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

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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.

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Figure B: Fox Float X Evolution Air Shocks

Figure C: Front Suspension set-up

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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

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Table C: Front Impact

For more in depth information see Appendix B.

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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

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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)

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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)

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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)

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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

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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

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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

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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)

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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)

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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

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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).