2 Building and Testing a
Gravity Cruiser Chassis
Build Knowledge
Introduction
What Students Do in This Activity
Students are organized into teams. Using only the given materials and no
direct instruction, teams build a gravity cruiser chassis that rolls freely.
Rationale
Students need to understand from the very beginning that design work
is a shared endeavor. This is important. Each team member takes on
specific tasks and follows what the others are doing, but each member
also has a chance to try all aspects of the work by rotating roles with
the other members of the team. Being able to work well in a team is an
important skill, not only for the classroom but also for both formal and
informal situations outside of school and in later life. The importance
of keeping detailed records of their experimentation is also emphasized
through the use of design logs.
Students are given no written assembly instructions for the chassis.
Their first engineering task is to create their own version of the vehicle
chassis through trial and error by looking at the demonstration material
(schematic and/or prototype) and experimenting with the given
materials. Design teams are given this time to focus on the chassis so
that they can address problems of friction and misalignment early in the
design process.
This activity is intended to build students’ design skills. The chassis has
limited options for design, but students should find that certain designs
work better than others. This activity will allow students to get their feet
wet in the design process.
Time
1–2 class periods
2 Building and Testing a Gravity Cruiser Chassis
49
Materials
for each student:
Design Team Roles and Badges, Reproducible Master 4
for each design team:
access to a test ramp
3-ring binder with paper to use as a design log
Gravity Cruiser Design Log Guidelines, Reproducible Master 5
Gravity Cruiser Chassis Test Data, Reproducible Master 6
scissors
shoe box, plastic bag, or other container for storing materials
a set of the gravity cruiser chassis materials (supplied in Gravity
Cruiser Materials Kit):
`` 1 corrugated pre-scored chassis platform sheet
`` 2 wooden dowels, 3/16-inch diameter, 20.3 centimeters
(8 inches) long
`` 4 wheels (students may select the size wheels they want)
`` plastic straws
`` binder clips
`` masking tape
for the teacher:
a working prototype of the gravity cruiser
large, flat piece of cardboard, fiberboard, or other stiff material
(at least 1 x 1 meter/3.3 x 3.3 feet) to use as a ramp
chart paper
colored markers
Preparation for the Activity
Refer to “Teaching the Gravity Cruiser Curriculum” in the Introduction
to A World in Motion, pages vii–xvi, for information about creating
and managing student design teams. Use Design Team Roles and
Badges, Reproducible Master 4, to make a sufficient number of badges
for students to wear to identify their roles. Make Consulting Engineer
badges for classroom visitors and yourself.
“Teaching the Gravity Cruiser Curriculum” also offers ideas on ways to
use student design logs. For you, the design log serves as a record of
students’ work and thinking; for students, it serves as a reference tool
of their own work. Decide how you want students to organize and use
the design logs during the challenge. The Gravity Cruiser Design Log
Guidelines, Reproducible Master 5, outline clear expectations for student
use and maintenance.
50
2 Building and Testing a Gravity Cruiser Chassis
In the During the Activity section, students will be testing the chassis
they have made. One of the problems they will likely face is friction in
the wheels. During the testing, you may choose to examine a chassis
that is not functioning well to introduce the idea of friction, or if you
are concerned that students might react negatively to problems in their
chassis being highlighted, you might want to build your own chassis
beforehand. If you choose this—to build a chassis beforehand—build a
chassis in which the axles are intentionally misaligned, and the wheels
rub against the sides of the chassis.
Test Ramp
Set up the test ramp for students to test how well the chassis platforms
roll. Choose a location with about 7 meters (23 feet) of clear space in
front of it. The ramp itself should be at least 1 meter by 1 meter (3.3
feet x 3.3 feet).
In order for teams’ data to be consistent, the test ramp must be stable
enough to stay at the same angle throughout multiple test runs.
Depending on the size of the ramp, an angle of 45–55 degrees will likely
work well. One way to keep the angle consistent is to prop the ramp
against a wall in the desired position and tape it in place. This method
has the additional advantage of keeping the ramp straight. A pile of
books underneath the ramp’s center will keep it from bowing.
You may wish to add a simple release mechanism to the ramp. You can
create the one shown in the illustrations above by taping a piece of
corrugated plastic near the top of the ramp. The corrugated plastic is
2 Building and Testing a Gravity Cruiser Chassis
51
much narrower than the gravity cruiser chassis, so the wheels do not
get caught. The release mechanism has a knife-cut parallel to the wall
that allows it to fold upward, as shown on the left. Using a release
mechanism helps students begin each trial run with the gravity vehicle
pointed straight ahead, and it also lets students start the vehicle without
any inadvertent pushes.
Measuring Travel Distance
You may want to measure and lay out equally spaced masking tape
markers (e.g., every 25 centimeters/9.8 inches) so that students can easily
tell how far their vehicle traveled. Also, a strip of masking tape running
down the middle of the ramp and aligned with a second strip running
the length of the testing space will help teams determine whether their
chassis is veering left or right as it travels.
Arrange for a place to test the cruisers. In many activities, students will
need access to a smooth (not carpeted), level surface, at least 2 meters
by 7 meters (6.6 feet by 23 feet), to use as a test track. In Activities 8,
9, and 10, students may need a wider test space so they can race their
cruisers against those of other teams, if so desired. A smooth wood or
tile floor is best. Teams should be able to present their final designs on
the same type of surface during design and testing. A quiet corridor or
the school gymnasium is an ideal test site. Alternatively, if the classroom
is large enough, desks can be moved in order to clear a test track.
Classroom Activity
Presenting the Activity
1. Review the discussion in the previous session about the letter
and what EarthToy Designs is requesting.
Explain to students that they will be working with a limited set of
materials because EarthToy Designs want to keep the kit simple
and inexpensive.
2. Show the class the materials they’ll be working with, and
make a list of the materials on a piece of chart paper or on the
board.
Formation and Responsibilities of Teams
3. Divide the class into teams of 3-4 students.
4. Distribute copies of Design Team Roles and Badges, Reproducible Master 4, and discuss the responsibilities of each role:
Project Engineer, Facilities Engineer, and Test Engineer. Explain
that you and any other classroom visitors/volunteers will be
Consulting Engineers.
Establish initial role assignments in each team.
52
2 Building and Testing a Gravity Cruiser Chassis
volunteer
opportunity:
The industry volunteer can
talk about the importance
of working as a team and
how teams are organized
in his or her workplace.
Let students know that their roles will be rotated from time to
time to ensure that all students get a chance to use a variety of
skills. Teams will be asked to report to the whole class at certain
points. Team members will need to share their ideas, especially
what they have learned from the performance tests they will be
doing, so that any team member can speak for the whole group.
All team members will be responsible for knowing what happened
during tests and what conclusions the team reached.
5. Give each team its own design log.
Remind students to add the copy of the letter from EarthToy
Designs to the log, as they may need to refer to it often during
the challenge. Ask students to use their logs to record information
about the process of putting their gravity cruiser together. Explain
that this information may be helpful to include in their final
presentation as advice for other people who will be assembling
a gravity cruiser for the first time. Although the responsibility
for recording this information falls to the Test Engineer, all team
members are expected to contribute their ideas.
6. Distribute the Gravity Cruiser Design Log Guidelines, Reproducible Master 5, to each design team.
Tell students that this sheet outlines what their design log should
contain at the end of the challenge and how it should look.
Explain that you will use this sheet to evaluate their design logs.
During the Activity
1. After the teams have been formed, tell students to take some
time to discuss with their teammates how they will proceed
and the design of the chassis they will assemble.
Encourage them to make rough sketches of possible designs. They
can also demonstrate their ideas using the materials.
2. While the teams are having this discussion, visit each team and
listen in on its discussion. You can use these observations as a
type of embedded assessment. Use the following questions to
structure your observations:
`` How are the members of the team working together?
`` Are they thinking creatively and carefully about their design?
Building the Chassis
3. Demonstrate again how your prototype gravity cruiser operates.
4. Ask students what the word chassis means. Point out the
chassis platform on either the schematic or the prototype.
5. Elicit ideas from the class about what characteristics make a
well-built gravity cruiser chassis.
Students should suggest the following:
`` The axles should spin freely.
`` The chassis should travel in a straight line.
2 Building and Testing a Gravity Cruiser Chassis
53
6. Give one set of gravity cruiser chassis materials to each team,
and challenge the teams to make their own rolling gravity
cruiser platform, using only the materials provided.
Allow the design teams to choose how they attach the axles to the
chassis platforms, as well as the diameter of the wheels.
7. Looking ahead, the platform must be strong enough to hold up
the lever arm and weights.
After students have a chassis that runs well, suggest that they add
weight to it and test its performance. Does the chassis sag, or are
the pennies supported easily?
Testing the Chassis
8. Distribute Gravity Cruiser Chassis Test Data, Reproducible
Master 6, to each team.
Tell the teams’ Test Engineers to use this data table to keep track
of their test results and to add the table to their design logs before the end of the session.
9. Once students have constructed a chassis, have them use the
test ramp to see how far the chassis rolls.
Most likely the chassis will not travel very far or in a straight line.
Encourage students to closely observe how their chassis moves
and to make adjustments to their model rather than completely
start over each time they carry out a test. Discuss with each team
how the team members can isolate problems they may have.
10.Use some of the following questions to help focus students on
finding ways to explore or improve the performance of their
toy vehicle:
`` How far does your chassis platform go?
Answers will vary. Some teams will find that their chassis
travel well and go 2.5-3 meters (8-10 feet) immediately.
Others will find that their chassis don’t travel very far
because they haven’t minimized friction and the wheels
rub, or the chassis don’t travel in a straight line.
`` Does it go the same distance every time? Why or why not?
Answers will vary. If their chassis do not perform
consistently, students may not have secured the parts
well; if so, they might be moving as the chassis is
returned to the ramp.
`` Does it go in a straight line? Why or why not?
Answers will vary. Many teams will find that their chassis
do not travel in straight lines. The most likely cause of
this is misalignment of the axles. Another cause might
be unequal amounts of friction, in which one wheel rubs
against the chassis.
54
2 Building and Testing a Gravity Cruiser Chassis
11.As teams are testing, choose a chassis that is not performing
well and gather the teams for a discussion. (Alternatively, you
could prepare a chassis ahead of time. See Preparation for the
Activity.) Demonstrate the chassis’ performance by launching it
on the test ramp.
12.As a class, discuss the chassis’ performance. What happened
when the chassis was launched?
The chassis will likely travel in an arc and/or not go very far. Ask
students what they think is causing the problem. Some students
may suggest that friction is the issue.
13.Ask students to define the word friction and explain why it’s a
good idea to reduce friction in their gravity cruiser chassis.
The following questions should help to focus the discussion:
`` Where is there friction in the chassis design?
Students should suggest that the friction occurs between
the wheels and the body of the chassis. Some students
may recognize that there is friction between the axles
and the axle housings as well.
`` How can they tell?
Friction can be located by turning the wheels by hand
and observing their performance. Students may also be
able to identify where wheels are rubbing by observing
the direction in which the chassis turns (friction is likely
occurring in the wheels that are on the inside of the arc).
`` How could you reduce that friction?
Students should understand that it is necessary to move
the wheels away from the body of the chassis to reduce
friction.
What to Look For
Friction is a force between all moving objects that tends to resist motion and dissipate energy. Friction
exists between moving parts, such as the axles and the straws, and the wheels and the chassis platform. Friction also exists at the points of contact between the gravity cruiser’s wheels and the floor.
Some friction is necessary for the gravity cruiser to move. Imagine the cruiser trying to move forward
on ice, for example: The wheels would simply spin in place. Too much friction, however, whether it’s
between the vehicle and the ground or between moving parts of the vehicle, will use up the available
energy, and the vehicle will move slowly or not at all.
Below are some common chassis construction problems and possible solutions:
The axles are not parallel, causing the vehicle to veer to the side. If the chassis is using straws
to hold the axles, students should remount the straws on the bottom of the chassis or reposition the
binder clips until the axles are parallel to each other and aligned with the chassis platform.
The wheels rub against the side of the vehicle, causing the vehicle to stop prematurely. If
this happens, students should adjust their wheel mount system in some way to stop or minimize
this problem (e.g., they might use straws or clips as spacers between the wheels and the chassis).
(continued on next page)
2 Building and Testing a Gravity Cruiser Chassis
55
(continued from previous page)
Something—a crimped straw, a bent paper clip, a tight binder clip—presses against the
axle, slowing the axle’s rotation. Students should check that their method of attaching the
axle to the vehicle body is not impeding the axle’s rotation.
Axle misalignment and lots of friction are two common problems that teams must overcome in order
to get their gravity cruisers functioning well. By focusing on the chassis only, this activity helps students
isolate these two problems and (hopefully) solve them before adding the more complicated lever arm
mechanism.
Teacher
Tip:
How students go about
isolating problems and
solving them is a chance
for an embedded
assessment. When
observing teams, the
following questions may
help pinpoint issues that
a team is having:
• Do the team members observe carefully
the movement of the
model?
• Do they attempt to
deal with all the problems at once or one at
a time?
• Are they persistent in
dealing with the different problems?
56
14.After discussing friction, have teams return to their chassis testing. If their chassis is only moving a few feet before it
stops, adjustments need to be made. Students should try to get
the chassis traveling at least 3.1 meters (10 feet).
Encourage teams to look for and fix sources of friction. Note how
effectively student teams are working together.
15.Ask students if they have ever heard the term
troubleshooting.
Explain that they are now troubleshooting their chassis platforms—improving their chassis’ performance by fixing problems
and analyzing the design and construction factors that affect
performance.
Sharing and Interpreting
16.Have each team demonstrate its gravity cruiser chassis before
the whole class by running it down the ramp and measuring its
travel distance.
The whole class should observe carefully how each chassis moves.
If a chassis doesn’t travel straight or only rolls a short distance,
ask students to make suggestions on changes that may improve
performance.
Those groups having problems should makes notes in their logs
about what changes they would like to make in the next session.
17.Keep track of the travel distances achieved on chart paper. Have
teams summarize what worked for their chassis and what didn’t.
Record these comments also on chart paper. Also have teams explain any problems they had building their chassis.
18.Discuss how the gravity cruiser chassis’ performed.
The following questions might help to prompt students to share
their findings:
`` What did you notice about the performance of your gravity
cruiser chassis?
`` How far did your cruiser travel?
`` How straight did it travel?
`` Did it go fast or slow or a steady speed?
2 Building and Testing a Gravity Cruiser Chassis
Teacher
Tip:
Some teams will reduce
friction in their chassis
and conduct the ramp
trials quickly. For those
teams that finish early,
have them conduct trials
with weight attached to
the chassis. How does the
additional weight affect
the chassis travel
distance? What does this
suggest about how much
additional weight they can
add to the chassis as they
build their lever arm
mechanism and support
tower?
`` How consistently did your gravity cruiser chassis perform?
Did it always go the same distance?
Once all teams have demonstrated their chassis, ask the class
to consider these questions:
`` What would you change to make your chassis roll farther or
faster? (Giving it a push at the top of the test ramp doesn’t
count!)
Students should suggest that they would try to look for
and minimize sources of friction. Some students might
suggest that adding weight to their chassis would make
it roll farther.
`` Based on all of the teams’ travel distance results, what do
you think is the best method for attaching axles to the
chassis? Why?
Students are likely to find that the best method for
attaching the axles to the chassis is by using straws as
axle housings. However, that method might not be the
clear winner without any weight attached to the chassis.
Whatever students answer, it is important that they back
up their assertions with evidence they have gathered
during testing or by watching the demonstrations.
You will return to many of these questions in later activities, so you
need not cover them exhaustively at this point or have students
come up with the “right” answers. However, these questions will
help students reflect on the work they have just completed.
Background information
Given the choice of materials available to the students, there are several ways that axles can be attached to the plastic cardboard. Below are examples of possible arrangements:
Binder
clip
Paper
clip
Hole
Straw
Each of these designs will allow a chassis to travel in a straight line and for a good distance. However,
when teams reach the point where they are adding weight in the form of a vial of pennies to the lever
arm, they may run into problems. For instance, binder clips may start to slip from the plastic cardboard, particularly when lots of pennies are added to the lever arm.
Some of the problems with different kinds of axle housings won’t become apparent until students start
testing the model with weight on the lever arm. It is reasonable to allow them to move forward with
their current designs, although it might be helpful to give them a warning that their axle housing design may need to change at a later time.
2 Building and Testing a Gravity Cruiser Chassis
57
19.If students have used different methods to attach their wheels
(straws vs. binder clips, etc.), discuss whether students think
the different designs affect performance.
Ask students, “Does there seem to be a difference in performance
between a chassis having a straw for an axle support compared to
binder clips, or a hole in the chassis?”
20.Reassure students that the whole class is working together to
create the best possible gravity cruiser designs.
If a team wants to change its axle attachments (or some other
chassis characteristic) after the whole-class discussion, the team
should feel free to do so. Acting on new information from shared
ideas and experimental data isn’t cheating.
21.Have teams store their gravity cruiser and materials in a shoe
box, plastic bag, or other container.
58
2 Building and Testing a Gravity Cruiser Chassis
Design Team Roles and Badges
1.Project Engineer: Responsible for helping members understand the team’s task, leading
team discussions, checking for safety at all times, and checking whether the team’s task
is complete.
2.Facilities Engineer: Responsible for collecting materials, directing model construction,
directing cleanup, and storing materials.
3.Test Engineer: Responsible for recording and organizing data in the team’s design log.
R
R
Consulting
Engineer
Project
Engineer
R
R
Test
Engineer
Facilities
Engineer
Reproducible Master 4
2 Building and Testing a Gravity Cruiser Chassis
59
Team Members______________________________________________Date___________
Gravity Cruiser Challenge Design Log
Guidelines
Possible
Points
Design Log Criterion
Content The design log is a complete record of the team’s work.
The required content is present:
 Letter from EarthToy Designs (Reproducible Master 1)
 Gravity Cruiser Schematic, Gravity Cruiser Test Data, including
Chassis, Wheel, Lever Arm, and Axle Tests (Reproducible
Masters 6, 8, 11, and 12) and Gravity Cruiser Graphs
(Reproducible Masters 9 and 13)
30
 Gravity Cruiser Design Specifications (Reproducible Master 14)
 Gravity Cruiser Presentation Planner (Reproducible Master 16)
 Other: ____________________________________________
 Other: ____________________________________________
Organization
The work in the team’s design log is arranged in the correct sequence.
It is easy to find information.
15
Gravity Cruiser Data Tables (Reproducible Masters 6, 8, 11, and 12) and
Gravity Cruiser Graphs (Reproducible Masters 9 and 13)
The information is easy to read and understand.
The information makes sense to the reader.
The information is complete, including the dates, data, and observations.
The information seems reasonably accurate.
Gravity Cruiser Design Specifications (Reproducible Master 14)
The design sheets and drawings reflect students’ work and thinking.
The drawings and design sheets are easy to interpret and understand.
Total:
Reproducible Master 5
60
2 Building and Testing a Gravity Cruiser Chassis
30
25
100
Team Members______________________________________________Date___________
Gravity Cruiser Chassis Test Data
Additional Pennies (Cargo) Attached: 0
Trial
Distance
(m/ft.)
Comments
1
2
3
4
5
Additional Pennies (Cargo) Attached: _______
Trial
Distance
(m/ft.)
Comments
1
2
3
4
5
Procedure
1.Conduct the first set of trials with no cargo attached to the chassis platform. Place your gravity
cruiser chassis at the top of the ramp and, without pushing, let it roll.
2. Measure the distance from the base of the ramp to the point where your chassis stops.
3.Write comments about the trial in the space provided. Did the chassis travel in a straight line?
Do you notice any significant friction between the wheels and the platform?
4.Troubleshooting: Make any necessary changes to reduce friction and/or make the chassis travel
straight, then test the chassis again.
If you finish early:
1.When you are satisfied that friction has been reduced as much as possible, and your chassis
travels the same distance consistently, try attaching some weight to the platform. Place 50 pennies in a vial.
2.Using masking tape, attach the penny vial to the chassis platform. Perform several trials with this
same number of pennies attached as cargo.
Reproducible Master 6
2 Building and Testing a Gravity Cruiser Chassis
61