2016 Baja SAE series frame design - UC DRC Home

2016 Baja SAE® Series Frame Design

A Baccalaureate thesis submitted to the

Department of Mechanical and Materials Engineering

College of Engineering and Applied Science

University of Cincinnati

in partial fulfillment of the

requirements for the degree of

Bachelor of Science

in Mechanical Engineering Technology

by

Jonathon Forrest

April 2016

Thesis Advisor: Professor Allen Arthur

2016 Baja SAE® Series Frame Design Jon Forrest

1

TABLE OF CONTENTS

TABLE OF CONTENTS .......................................................................................................... 1

LIST OF FIGURES .................................................................................................................. 2

LIST OF TABLES .................................................................................................................... 2

INTRODUCTION .................................................................................................................... 3

ABSTRACT ...................................................................................................................................................... 3 PROBLEM STATEMENT ........................................................................................................................................ 3 BACKGROUND .................................................................................................................................................... 3

RESEARCH .............................................................................................................................. 4

PAST DESIGNS .................................................................................................................................................... 4 DRIVER ERGONOMICS ........................................................................................................................................ 4 MAINTENANCE ACCESS ...................................................................................................................................... 5 OVERALL FRAME WEIGHT ................................................................................................................................. 5

DESIGN .................................................................................................................................... 6

MATERIAL SELECTION ....................................................................................................................................... 6 DESIGN ANALYSIS .............................................................................................................................................. 8 FORCE CALCULATIONS ....................................................................................................................................... 9 FINITE ELEMENT ANALYSIS ............................................................................................................................. 11

MANUFACTURING ............................................................................................................. 13

BENDING AND PROFILING ................................................................................................................................. 13 WELDING ......................................................................................................................................................... 14 OTHER COMPONENTS ....................................................................................................................................... 14

CONCLUSION ....................................................................................................................... 15

ACKNOWLEDGEMENTS .................................................................................................... 15

WORKS CITED ..................................................................................................................... 16

APPENDIX A – NAMED ROLL CAGE POINTS ................................................................ 17

APPENDIX B – VEHICLE BUDGET ................................................................................... 18

APPENDIX C - SCHEDULE ................................................................................................. 19

APPENDIX D – DRAWINGS ............................................................................................... 20


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LIST OF FIGURES Figure 1 - 2012 Frame............................................................................................................... 4

Figure 2 - 2013 Frame............................................................................................................... 4

Figure 3 - 2014 Frame............................................................................................................... 4

Figure 4 - UMW Baja Car ........................................................................................................ 5

Figure 5 - Revision 1 Wireframe .............................................................................................. 8

Figure 6 - 2014 to 2016 Objective Comparison 1..................................................................... 8

Figure 7 - 2014 to 2016 Objective Comparison 2..................................................................... 9

Figure 8 - Front Impact FEA Results ...................................................................................... 11

Figure 9 - Side Impact FEA Results ....................................................................................... 12

Figure 10 - Rear Impact FEA Results ..................................................................................... 12

Figure 11 - Top Impact FEA Results ...................................................................................... 13

Figure 12 - Manual Tube Bending .......................................................................................... 14

Figure 13 - TIG Welding Frame ............................................................................................. 14

Figure 14 - Ergonomic Comparison 2014 vs. 2016 ................................................................ 15

LIST OF TABLES Table 1 - Material Selection 7


3

INTRODUCTION

ABSTRACT

Each year the Society of Automotive Engineers (SAE) hosts three intercollegiate

competitions in the United States for the Baja SAE Series. The cars that compete in this

design challenge utilize a common 10 horsepower Briggs and Stratton horizontal shaft motor

and must abide by all design rules as published by the SAE. Competitions include design,

cost and sales judging as well as several vehicle function tests. These tests include but are not

limited to brake and acceleration tests, hill climbs, sled pulls, suspension and traction and

finally a four-hour endurance race.

PROBLEM STATEMENT

Jon Forrest (MET) and Devon Dobie (ME) propose to build a new frame which is focused on

resolving three main problems: improving driver ergonomics, improving maintenance access

and maintaining or reducing weight while maintaining proper structural strength. Since 2012

the club has made significant strides in design and manufacturability. These three problems

however seem to be reoccurring issues that 2016 frame team wishes to resolve which will

overall make a more comfortable ride, faster repair times and a more competitive car.

BACKGROUND

The University of Cincinnati’s Baja SAE team has been competing continuously for the last

four years in which three of those years we have built new cars (2012, -13, -14). The 2013

and 2014 cars were the teams most competitive and innovative cars but each of them had

their own shortcomings. The 2013 car made a huge step in weight reduction, ergonomics and

manufacturing while the 2014 car mostly focused on ergonomics and integrating our custom

designed and built gearbox. However, the 2013 and 2014 cars both failed to meet some basic

ergonomic requirements that made customer ride uncomfortable and maintenance very

difficult.


4

RESEARCH

The research for the construction of a new Bearcats Baja frame is stemmed off of three main

sources. These sources include and are not limited to past Bearcats Baja frame builds, past

competing teams concepts and the SAE Baja Rules [4]. The problem statement spells out that

the frame team would like to focus on three major aspects which include driver ergonomics,

maintenance access, and frame weight maintenance or reduction. All of these are stemmed

from issues that have risen from previous frame design or have been previous frame design

goals.

PAST DESIGNS

The research from our past teams comes from the three senior design reports [1][2][3] for

Baja from 2012, 2013 and 2014. The 2012 car frame (Figure 1) was oversized, heavy and

very poorly put together. This was essentially the first car the Baja team put together and it

was manufactured using manual tube benders, manual pipe notchers and MIG welding which

caused a lot of inconsistencies and excessive weight when assembled. This was improved

upon in 2013 (Figure 2) with basic goals of dropping weight and improving the

manufacturing processes. The weight goals were met by slimming the frame up, removing

unnecessary tubing, outsourcing tubing bending and profiling to a company with CNC

capabilities and moving to a TIG welding process. The 2014 car (Figure 3) sought to improve

upon this car by lowering weight again and improving driver ergonomics. The 2014 team

however had a larger hurdle to overcome which arose from having larger drivers for

competition that year. This led to a wider frame to meet SAE rules and increased weight

overall. Poor attention to driver ergonomics caused discomfort to driver’s legs as well as

poor egress times per SAE rules.

Figure 1 - 2012 Frame



DRIVER ERGONOMICS

Most of the ergonomics are governed by pre-established Baja SAE rules [4] based purely on

the safety of the driver. Additional ergonomics such as length and width have maximum

values but are largely determined by the team’s research. In the 2014 car, due to limited leg

room and short cockpits, the driver’s shins were pressed against the DF members causing

bruising under normal operating conditions as well as interference with the steering wheel

and drivers knees. When observing several members of the team sitting in the past cars, it


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was noted that everyone’s knees were bent and pointed toward the side impact member while

their legs enter the nose of the frame at an angle as if doing a butterfly stretch. A primary

goal is to make the cockpit area slightly longer which would bring the drivers knees down

and out of the way of the steering wheel while reducing the angle of your legs into the nose

area thus relieving shin pressure.

MAINTENANCE ACCESS

The 2012 car was mostly easy to maintain due to its large size, but fell short in the drivetrain

area where some parts could only be removed by completely disassembling the rear end of

the car. The 2013 and 2014 cars were more compact with the focus on driver ergonomics but

also fell just barely short on maintenance access due to small hand clearances and difficult

tool clearances in the drivetrain and cockpit areas. The 2016 team is looking into a front end

design similar to UMW’s (Figure 4) to improve master cylinder and steering access. The

focus here is to design a vehicle that can be easily maintained by allowing proper hand and

tool clearances for fast repairs.

Figure 4 - UMW Baja Car

OVERALL FRAME WEIGHT

The 2013 frame was the first year that the frame weight was lowered significantly. The 2014

car improved several parts of the frame too but decided to have a goal to maintain or lower

weight if possible, which they were able to maintain. For 2016, the goal is to maintain and

possibly even lower the weight of the vehicle by removing unneeded bracing in the nose and

moving to a smaller outside diameter tubing for the fore aft bracing since it does not have to

be made of primary tubing.


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DESIGN

MATERIAL SELECTION

The Baja SAE rules set two basic requirements for material selection. The first requirement

is that primary tubing must conform to rule B8.3.12 which states:

“The material used for the Primary Roll Cage Members must be:

(A) Circular steel tubing with an outside diameter of 25mm (1 in) and a

wall thickness of 3 mm (0.120 in) and a carbon content of at least 0.18%.

OR

(B) A steel shape with bending stiffness and bending strength exceeding

that of circular steel tubing with an outside diameter of 25mm (1 in.) and a

wall thickness of 3 mm (0.120 in.). The wall thickness must be at least

1.57 mm (0.062in.) and the carbon content must be at least 0.18%,

regardless of material or section size. The bending stiffness and bending

strength must be calculated about a neutral axis that gives the minimum

values.

Bending stiffness is considered to be proportional to:

𝐸𝐼

E Modulus of elasticity (205 GPa for all steels)

I Second moment of area for the structural cross section

Bending strength is given by:

𝑆𝑦𝐼

𝑐

where:

Sy Yield strength (365 MPa for 1018 steel)

c Distance from neutral axis to extreme fiber[4]”

The second requirement is from rule B8.3.1 stating that all secondary tubing must “be steel

tubes having a minimum wall thickness of 0.89 mm (.035 in) and a minimum outside

diameter of 25.4 mm (1.0 in)[4]”.

In order to properly select primary and secondary members, an Excel chart (Table 1) was

made to compare tubing properties of SAE minimum requirements compared to several other

tubing selections.


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Table 1 - Material Selection

4130 chromoly steel tubing was chosen for the primary and secondary members due to past

frame success, favorably higher ultimate strength and lower weight per unit length. When

compared to the rule material of 1018 cold drawn steel with 1” OD x 0.120” wall thickness,

the primary members met and exceeded rule B8.13.12-B by using 1.25” OD x 0.065” wall

thickness. The selected tubing has a calculated bending strength of 4301.1 in-lb (837.8 in-lb

greater than rule material) and a calculated bending stiffness of 1,267 kip-in2 (294 kip-in

2

greater than rule material).

𝐵𝑒𝑛𝑑𝑖𝑛𝑔 𝑆𝑡𝑟𝑒𝑛𝑔𝑡ℎ = 𝑆𝑦𝐼

𝑐=

(63,100 𝑝𝑠𝑖)(0.043 𝑖𝑛4)

0.625 𝑖𝑛= 4,301.1 𝑖𝑛 − 𝑙𝑏

𝐵𝑒𝑛𝑑𝑖𝑛𝑔 𝑆𝑡𝑖𝑓𝑓𝑛𝑒𝑠𝑠 = 𝐸𝐼 = (29,732.7 𝑘𝑠𝑖)(0.043 𝑖𝑛4) = 1,267 𝑘𝑖𝑝 − 𝑖𝑛2

The secondary members were chosen to be 1.00” OD x 0.065” wall thickness, which also

exceeded rule B8.3.1.


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DESIGN ANALYSIS

The design of the vehicle was completed using Solidworks 3D sketch and weldment features.

We began by setting up a series of construction lines and planes to get a general vehicle

layout. Each plane was carefully set to ensure that revisions to the frame’s angular and linear

dimensions could be made easily. We began by drawing out a wireframe (Figure 5) that met

all of the basic roll cage requirements spelled out in article 8 of the rules.

Per the problem statement the frame team

wanted to focus on three main goals: driver

ergonomics, maintenance access, and weight

maintenance or reduction. The 2013 and 2014

car frames were great improvements upon each

other but suffered to meet basic driver

ergonomic requirements, maintenance

accessibility and even a few roll cage

requirements.

Starting with driver ergonomics, the first goal

was to improve legroom and shin to frame

clearance. In both the 2013 and 2014 frames, the

DFL and DFR members are vertical and are 14

and 13 inches apart. The short cockpit area and

the small width between the DFL and DFR was the major reason why legroom and shin

clearance was so poor. The final design included angling the DFL and DFR out slightly for

better shin clearance. This change left the DL to DR distance to be 17 inches and FL to FR

distance to be 12 inches. This would leave the driver’s shins 14 to 15 inches of shin clearance

once the raised floor is added for brake and steering components. The cockpit area was also

extended roughly 2.5 inches to allow the drivers knees to be lower and out of the way of the

steering wheel (seen objectively in Figure 6 and 7, where the red frame is 2014 and grey

frame is 2016).

The shape of the nose was also changed to help

improve vehicle egress times. A driver must be able

to egress a vehicle in less than five seconds per point

B9.2 of the 2016 BAJA SAE Technical Inspection

sheet [5]. It was observed at the 2015 Baja SAE

competition in Auburn that our driver’s feet were

getting caught on the top bar of the nose during

egress that connected points DL and DR. According

to rule B8.3.2, the frame is not required to have a

lateral cross member between points DL and DR

because our car is classified as a ‘Nose’ car. Also,

along with members DFL and DFR being angled

outwards, they were also angled forward. This did

two things; shortened up the top of the nose

Figure 5 - Revision 1 Wireframe

Figure 6 - 2014 to 2016 Objective

Comparison 1


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allowing more room for the driver to egress the vehicle between points D and S while leaving

adequate room for steering and front suspension mounting (seen objectively in Figure 7,

where the red frame is 2014 and the grey frame is 2016).

At this point in the design the seat and

steering components were modeled

and loosely placed within the frame to

ensure adequate room. The final

positioning of these parts would be

determined when the frame was

completely assembled so steering

angle and seat height could be tuned to

a more comfortable position based off

of an actual setting rather than

predicted in a model.

The idea behind designing

maintenance access into the frame was

to improve maintenance time. The frame was designed so that minimal support tabs were

used to secure the firewall and skid plate as well as left adequate room under the seat to

access the seat bolts if need be. There also was considerable discussion with extending this

initiative onto other systems such as drivetrain, steering and braking where maintenance has

been problematic to impossible to complete in a timely manner. Unfortunately due to time

constraints for completing manufacturing, final brake components and final front suspension

components could not be given maintenance access thought due to unforeseen subsystem

design delays. However, adequate room was left for both systems with the idea that the frame

would be “blank” for future Bearcats Baja teams to could easily adapt new designs.

Lastly, the improvements to weight were based on removing unneeded structural support

members around the nose area and changing the OD of the fore aft bracing tubing from 1.25”

OD x 0.065” wall thickness to 1” OD x 0.065” wall thickness. It was observed that the

majority of the front end tubing support was not needed based off of past finite element

analysis’ worst case scenario data and they were excluded from this design as seen in Figures

6 and 7.

FORCE CALCULATIONS

As with past designs, the force calculations were based on an assumed overall vehicle weight

of 600 lbs. (18.63 slug), which would include a 350 lb. vehicle and a 250 lb. driver. The

biggest difference for 2016 focused on calculating an impact force based off of published

crash data impulse times rather than derive an impulse time from a previously accepted

deceleration force of 9 g.

The drivetrain team predicted a maximum theoretical top speed of 30 mph (44 ft/s) when the

vehicle was complete. The forces were calculated based on worst case scenarios which were

decided to be a front impact at 30 mph into an immovable object, side and rear impacts by

Figure 7 - 2014 to 2016 Objective Comparison 2


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another 600 lb. vehicle moving at 30 mph and top impact if the car were to drop six feet onto

one corner.

After reading through Matthew Huang’s book on Vehicle Crash Mechanics [6], graphical

results of crash test data for a truck in a 31 mph barrier crash was found to have an impulse

time of 0.075 seconds. The following calculations were made to calculate the impact forces:

Stopping Distance

∆𝑉 =𝑑

𝑡→ 𝑑 = ∆𝑉 ∙ 𝑡

𝑑 = (44𝑓𝑡

𝑠− 0

𝑓𝑡

𝑠) ∙ 0.075 𝑠

𝑑 = 3.3 𝑓𝑡

Deceleration

𝑑 =𝑉2

2𝑎→ 𝑎 =

𝑉2

2𝑑

𝑎 =44 𝑓𝑡/𝑠2

2(3.3 𝑓𝑡)

𝑎 = 293 𝑓𝑡

𝑠2⁄ = 9.1 𝑔′𝑠

Front, Side and Rear Impact Force

𝑓 = 𝑚𝑎

𝑓 = (18.63 𝑠𝑙𝑢𝑔) ∙ (32.2 𝑓𝑡

𝑠2⁄ ) ∙ (9.1 𝑔)

𝑓 = 5470 𝑙𝑏𝑓

Six Foot Drop Velocity

𝑚𝑔ℎ =1

2𝑚𝑣2 → 𝑣 = √2𝑔ℎ

𝑣 = √2 ∙ (32.2 𝑓𝑡

𝑠2⁄ ) ∙ (6 𝑓𝑡)

𝑣 = 19.66 𝑓𝑡

𝑠⁄

Top Impact Force

𝑓 = 𝑚∆𝑉

∆𝑡

𝑓 = (18.63 𝑠𝑙𝑢𝑔) ∙

(19.66 𝑓𝑡

𝑠⁄ )

(0.075 𝑠)

𝑓 = 4884 𝑙𝑏𝑓


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FINITE ELEMENT ANALYSIS

Force analysis was completed on the frame using Solidworks built in finite element analysis

(FEA) capabilities. The four analysis’ (front, side, rear and top impacts) were all completed

using static load studies and joint fixtures were chosen case by case to yield the most realistic

force distribution through the frame. It should be noted that finite element analysis is

subjective and prior to testing is largely theoretical with respect to design. The following is

the results of the FEA:

Front Impact (Figure 8)

Impact Force: 9.1 g’s or 5470 lbf

Max Stress: 75.9 ksi

Factor of Safety: 1.28

Figure 8 - Front Impact FEA Results

Side Impact (Figure 9)





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Figure 9 - Side Impact FEA Results

Rear Impact (Figure 10)




Figure 10 - Rear Impact FEA Results


13

Top Impact (Figure 11)

Impact Force: 4884 lbf



Figure 11 - Top Impact FEA Results

Questioning of the driver safety did arise from the results of the worst case scenarios low

factors of safety. The reason why the design was deemed acceptable by the author could be

perceived as a grey area in safety versus performance that can be seen in any real world

design application of reliability versus performance. If the frame was designed to withstand a

50 mph front impact, the car would be overdesigned to the point that its weight would

negatively impact overall vehicle performance. Sport side-by-sides currently on the market

have the ability to do upwards of 80 mph which would undoubtedly total the vehicle in a

collision at that speed. There is an inherent danger that should be understood and accepted

with off-road racing and the 2016 frame FEA proves that it will be safe all the way up to its

top speed.

MANUFACTURING

BENDING AND PROFILING

The 2016 frame subsystem team made the decision from the beginning of design to

manufacture this prototype frame at minimal cost while still delivering comparable quality to

previous years frames. The most successful decision made towards this goal was deciding to

not outsource CNC bending and laser profiling at VR3 (located in Ontario, Canada), which is

a savings of nearly $2,000. Instead the frame tubing was cut to appropriate lengths and bent


14

at Ohio Hydraulics located in Sharonville,

Ohio and was profiled using a simple logical

technique that involved a Sharpie, precision

angle grinding and calibrated eye balls.

There was an accepted risk with this

approach largely due to the high probability

of making a bad cut, which only turned into

scrapping two tubes out all 39 tubes that

made up the frame. In this case, the in house

manual tube bender was used to replace the

scrapped tubing (Figure 12).

WELDING

The welding process chosen for the frame was Tungsten Inert Gas (TIG) welding (Figure

13). The decision was made based off of research for welding 4130 Chromoly tubing,

availability of proper filler wire and cleanliness of the process. 4130 tubing can be MIG or

TIG welded and both are commonly used in the aerospace industry; however the author

chose TIG welding due to the higher ability

to control the weld puddle and penetration of

the weld to prevent weld burn through from

excessive heat. The filler material selected

for the fabrication of the frame was ER70S-2

for it high strength and favorable fatigue

properties. Any non-structural tubing

members were MIG welded to cut down on

fabrication time. It was also deemed

unnecessary to have the frame heat treated

due to the thin wall thicknesses that had been

used but typically any 4130 over 0.120” wall

thickness would require weld stress relief to

prevent cracking. Due to this manual method

of construction, the vehicle was assembled using a variety of tools to ensure proper

construction such as levels, digital angle finders, a square and measuring tape.

OTHER COMPONENTS

Firewall

Skid plate

Fire extinguisher

Seat

Restraint system

Figure 12 - Manual Tube Bending

Figure 13 - TIG Welding Frame


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CONCLUSION

In review, the frame team was able to achieve all of the three main problems addressed in the

problem statement which were improving ergonomics, maintenance access and maintaining

or reducing weight.

Compared to the 2014

frame, it can be seen in

Figure 14 that ergonomic

improvements were made

when one of the team’s

drivers sat in the car. It can

be noted that there are

improvements of forearm

to leg space, steering wheel

to knee space, leg angle

and shin to DFL and DFR

member relief. The overall

weight of the 2016 frame

came out to 76 lbs, which

is +1 lb compared to the

2014 frame which came in

at 75 lbs. Overall cost

savings from 2014 to 2016

frame were approximately

$1,650. On top of these successes, it can be concluded that the 2016 frame is a competitive

platform for future University of Cincinnati Baja teams to use and compete in for years to

come.

ACKNOWLEDGEMENTS

I would like to thank Dean Allen Arthur for his continuous support of this team over the

years. I would like to thank Devon Dobie (ME) for being a fantastic partner through the

design and manufacturing of the 2016 Frame. A big thanks to TW Metals for the very

generous donation of material. Finally I would like to thank my parents for their continuous

support and to the alumni that I’ve met through the Bearcats Baja program for mentoring me

throughout the years and encouraging me to pursue this project for Senior Design.

Figure 14 - Ergonomic Comparison 2014 vs. 2016


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WORKS CITED [1] Biteman, Brooks. Baja SAE Frame Design. Thesis. Cincinnati: University of Cincinnati,

2013.

[2] Kobs, Joe. 2014 University of Cincinnati Baja SAE Chassis. Thesis. Cincinnati:

University of Cincinnati, 2014.

[3] Ratliff, Michael. 2012 Baja Frame and Chassis. Thesis. Cincinnati: University of

Cincinnati, 2012.

[4] —. "2016 Collegiate Design Series Baja SAE Rules." 2015. SAE International.

<https://www.bajasae.net/content/2016_BAJA_Rules_Final-9.8.15.pdf>.

[5] SAE. "2016 BAJA SAE Technical Inspection Sheet." 3 4 2016. Baja SAE. 10 4 2016

<http://www.bajasae.net/content/TechSheet04-03-2016_Rev2-3.pdf>.

[6] Huang, Matthew. Vehicle Crash Mechanics. Boca Raton, FL: CRC, 2002. Print.

17

APPENDIX A – NAMED ROLL CAGE POINTS

“All named points are implied to have a Left and Right hand side, denoted by subscript L or R

(e.g. AL and AR)”[4]

18

APPENDIX B – VEHICLE BUDGET

19

172

431

714

21

28

512

192

62

916

23

307

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12

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APPENDIX C - SCHEDULE

20

APPENDIX D – DRAWINGS

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22

23

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