Off Road Innovations
Design of an Off-Road Suspension and Steering System
EN 8926 - Mechanical Design Project II - Progress Report 2
Andrew Snelgrove 200832467
Calvin Holloway 200814416
Jeremy Sheppard 200907756
Kathleen Price 200735017
1
Acknowledgements
Off-Road Innovations would like to thank the following individuals within the Engineering
Department of Memorial University of Newfoundland whose time, assistance, and enthusiasm
for our design project helped make this possible.
Professor Andy Fisher
Dr. Geoff Rideout
Taufiqur Rahman
M. Raju Hossain
For your expertise and guidance throughout our project, thank you.
1
Contents
1 Introduction ............................................................................................................................ 3
1.1 The Baja Society of Automotive Engineers (SAE) Series .................................................. 3
1.2 The Memorial Baja Team ................................................................................................. 3
1.3 Off-Road Innovations ....................................................................................................... 3
2 Project Management Plan ...................................................................................................... 4
2.1 Project Goals .................................................................................................................... 4
2.2 Project Constraints ........................................................................................................... 4
2.3 Member Responsibilities .................................................................................................. 4
2.4 Project Schedule ............................................................................................................... 5
2.5 Team Communications .................................................................................................... 5
2.6 Project Risks ..................................................................................................................... 5
3 Suspension and Steering Design ............................................................................................. 6
3.1 Redesign Scope ................................................................................................................ 6
3.2 Suspension and Steering Design Methodology ............................................................... 7
3.2.1 Wheel Alignment ...................................................................................................... 7
3.2.2 Steering Geometry .................................................................................................. 10
3.3 Design Targets ................................................................................................................ 11
4 Steering Enhancement .......................................................................................................... 12
4.1 Front Mounted Rack and Pinion System ........................................................................ 12
4.2 Minimizing Bump Steer .................................................................................................. 12
4.3 Mud Protection .............................................................................................................. 13
5 SolidWorks ............................................................................................................................ 14
5.1 Toe .................................................................................................................................. 16
6 SimMechanics ....................................................................................................................... 18
6.1.1 Mating Joints ........................................................................................................... 18
6.1.2 Shock Absorber Model ............................................................................................ 18
6.1 Initial Results .................................................................................................................. 21
7 Moving Forward .................................................................................................................... 23
8 Budget ................................................................................................................................... 26
9 References ............................................................................................................................ 27
2
List of Figures
Figure 1: Wheelbase and Track (Wheelbase, 2013) ....................................................................... 7
Figure 2: Example of Positive and Negative Camber ...................................................................... 8
Figure 3: Example of Steering Axis and Scrub Radius (Front View) ................................................ 8
Figure 4: Toe-in and Toe-out (View from Top of Vehicle) .............................................................. 9
Figure 5: Negative and Positive Caster ......................................................................................... 10
Figure 6: Independent Front Suspension and Steering Geometry ............................................... 10
Figure 7: Determining Ideal Centre of Steering Ball Travel .......................................................... 12
Figure 9: Static Ride Height and Static Camber (Front View) ....................................................... 15
Figure 11: Range of Caster from Uncompressed (Left) to Compressed (Right) Shock Travel ...... 16
Figure 13: Spherical Joint Mate .................................................................................................... 18
Figure 15: Progressive Damping Curve (Left) (Factory, 2009) and Curve Fit Data (Right) ........... 20
Figure 17: Shock Compression (m) with respect to Time (s) ........................................................ 22
Figure 19: Loop 1 for Motion Analysis .......................................................................................... 23
Figure 21: Loop 3 for Motion Analysis .......................................................................................... 24
List of Tables
Table 1: Design Targets ................................................................................................................. 11
Table 2: Estimated Cost of Parts to be Purchased ........................................................................ 26
Table 3: Cost to Mass Produce the Upper A-arm ......................................................................... 26
3
1 Introduction
The purpose of the term 8 Mechanical Design project is to provide students with an opportunity
to pursue an open-ended design project from start to finish. Students are required to develop a
strategy for solving a problem, plan and manage, evaluate system design variations and
develop a complete design documentation package. (Fisher 14)
For this design challenge, Off-Road Innovations was formed to redesign the Memorial Baja’s
front suspension and steering systems.
1.1 The Baja Society of Automotive Engineers (SAE) Series
The Baja SAE series is an international competition which test the limits of minimalistic racecars
(Baja’s), designed and built by engineering students from over 100 universities. The
competitions are geared toward simulating real-world engineering design projects and the
challenges faced within them.
The Baja’s compete in static and dynamic events. Static events include ranking the students
based on their design and vehicle costs. Dynamic events incorporate acceleration, suspension
and traction, maneuverability, hill climb, rock crawl, mud pits and a four hour endurance race
designed to push the vehicle to its limits through rough terrains.
All vehicles are required to use an identical ten-horsepower Intek Model 20 engine donated by
Briggs & Stratton Corporation. Having all teams use the same engine creates a more challenging
engineering design test. (Society of Automotive Engineers, 2014)
1.2 The Memorial Baja Team
The Memorial Baja team is a group of engineering students from Memorial University that have
been competing in the Baja SAE series since 2010. Over the past four years the Baja has
developed and undergone many design changes with the key focus of maintaining a light,
durable and competitive car.
1.3 Off-Road Innovations
This design team of four Memorial University Engineering Students are each a part of the
Memorial Baja competitive team. With past experience, passion for engineering and a
commitment to the team, Off-Road Innovations is an unparalleled design group with the
determination and capability to guarantee success.
To improve on previous Memorial Baja car performance, Off-Road Innovations has taken on the
challenge of redesigning the front suspension and steering systems.
4
2 Project Management Plan
Off-Road Innovations has selected a free form design group structure where a lead defines
global milestones and objectives and responsibilities are given at a task-by-task basis. With this
structure each member has the opportunity to contribute equally to all aspects of the project
design where revisions are made as needed.
2.1 Project Goals
Off-road Innovations goal is to develop an enhanced front suspension and steering system for
the Memorial Baja team. The improved system will:
Provide more driver room
Give a raised ride height
Lower forces transmitted in frontal impacts through recessional travel
Maintain previous reliability and low weight
These goals have to be met while still maintaining the reliability and low weight of the 2013
Baja design. In order for this goal to be met successfully all members of the team will play an
intricate role to see the project through.
2.2 Project Constraints
Design Constraints:
Improve on original design as described in section Error! Reference source not found.
Suspension must mount to 4130 steel tubing with 1¼” diameter (SAE regulations)
Project Constraints:
Must be in line with previous Memorial Baja suspension costs.
Project deliverables are to be completed on or before April 4, 2014
2.3 Member Responsibilities
Each member of Off-Road Innovations is committed to the following standards:
Contribute an average of 5+ hours per week
Participate equally in the writing of all reports and presentations
Engage with enthusiasm
Offer feedback and criticism where necessary
Play an active role by contributing at weekly review and design group meetings
5
2.4 Project Schedule
A Gantt chart is used to facilitate easy viewing of the schedule and to ensure timely completion
of all tasks. It provides understanding on how tasks are interrelated and helps to effectively
allocate resources. The most recent Gantt chart can be seen in Appendix A.
2.5 Team Communications
The team keeps an updated website (www.OffroadInnovations.weebly.com), giving an
overview of the members, projects and team goals. The website provides access to meeting
minutes and an updated Gantt chart.
2.6 Project Risks
The two main risks that Off-Road Innovations is faced with include driver safety and failure to
complete the project by the required date. To mitigate the risk to the driver’s well-being, the
development of the steering and suspension system will closely follow the standards set forth
by the SAE competition governing body. To ensure the project is completed by the required
time, the project management plan will be reviewed weekly and the updated Gantt chart will
be used to track progress.
6
3 Suspension and Steering Design
Off road vehicles pose an interesting design problem for engineers due to the long suspension
travel and low wheel rates. Baja vehicles introduce a number of design difficulties merited by
many parameter and behavioral interactions. In context, the amount of suspension travel for
the Baja vehicle is over twice that of typical passenger cars. With such significant travel, strong
consideration must be given to how the tire is moving relative to the ground during travel. The
very small engine type causes any inefficiency that are present to greatly affect the
performance of the vehicle. For information on the suspension and steering systems used in
previous years by Memorial Baja refer to Appendix C.
After reviewing and analyzing various suspensions and steering systems it was determined that
the double a-am suspension and the rack and pinion steering systems remain the best suited
designs for our application. For a more detailed review of each system and to view the selection
matrices used see Appendix D.
3.1 Redesign Scope
Using the collective information and field experience that the team has developed, a new
suspension and steering system will be engineered with enhancements including more
recessional travel and caster angle, greater driver ergonomics and other general performance
enhancements. The redesign work will be focused on system geometry and placement as well
as ensuring previous reliability and performance targets are met or exceeded.
Due to time and financial limitations, some components will be outside the scope of the
redesign work and include:
Front wheels
Hubs
Knuckle assembly
Shocks
A-arm bushings of the suspension system
Rack and pinion aluminum mount
With these restrictions in place the components to be designed include:
A-arm structural member types and material
Upper and lower A-arm geometry
Tie rod material and geometry
Steering column
Chassis mounting locations
Rack and pinion geometry and locations
Rack and pinion bushing material
Rack and pinion bushing design
7
3.2 Suspension and Steering Design Methodology
The first objective is to have all the tires turn around a centurial point. This is important
because it prevents the tire from scuffing and creating premature tire wear. To get the proper
geometry the Ackerman angle needs to be considered. Below is the formulas used to calculate
the offset of the outside front tire and inside front tire:
δo = 𝐿
𝑅+𝑡
2 (1)
δi = 𝐿
𝑅−𝑡
2 (2)
δo – Ackerman angle outside tire
δi – Ackerman angle inside tire
L – wheelbase
R – turn radius
t – track width
(Stone & Ball, 2004)
Figure 1: Wheelbase and Track (Wheelbase, 2013) (Date Accessed: February 1st, 2014)
3.2.1 Wheel Alignment
Wheel alignment is very important for the handling of the Baja. If you have the tire aligned
properly the car should drive straight without any input from the driver. The components of
wheel alignment include:
Camber
Steering axis inclination
Toe
Caster
8
Camber is the angle of the tire in the vertical direction. A car can have positive camber, this is
when the top of the tire is farther away from the car. Alternatively a car can have negative
camber, when the top of the wheel is closer to the car. Negative camber is normally used in off-
road vehicle because it enhances tire engagement with the ground when maneuvering around
turns. (Stone & Ball, 2004)
Figure 2: Example of Positive and Negative Camber (Adapted from: (July Subaru of Keene Service Specials), Date Accessed: February 5th, 2014)
Steering axis is the vertical axis that is generated from the upper and lower joints. The steering
axis to the center of the tire is called the scrub radius. This creates scrubbing forces when the
driver is turning the wheel. Ideally this would be as small as possible so it creates less wear on
the tires. The upper joint is normally closer to the car and the lower joint is farther away.
Having the steering axis on an angle to reduce the scrub radius in turn makes it easier to steer
the car and causes less wear on the tires. (Stone & Ball, 2004)
Figure 3: Example of Steering Axis and Scrub Radius (Front View)
(Stone & Ball, 2004)
9
Toe is the difference from the front of the tire to the back of the tire when looking from the top
view. Toe in is when the front of the tire is closer to the vehicle and toe out is when the rear of
the tire is closer to the car. Both toe in and toe out reduce the efficiency of the car by
introducing scrubbing forces. (Stone & Ball, 2004)
Figure 4: Toe-in and Toe-out (View from Top of Vehicle)
(Stone & Ball, 2004)
Caster is the angle of the steering axis viewed from the side. Positive caster is when the upper
joint is farther to the rear of the car. Negative caster is when the steering axis is inclining
towards the front of the car. Having positive caster is desirable because it provides a more
stable ride and helps with aligning the wheels to drive in a straight line. When designing the
caster of the system, the steering axis should intersect with the ground before the tire contact
patch. (Stone & Ball, 2004)
10
Figure 5: Negative and Positive Caster (Adapted From: (July Subaru of Keene Service Specials)) (Date: Accessed: February 5th, 2014)
3.2.2 Steering Geometry
One of the biggest challenges when designing a steering and suspension system is to get the
wheel alignment to stay in place during the travel of the suspension. There is a relationship
between the suspension linkages and the tie-rods. As seen in the figure below the intersection
of the point IC and I’C’ is the correct placement for the rack and tie-rod connection. Having the
connection in this position will limit any bump steer. Bump steer is when the toe changes when
the shocks are compressed. As discussed in previous section, if the car has too much toe in, this
will reduce the efficiency of the car. (Stone & Ball, 2004)
Figure 6: Independent Front Suspension and Steering Geometry (Stone & Ball, 2004)
11
3.3 Design Targets
Using the overall results from the previous Memorial Baja car new design targets have been
discussed and decided upon by the team. The targets set include:
Table 1: Design Targets
12
4 Steering Enhancement
4.1 Front Mounted Rack and Pinion System
To increase space while improving the current system, the steering knuckle was flipped so the
tie rods and the rack and pinion gearbox are positioned in front of the centre of the hub.
Moving the rack and pinion gearbox forward means that the steering column will no longer
interfere with driver entry and exit of the vehicle. Steepening the angle of the steering column
will also enable an acceptable angle to be created so that the universal joint that couples the
column can be eliminated. At this time, the universal joint is still present in the SolidWorks
model as the final position and mounting of the rack are in development. Removing the
universal joint removes a potential mode of failure, reduces cost and weight, and increases
overall system efficiency.
4.2 Minimizing Bump Steer
To minimize bump steer, the connection between the rack and tie rod must lie in the ideal
centre of steering ball travel (Stone & Ball, 2004). To determine this point, lines were drawn in
SolidWorks through the upper and lower A-arms when the steering wheel was in a neutral
position with an uncompressed shock (AI and BI) and when the wheel is moved to its maximum
height when the shock is fully compressed (A’I’ and B’I’). Another line was drawn from the
connection point between the steering knuckle and the tie rod, to the previous intersection
points at I and I’ (CI and C’I’). The intersection point between lines Cl and C’l’ will be the ideal
centre of steering ball travel, and so the length of the rack was modified to move the joint to
this ideal point. This can be seen in Figure 7.
Figure 7: Determining Ideal Centre of Steering Ball Travel
l
l’
13
4.3 Mud Protection
Mud located inside the rack-and-pinion gearbox reduces the efficiency of the steering system,
and was a problem on the previous model of the car. As shown in Figure 8, by extending the
bushings farther outside of the gearbox, mud-protective boots covering the rack can be tied
more securely to the exposed lips, and would therefore reduce the potential for mud to seep
into the gearbox.
Figure 8: Extension of Brass Bushings at Rack and Pinion Gearbox
14
5 SolidWorks
SolidWorks is a 3-Dimensional modeling software that allows a user to quickly conceptualize
their design idea. The software allows the user to create annotations to show relevant
measurement readings on the model. These annotations can update automatically, and are
useful in determining how manipulating the model geometry can affect wheel alignment as
well as other measurements of interest. The critical parameters or measurements of interest
are five main design targets taken from Table 1 and are listed below:
Static ride height
Camber through the range of motion
Caster through the range of motion
Toe through the range of motion
Recessional travel
With the obstacles that will be encountered at a Baja SAE Competition, static ride height is very
important to ensure minimal ground interference without sacrificing the Baja’s stability. To
change the ground height of our Baja in SolidWorks the mounting points of the shock on the
lower a-arm is adjusted. Ground clearance was shown through an annotation that measured
the distance between the bottom of the chassis and the bottom of the tire. The models current
static ride height of 289.3mm making it within an 8mm tolerance of the 281mm design target.
When the shock is fully compressed the Baja has a ground clearance of 63.2mm preventing the
car from bottoming out.
Through research and previous experience negative camber was determined to be desirable. To
manipulate camber the multiple links and tab locations of the upper and lower A-arms were
adjusted. The mounting location of the upper a-arm had a significant effect on the camber.
Camber was shown on the model through an annotation that measures the angle of vertical
line that was sketched on the wheel rim to the right plane, which runs through the centre of the
car. In the current model the static camber is 4.27˚which reasonably close to the 4˚design
target that was set earlier in project. The camber change through the range of motion is
4.45˚which is under the design target of 6˚. Ground clearance and this camber range is shown in
Figure 9, and Figure 10.
15
Figure 9: Static Ride Height and Static Camber (Front View)
Figure 10: Compressed Shock Ride Height Change and Camber Change (Front View)
Caster is the alignment of the steering axis which helps with the self-aligning characteristic of
steering and it changes camber while turning. Having negative caster enables the car to lean
16
into the turn. For the model the caster was manipulated by changing the mounting of the
bottom A-arm to the chassis. To track this parameter there was a sketch created on the
steering axis that was measured from the front plane. The steering axis is the two point where
the A-arms connect to the steering knuckle. The design target for caster was set at negative six
degrees. The SolidWorks model currently has 5.65˚which does not change through the range of
motion, see .
Figure 11: Range of Caster from Uncompressed (Left) to Compressed (Right) Shock Travel
The next parameter that was considered was the recessional travel of the wheel. This is when
the wheel travels towards the back of the car through its range of motion. This helps to absorb
frontal impact by transferring some of the force through shocks. In the model the recessional
travel is measured from the spindle to the front plane. Currently the recessional travel is 29.5
mm while the design target is set for 51 mm. This is something that could change in the future.
To increase recessional travel, the angle of the bottom chassis member that the lower A-arms
are mounted is increased.
5.1 Toe
The final parameter that was evaluated was the toe of the Baja. This is the angle of the tire
when looking at the top of the car, see Figure 12. To display toe-in and toe-out a horizontal line
was sketched on the rim of the wheel. This enabled us to measure the angle made between the
line and the right plane which runs through centre of the car. The current model has 3˚ of toe in
at the static ride height and 2.1˚ when the shock is compressed. Having one degree of travel
through the range of motion is reasonable for a steering system that has such a steep incline in
there tie rods. It was decided that having a little toe in was acceptable because the caster will
create a moment around the center of the tire. This moment will straighten the wheels once
the Baja get up to speed.
17
Figure 12: Range of Toe from Uncompressed (Left) to Compressed (Right) Shock
18
6 SimMechanics
After the geometry of the front suspension and steering was established in SolidWorks, it was
necessary to import the model into SimMechanics and Simulink in Matlab to perform a dynamic
analysis. It is difficult to create the characteristic of the shock in SolidWorks, but SimMechanics,
combined with Simulink, allows the user to perform these tasks. SimMechanics provides the
“multi-body simulation environment for 3D mechanical systems” (Math Works, 2014), and
Simulink allow the user to modify and view the parameters of the system, such as establishing
force inputs, and determining reaction forces at joints.
To import the SolidWorks model into SimMechanics/Simulink, a SimMechanics Link was created
between Matlab and SolidWorks. The SolidWorks model was saved as an “.xml” file, and the
.xml file was imported into SimMechanics.
6.1.1 Mating Joints
The team experienced one initial problem importing the SolidWorks model into SimMechanics.
When the SolidWorks model was first created, a coincident joint was generated between the
inner surface of the cup, and the outer surface of the ball, for all of the ball and cup spherical
joints in the model. SimMechanics did not recognize this mate and defaulted all spherical joints
to welded joints. After consulting Mr. Rahman, it was later determined that a coincident mate
between the centers of the ball and cup would create the required spherical joints in Simulink.
Figure 13 depicts the blue spheres, at the centre of each part, to be mated.
Figure 13: Spherical Joint Mate
6.1.2 Shock Absorber Model
As no mathematical model was available for the Fox Float X Evol shock absorbers, the team was
able to derive one using empirical data that was available in the unit’s manual. The data was
19
provided in the form of progressive spring curves that showed the effect of varying the
pressure in the main air chamber of the shock, and progressive damping curves that showed
the effect of adjusting the High Speed adjuster. Using median data, a curve was plotted in excel
and using the method of least squares for a third order polynomial an equation for each curve
was found. Spring and damper empirical data and curve fits are shown below in Figure 14 and
Figure 15.
Figure 14: Progressive Spring Curve (Left) (Factory, 2009) and Curve Fit Data (Right)
The spring characteristic from this curve was found to be:
𝐹𝑆 = −17.52 + 48434𝑥 − 915122𝑥2 + 990480𝑥3 (3)
Where spring force (Fs) is in Newtons and Travel (x) is in meters. The derivative of this equation
provides the shock coefficient Ks as a function of travel in Newton’s per meter:
𝐾𝑆 = 48434 − 1830244𝑥 + 2971440𝑥2 (4)
-1000
0
1000
2000
3000
4000
5000
6000
7000
0 0.05 0.1 0.15
Forc
e (
N)
Travel (m)
20
Figure 15: Progressive Damping Curve (Left) (Factory, 2009) and Curve Fit Data (Right)
The damping characteristic from this curve was found to be:
𝐹𝐷 = 42.12 − 1116.9�� + 393.1��2 − 78.4��3 (5)
Where derivative of this equation produces the function of the damping coefficient “BD”:
𝐵𝐷 = 1116.9 + 786.2�� − 235.2��2 (6)
Using the above mathematical models a Simulink model was created for the shock (Figure 16).
The model first takes body coordinates from the lower and upper shock bodies and generates a
length signal. The length signal is inputted directly into the spring channel and is differentiated
to achieve a velocity signal for the damping model as shown.
-2500
-2000
-1500
-1000
-500
0
500
Forc
e (
N)
Velocity (m/s)
21
Figure 16: Shock Absorber Simulink Model
6.1 Initial Results
Outputs of the model are quite promising with shock displacements (Figure 17) and forces ()
behaving as expected. Further development is required to better represent the boundary
conditions of the shock as it reaches its maximum or minimum travel. To simplify the model for
initial development very stiff springs were used to represent end stops and has caused the
jolting forces as shown in the force plot (Figure 18). With the addition of a damper in parallel to
the stiff spring to account for losses at these end stops the repeated jolting forces in the joints
will be smoothed out and quickly dissipate. To achieve these initial results, a simple pulse force
was applied vertically to the center of gravity of the steering knuckle for a short period of time,
then released.
22
Figure 17: Shock Compression (m) with respect to Time (s)
Figure 18: Reaction Force (N) experienced by Top A-Arm Spherical Joint over Time (s)
23
7 Moving Forward
The team will continue to fine-tune the SimMechanics Model. This will include dampening the
stiff boundary springs and applying varying force and position inputs to simulate obstacles
encountered on the track. Motion Analysis will be used to validate the results. Vertical
movement of the tire can be related to the compression of the shock with three loop vectors.
This can be viewed in Figure 19, Figure 20, and Figure 21. Forces can be determined at any of
the joints along the loop vectors.
Figure 19: Loop 1 for Motion Analysis
𝑟𝑦 + 𝑟𝑥 + 𝑟1 = 𝑟2 + 𝑟3
Known: 𝑟1, 𝑟2 , 𝑟3
Unknown: 𝜃2, 𝜃3, 𝑟𝑥
Input: 𝑟𝑦 , 𝑟�� , 𝑟��
24
Figure 20: Loop 2 for Motion Analysis
Figure 21: Loop 3 for Motion Analysis
𝑟2 + 𝑟3 = 𝑟4 + 𝑟5
Known: 𝑟2, 𝑟3 , 𝑟4 , 𝑟5
Unknown: 𝜃5
𝑟5𝑎 + 𝑟6 = 𝑟4𝑎
Known: 𝑟4𝑎 , 𝑟5𝑎
Unknown: 𝑟6, 𝜃6
25
Finite Element Analysis will be used to assist in choosing material and hardware based on the
stresses experienced in the joints and members.
26
8 Budget
A preliminary budget of parts to be purchased can be seen in Table 2. Hardware is subject to
change pending simulation results. A cost analysis will be completed for all parts that are being
fabricated in the context of mass-production. Expenses relating to labor and material will follow
guidelines as specified by the SAE. Cost to fabricate the upper a-arm can be seen in Table 3. This
is based on the assumption that ¾” 1020 steel with a wall thickness of 1/16” will be used.
Table 2: Estimated Cost of Parts to be Purchased
Table 3: Cost to Mass Produce the Upper A-arm
27
9 References
AGCO Automotive. (2014). AGCO. Retrieved January 20, 2014, from
http://www.agcoauto.com/content/news/p2_articleid/214
Bauer, H. (Ed.). (2000). Automotive Handbook. Robert Bosch GmbH.
Ciulla, V. T. (2002). Power Steering. Retrieved February 6, 2014, from About.com:
http://autorepair.about.com/cs/generalinfo/l/bldef_628.htm
Crolla, D. A. (Ed.). (2009). Automotive Engineering: Powertrain, Chassis System and Vehicle
Body. Amsterdam: Butterworth-Heinemann (Elsevier Science & Technology Books, Inc./Elsevier
Inc.).
Fisher, A. (2014). ENGI 8926 Course Outline. Memorial University.
How Stuff Works. (2014). How Car Steering Works. Retrieved January 20, 2014, from
http://auto.howstuffworks.com/steering3.htm)
Inc, F. F. (2009). ATV Float X EVOL Owner's Manual. Watsonville, CA, United States: Fox Factory.
Isaac-Lowry, J. (2004, August 22). Suspension Design: Types of Suspensions. Retrieved from
Automotive Articles:
http://www.automotivearticles.com/Suspension_Design_Types_of_Suspensions.shtml
July Subaru of Keene Service Specials. (n.d.). Retrieved February 5, 2014, from Subaru
(subaruofkeene.com): http://www.subaruofkeene.com/specials/service.htm
Levine, M. (2010, May 31). Driving a Pickup with Electric Power Steering. Retrieved February 1,
2014, from PickupTrucks.com: http://news.pickuptrucks.com/2010/05/driving-a-pickup-with-
electric-power-steering.html
Math Works. (2014). SimMechanics. Retrieved March 5, 2014, from Model and Simulate
Multibody and Mechanical Systems:
http://www.mathworks.com/products/simmechanics/?nocookie=true
Society of Automotive Engineers. (2014). SAE Collegiate Design Series. Retrieved January 26,
2014, from SAE International: http://students.sae.org/cds/bajasae/about.htm
Stone, R., & Ball, J. K. (2004). Automotive Engineering Fundamentals. Warrendale: SAE
International.
Toboldt, W. K., Johnson, L., & Gauthier, W. S. (2000). Automotive Encyclopedia: Fundamental
Principles, Operation, Construction, Service, and Repair. Tinley Park: The Goodheart-Willcox
Company, Inc.
Wheelbase. (2013, December 17). Retrieved February 1, 2014, from Wikipedia:
http://en.wikipedia.org/wiki/Wheelbase
i
Appendix A – Project Gantt Chart
ii
Appendix B – Detailed Memorial Baja Specifications
Suspension Parameters Front
Suspension Type Dual unequal length A-Arm, Fox Float X EVO air shocks.
Tire Size and Type *24x6-10 MAXXIS R2
Wheels (width, construction) *6" wide, Forged Al, 4/2 offset
Center of Gravity Design Height 17"-18" ( 442mm) above ground
Vertical Wheel Travel (over the travel) 7" (178 mm) jounce/ 5.4" (137 mm) rebound
Recessional Wheel travel (over the travel) 0" (0mm)
Total track change (over the travel) 1.2" (30.5mm)
Wheel rate (chassis to wheel center) 58 lbs/in (10 N/mm) initial;
288 lbs/in (50 N/mm) high impact (adjustable);
(progressive airshock, 2 linear approximations)
Spring Rate 200 lbs/in (35 N/mm) Initial;
800 lbs/in (140 N/mm) High Impact (adjustable);
(progressive airshock, 2 linear approximations)
Motion ratio / type 0.57 average 0.54-0.6 actual progressive rate
Roll rate (chassis to wheel center) 6.4 degrees per g
Sprung mass natural frequency 1.2 Hz (Fully Adjustable)
Type of Jounce Damping Low speed adjustable
Type of Rebound Damping Low Speed adjustable
Roll Camber (deg / deg) 0.86 deg / deg
Static Toe 2 deg
Toe change (over the travel) 1 deg
Static camber and adjustment method 5 deg inward, adj. via outboard rod end on A-arm
Camber Change (over the travel) 6 degrees
Static Caster Angle 7 deg
Caster Change (over the travel) 0 deg
Kinematic Trail 1.7"
Static Kingpin Inclination Angle 8 degrees non-adjustable
Static Kingpin Offset 0.92" (23.4mm)
Static Scrub Radius -1.7" (-43.2mm)
Static Percent Ackermann 40%
Percent Anti dive / Anti Squat 0% Anti dive
Static Roll Center Position 6.3" (160mm) above ground
Number of steering wheel turns lock to lock 2
Outside Turn Radius 11' ( 3.3m) to right
i
Appendix C – Previous Generation Suspension and Steering Systems
The front suspension design of the Memorial Baja car has been consistent over the past few
years since it has never caused problems due to its simplistic and practical design. However,
since the Memorial Baja team has made such radical improvements to each aspect of the car
each year, the current system requires optimization in order for the team to be an even more
aggressive competitor.
Design
The design of the 2013 Memorial Baja front suspension/steering systems is a fixed length
double A-arm design with rack and pinion steering. The suspension system utilized the steering
knuckle from a Polaris Outlaw 525 IRS and Fox Float Evolution shocks with 7” of travel.
Front Suspension Arms
The bottom A-arms are comprised of 1” OD x 1/16” wall AISI 4130 ‘chromoly’ tubing. There are
lower stresses in the top A-arm due to the shock mounting position so smaller tubing was
selected, 3/4” x 1/16” AISI 1020 steel tubing.
The 1” OD A-arm pivots feature Delrin™ bushings, a spacer made of high-grade pre-ground drill
rod (AISI A2 tool steel), and M8x65 grade 8.8 bolts. To reduce friction and maintenance a M6
grease fitting is used to allow lubrication of the sliding interface between the Delrin™ bushings
and the polished drill rod spacer. This is modeled in Figure A. The A-arms are mounted to the
chassis with 1/8” 44W grade steel plate (similar to A36), cut out with a water jet cutter. All tabs
were made identical in order to simplify fabrication and mass production
Figure A: Suspension Mounting and Bushing Internals
Shock Absorbers and Steering Knuckles
The current front suspension utilizes Fox Float X Evolution air shocks see Figure B. These shocks
are lightweight and have adjustment capabilities. The shocks are inclined to give a wheel rate
ranging from 58 lb/in up to 288 lb/in through its travel and mount to the bottom A-arms close
to the steering knuckle. To mount the shocks, tabs were fabricated from 1/4” 44W grade steel
and cut out with a water jet cutter. The steering knuckle used from the Polaris Outlaw 525 IRS
can be viewed in Figure C.
ii
Figure B: Fox Float X Evolution Air Shocks
Figure C: Front Suspension set-up
iii
Rack and Pinion System
The rack and pinion steering system consists of two durable 19mm (3.4”) 1020 steel tie rods
with a wall thickness of 1.5875mm (1/16”). These tire rods link the steering rack output to the
front knuckles via ball joint and clevis connectors.
The steering column uses a double universal set up that bolts to the steering wheel and is
coupled to the pinion shaft on the steering rack. The column is made of 25.4mm (1”) diameter
1020 steel tubing with a wall thickness of 1.5875mm (1/16”).
The rack has been designed with a lock to lock distance of 139.7mm (5.5”) achieved by a gear
rotation of 530 degrees. To counteract thrust on the input shaft brought on by driver
movement over rough terrain, the input shaft is held in place with internally pressed bushings.
A grease fitting is used to allow for lubrication.
Suspension and Steering Modeling and Simulation
The SolidWorks Simulation package has been used to identify stress concentrations and the
factors of safety for each element, revised structural members and support have been selected
based on results.
Car specifications, drop test simulation results and frontal impact simulation results are
depicted in tables A, B and C below. All results were achieved using the SolidWorks application
package.
Table A: Car Specifications
Table B: Drop Test
iv
Table C: Front Impact
For more in depth information see Appendix B.
i
Appendix D – Concept Generation and Selection – Suspension and Steering
To ensure that previous decisions regarding the suspension and steering system types were still
preferable, a concept generation and selection phase of the project was carried out. After a
number of systems were researched, weighted ranking was assigned to each to validate that
the chosen system would best fit our needs.
Suspension Concept Generation
The initial concept generation process resulted in four different types of suspension
configurations. The following are the most commonly used in off-road applications:
Swing Arm
Double A-arm (Double Wishbone)
MacPherson Strut
Trailing Link
Swing Arm
This independent suspension is positioned in the front of the vehicle and causes the axle to
pivot about the center of the car. Each wheel can travel without affecting the other side. (Isaac-
Lowry, 2004)
The following table describes the advantages and disadvantages of the swing arm:
Table D: Advantages and Disadvantages of a Swing Arm Suspension
Advantages Disadvantages
Manufacturability
Robust
Relatively durable
Improves steering
Heavy due to axle and pivot
Does not handle big bumps
Rough ride
ii
Figure D: Swing Arm Suspension (Isaac-Lowry, 2004) (Date Accessed: Feb. 1st, 2014)
Double A-arm:
The Double A-arm consists of two triangulated arms that connect to the top and bottom of the
wheel hubs. These A-arms are different lengths to create the appropriate negative camber.
This design is normally used in the front suspension of off-road vehicles. The table below shows
some advantages and disadvantages of this suspension type:
Table E: Advantages and Disadvantages of a Double A-arm Suspension
Advantages Disadvantages
Easy to adjust camber
Large range of deflection
Versatile
Camber should change when hitting a bump
Camber changes when turning
Expensive
Very complex
(Isaac-Lowry, 2004)
iii
Figure E: Double A-arm on 2013 Memorial Baja
MacPherson Strut
In a Macpherson strut the shock is mounted directly to the wheel hub and acts as the top link of
the suspension. This independent suspension is normally used in small compact vehicles that
mounting an engine in the front of the car. The following table notes the advantages and
disadvantages of this suspension type:
Table F: Advantages and Disadvantages of a MacPherson Strut Suspension
Advantages Disadvantages
Low maintenance
Compact
Simplicity
Improve ride quality
Handling
Cannot change the position vertically without changing camber
Hard to increase the width of the tires
(Isaac-Lowry, 2004)
iv
Figure F: Example of a MacPherson Strut (Isaac-Lowry, 2004) (Date Accessed: Feb. 1st, 2014)_
Trailing Arms
In a Trailing Arms suspension, the links are ahead of the tire. This type of suspension is normally
used in the rear of the car because it is hard to mount the links ahead of the tires in the front.
Table G outlines some advantages and disadvantages of a trailing arms suspension:
Table G: Advantages and Disadvantages of a Trailing Arms Suspension
Advantages Disadvantages
Low cost
Small space requirements
Moves up and down with the bumps in the road
Ride quality
Normally used in rear suspension
Does not allow lateral or camber change
Very bulky supports
Links bend when under significant loading
(Isaac-Lowry, 2004)
v
Figure G: Example of a Trailing Arms Suspension (Isaac-Lowry, 2004) (Date Accessed: Feb. 1st, 2014)
Suspension Concept Selection
To select the best system, the relative importance of each criteria was weighted between
themselves out of 100%. Priority was given first to weight, then cost and manufacturability, then
performance and maintenance. Then, each criteria was judged on a scale of 1 (Poor) to 5 (Great).
Then, these rating were weighted, and the results were totaled for each system type. The system
that had the highest weighted totals would be the one that the team designed.
Table H: Constraint Description and Weights for Suspension Selection
Constraint Description Weight
Cost Total Cost of Implementation 0.1
Durability Endure Competition 0.2
Weight Relative Weight of System 0.2
Manufacturability Ease of Manufacture 0.15
Performance Relative Performance 0.35
Maintenance Ease Perform Maintenance 0.1
vi
Table I: Suspension Selection Matrix
Criteria
Swing Axle Double A-arms MacPherson
Strut Trailing Link
Score Weighted
Score Score
Weighted
Score Score
Weighted
Score Score
Weighted
Score
Cost (0.1) 1 0.1 2 0.2 2 0.2 4 0.4
Durability (0.2) 4 0.8 5 1 3 0.6 2 0.4
Weight (0.2) 4 0.8 4 0.8 3 0.6 4 0.8
Manufacturability (0.15) 2 0.3 5 0.75 2 0.3 1 0.15
Performance (0.35) 2 0.7 5 1.75 3 1.05 1 0.35
Maintenance (0.1) 3 0.3 4 0.4 2 0.2 4 0.4
Weighted Total 3 4.9 2.95 2.5
After carrying out the selection process it was determined that the Double A-arm will be the
concept selected. This is the concept that best meets the requirements. The Double A-arm
scored well in the performance and weight constraints, which were the most important for the
Baja application.
Steering Concept Generation
To ensure that the team chose a suitable steering system that met the requirements of the
design project, four common steering systems were examined. These steering systems
included:
Manual rack and pinion
Manual recirculating ball
Hydraulic power-assisted
Electric power-assisted
Manual steering uses only the energy of the driver to turn the wheels (Bauer, 2000). Power-
assisted steering is also known as power steering (Toboldt, Johnson, & Gauthier, 2000). Power
steering has been developed to reduce the amount of effort the required by the driver to steer
the vehicle (Stone & Ball, 2004). It uses two energy sources, the force of the driver turning the
vii
steering wheel, and another source of energy, such as hydraulics or electricity. Both types of
power-assisted steering were examined.
9.1.1 Manual Rack and Pinion
The manually operated rack and pinion steering configuration is an inexpensive, simple, and
relatively compact system. As one can observe from Figure H, when the steering wheel is
turned, it rotates a pinion gear that meshes with the teeth embedded in a rack. This rack moves
laterally, pushing and pulling tie rods, causing the tires to rotate about the kingpins. (Stone &
Ball, 2004)
Figure H: Manual Rack and Pinion on the Memorial Baja 2013 Car
Table J: Advantages and Disadvantages of a Manual Rack and Pinion
Advantages Disadvantages
Inexpensive
Simple Design
Relatively compact
Manufacturability
Driver experiences feedback and “feeling” from steering system as they steer (Stone & Ball, 2004)
Proven steering system in previous competitions with Memorial Baja
Higher impact sensitivity
System can experience greater stresses due to forces exhibited by tie rods
Memorial Baja observed at the past competition that tie rods joints were backing off, causing toe in and therefor tire scrubbing
viii
9.1.2 Manual Recirculating Ball
Figure J - Cross-Section of a Recirculating Ball Gearbox Example
(How Stuff Works, 2014) (Date Accessed: February 1st, 2014)
Figure K - Example of Complete Recirculating Ball Steering System with Pitman Arm (How Stuff Works, 2014) (Date Accessed: February 1st, 2014)
Another steering system configuration that was considered was the manual operation of the
recirculating ball type. This configuration uses a combination of a nut and a worm gear. The nut
moves up and down the worm gear as the worm gear turns from the steering column. Ball
bearings inside the box “recirculate” around the worm gear, reducing wear on the gear. (Stone
& Ball, 2004)
ix
As the nut moves up and down the worm gear, it causes the pitman arm to rotate left or right
about a fixed axis, therefore pushing and pulling the track and tie rods to turn the wheels
appropriately. (How Stuff Works, 2014)
Table K: Advantages and Disadvantages of a Recirculating Ball System
Advantages Disadvantages
Steering effort by driver is reduced. More complicated than rack and pinion.
More expensive than rack and pinion.
No feedback or steering “feeling” experienced by driver. (Stone & Ball, 2004)
9.1.3 Hydraulic and Electric Power-Assisted Steering
Hydraulic and electric energy are examples of alternative sources of energy that can assist a
driver in turning their front wheels. Hydraulic power-assisted steering uses fluid from a
reservoir and a pump to assist in pushing the tire wheels (AGCO Automotive, 2014).
Alternatively, in electric power-assisted steering, an electric motor can assist in turning the
wheels (Levine, 2010). As shown below in Figures L and M, they can be used in combinations
similar to a rack and pinion setup.
Figurer L: Example of a Hydraulic Power-Assisted Steering Configuration (AGCO Automotive,
2014)
x
Figure M: Example of an Electric Power-Assisted Steering Configuration (Levine, 2010)
Table K: Advantages and Disadvantages of Power Assisted Steering
Advantages Disadvantages
Less effort by driver to turn
steering wheel
Both types are more complicated
Expensive
Noise and leaking from hydraulic
systems
Requires maintenance
Difficult to repair
Increase weight
Steering Concept Selection
As with the front suspension concept selection, the four types of steering underwent a design
matrix selection to determine which steering system was suitable for the requirements of the
team. The selection criteria included financial cost to construct, the weight of the system, the
ease of fabrication, steering performance, and ease of maintenance should the steering system
break during competition.
TableL –Constraint Description and Weight
Constraint Description Weight
Cost Total Cost of Implementation 0.2
Weight Relative Weight of System 0.3
xi
Manufacturability Ease of Manufacture 0.2
Performance Relative Performance 0.15
Maintenance Ease of Maintenance 0.15
Table M – Steering Selection Matrix
Constraint
Rack and Pinion
Recirculating Ball
Hydraulic Power-Assisted
Electric Power-Assisted
Score Weighted
Score Score
Weighted Score
Score Weighted
Score Score
Weighted Score
Cost (0.1) 5 1 4 0.8 2 0.4 2 0.4
Weight (0.2) 5 1.5 4.5 1.35 1.5 0.45 4 1.2
Manufacture (0.2) 4 0.8 2.5 0.5 0.5 0.1 1.5 0.3
Performance (0.35) 2.5 0.375 2.5 0.375 3.5 0.525 3.5 0.525
Maintenance (0.1) 4 0.6 3.5 0.525 1 0.15 2 0.3
Weighted Total 4.275 3.55 1.625 2.725
The final selection was the manual rack and pinion steering, with a weighted total of 4.275. The
remaining options, ordered from descending scores, include the manual recirculating ball
steering (3.55), electric power-assisted steering (2.725), and hydraulic power-assisted steering
(1.625).