Final Report:
Team 16
Supermileage
May 8
2013
Andrew D’Agostino (ME)
Eric Bixler (ME)
Santiago Tinholt-Chiza (ME)
© 2012, Team Super-mileage and Calvin College
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Abstract
This document summarizes the design and manufacturing process taken by the team to build a
high fuel economy vehicle to compete in the Society of Automotive Engineers Supermileage®
competition. The purpose of the Supermileage® competition is to bring together colleges from
the United States and Canada to attempt to achieve the highest possible fuel economy. The
competition revolves around good engineering practices. The following design will perform
well in the competition.
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Table of Contents
Abstract .....................................................................................................................................................1
1 Project Proposal and Feasibility Study .......................................................................................3
1.1 Supermileage® Competition ................................................................................................................. 3
1.1.1 Competition Constraints ............................................................................................ 3
1.2 Project Management ................................................................................................................................. 4
1.2.1 Team Organization..................................................................................................... 4
1.2.2 Schedule ..................................................................................................................... 4
1.2.3 Budget ........................................................................................................................ 5
1.2.4 Method of Approach .................................................................................................. 6
1.3 Research ........................................................................................................................................................ 6
1.4 Design Process ............................................................................................................................................ 7
1.4.1 Aerodynamic Analysis ............................................................................................... 7
1.4.2 Computer Analysis..................................................................................................... 8
1.4.2.1 Computer Aided Design ......................................................................................... 8
1.4.2.2 Computational Fluid Dynamics .............................................................................. 9
1.4.2.3 FEA Stress Analysis ............................................................................................. 12
1.4.3 Stress Analysis ......................................................................................................... 12
1.4.4 Electrical .................................................................................................................. 16
1.4.5 Engine Modifications ............................................................................................... 17
1.5 Design Alternatives .................................................................................................................................18
1.5.1 Shell Material/Shape ................................................................................................ 18
1.5.2 Frame Material/Shape .............................................................................................. 18
1.5.3 Engine Tuning Options ............................................................................................ 21
1.5.4 Steering .................................................................................................................... 22
1.5.5 Wheels / Tires .......................................................................................................... 23
1.5.6 Drivetrain ................................................................................................................. 24
1.6 Manufacture ...............................................................................................................................................24
1.6.1 Shell Manufacture .................................................................................................... 24
1.6.2 Frame Manufacture .................................................................................................. 30
1.7 Conclusion .................................................................................................................................................32
Works Cited .......................................................................................................................................................33
Appendix A: Frame Calculations................................................................................................................34
Appendix B: CFD Data………………………………………………………………………………………………………36
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1 Final Design Report
1.1 Supermileage® Competition
The Society of Automotive Engineers sponsors an annual competition in Marshall, Michigan as
part of the Collegiate Design Series. The Supermileage® Competition brings in teams from
colleges and universities from not only the United States but Canada as well. Supermileage®
began in 1980, but recorded data about the competition begins in 1996. Teams from Calvin
College have competed in the competition for two years in a row now. Results from these two
teams are presented in the following section.
1.1.1 Competition Constraints
In order to promote fair competition between all of the teams, many constraints are put on the
vehicles entered into the competition. The complete list of rules and regulations is 28 pages
long, so a brief synopsis will be presented here. These rules are presented in the SAE
Supermileage® document (http://students.sae.org/competitions/supermileage/rules/rules.pdf)
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All power used to propel the car must come from the engine provided by the SAE
competition.
The driver must have a forward field of vision ± 80o from the longitudinal axis of the
vehicle (perpendicular to the length).
The vehicle needs to be able to maneuver with a 50 foot turning radius
The engine used in the competition must be the 1 cylinder, 3.5 horsepower engine
provided by the competition. Modifications are allowed.
The engine must be able to run without powering the vehicle.
The fuel bottle provided by the competition must be able to be attached and ready to run
in 45 seconds or less.
Batteries may only be used for instrumentation, ignition, and other activities that do not
directly power the vehicle.
There must be 3 kill switches on the vehicle to ground the engine. (One on each side of
the vehicle, and one within reach of the driver)
The roll bar must be 2 inches or more above the driver’s helmet and be able to support a
250 lb force from any direction. The roll bar must also be at least as wide as the driver’s
shoulders.
A .032 inch thick firewall made of steel or aluminum must separate the driver from the
engine. There are no gaps greater than 0.5 inches allowed in the firewall.
Driver must be able to exit the vehicle in 15 seconds, and driver must be able to be pulled
from the vehicle in 20 seconds.
The vehicle must have a 13.2 foot braking distance when traveling at a speed of 15 mph.
There must be a brake light on the car that is clearly visible to other drivers.
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These are just a few of the important constraints that the team must consider during the entire
design process. The rules are checked and enforced during a technical inspection at the
beginning of the competition.
1.2 Project Management
1.2.1 Team Organization
Four mechanical engineering students make up the senior design team from Calvin College
Engineering department. The scope of the project was divided into four main categories: frame,
aerodynamics, powertrain, and engine. Work was distributed by personal interest and individual
skills. The team break down is shown below.
Students Members
Eric Bixler: Frame design and electrical system
Santiago Tinholt-Chiza: Aerodynamic modeling and design
Andrew D’Agostino: Drivetrain, steering assembly, and material selection
1.2.2 Schedule
To manage time, team member Eric Bixler was appointed to maintain the schedule and inform
the team of upcoming due dates. When a date for a deliverable was approaching, or if there was
an important meeting approaching, the item was added to our senior design station whiteboard.
These dates were updated or recognized every team meeting. When a scheduling conflict arose,
work was divided and then discussed next team meeting. For long term planning and scheduling,
a Gantt chart was created in Microsoft Project. The following figure shows the work plan for the
project.
Figure 1: Team Schedule
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Table 1: Hours Per Person
Task
Hours
Research previous design team work
16
Fuel injection vs. carbouration
8
Economic Analysis
8
Plan engine modifications
8
Tire and rolling resistance research
10
Compression ratio research
8
Engine testing
8
Ongoing engine modifications
20
Research of areodynamic options
8
Design of shell
8
CAD modeling of shells
6
CAD areodnamic testing
4
Construction of models for wind tunnel
8
Wind tunnel testing
3
Analysis of shell shapes
8
Find driver and measure for chasis
4
Design chasis
8
Research shell material options
8
Research stearing assemblies
8
Integrate stearing with chasis
8
Integrate chasis with shell
8
Aquire parts
8
Assemble vehicle
550
Prepartion of PPFS
10
Verbal presentations
4
Final report preparation
10
Compete in SAE competition
20
Total Hours
777
1.2.3 Budget
Management of the budget was a responsibility of all team members. To keep track of the
budget there was a Microsoft Excel spreadsheet that all members had access to. This recorded
the monetary amount, description of what was purchased, name of purchaser and the date of
purchase. Budget issues are a given in any engineering project. When a budget issue arose (ex.
going over budget), the decision on seeking more funds will be based on improvement of
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performance versus cost. The specified budget for the Supermileage entry was roughly $2100.
Table 2 Team 16 Budget
Team Budget
$
SAE Registration
$ 650.00
Engine Modifcation
$ 100.00
Shell Materials
$ 605.00
Gas
$ 20.00
Plug for Molding
$
Plexiglass
$
Wheels
$ 190.00
Tires / Tubes
$ 98.00
Aluminum Tube / Channel $ 70.00
Aluminum Sheet
$ 25.00
Transmission
$ 140.00
Plastic
$ 40.00
Paint
$ 80.00
Misc
$ 96.00
Electronics
$ 70.00
3-D Printing
$ 80.00
Total
$2,264.00
Vendor
SAE
West Systems
Betz Ind
Shop
Velocity
Alger Bike
Alro Metals
Alro Metals
McMaster Carr
Lowes
Physical Plant
Calvin
1.2.4 Method of Approach
There are numerous modifications that can be made to a vehicle to increase its fuel efficiency.
Our approach to designing a vehicle more efficient than past Supermileage vehicles was to focus
on changes that will provide the greatest increase in efficiency. From researching and studying
previous team’s reports, we learned from others mistakes and avoided wasting time on changes
that have a low success rate or ideas that did not improve the vehicle performance.
1.3 Research
This project required constant research regarding various topics that need to be addressed
throughout the entire design and construction phase. Reading and gaining knowledge for every
component of the vehicle was vital for the success of the project. Many of the design aspects that
came into the development of the vehicle were new to the members on the team. Topics that
required hours of inquiry are as follows: four-stroke Briggs and Stratton engines, aerodynamics,
design norms, electrical power distribution, frame FEA analysis, exhaust heat reduction, gearing
and transmission techniques, ICE maintenance, and steering assembly design.
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1.4 Design Process
1.4.1 Aerodynamic Analysis
The drag forces acting on the vehicle during operation can be lessened by designing a more
aerodynamic profile for the outer shell. The most important factors regarding drag reduction
considered during the design process are frontal area, frontal pressure, and rear vacuum effects
on the vehicle. One method for minimizing the air pressure at the front of vehicle is to decrease
the frontal surface area. This reduction in surface area for the air to hit will result in less air
molecules compressing. It was imperative that the frontal profile was aerodynamically shaped to
prevent large pressure differences on the surface of the shell. As a designer, you want to create a
streamline that cuts through the air at speed, and prevent turbulent flow. Another consideration
was the effect of flow detachment from the rear profile of the vehicle. Flow detachment is the
rear vacuum that occurs when vehicles produce holes left in the air when they operate. More
aerodynamic shapes will reduce the vacuum area behind the vehicle. The designs we are
considering all have the rear profiles converging to one point. Since the SAE regulations require
the competitors to limit the top speeds during competition, lift and down-force effects will be
negligible for the final design process. Down-force is necessary for race car design due to the
high cornering and straight line speeds, but the competition for our vehicle is not built for speed.
Lift effects are not necessary either, because the speeds are not high enough to produce sufficient
lift forces to overcome normal forces. Goals that the design team will focus on achieving are as
follows: minimizing frontal area to reduce frontal air pressure, lowering vehicle frame and
profile to minimize air flow under the car, converging bodywork slowly so to avoid flow
turbulence and reduce drag, covering open wheels, and using a material for outer shell that
reduces surface friction. All of these considerations will ultimately affect the coefficient of drag
associated with the shape and improve the fuel economy. One method for obtaining the
coefficient of drag of various vehicle designs is utilizing the wind tunnel located at Calvin
College. This requires physical models of the designs considered which are scaled down to about
1:15 size. The wind tunnel test was conducted with the scaled down model created from rapid
prototyping on campus. The results gathered from the force gages and various air velocities are
seen in Table 3. The values from the physical wind tunnel and virtual CFD analysis differ as a
result of error in the system gages and as a result of the scale of the wind tunnel. In order to
properly scale up the wind tunnel test to the same conditions of the CFD analysis, the air
velocities would need to be increases by a large factor to compensate for the smaller vehicle
model. The tunnel on-campus has a limited air velocity, so the desired conditions could not be
used.
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Table 3: Physical Wind Tunnel Test
Air Speed (m/s)
18
26
35
43
45
Force Drag (N)
Drag C
0.012
0.166
0.017
0.171
0.023
0.126
0.028
0.156
0.030
0.152
1.4.2 Computer Analysis
1.4.2.1 Computer Aided Design
The second method used for the project is Solidworks CAD software to model the shell. Using
the Solidworks model, a virtual flow simulation will then be performed using Autodesk CFD
2013. Designing and rendering the CAD models in Solidworks has proven to be a useful method
as opposed to creating/purchasing physical models. The method for correctly sizing the
dimensions of the shell involves importing the CAD frame design and forming a shape around it.
The shell designs in Solidworks were modeled as two separate parts. The top and bottom halves
of the shell can be seen for prototypes (1-7) in Figure 3. The corresponding drag coefficients and
drag forces for each prototype are displayed in the figure below as well. Last year’s shell was
designed as well and run through the CFD. This allowed the team to compare data with last
year’s car. The team goal of reducing the frontal area of this year’s shell from last year was
achieved. The goal set was to reduce frontal area by 15%. All seven prototypes have achieved
this goal. The final design chosen P8 has a 30% reduction in frontal area from last year. This
design is shown further into the report.
The final optimized shell design P-8 is shown in Figure 2-A, along with the frame oriented
inside.
Figure 2-A: Calvin College SM-13 P8 chassis
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Figure 2-B: Prototypes 1-7 top and bottom shell
1.4.2.2 Computational Fluid Dynamics
Computational fluid dynamics software will be utilized for the aerodynamic properties
associated with each prototype design for the shell. The package that the team is using this year
will be Autodesk CFD 2013. This program will serve as the primary method to obtain drag
forces and drag coefficients of the considered designs. Validation of the software for virtual wind
tunnel testing was necessary before any of the new shell designs could be tested. The initial test
method done to validate the software was an external flow analysis over a sphere using air as the
active fluid. A solid sphere was modeled in Solidworks and then imported into CFD program,
where a full range of Reynolds numbers was tested. The range of Reynolds numbers in the flow
trials (0.4 < Re < 640,000) allowed for a dataset of drag coefficients and pressure levels to be
determined based on air velocity. Numerous iterations were solved for in the full analysis, and
using a refined mesh zone consisting of over 900,000 nodes, accurate readings for frontal and
rear pressures were determined. Global velocity and pressure profiles are shown in Figure 3. A
vital goal that the team will hope to achieve while designing the shell is to reduce the frontal and
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rear pressure levels. This would result in a reduced drag force on the vehicle, and thus better fuel
economy.
Figure 3: CFD velocity and pressure profiles (velocity of air = 50 m/s)
Comparison of the CFD and existing external flow studies was the next step. It was necessary to
construct a plot that represented the average drag coefficient of the sphere as a function of the
Reynolds number of flow. Data for the average drag coefficient for flow over a sphere was taken
from Çengel, Yunus A., and Michael A. Boles. Thermodynamics: An Engineering Approach. This data
is shown in Figure 4 as a plot. The CFD data compounded was then used to create similar data
plot including the same parameters. The data from the CFD software is shown in Figure 6. It was
concluded that the two sets of data were consistent with each other and that validation for the
software was successful. The section of the plots to focus on are the region of (10 < Re < 1000).
This is the region where air over the sphere is not yet in the turbulent boundary and the
phenomena of flow wake occurs.
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Figure 4: Average drag coefficient for flow over cylinder and sphere (Cengel Data)
Figure 5: Average drag coefficient for flow over sphere (CFD data)
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1.4.2.3 FEA Stress Analysis
In order to further analyze the frame as a whole, it was drawn using Autodesk Inventor
Professional 2012. The 3D model was then exported into Autodesk Simulation Multiphysics
2012 to run a stress analysis with forces applied to the frame based on weights. The program
created a mesh that connected all the members, and then made it easy to select the nodes. The
sections that were connected to wheels were considered the fixed nodes (circled sections on
figures). The results are shown in the figure below. The range on coloring is as follows: dark
blue=0 psi, red=30000 psi.
Figure 6: FEA stress analysis results (Rollbar force downward)
As shown, the maximum stress in the entire frame is about 20000 psi, which is a factor of 2
under the yield stress limit for aluminum 6061-T6. This proves that the frame is safe for use in
the vehicle. As is evident in both previous figures, the higher stresses are not across the entire
frame, but in different localized sections. Most sections of the frame undergo stresses under
5000 psi.
1.4.3 Stress Analysis
The frame has many constraints revolving around it. As was presented in section 1.4.2, the roll
bar must support a 250 pound force from any direction, be wider than the driver’s shoulders, and
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extend at least 2 inches above the driver’s helmet. The frame must also be able to support the
weight of the driver, engine, shell, and any other components of the vehicle. The final result is
that the frame must support a weight of over 400 lb.
In the current frame design, the majority of the load will be placed on the four beams at the base
of the frame. The beams are shown on a 3D model of the frame in figure 9. For simplified
calculations as a material comparison tool, these four beams were combined into two beams that
would run the length of the vehicle. The load profile of these simplified beams is shown in
figure 9.
Figure 8: Vehicle Frame
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Figure 9: Force distribution along the main support beam
A stress analysis was then done in the program EES for the simplified system. This was only to
get an idea of what material to use and what thickness. This calculation sheet is presented in
appendix A. In the model, all of the forces were combined into one equivalent force at a specific
length along the beam. The forces are presented in table 4 below.
Table 4: Forces on the frame
The forces in table 1 were calculated using two simple laws of physics. The first is that the
forces on the beam must sum to zero. The second is that the moments must also sum to zero.
These two equations were used to calculate the resultant forces, R1 and R2. Then, using the same
two principles, the equivalent force and position was calculated for Fequivalent. Using the position
for Fequivalent, the maximum moment was calculated. Then, the maximum moment (𝑀𝑚𝑎𝑥 ) was
used to calculate the maximum stress (𝜎𝑚𝑎𝑥 ) in the beam using equation 1 (Riley 481).
𝑀𝑚𝑎𝑥 𝑐
𝜎𝑚𝑎𝑥 =
𝐸𝑞𝑛 1
𝐼
The maximum moment is simply the moment calculated at the position of the equivalent force.
The term ‘c’ refers to the furthest distance from the neutral axis, which happens to be half the
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height of the beam. The moment of inertia is denoted by I, which is calculated using equation 2
(Riley 660-661,677).
3
𝑏ℎ3 𝑏𝑖𝑛𝑠𝑖𝑑𝑒 ℎ𝑖𝑛𝑠𝑖𝑑𝑒
𝐼=
−
𝐸𝑞𝑛 2
12
12
In equation 2, b is the base length on the outside of the rectangular tubing, and b inside is the base
length on the inside of the tube. The height is denoted by h, and the inner height is given by
hinside. A brief diagram is shown in figure 10.
b
binside
h
hinside
Figure 10: Beam moment of inertia variables
In EES, a system of equations set the maximum stress in the beam to have a safety factor of 2.
Then, given tube wall thickness, a side length of the beam was calculated. The beam dimensions
were then turned into a weight, which is presented in table 5.
Table 5: Calculated beam thicknesses to achieve a safety factor of 2
Beam
Material
Thickness
Beam Mass
(in)
(lbm)
Aluminum 6061-T6
0.110
2.6
Steel ASTM A-500
0.027
2.06
304 Stainless Steel
0.160
10.5
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Going by the results in table 5, the steel appears to be the best choice for the material in the
frame. These values are based on a beam base of 1 inch and a height of 1.5 inches. However,
when considering the purchase of the tubing, not all sizes are available. The closest size for steel
that is readily available is about .0625 inches, and aluminum can be purchased at a thickness of
.125 inches. When accounting for these thicknesses, the aluminum becomes the better choice.
The updated table is shown below.
Table 6: Updated beam thickness comparison table
Material
Beam
Thickness
(in)
Beam Mass
(lbm)
Aluminum 6061-T6
0.125
2.96
Steel ASTM A-500
0.048
3.64
After the analysis, it is decided that the aluminum frame will be the best choice. This is to
minimize the weight, which will in turn boost the fuel economy.
1.4.4 Electrical
Per SAE Competition requirements, the Supermileage vehicle must be equipped with:
 Electric Start
 Brake light
 3 Kill Switches
 Fused Circuit
The following schematic shows the electrical layout for our vehicle:
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Figure 11: Electrical Schematic
The kill switches are wired so that if any of them are set to the “kill” position, the magneto is
immediately grounded and cannot transmit a current to the spark plug, causing the engine to die.
The electric start is connected through a push button to a motor that spins a gear meshed with the
flywheel. Assuming all kill switches are set to “run,” this should start the engine almost
immediately. The brake light is connected via a switch that connects the circuit when the brakes
are activated. Since there are 2 sets of brakes, there are 2 switches that need to be in this circuit.
If either one trips, the brake lights turn on. A basic circuit layout is shown in figure 11.
1.4.5 Engine Modifications
Modifications to the engine hold vast potential in improving mileage. Engine modification can
also be a highly time consuming effort that may result in decreased mileage and reliability.
Balancing the risks with the rewards is essential in this endeavor. It is also important to keep
engine modifications within budget. These considerations were kept in mind as we researched
possible engine modifications. To see a description of the engine as well as alternatives see the
engine tuning options section.
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1.5 Design Alternatives
1.5.1 Shell Material/Shape
The desirable material for this design has not only strength-to-weight properties, but also a
reasonable application time and purchase cost. Table 7 shows the initial shell material
considerations.
Table 7: Summary of potential shell materials
Monokote is a plastic shrink wrap that is used to cover remote controlled airplanes. This
material is extremely lightweight and fairly puncture resistant however, it requires an internal
structure to support it. With the curvature of our final shell, fabricating a structure to support the
material would be difficult therefore this option was not chosen. Vacuum forming a plastic such
as PETG was also looked into. This would dramatically shorten shell fabrication time, but the
need of a high temperature susceptible mold, ruled out vacuum forming because of costs. In
order for the team to create the outer shell using Carbon Fiber, Dacron, or Fiberglass, a
Styrofoam plug scaled 1:1 to our desired exterior design was necessary. Betz Industries, a local
company, donated the plug. Betz Industries uses a full mold casting process to produce iron
castings created from Styrofoam CNC machines. Betz Industries requires a solid model of the
desired part the customer wants. A single CAD file was enough to create the plug for our
vehicle. The plug can be used to form that shape of the car when a composite fabric is applied
over it. The fabric chosen for the shell material was 4 oz. fiberglass. The choice was a function
of the material having a necessary strength to weight ratio as well as cost. Carbon Fiber offered
unnecessary amounts strength at a high price. Dacron was calculated to be more expensive than
fiberglass by $6 / yard.
1.5.2 Frame Material/Shape
When designing the frame for a vehicle, the material is one of the main decisions to affect the
final product. The main point of the frame material is to provide stability, yet be as low weight
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as possible. The comparison tool for the materials that was used before analysis is something
referred to as the strength density. This is the yield strength divided by the density, so as to give
some sort of number to strength per pound of the material. The values for Aluminum 6061-T6,
SAE Steel 4340 and 304 Stainless Steel were analyzed. Results are presented in table 8.
Table 8: Mechanical properties of metals
Density, modulus of elasticity, rigidity, and yield strength were all found in tables (Riley 700,
http://asm.matweb.com/search/SpecificMaterial.asp?bassnum=MQ304A). As is shown in table
7, the strength density seems to point to steel as being the best choice. However, after analysis,
the aluminum tubes proved to have the greater advantage. This choice was described in section
1.5.3.
For the frame shape, the basic outline comes from the 2011-2012 team design. However, to
achieve less aerodynamic drag, the shape is about 8 inches shorter than the previous design. In
order to achieve this design, we realized that the only thing keeping the driver sitting more
upright was the steering assembly. Even though the driver could slide further down into the
vehicle, the steering assembly was in the way. In order to combat this problem, the steering
assembly was moved up slightly and forward. The next issue was wasted space beneath the
driver. In order to combat this problem, the front beams were moved apart to allow the driver to
be seated in between and below the two members.
The final design for the roll hoop is presented in figure 12. The height from bottom to top is 21
inches, and from the very right side to the very left side is 17 inches. This fits in the constraints
of the competition, as the shoulder width of the driver is only 16.5 inches, and in driving
position, the driver’s head is 18 inches above the seat. This gives 3 inches of freedom before
running into the roll bar height constraint.
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Figure 12: Roll bar geometry to fit in the competition constraints.
In order to make the roll bar, there was a choice between bending the tubing or welding. After
some decisions, the decision was made that bending the tube would retain the highest strength in
the material.
The rest of the frame is designed to be lightweight and house all components effectively. The
side view of the entire frame is shown in figure 13. The overall length of the members will be
just under 6 ft. This provides enough room in the back for the wheel, engine, and gearing
system, while also leaving plenty of room in the front for the driver.
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Figure 13: Side view of the frame geometry
1.5.3 Engine Tuning Options
The engine of our vehicle is mandated to be a Briggs & Stratton Model 091202 Type
1016E1A1001. This engine is 148cc, produces 3.5 hp at 3600 rpm, and 5.25 ft-lbs of gross
torque per SAE J1940. This engine is provided to every team entering in the competition. Any
modifications are allowed to the engine so long as the crankcase and cylinder are original. Fuel
provided at the completion will be iso-octane (2,2,4-Trimethylpentane). Iso-octane is the
standard 100 octane fuel on the 0-100 scale seen by motorist at the pump.
We are currently planning on using an aftermarket carburetor scrapped from last year’s engine to
provide air to the combustion process. The carburetor is a Mikuni VM model and allows for
easier air-fuel mixture optimizing and idling speed of the motor. The desired idle performance of
the engine will need to be as lean as possible, to conserve fuel during start-up and low speed
operation.
The best mileage is obtained when the spark plug fires just as the piston has completed its
compression stroke and is beginning its power stroke. Adjustments to the ignition timing can
result in reliability issues and engine vibrations. Valve timing is closely related to ignition firing.
The Briggs & Stratton engine provided to the competitors controls valve timing via an adjustable
gear tied to a camshaft housed in the crankcase. The stock valve timing keeps the exhaust and
intake stokes completely separate. There is a potential for higher mileage when the intake valve
opens slightly before the exhaust valve closes. This flushes out remaining waste gas.
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Modification to the cam profile can advance the opening of the intake valve and keep the valve
open until the piston reaches bottom dead center (BDC).
Finally, we will increase the compression ratio of the stock engine. When the compression ratio
is increased the pressure ratio also increases. The result of these increases is that the potential for
expansion in the power stroke is greater. We can increase our compression ration by adding a
sleeve to the cylinder or by welding material to the head or piston. The stock head ratio is 6.6:1;
we are increasing the ratio to 7.2:1 by adding material to the head.
In summary, we will be running our Briggs & Stratton engine carbureted at an increased
compression ratio.
1.5.4 Steering
To make the make the manufacture of the steering column easier several changes were made
from the previous years this can be seen in figure 14.
Figure 14: Comparison of steering assemblies (Left: 2012, Right: 2013)
By changing the design of the steering column it allowed for a reduced amount of welds as well
as more simple weld geometry- the only angle that has to be accounted for was were the square
stock was welded to the cylinder that housed the steering wheel rod. The steering wheel itself
was also dramatically changed for this year. On the 2012 car the steering wheel only allowed the
driver to steer, brake and accelerate. For the driver to start or kill the engine he would have to
take his hands of the steering wheel. For 2013 these controls were added to the steering wheel
itself as shown in the following figure. The wheel this year includes the following functions for
the driver to engage without changing his eye position or taking his hands off wheel. Ignition,
brake, throttle, master kill switch, tachometer, speedometer.
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Figure 15: Steering Wheel Controls
1.5.5 Wheels / Tires
To minimize the frontal area of the vehicle, the front wheels will be sized at 20 inches and the
rear wheel at 27 inches. Rolling resistance was a sacrifice, for aerodynamic efficiency. The
front tires are Maxxis Hookworm rated at 85 to 110 psi. The rear tire will be a 27 inch tubeless
Continental Podium rated at 120 psi. The wheels will be mounted to the vehicle using a 20 mm
thru axle hub custom axle assemblies. Figure 15 shows the axle assemblies.
Figure 16: Custom Axle Assemblies
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1.5.6 Drivetrain
The following figure shows a picture of the drivetrain used by last year’s Supermileage team.
Figure 17: 2012 Drivetrain Assembly
For 2013 the drivetrain will consist of a single gear reduction. For engine speeds between 2300
to 4000 rpm the gear reduction will range from 12 to 18. This was calculated using simple
equations of motion and gear reduction. Firstly, it was assumed that a vehicle speed of 18 miles
per hour was necessary for the competition. This is because we did not want to go to close to the
lower bounds of the competition speed restraints, and stay away from higher speeds as they will
rob the engine of the necessary fuel economy. Using geometry, the circumference (c) of our 27
inch diameter rear wheel is about 85 inches. A speed of 18 miles per hour translates into roughly
317 inches per second.
𝑣
𝑠𝑒𝑐
60
=𝑁
𝐸𝑞𝑛 3
𝑐
𝑚𝑖𝑛
In equation 3, N is the required rotational speed in rpm of the drive wheel. Then, dividing the
rotational speed of the engine by the necessary rotational speed of the drive wheel, the necessary
gear reduction of 12 to 18 is calculated.
1.6 Manufacture
As with any design, there is always a build stage. This next section details the larger portions of
the manufacturing of the vehicle.
1.6.1 Shell Manufacture
The most important part of the shell manufacturing process was the Styrofoam plug donated to
the team by Betz Industries. This was made using CNC milling machines that cut out different
Calvin College Engineering 25
blocks of Styrofoam, which were then glued together to form the shape of the shell that was
presented in section 1.4.2.1. The employees at Betz took the 3D CAD file that we sent them and
made the Styrofoam mold from it. The mold is shown in figure 18.
Figure 18: Styrofoam Mold donated by Betz Industries
The next step was to slice the mold in half so that we could make the shell in 2 halves. This was
done using a hot wire mounted in between two wooden posts. These posts were clamped to
heavy steel pieces, so that pushing the Styrofoam through the hotwire would not drag the
wooden frame with it. The hotwire was set at the height we desired, and then the Styrofoam was
slowly pushed through the hotwire.
After the mold was in two pieces, the first step was to make sure that the fiberglass would release
from the mold. In 2012, the Supermileage team spent a week grinding the Styrofoam out of their
shell. This was a time consuming process that we wanted to avoid. The reason that the
fiberglass sticks to the Styrofoam is the resin is absorbed by the Styrofoam, creating a bond
between the fiberglass and the mold. This is what we didn’t want. After consulting with Phil
Jasperse and Professor Nielsen on many occasions, as well as testing ideas we settled on
covering the Styrofoam in shrink wrap to create a barrier between the Styrofoam and resin. This
was done using standard shrink wrap purchased from Lowe’s hardware store. The wrap was put
on the mold, taped down, and blown on with a heat gun to pull the wrap tight and remove any
imperfections. Figure 19 shows the top half of the mold halfway through being wrapped. The
left half of the mold is complete, and shows a smooth surface on which to lay the fiberglass.
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Figure 19: Top half of mold after shrink wrap
After shrink wrapping was complete, the first fiberglass layer needed to go on. We found that
the best way to do this was to lay the cloth down completely dry before actually putting resin on
it. The dry cloth was cut into 4 smaller pieces that covered the top half of the mold. This
allowed us to formulate a plan for the best way to lay down the material, especially to avoid
wrinkles. After laying the dry fiberglass down, we knew how we needed to proceed, and the first
layer of fiberglass began. A picture of the dry layup is shown in Figure 20.
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Figure 20: Dry fiberglass laid out for epoxy coating
After the dry layer was laid down, 3 of the cloths were removed and the epoxy coating was laid
down starting back to front. During this process, knowing where to place the next piece of cloth
sped up the process a lot.
After the first layer of fiberglass was completely finished, a foam layer was added to increase the
rigidity of the shell. The foam was purchased from Lowe’s at $35 for 200 square feet, which
was much more than required. Laying the foam layer required a lot of patience. The foam was
cut into small strips that were able to follow the contours of the shell. This was more of a trial
and error process rather than a planned process. Any strip that required more than adhesion
between the epoxy and the foam was tacked into the Styrofoam using nails. After a section was
completed, the outside was stretch wrapped to hold all of the foam pieces in place. After the
stretch wrap went on, it was important to remove every single nail; otherwise the fiberglass
would stick to the mold because of the nails rather than the epoxy. A picture of the top half is
shown during the foam laying process in figure 21.
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Figure 21: Foam layer of the shell
In the back of figure 21, the stretch wrapping is visible. In the front, the foam laying process is
visible. The aluminum blocks are there to keep the shrink wrap underneath from lifting up and
deforming the fiberglass. After the foam layer was complete, it was necessary to let the layer
cure before removing the stretch wrap. We waited about 15 hours for this, and then removed the
stretch wrap and any remaining nails. Then, the final layer of fiberglass was laid down in a
similar fashion to the first layer. The top half of the shell (after the final fiberglass layer) is
shown in figure 22.
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Figure 22: Shell after final layer of fiberglass
After this final layer of fiberglass was laid and cured, the excess was trimmed off the bottom.
Then, a flat head screwdriver was used to slightly pry the edges away from the mold. Handles
were melted into the mold itself so that we could get a better grip while trying to remove the
Styrofoam. After about 15 minutes of rocking the mold and working it, the shell slid right out.
After the shell came out, it was sent to the Calvin Physical Plant to be painted. After painting, a
strip of aluminum was riveted along the bottom of the top half of the shell. This provided a way
to keep the top half from sliding off of the bottom during operation. The final shell (without
plexiglass in the windows) is shown in figure 23.
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Figure 23: Final shell shape
1.6.2 Frame Manufacture
As presented in section 1.4.3, aluminum 6061-T6 was the choice of material for the frame. This
is a particularly difficult material to weld, so a lot of time was taken to learn how to TIG weld.
TIG welding is an arc welding process where the heat is controlled by a foot pedal and the filler
material is fed in by hand. It takes a lot of practice to master, and Andrew D’Agostino became a
proficient welder. This made him our group welder, so when pieces needed to be welded,
Andrew was the guy who did it.
Our material was ordered in 2 sizes: 1”x1.5” tubing and 1”sq tubing. Both tube sizes were 1/8”
wall thickness. Then, .75”square L-channel was used for the supports in the rear. Most of the
frame was welded; however bending was taken advantage of wherever it was possible. The roll
bar was bent using a tube bender that was fabricated by Phil Jasperse. This proved to be a
difficult task, as the aluminum was not flexible enough to make the bends without cracking. In
order to combat this problem, the material was heated in the bend areas with an acetylene torch,
which brought some flexibility to the material. Once the roll bar was completed, the material
was placed in an oven overnight to regain the original strength properties.
The rear L-channels were also bent, after cutting a v-shaped area out of the bend area. This
allowed the material left in that section to fold in nicely, and the resulting crack in the channel
could be welded closed. All other pieces of the frame were welded. An image showing the final
frame is shown in figure 24.
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Figure 24: Final frame after fabrication (front)
Figure 25: Final frame after fabrication (back)
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1.7 Conclusion
After completing this project, there are quite a few things we have learned. There is also quite a
bit that we would do differently if there was a next time. The main things we learned were:




Budget your time well, and stick to any schedule you make at the beginning.
Everything takes longer than you expect it to.
There will be problems, so budget extra time at the end of the project.
Be looking to save money at any time, because you might need it later.
We believe that this vehicle will perform competitively, and we are excited to see how it matches
up against the 27 other teams involved in the competition. We have enjoyed the design and
manufacturing processes, and we hope that a team in 2013-2014 will try to take our design and
make their own improvements to it.
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Works Cited
Riley, William F., Leroy D. Sturges, and Don H. Morris. Mechanics of Materials. Hoboken, NJ:
John Wiley, 2007. Print.
"ASM Material Data Sheet." ASM Material Data Sheet. Aerospace Specification Metals, n.d.
Web. 18 Nov. 2012. <http://asm.matweb.com/search/SpecificMaterial.asp?bassnum=MQ304A>.
Çengel, Yunus A., and Michael A. Boles. Thermodynamics: An Engineering Approach. New
York: McGraw-Hill, 2011. Print.
"2013 SAE SUPERMILEAGE® RULES." SAE International. Web. 17 Sep. 2012.
<http://www.sae.org/domains/students/competitions/supermileage/>.
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Appendix A: Frame Calculations
g=32.2 [ft/s^2]
"Steel"
sigma_y=135 [ksi]
rho=.283 [lbm/in^3]
E=29000 [ksi]
"Aluminum 6061-T6"
{sigma_y=40 [ksi]
rho=.098 [lbm/in^3]
E=10000 [ksi]}
"304 Stainless Steel http://asm.matweb.com/search/SpecificMaterial.asp?bassnum=MQ304A"
{sigma_y=31.2 [ksi]
rho=.289 [lbm/in^3]
E=12500 [ksi]}
"Analysis of 2 main support beams"
L_beam=5.75 [ft]
L_tot_rollbar=55 [in]
L_side_beam=1 [in]
t_beam=.0625
L_inside_beam=L_side_beam-2*t_beam
A_xs=L_side_beam^2-L_inside_beam^2
V_rollbar=L_tot_rollbar*A_xs
m_rollbar=V_rollbar*rho
m_engine= 30 [lbm]
m_shell= 25 [lbm]
m_driver=140 [lbm]
F_rollbar=260 [lbf]
F_engine=m_engine*g*convert(lbm-ft/s^2,lbf)
F_shell=m_shell*g*convert(lbm-ft/s^2,lbf)
F_driver=m_driver*g*convert(lbm-ft/s^2,lbf)
x_shell_1=.5 [ft]
x_R1=0 [ft]
x_driver= 2.5 [ft]
x_rollbar=3.6 [ft]
x_engine=4 [ft]
x_shell_2= 5.5 [ft]
x_R2= 5.75 [ft]
"Sum of the forces equal 0"
R_1+R_2-.5*(F_shell+F_driver+F_engine+F_rollbar)=0
R_1+R_2-F_apparent=0
"Sum of the moments equal 0"
.5*((F_shell/2)*(x_shell_1-x_R1)+F_driver*(x_driver-x_R1)+F_rollbar*(x_rollbarx_R1)+F_engine*(x_engine-x_R1)+(F_shell/2)*(x_shell_2-x_R1))-R_2*(x_R2-x_R1)=0
R_2*(x_R2-x_R1)-F_apparent*(x_apparent-x_R1)=0
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"Maximum Deflection: Treated as a single equivalent force"
base=1 [in]
h=1.5 [in]
I_beam=(base*h^3)/12-((base-2*t_beam)*(h-2*t_beam)^3)/12
a=x_apparent*convert(ft,in)
b=(L_beam-x_apparent)*convert(ft,in)
L=L_beam*convert(ft,in)
delta_max=(-F_apparent*b*(L^2-b^2)^(3/2))/(9*3^(.5)*L*E*convert(ksi,psi)*I_beam) "Pg 702 Mechanics
of Materials"
M_max=R_1*X_apparent
c=h/2
sigma_max*convert(ksi,psi)=M_max*convert(ft,in)*c/I_beam
{sigma_max*2=sigma_y}
W_beam= A_xs*L_beam*convert(ft,in)*rho
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Appendix B: CFD Data
Frontal Area
Prototype
Force Drag (N)
[m^2]
2012
9.7146
0.287
2
8.06
0.241
3
9.367
0.262
4
6.62
0.258
5
6.615
0.245
7
6.37
0.23935
8
6.67
0.247
Drag C
Speed (m/s)
0.314182636
0.31042533
0.331847371
0.238164724
0.250612609
0.247027415
0.250650187
13.4
13.4
13.4
13.4
13.4
13.4
13.4