Saluki Engineering Company
Team 31
SAE Baja Suspension
Literature Review
Team Leader:
Brett Probst
Team Members:
Joe Antonaci
Harjot Singh
Philip Thompson
Josh Walker
Table of Contents
Abstract
Review of Literature

Rules

SIUC Baja

General Dimension – JA

Suspension Overview – JW & HS

Front Suspension - JW

Rear Suspension - HS

Steering/Spindles – PT
Conclusion
Works Cited
Abstract
SAE Baja hosts annual collegiate competitions where teams design, build, and race a Baja. SIU’s
Baja team competed in 2012 and will do so again in 2013. Saluki Engineering Company Team
31, Baja Suspension, has the task of developing the suspension for this year’s vehicle. This will
consist of designing the front suspension, rear suspension, steering system, and spindles. The
final aspect of the suspension is the shocks. Unfortunately, there will not be enough funding to
purchase custom shocks that will fit the suspension design. Therefore, the team will need to
create a design with a geometry and weight distribution that can incorporate last year’s shocks.
The team must also design the suspension to conform to strict SAE Baja rules.
SAE Baja Competition Rules
The Baja competitions are hosted and strictly regulated by SAE Baja. At the beginning of all
SAE Baja competitions, teams are required to submit to a technical inspection. During this
technical inspection judges must determine that a team’s Baja is safe, was built with quality
engineering practices, and conforms to all competition rules. If a team is found to not meet these
requirements, the team is not allowed to compete. There are few limitations with regards to the
suspension, but basic design requirements shown below are taken the 2013 SAE Baja Rules:
Figure 1: SAE Mini Baja Technical Requirements
Saluki Baja Background
The Saluki Baja team submitted its first vehicle in the 2012 SAE Baja Midwest Competition.
The analysis of the previous submission will serve as an important reference in the design of the
2013 submission. For the SIUC team, as well as many others, the suspension presented the
largest room for improvement. Key issues with the suspension included low ride height, ball
joint failure, insufficient suspension travel, a high center of gravity, and loose steering. The low
ride height and travel constraints made it difficult to navigate the rough terrain of the course.
Because the suspension often moved through its entire range of motion, various parts of the
vehicle were subjected to large stresses, severely increasing the potential for component failure.
One such example was the ball joints in the front suspension, where improper geometry resulted
in multiple breakdowns. The high center of gravity meant that vehicle was prone to roll-overs.
This was not only a safety concern but also created the potential for damage to the vehicle and
disqualification from the race. Though the suspension was certainly a factor with regard to the
center of gravity, the low ride height suggests that the source of the issue was largely related to
mass distribution within the frame. The last major issue was the durability of the steering where
insufficient support caused the steering shaft to bend. In addition, the tie rod ends were subjected
to high stress, which resulted in failure. The extreme stress was caused by the geometry of the
spindle. The vehicle utilized an OEM spindle which was not appropriate for that application. The
spindle was too narrow, causing alignment issues and a tendency to shear. Devising solutions to
these design issues represent the main challenges of the 2013 suspension design team.
General Dimensions
As noted above, the SIUC 2012 Mini Baja submission will serve as a reference for the design
and construction of the 2013 Baja. During the design phase, the SIUC team will also consider
data collected from other teams when available. Design reports are available from India-Purdue
University at Fort Wayne [1], the University of Tennessee at Chattanooga [2], the University of
Wisconsin at Platteville [3], and Brigham Young University – Idaho [4]. Some of the general
data from these reports has been compiled below.
University
SIU-C
IPU-FW
IPU-FW
Year of Submission
2012
2008
2007
Competition
Midwest
Midwest
Midwest
Empty Weight
597
472
NA
Weight w/ Driver
700
NA
NA
Overall Length
93
88
92
Maximum Width
52
53
62
Wheel Track
NA
NA
NA
Static Ride Height - Front
10.5
12
12
Travel - Front
8
7
7
Static Ride Height - Rear
10.5
12
12
Travel - Rear
7
3.5
7
Weight Distribution
36:64
36:64
NA
Table 1:General Dimensions of Various Baja Submissions
UT-C
2006
Midwest
430
600
92.5
54
48.5
10
10
10
8
40:60
UW-P
2009
Midwest
NA
500
NA
NA
56
12
NA
12
NA
NA
BYU-I
2005
West
NA
NA
NA
NA
NA
11.5
10
14
12
NA
BYU-I
2004
West
NA
NA
NA
NA
NA
8
5
10
8
NA
AVG
NA
NA
500
600
91.38
55.25
52.25
10.86
7.83
11.50
7.58
37:63
Suspension Overview
The purpose of the suspension for conventional land vehicles is to dampen the effects of the
terrain it encounters and to assist in braking. A suspension system generally consists of
dampener or shock absorber, springs, and multiple linkages. The dampener ensures that the
forces generated between the ground and wheels are dissipated before reaching the frame of the
vehicle. The purpose of the spring is to maintain the contact between the tires and the ground.
Most Baja submissions employ an independent front and rear suspension. This design allows
each tire in the suspension to move separately from another other. An independent suspension
creates a more comfortable ride and allows for better handling on uneven terrain.
Steering responsiveness can be greatly affected by the spring rate of the coil/shock setup. Spring
rate is defined as the amount of force it takes to compress a spring 1 inch [5]. The spring rate is
usually specified by the manufacture and can be used to determine the force exerted by the
spring according to the following equation:
𝐹 = −𝑘∆𝑥 (𝐹𝑜𝑟𝑐𝑒 𝑜𝑓 𝐴 𝑆𝑝𝑟𝑖𝑛𝑔) Equation 1
Where k is the spring rate, Δx is the length of compression (or extension) of the spring, and F is
the resulting force. This force is what "pushes" the tire toward the ground in order to maintain
contact. The weight (force exerted) by the shock setup greatly contributes to the handling of the
vehicle. The weight of the body and all the parts mounted on it are considered sprung weight.
The tires, wheels, and suspension parts, which will move up and down with the wheels, are
underneath the springs and will not be insulated from the road conditions. These moving
suspension parts are considered un-sprung weight. The SAE Baja Competition requires a design
concept for a durable vehicle that could be mass produced as a production All-Terrain Vehicle.
Due to the rugged nature of the courses, a high sprung-to-un-sprung weight ratio is desirable to
maintain contact with the ground. This relationship can be calculated for the vehicle by dividing
the total sprung weight by the total un-sprung weight of all four wheels.
The figure to the right gives an
example of a weight ratio breakdown
for the individual suspension
components. The wheel, tire, stub
axle, and upright all move together as a
rigid unit, and thus are all 100% unsprung weight. The axle shaft and the
lower suspension arm are both 50%
un-sprung, an average between the two
ends. The shock is 36% un-sprung
because the lower end moves 72% of
the un-sprung motion. The 36% is
taken from the average of the two ends
of the shock. The coil spring mounted
on the shock is also 36% un-sprung
weight.
Figure 2: Weight Ratio Illustration
There are many types of materials that can be used to form the members of the suspension
components. The most widely used materials different varieties of alloy steel. The steel is
generally stamped, rolled, or square tubing. Steel has relatively high yield strength, allowing it
to withstand repeated impact without deforming. Steel also has a rather large area within the
plastic region once elongation surpasses the yielding point. This allows a significant amount of
deformation to occur before fracture, meaning damage can be identified before complete
member failure occurs. Composite materials are also available, but these material are generally
cost prohibitive. The Stress versus Strain curve for typical alloy steels is provided below.
Figure 3: True Stress Versus True Strain for Alloyed Steels
Front Suspension
The 2011-2012 Baja front suspension incorporated an independent front suspension. It
consisted of a double A-Arm construction that connected to the frame with Heim joints. The
Heim joints were an excellent choice since they allow for some error in frame-to-arm alignment.
They also tend to outlast the traditional rubber and urethane bushings. The Heim joints were
threaded into the A-Arms which allowed for caster and camber adjustments. The spindles
connected to the A-Arms through ball joints. King UTV coil-over shocks were used and
mounted to the lower control arms. This style of design has been used by many other teams and
in production vehicles for many years. However, problems did arise from improper geometry of
the system. This included multiple ball joint failures, insufficient wheel travel, overall vehicle
weight, and rollovers. Pictures of a typical A-Arm setup, the previous front suspension,
caster/camber adjustment, and Heim joints are below.
\
Figure 4: A-Arm Suspension
Camber Adjustment
Heim Joint
Location
Figure 5: Camber/Caster Adjustment [7]
Figure 6: Heim Joints
Some variations of the front A-Arm suspension are pictured below. The pictures were taken at
the 2012 SAE Baja Competition in Alabama. Notice that all of these have the same concept, but
very in dimension and orientation. The coil/shock location, A-Arm length, ride height, and tierods are some of the main differences.
Figure 7: Various A-Arm Suspension Designs
There are other ways to configure the front
suspension. One of which was designed by
the 2011 Senior Design students at The
University of Texas at El Paso (UTEP) as seen
in the figure below. It eliminated the use of
the double A-Arm suspension and instead used
a single, larger, lower control arm and a solid
adjustable shaft for the upper control arm. The
lower control arm was reinforced to withstand
the increased loads it would be subjected to.
The advantage is that there is less material
present in the upper control arm and a much
cleaner design. However, it seems that the
single upper control arm contact point would
allow unwanted movement at the spindle.
Also, the material removed from the upper
Figure 8: UTEP Front Suspension
control arm has to be compensated for with
the increased lower control arm size. [6]
Rear Suspension
A variety of mounting systems for rear suspension have been used in competition with the two
most prevalent being double A-arms and trailing arms or a combination of the two. Unlike the
front suspension, the rear suspension will have to integrate a drive-train in the design as well.
Trailing arms can also be used as a rear suspension links. The main characteristic is the use of a
single arm to connect the frame to the knuckle making this kind of suspension the simplest to
setup as well as a lighter weight benefit. Trailing arms also have little room for failure due to the
fact it rotates on the same axis of rotation as the tires will.
An A arm design in the rear suspension is composed of two link arms mounted one on top of the
other. The arms can be either equal in length or have different lengths. This system does increase
the weight of the vehicle comparative to having a single trailing arm as well as being more prone
to failure under heavy stresses during the race.
Figure 9: Various A-Arm Designs
Figure 10: Standard Trailing Arm
Steering
The steering apparatus is considered an independent system, but as it shares a dynamic
relationship with the suspension, a general overview is given. The spindle is the central
component of the front suspension. The spindle is where the steering mechanism connects to the
suspension and it is also the where the suspension arm(s) are mounted and the component about
which the wheel rotates. The spindles must withstand the loads transferred from each of the
components in varying directions and magnitudes. Thus it is important to conduct a rigorous
stress analysis when designing the spindles. For this reason, the material properties of the
spindles must be carefully considered as well. The table below shows a compares key properties
and cost estimate for prospective materials.
Material
Density
6061 Aluminum
Ti 6-4 (Titanium)
304 Stainless Steel
0.0975 lb/in3
0.16 lb/in3
0.289 lb/in3
Max Yield
Strength (psi)
8,000
141,000
73,200
Cost/Pound
$2.00 to $5.00
$25.00 to $50.00
$2.00 to $4.00
Table 2: Material Property Comparison
A common type of steering system used in the Baja Competition is a rack and pinion. A pinion is
a circular gear that, when rotated by the steering shaft, causes linear movement of the rack. A
basic rack and pinion mechanism is shown below. This linear motion is translated to the
individual wheels by the long thin members
called tie rods. Tie rods must be able to withstand
both tensile and compressive stress with minimal
deformation for responsive handling. The tie rods
connect to the feature of the spindles called the
steering arms. The steering arm acts as a lever to
rotate the spindles about their connection to the
suspension arm, thus turning the wheels.
Figure 11: Rack and Pinion
The previous Baja design utilized a rack and
pinion mechanism that was walled off from the
acceleration and brake pedals. The location of the
steering components increased the size of the
front end, adding extra frame weight, and
increasing the turning radius of the Baja.
When selecting the size of the rack and pinion, several factors will be must be considered. The
first is the size of the rack. To utilize the full range of motion of the rack, the frame must be
nearly twice as wide as the length of the rack. A larger rack will offer a wider and more precise
range of motion in the spindles but will increase the size of the front end and decrease the turning
radius. As the size of the rack decreases, so will the size of the frame, but the result will be a
decrease in handling. Another factor is the size of the pinion. For a given torque in the steering
shaft, the amount of force delivered to the spindles will be determined by the ratio of the rack to
the radius of the pinion. Decreasing pinion size will increase the precision of the steering but will
apply less force. Also the relative size of the steering wheel will determine the torque that can be
applied to rack. Finally the distance from the tie rod connection to the pivot point of the spindle
will determine the torque applied to turn the wheels.
Summary
After conducting research and assessing different designs, the Baja Suspension Team has a better
understanding of their goals. The ultimate goal of the project is to develop a suspension system
that will dramatically increase performance. However, the design must remain cost effective,
easy to fabricate, and adhere to SAE Baja rules. Given the budget, manufacturing capabilities,
and other considerations, the following components show potential for the suspension design:
double A-arm front suspension, rear trailing arm, and rack and pinion steering.
Works Cited
[1] Colone, Mike, et al. SAE Mini-Baja® 2008 – Suspension and Frame Design: Senior Design
Report. Rep. N.p.: n.p., n.d. Web. 26 Sept. 2012. <new.ipfw.edu/dotAsset/239508.pdf>.
[2] Gilbert, Mark, and Jeremy Goodman. 2006 Midwest Mini Baja Team. Rep. SAE
International, 2006. Web. 27 Sept. 2012. <http://www.utc.edu/Departments/engrcs/UTCMWMB.pdf>.
[3] Behrens, Brendan, and Kyle Droessler. Mini Baja Team: 2009 Design Report. Rep. SAE
International, 2009. Web. 27 Sept. 2012. <http://www.uwplatt.edu/org/sae/Baja/20082009/University%20of%20Wisconsin%20Platteville_Baja%20Wisconsin.pdf>.
[4] Johnson, Ben. SAE Mini-Baja West Design Report. Rep. SAE International, 2005. Web. 26
Sept. 2012. <http://www2.byui.edu/Societies/SAE/2006%20Buggy.pdf>.
[5] Huber, A., "All About Spring Rates", in Chassis and Suspension Handbook, First Edition, HP
Books, Division of Penguin Group (USA) Inc., New York, New York, pg. 1.
[6] "MiniBaja Vehicle Suspension Design". 2011. [Online]. Available:
me.utep.edu/mrkhan/Documents/FrontSuspentionDesignMiniBaja.pdf. [Accessed: September
25, 2012]
[7] Bolles, B., "Caster and Camber Settings" , Jan. 2008, [Online] Available at
http://www.circletrack.com/chassistech/ctrp_0801_caster_camber_settings/photo_09.html,
[Accessed Sept. 29, 2012].
[8] Edmondson, Chuck, “Fast Car Physics,” Baltimore, Maryland: The Johns Hopkins University
Press, 2011.
[9] Smith, Carroll, “Engineer to Win,” Osceola, Wisconsin: MBI Publishing Company, 1984.