-inch and a wall thickness of 0.120-inch and a carbon
content of 0.18%. The wall thickness must be at least 0.062-inch, regardless of material or
section size” [1, p26]. Secondary members which consist of Lateral Diagonal Bracing (LBD),
Lower Frame Side (LFS), Side Impact Member (SIM), Fore/Aft Bracing (FAB), Under Seat
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Member (USM), any tube that is used to mount the safety belts, and all other required Cross
Members must have a minimum wall thickness of 0.035-inch and a minimum outside diameter
of 1-inch. Also the roll cage members that are bent must not exceed 28 inches between supports
on the straight sections [1].
A frame may be constructed from many different materials. Some larger vehicles are
made of square tubing. Although square tubing is stronger in members that are at angles, it has
more short-comings in the application of building a SAE Baja. Square tubing will kink because
the excess material in the corners. This means all of the square tubing would have to contain
mitered angles and then be welded to make one solid piece. Round tubing can easily be bent, and
to a much greater angle with only minimal kinking. Since, square tubing uses more material than
the standard round tubing, the overall weight of the car increases and the price is higher. The
goal of the project is to build a light weight frame, hence, the round tube would be preferred, not
only because of weight, but for economic reasons.
A. Frame Design Overview [TT] [SB] [KL]
To develop a preliminary design, the design guidelines should be set first. The design
guidelines will include not only design features, but also the limitations of tools used during the
build process. The suspension and steering design as well as intended fabrication methods must
be taken into account. Rules referring to frame geometry and safety of the driver must be
considered as well. The design process begins with the selection of major components such as
the overall dimension, ride height, wheels, suspension geometry, and drivetrain [2]. The most
common design for a Baja vehicle frame is a tubular space frame which has a series of tubes
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connected together in different ways to form a support structure [3]. An efficient frame must be
stiff enough to handle the loads and light enough to deliver good performance.
Research has shown that for loads higher than 9 times the force of gravity or 9 G’s, the
human body will lose consciousness. Therefore, 10 G’s is an extreme worst-case collision [3].
By calculation, a value of 10 G’s is equal to a static force of 26,698 N or 6,000 lbf load on the
vehicle. The estimated maximum g-force that a SAE Baja will experience is 7.9 G [4].
Therefore, the designed frame must protect and keep a driver alive during a 7.9 G front, 7.9 G
side impact and 7.9 G roll over situation.
B. General Baja Car Specification [TT]
Given the past data from table I, Baja’s that typically do well in all events are relatively
similar in most aspects.
Table I [5]
General Dimensions of Various Baja Submissions
University
SIU
SIU-C
IPU-FW
IPU-FW
UT-C
UW-P
BYU-I
BYU-I
AVG
Year of Submission
2013
2012
2008
2007
2006
2009
2005
2004
NA
Competition
Midwest
Midwest
Midwest
Midwest
Midwest
Midwest
West
West
NA
Empty Weight
550
597
472
NA
430
NA
NA
NA
512
Weight w/ Driver
NA
700
NA
NA
600
500
NA
NA
600
Overall Length
86
93
88
92
92.5
NA
NA
NA
90.3
Maximum Width
54
52
53
62
54
NA
NA
NA
55
Wheel Track
NA
NA
NA
NA
48.5
56
NA
NA
52.25
Static Ride Height -Front
12
10.5
12
12
10
12
11.5
8
11
Travel - Front
10
8
7
7
10
NA
10
5
8.14
Static Ride Height -Rear
12
10.5
12
12
10
12
14
10
11.56
Travel - Rear
12
7
3.5
7
8
NA
12
8
8.2
Weight Distribution
40:60
36:64
36:64
NA
40:60
NA
NA
NA
38:62
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C. Problem Statement of Frame Design [TT]
There are two main types of stiffness in vehicle frame design: bending and torsional
stiffness. Bending is not a concern for a Baja frame because midpoint bending does not affect
suspension performance [2]. Torsional stiffness has resistance in the frame to twisting loads [2].
Any misalignments in the suspension geometry can create moments within the frame structure
that could cause catastrophic failure of the frame, resulting in driver injury [2].
D. Solution [TT] [SB]
To increase torsional stiffness, two methods have been used by most Baja teams. One
method is to increase the amount of material to the frame structure, which means increasing the
total weight of the frame. Hence, the overall performance of the car would decrease. The other
method to increase torsional stiffness is triangulation, which is the more efficient technique. In
this method, several frame members are connected and combined to form triangles. This
technique will significantly strengthen the frame without adding unnecessary weight and will be
used in this project [2].
E. Triangulation [TT] [SB]
Nodes can be made significantly stronger by using the triangulation, a method capable of
handling large forces. Because these nodes can accommodate large loads, they are an excellent
location to mount the suspension points. In addition, triangulation creates a clear load path within
the chassis. With a coherent load path in a structure, forces are distributed evenly over the many
interconnected members. Thus triangulation reduces the force and stress felt by any individual
member. In contrast, without a coherent load path, the structure will fail due to the load being
concentrated on one element [2].
To reduce the weight of the frame, thin walled tubes may be used. These tubes perform
greatly in compression and tension. However, they do not perform well in bending. In order to
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help prevent the members from bending, the frame should be constructed from multiple shorter
members [2].
Material Selection [PM]
Some teams that compete at a SAE Baja sanctioned event use 1018 steel for a majority of
their vehicles frame fabrication. Typically, 1018 is the most readily available cold-rolled steel
and is very durable. The carbon content is quite low, at only 0.15 to 0.2 percent, the phosphorous
content of 0.04 percent maximum and sulfur content of 0.05 percent maximum are low enough
that they have little impact on the material's physical properties. Like other, more complex steels,
1018 contains trace elements such as chromium, tungsten and silicon, which give tool steels
added corrosion resistance and toughness [6]. The lack of a moderate chromium content and
simple chemistry, however, make 1018 prone to oxidation, necessitating an air-tight coating to
protect the steel from rusting too quickly [7].
The most commonly used steel for frame design is 4130, also known as Chromoly. The 4130
consist of two main alloying materials: chromium and molybdenum with a carbon content of
0.3%. Chromoly is widely used in aircrafts, bicycles, and drag race vehicles for its strength,
toughness, and ductility. It has great strength to weight ratio as well as excellent heat treating
properties. With a 100,000 psi normal tensile strength, this steel is ideal for off-road applications.
Its alloying also makes it considerably more rust resistant. The 4130 is available in two subsets:
4130A (annealed) and 4130N (normalized). Annealed indicates the softest form and means it can
be easily formed [6]. Normalized means the material is in its neutral, non-heat treated form. Its
nominal hardness gives 4130N a far greater strength than 4130A [7].
Table II gives a side by side comparison of the 1018 steel and 4130(N) steel. Clearly the
4130(N) meets or exceeds in all aspects necessary for an off-road vehicle.
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A. Material Comparison [PM]
Table II
Comparison of 4130(N) & 1018
4130(N) vs. 1018
4130(N)
1018
Density (g/cm3)
7.8
7.8
% Elongation at Break
26
15
Hardness Brinell
197
126
Strength to Weight Ratio Tensile, Ultimate (kN-m/kg)
85
56
Strength to Weight Ratio Tensile, Yield (kN-m/kg)
55
47
Tensile Strength Ultimate (MPa)
670
440
Tensile Strength: Yield (proof) (MPa)
435
370
Finite Element Analysis [AL] [KK]
A. Strategy of FEA [KK] [AL]
Finite Element Analysis has been a useful tool used to solve problems concerning chassis
and frame design since the mid 1960’s. FEA is a computerized process that enables engineers to
place calculated loads at certain “nodes,” or points of interest where a load would be
concentrated in the real world. Advantages of using FEA include improving safety, durability,
reducing material waste, and most importantly weight reduction [8].
B. Calculating Forces [KK]
It is important to calculate the limits of any product for safety reasons. It is also
imperative to incorporate a safety factor, which ensures a certain material will not fail even if
forces to a certain percentage above calculated limits are applied to a prototype [9]. Some of the
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primary calculations that need to be completed when designing a frame include area, force,
angles, acceleration, torque, stress, strain, and strength. Several of these calculations may be
computed through the FEA program itself, while some are found independently of the program.
C. Force Application Points [KK]
The ability to apply simulated forces to a frame, and view the reactions of every member
within a frame proves to be extremely critical in frame design. However, an individual must still
be knowledgeable on where to position forces, and how to calculate certain forces. The joints
supporting the wheels and suspension are obviously important because these particular joints
receive the most impacts. More than four actual joints are associated with the linking of the
wheels and suspension to the frame, but each individual joint is equally important. Other
essential force application points on a frame include front, side, rear, top, and even bottom
impact forces. These particular impact forces are the most important when considering the safety
of the operator. A favorable frame design will come from the proper load placement, calculation,
and utilization of FEA to optimize the weight, durability, and of course the safety of the vehicle.
D. The FEA Standard [KK] [AL]
The purpose of FEA regarding Baja is structural optimization. The goals of applying the
FEA process include producing more lightweight, durable, and efficient vehicles. The program
calculates if a member’s length or wall thickness can be downsized without compromising the
integrity. One strategy used by the Baja team at the University of Florida is an iterative method
consisting of tracking the stress throughout the frame over a period of 0.25 seconds. The team
used Chromoly tubing 1" in diameter with a wall thickness of 0.065”. The member at this size
left the Von Mises stress to be too high in regards to the desired safety factor; thus the size of the
member was increased to further reduce the stress at that node. This process was repeated until a
suitable size member that would absorb the loading at that point in time was found [4].
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There are a set of transient impact tests that are held as the standard for producing safe
vehicles for the general public. The tests include frontal, rear, and side impacts. For each of
these situations there are standards of the crash procedure. For example, the velocity of a frontal
and side impacts are evaluated at 40 mph and 30 mph, respectively [4]. Although these do cover
many daily driver incidents, it does not cover the typical roll over events seen in the SAE Baja
competitions. This area of impact to the top of a roll cage has been neglected because many
passenger cars rarely see this type of incident.
E. Analyzing FEA Results [KK] [AL]
FEA simply performs the advanced calculation of forces and stresses at different points
in a frame. When a node’s load exceeds its stress handling capabilities, the designer must be able
to adjust the design in the most efficient way possible. When considering the SAE Baja vehicle’s
frame, the most likely option for correcting an error would be to add or move a support member
to lessen the stress on the overloaded node. After one member or node is adjusted, the simulation
is repeated, and the iterative process continues [4]. The Federal University of Minas Gerais,
conducted a modal vibration analysis of a tubular structure vehicle. From testing six modes of
vibration including torsional loading and drivetrain vibration, the following conclusions were
reached [10]:

Triangulating chassis regions with braced members improves overall frame stiffness

Square cross section members applied to lower spars can improve fore roll cage stiffness

Vibrations from the engine operating at low speeds will excite the frame vibration mode

Severe torsional loads applied to fore and aft sections of frame can cause suspension
misalignment, and can be fixed with cross bracing.
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Conclusion
The main goal of this project is to produce a lightweight, durable, and marketable off road
vehicle capable of being sold to the public. Throughout all the research there were several final
conclusions that could be made about each of the three areas: frame design, material selection,
and FEA. Frame design for the SAE Baja is mainly governed by the guidelines set out in the
2014 SAE Baja Rulebook. All of the design is focused around producing the lightest weight car
possible, while maintaining a necessary level of safety. Material selection concluded that the
optimum choice for primary frame members as far as strength, durability, and weight are
concerned is 4130 Chromoly tube. Member size selection is very important in this stage so that
the roll cage will maintain its rigidity in various impact scenarios. What can be concluded from
FEA is that testing should be completed for static, as well as dynamic situations. Tests should be
ran for all typical car crash circumstances, as well as the often forgotten roll over situation. The
research of what has already been done in concerns to a particular project is always important
because there is no need to “reinvent the wheel.” This research is also beneficial in finding gaps
in others’ design processes, and gives a great place to start in terms of producing faster, lighter,
higher performance products.
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References
[1] SAE. 2014 Baja SAE Series Rules. SAE International 2014. 12 Sept. 2013.
<http://www.sae.org/students/2014_baja_rules_8-2103.pdf>
[2] Gaffney III, Edmund, and Anthony Salinas. "Introduction to Formula SAE Suspension and
Frame Design”. University of Missouri, Rolla, 971584.
[3] N. Noorbhasha, “Computational analysis for improved design of an SAE BAJA frame
structure,” University of Nevada, Las Vegas., Las Vegas, 736, Dec. 2010. Available:
http://digitalscholarship.unlv.edu/cgi/viewcontent.cgi?article=1737&context=thesesdissertations
[4] Mini Baja Florida Tech, “Chassis” Florida Institute of Technology, pp. 30-45. 2008.
Available: http://forums.bajasae.net/forum/uploads/130/FL_TECH_FRAME.pdf
[5] B. Probst et al., “SAE Baja Suspension Literature Review”, SEC., Carbondale., IL, Tech, Oct
2012.
[6] C. Shelton, “The Steel Age,” DIRTsports, Issue 9, p35-40, 6p. Sept. 2005.
[7] A. M. Soo, “Design, Manufacturing, and Verification of a Steel Tube Space frame Chassis
for Formula SAE,” Dep. Of Mech. Engr., Massachusetts Institute of Technology, Cambridge,
Jun. 2008.
[8] J. Pemegger. (1997, December 22). Finite Element Analysis of Bolted Flange Connections.
[Online]. Available: http://www.hydrocarbononline.com/doc/finite-element-analysis-of-boltedflange-conn-0001?VNETCOOKIE=NO
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[9] A. Purushatham. (2013, May 5). Static Stress and Deflection Analysis of a Three-wheeler
Chassis. [Journal]. Available: http://web.ebscohost.com/ehost/detail?vid=2&sid=b0e20a6a-e67341fa-9943-cd8ce4208798%40sessionmgr104&hid =118&bdata=JnNpdGU9ZWhvc3QtbGl2ZSZ
zY29wZT1zaXRl#db=ofs&AN=89631145
[10] Frederico Mol Alvares da Silva, “Modal Analysis of a Tubular Structure Vehicle Chassis”
Federal University of Minas Gerais – UFMG 2004-01-3423
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