ME 224 – Final Project: SunRayce Car Suspension

Analysis

Alexander Ellis Ian Harrison Lars Moravy

Jonathon Walker

December 12, 2001 Prof. Espinosa, ME224 1:00

Table of Contents

Introduction 1

Experimental Setup and Procedure 2

Theory and Analysis 4

Data and Results 6

Conclusions 11

Appendix A: LabVIEW Programs i

Appendix B: Circuit Diagrams ii

Appendix C: Biographical Sketches iii

Appendix D: Mechanical Calculations v

- 1 -

Introduction

SunRayce is a nation wide competition that allows college teams to design, build

and race solar cars. The Northwestern Solar Car Team built a car that competed during

the summer of 2001. Currently the team is preparing to build the second-generation car,

improving on previous efforts. After benchmarking other teams, Northwestern

determined that the key strategy to producing a more successful car is to significantly

reduce the car’s weight. The team asked our group to assist them by collecting data on

the forces of the suspension. With this information, a future design can be optimized for

lighter weight.

As the car drives, various forces act on the wheels of the car. These loads are

created by bumps or other imperfections in the road and by acceleration of the car. The

former primarily produces forces that are normal to the road surface, and the latter

produces forces that are primarily tangential to the road surface. Since the tangential

forces are always produced at the tire contact patch, their magnitude is limited by the

following:

Fnormal = µtire * Ftangential

Knowing the normal forces on the wheel will tell us the maximum tangential forces on

the wheel. Therefore, the magnitude of the normal forces on the wheel is of primary

interest to the solar car team.

The purpose of this experiment to to determine the magnitude and frequency of

forces acting on the front suspension of the solar car. To do our experiment, we utilized

each of the tools learned in ME 224 form LabVIEW programming to circuit setup. Our

experiment will improve the SunRayce vehicle and hopefully contribute to a strong

Northwestern finish at this year's competition.

- 2 -

Experimental Setup and Procedure Experimental Setup:

We have set out to measure the displacement of the shocks and axial strain for the

Northwestern N’Ergy solar car vehicle under a variety of driving conditions. We have

written a LabVIEW program that takes data from a potentiometer and strain gauges

placed strategically on the vehicle and converts the changes in voltage to give us real-

time data of the displacement of the shock system.

The current suspension design is shown to

the right. This drawing shows that the normal

forces on the tire are transmitted through the

wheel to a push rod, which turns a bell crank that

pushes or pulls on a spring and damper. The

easiest way for our group to measure the normal

forces on the wheel is to measure the strain on the

push rod.

In order to measure the strain on the shock

system, a Wheatstone bridge of four strain gages

was used. This was placed on the end of the push

rod, as shown below. In order to attach the strain

gages, an M-bond kit was used. M-bonding is the standard for attaching strain gages to

materials. The voltage across the bridge of strain gages was then wired to the DAQ of the

Solar Car, which sits near the driver, using shielded wire with one end grounded to

protect from the electrical interference the car might produce. The DAQ was then

attached to a laptop computer and the LabVIEW program written for data acquisition and

analysis.

The next step in setting up this experiment is attaching

the potentiometer to measure the travel of the shock. At first,

a linear potentiometer was considered, but was deemed too

excessive. Instead, measuring the angle change was chosen as

the best option. A 5 kΩ rotational potentiometer was attached

to the fixed pivot point of the shock and a bracket was

- 3 -

attached to a bolt on the shock attachment fixture. A

picture is seen to the right. Taking the resistance at

several known displacements, a calibration was

found for Ohms / inch. This calibration is 224.6 Ω /

inch. The voltage across the potentiometer was wired

to the DAQ using the shielded wire with one end

grounded, again to reduce interference. Using the

calibration factor, we were able to measure the displacement of the shock very accurately

through the LabVIEW program.

Procedure: SunRayce requested us to find the strain on the shock under various conditions so that

they might use the data to better the car's suspension design. With this in mind we ran

tests under various conditions. The testing conditions were chosen to closely model a

variety of situations that the car might encounter during competition. Since our setup

was complete to the point where the only thing we needed to do was run the LabVIEW

program and drive the car, the program was run while the Solar Car was driving in the

following scenarios:

1. Straight down a smooth road

2. Turning left on a smooth road

3. Turning right on a smooth road

4. Straight down a bumpy road

5. Running over a pipe .5'' diameter

In each of these situations, care was taken that car speed, weather conditions and other

factors were kept as constant as possible. The data from these seven situations was saved

into 5 different files so that the data may later be analyzed.

- 4 -

Theory and Analysis Potentiometer Circuit:

The potentiometer resistance changes linearly as the knob is turned. We can therefore

relate the travel of the shock system to a voltage and use a LabVIEW program to analyze

our results in real time. We will use a second resistor (R1) in series with our

potentiometer as is pictured in the diagram below. Then, the equation for the resistance

in our potentiometer as a function of its voltage is as follows:

Knowing that …

I = V / R Eq. 1

Then, for the circuit to the left …

I = V / (R1 + R2) Eq. 2

Using Eq. 1 for the voltage across R2 …

V2 = I * R2 Eq. 3

Subbing Eq. 2 into Eq. 3 …

V2 = (V * R2) / (R1+ R2)

Solving for R2 …

R2 = (R1 * V2) / (V - V2) Eq. 4

Using Eq. 4, we can convert it with the calibration factor (224.6 Ω / inch) to inches of

travel.

Strain Gage Circuit:

This circuit involves an op-amp

since the change in voltage on

the strain gage is so minor that it

is very hard to measure. We will

use an amplification factor of

about 100 to get a range of a ½

Volt or so for the DAQ input.

- 5 -

This means that the two resistor values R2 and R3 will have to be related by the 100 = R2 /

R3 . The easiest way to do this is to make R2 = 100 kΩ and R3 = 1 kΩ. Converting the

voltage the DAQ reads to actual strain gage voltage using this amplification factor and

then the strain can be calculated using:

ε = (4*Vo) / (Vs* Sg) Eq. 5

Where Vo = Voltage of strain gauge = Function of strain

Vs = Volatge source = 12 V

Sg = Gauge Factor = 2.08

ε = Strain = Variable

- 6 -

Data and Results

(Note different scale for each graph)

1. Smooth Road

The strain on the rod while driving on a smooth road varies + and - .004 um/m from

about .08 um/m. We therefore conclude that the resting strain is -.08 um/m. The shock

worked well as it only let the bar displace about 1/20 of an inch each way. At the same

time, the wheel itself traveled .05” up and .1” down. These values are to be expected

since the road was relatively flat.

Smooth Road

-0.086

-0.084

-0.082

-0.08

-0.078

-0.076

-0.074

Time (sec)

Str

ain

(u

m/m

)

Smooth Road

-0.15

-0.1

-0.05

0

0.05

0.1

Time (sec)

Dis

tan

ce (

in)

- 7 -

2. Left Turn

As the car turned left, the strain decreased on the bar. The left strut also lengthened.

These results are due to the car rolling slightly as it turns. This shifts weight from the

wheel on the inside of the turn to the wheel on the outside. It is also important to note

that the wheel displacement in this situation is significantly larger than the displacement

while driving straight on a smooth road.

Left Turn

-0.09-0.08-0.07-0.06-0.05-0.04-0.03-0.02-0.01

0

Time (sec)

Str

ain

(u

m/m

)

Left Turn

-0.2

0

0.2

0.4

0.6

0.8

1

Time (sec)

Dis

tanc

e (in

)

- 8 -

3. Right Turn

The right turn is almost exactly the opposite of the left turn, which makes sense.

Right Turn

-0.16

-0.14

-0.12

-0.1

-0.08

-0.06

-0.04

-0.02

0

Time (sec)

Str

ain

(u

m/m

)

Right Turn

-1

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

Time (sec)

Dis

tan

ce (

in)

- 9 -

4. Bumpy Road

The graphs still oscillate about the restings values as they did on the smooth road.

However, they bounce sharper and with greater magnitude.

Bumpy Road

-0.12

-0.1

-0.08

-0.06

-0.04

-0.02

0

Time (sec)

Str

ain

(u

m/m

)

Bumpy Road

-0.5-0.4-0.3-0.2-0.1

00.10.20.30.4

Time (sec)

Dis

tan

ce (

in)

- 10 -

5. Running Over a Pole

Running Over Pole

-0.14

-0.12

-0.1

-0.08

-0.06

-0.04

-0.02

0

Time (sec)

Str

ain

(u

m/m

)

c

Running Over Pole

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

Time (sec)

Dis

tanc

e (in

)

As you can see here, the car behaved as it did on a smooth road, but then there is a spike

in compressive strain that we can only assume is when we hit the pole. The strain dove

from its at rest value of -.08 to -.12 and then went back up to about -.05. This is about

what is expected. The travel also behaved as it did on a smooth road but then spiked to

about -.6 in and then oscillated before returning to normal.

Bar impact

Bar impact

- 11 -

Conclusions The objective of this laboratory was to determine the magnitude and frequency of

the loads impacting the front suspension on the Northwestern University solar car so that future suspension designs can be optimized.

The car was driven in several situations that were meant to simulate typical racing conditions:

n flat road surface n bumpy road surface n turning n road hazard

Surprisingly, it was found that the highest stress occured when the car went through a turn. However, this information should be used cautiously as several sources of error may have produced this result. It is very likely that the bumpy road or the road hazard actually caused the greatest strain in the push-rod, but either LabView or the strain guages did not have the capacity to measure such a brief event. In contrast, turning occured over a comparatively longer amount of time, giving the sensors and PC more time to respond. Data analysis revealed that the solar car was of robust design. Several calculations were conducted to determine the fatigue, yield and impact limits (Appendix D). The existing design exceeded all requirements with some safety factors of up to 60 times the expected loads. As might be expected, impact loading had the lowest safety factor. Future designs should aim to push these saftey factors to much lower levels as excess weight is the greatest consideration. Other material choices such as aluminum or magnesium alloys could provide the neccessary strength without sacrificing weight.

The most important conclusion from this experiment is that further testing of the solar car is necessary. There are so many varibles that could affect the loads on the wheels, and our group just touched on a few of them. For example, the team should strongly consider running the tests in this laboratory for various vehicle speeds as well as a more detailed test on the effect of turning radius.

Appendix A: LabVIEW Programs

- i -

This is the LABView screen that gave us a real-time view of the data as we drove and saved it to an Excell spreadsheet.

Appendix B: Circuit Diagrams

- ii -

Potentiometer

R1 = 5100 Ohms V = 12 Volts R2 = 0 to 3400 Ohms

Strain Gauges R1 = Vary linearly with strain R2 = 1000 Ohms R3 = 100000 Ohms V = 12 Volts Voffset = 0

Appendix C: Biographical Sketches

- iii -

Ian Harrison

Height: 6’9”

Weight: 95 lbs

Major: Mechanical Engineering

Hometown: Bumbleville, MO

Hobbies: Building SunRayce cars, testing

Sunrayce cars, thinking about SunRayce

cars, drawing SunRayce cars in his notebook,

needlepoint.

Fav. TV Show: Cops

Quotation: “Never put off until tomorrow what you can put off even longer.”

Bio: Ian is a senior who participates in many school-related engineering activities.

He helped build and design the SunRayce car, competed in the robotics design

competition, and held office in his fraternity. Ian interned for General Electric

Power Systems.

Alexander “Sasha” Ellis

Rank: Second Luitenant upon graduation

Major: Mechanical Engineering

Fav. Show: “History’s Greatest Military Blunders”

Hometown: Evanston, IL

Hobbies: Flying, bossing Ian around, pull-ups,

working out

Quotation: “I can’t believe the government is going to

let me fly 40 million dollar jets.”

Bio: Sasha, son of Northwestern Professor Donald Ellis, is a training to be a

pilot in the Marines. He participates in NU design competition and will be

a vital member of the US armed forces when he graduates this Spring.

Sasha’s life long goal is to be a jet pilot and his skills as a mechanical

engineer will no doubt help him in the future.

Appendix C: Biographical Sketches

- iv -

Lars Moravy

Ethnic Background: Swedish

Major: Mechanical and Manufacturing

Engineering

Year: Junior

Hometown: Arlington Heights, IL

Hobbies: Fixing things, buildings things,

making things, hanging out.

Quotation: “Da Bears”

Bio: Lars’ resume reads like a manual of how to become an engineer. He is co-

president of the Society for Automotive Engineers, his robot team took third place

at last years competition, is a member of ASME, and is in charge of his

fraternity’s house management and repairs. Lars plays tennis and likes cars a lot.

His brother also attended Northwestern.

Jonathan Walker

Nicknames: J-Walker, Walky-Talky, J Dubs,

Jonny Walker

Year: Junior

Hometown: Boulder, CO

Hobbies: Chess, studying, computer games, soccer

Favorite Band: Pearl Jam

Quotation: “What’s the deal with water? I

mean, it doesn’t taste like anything, so why

are so many people drinking it?”

Bio: Jon grew up in Colorado and loves the outdoors. He Participated in NU design

competition and is studying to join the chess club. As vice-president of his

fraternity, Jon must manage a large group of people. He is currently a Co-Op

student working for Moog in East Aurora, NY. Jon wants to be an astronaut

when he grows up.

Appendix D: Mechanical Calculations

- v -

FATIGUE ANALYSIS:

Endurance limit:

Se’ = 0.45 Su

Se’ = 363 Mpa

Modified endurance limit:

Se = kf * ks * kr * kt * km * Se’

kf = ks = kt = km =1

kr = 0.9 for 90% survivability

Se = 0.9 Se’ = 327 MPa

[Se = 327 Mpa] > [σmax = 1.01E7]

ð infinite life without fatigue failure

FATIGUE SAFETY FACTOR:

σa = max. expected amplitude of stress on the push-rod

σm = mean expected stress on the push-rod

kf * σa / Se + σm / Sut = 1 / ns

7.83ε-3 + 9.29Ε-3 = 1 / ns

0.0171 = 1 / ns

ns = 58 <= too high!

YIELD ANALYSIS:

σmax = 1.01ε7 Pa

σy = 600Ε6 = 6Ε8 Pa for 4140 steel

[σy = 6Ε8 Pa] > [σmax = 1.01Ε7 Pa]

ð will not yield

YIELD SAFTEY FACTOR:

ns = 6Ε8 / 1.01Ε7 = 60

ð too high

Appendix D: Mechanical Calculations

- vi -

IMPACT LOADING:

Impact Factor, Im = Pmax / Pavg = 1.69 E3 N / 5.07 E2 N = 3.333

Impact stress, σi = Pmax / Area = 1.69 E3 N / 1.576 E-4 m2 = 1.07 E8 Pa IMPACT LOAD SAFETY FACTOR:

Yield Stress, Sy = 8.07 E8 Pa

ns = Sy / σi = 8.07 E8 Pa / 1.07 E8 Pa ~ 8